\input texinfo INTERNALS INTERNALS of the GNU compiler. Copyright (C) 1988, 1989, 1992 Free Software Founda- tion, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the section entitled ``GNU Gen- eral Public License'' is included exactly as in the origi- nal, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute transla- tions of this manual into another language, under the above conditions for modified versions, except that the section entitled ``GNU General Public License'' and this permission notice may be included in translations approved by the Free Software Foundation instead of in the original English. INTERNALS Using and Porting GNU CC INTERNALS Using GNU CC Richard M. Stallman last updated 15 February 1992 for version 2.0 (preliminary draft, which will change) Copyright (C) 1988, 1989, 1992 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the section entitled ``GNU Gen- eral Public License'' is included exactly as in the origi- nal, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute transla- tions of this manual into another language, under the above conditions for modified versions, except that the section entitled ``GNU General Public License'' and this permission notice may be included in translations approved by the Free Software Foundation instead of in the original English. 2 Using GNU CC INTERNALS This manual documents how to run and install the GNU C compiler, as well as its new features and incompa- tibilities, and how to report bugs. It corresponds to GNU CC version 2.0. INTERNALS INTERNALS GNU GENERAL PUBLIC LICENSE Version 2, June 1991 Copyright (C) 1989, 1991 Free Software Foundation, Inc. 675 Mass Ave, Cambridge, MA 02139, USA Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed. Preamble The licenses for most software are designed to take away your freedom to share and change it. By contrast, the GNU General Public License is intended to guarantee your freedom to share and change free software---to make sure the software is free for all its users. This General Public License applies to most of the Free Software Foundation's software and to any other program whose authors commit to using it. (Some other Free Software Foundation software is covered by the GNU Library General Public License instead.) You can apply it to your programs, too. When we speak of free software, we are referring to freedom, not price. Our General Public Licenses are designed to make sure that you have the freedom to distri- bute copies of free software (and charge for this service if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs; and that you know you can do these things. To protect your rights, we need to make restrictions that forbid anyone to deny you these rights or to ask you to surrender the rights. These restrictions translate to cer- tain responsibilities for you if you distribute copies of the software, or if you modify it. For example, if you distribute copies of such a pro- gram, whether gratis or for a fee, you must give the reci- pients all the rights that you have. You must make sure that they, too, receive or can get the source code. And you must show them these terms so they know their rights. Using GNU CC 3 We protect your rights with two steps: (1) copyright the software, and (2) offer you this license which gives you legal permission to copy, distribute and/or modify the software. Also, for each author's protection and ours, we want to make certain that everyone understands that there is no warranty for this free software. If the software is modi- fied by someone else and passed on, we want its recipients to know that what they have is not the original, so that any problems introduced by others will not reflect on the origi- nal authors' reputations. Finally, any free program is threatened constantly by software patents. We wish to avoid the danger that redis- tributors of a free program will individually obtain patent licenses, in effect making the program proprietary. To prevent this, we have made it clear that any patent must be licensed for everyone's free use or not licensed at all. The precise terms and conditions for copying, distri- bution and modification follow. TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION 1. This License applies to any program or other work which contains a notice placed by the copyright holder saying it may be distributed under the terms of this General Public License. The ``Pro- gram'', below, refers to any such program or work, and a ``work based on the Program'' means either the Program or any derivative work under copyright law: that is to say, a work containing the Program or a portion of it, either verbatim or with modif- ications and/or translated into another language. (Hereinafter, translation is included without lim- itation in the term ``modification''.) Each licensee is addressed as ``you''. Activities other than copying, distribution and modification are not covered by this License; they are outside its scope. The act of running the Program is not restricted, and the output from the Program is covered only if its contents constitute a work based on the Program (independent of having been made by running the Program). Whether that is true depends on what the Program does. 2. You may copy and distribute verbatim copies of the Program's source code as you receive it, in any medium, provided that you conspicuously and ap- propriately publish on each copy an appropriate 4 Using GNU CC copyright notice and disclaimer of warranty; keep intact all the notices that refer to this License and to the absence of any warranty; and give any other recipients of the Program a copy of this License along with the Program. You may charge a fee for the physical act of transferring a copy, and you may at your option offer warranty protection in exchange for a fee. 3. You may modify your copy or copies of the Program or any portion of it, thus forming a work based on the Program, and copy and distribute such modifi- cations or work under the terms of Section 1 above, provided that you also meet all of these conditions: 1. You must cause the modified files to carry prominent notices stating that you changed the files and the date of any change. 2. You must cause any work that you distribute or publish, that in whole or in part contains or is derived from the Program or any part thereof, to be licensed as a whole at no charge to all third parties under the terms of this License. 3. If the modified program normally reads com- mands interactively when run, you must cause it, when started running for such interactive use in the most ordinary way, to print or display an announcement including an ap- propriate copyright notice and a notice that there is no warranty (or else, saying that you provide a warranty) and that users may redistribute the program under these condi- tions, and telling the user how to view a copy of this License. (Exception: if the Program itself is interactive but does not normally print such an announcement, your work based on the Program is not required to print an announcement.) These requirements apply to the modified work as a whole. If identifiable sections of that work are not derived from the Program, and can be reason- ably considered independent and separate works in themselves, then this License, and its terms, do not apply to those sections when you distribute them as separate works. But when you distribute the same sections as part of a whole which is a Using GNU CC 5 work based on the Program, the distribution of the whole must be on the terms of this License, whose permissions for other licensees extend to the en- tire whole, and thus to each and every part re- gardless of who wrote it. Thus, it is not the intent of this section to claim rights or contest your rights to work writ- ten entirely by you; rather, the intent is to ex- ercise the right to control the distribution of derivative or collective works based on the Pro- gram. In addition, mere aggregation of another work not based on the Program with the Program (or with a work based on the Program) on a volume of a storage or distribution medium does not bring the other work under the scope of this License. 4. You may copy and distribute the Program (or a work based on it, under Section 2) in object code or executable form under the terms of Sections 1 and 2 above provided that you also do one of the fol- lowing: 1. Accompany it with the complete corresponding machine-readable source code, which must be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software interchange; or, 2. Accompany it with a written offer, valid for at least three years, to give any third par- ty, for a charge no more than your cost of physically performing source distribution, a complete machine-readable copy of the corresponding source code, to be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software inter- change; or, 3. Accompany it with the information you re- ceived as to the offer to distribute corresponding source code. (This alternative is allowed only for noncommercial distribu- tion and only if you received the program in object code or executable form with such an offer, in accord with Subsection b above.) The source code for a work means the preferred form of the work for making modifications to it. For an executable work, complete source code means 6 Using GNU CC all the source code for all modules it contains, plus any associated interface definition files, plus the scripts used to control compilation and installation of the executable. However, as a special exception, the source code distributed need not include anything that is normally distri- buted (in either source or binary form) with the major components (compiler, kernel, and so on) of the operating system on which the executable runs, unless that component itself accompanies the exe- cutable. If distribution of executable or object code is made by offering access to copy from a designated place, then offering equivalent access to copy the source code from the same place counts as distri- bution of the source code, even though third par- ties are not compelled to copy the source along with the object code. 5. You may not copy, modify, sublicense, or distri- bute the Program except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense or distribute the Program is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses ter- minated so long as such parties remain in full compliance. 6. You are not required to accept this License, since you have not signed it. However, nothing else grants you permission to modify or distribute the Program or its derivative works. These actions are prohibited by law if you do not accept this License. Therefore, by modifying or distributing the Program (or any work based on the Program), you indicate your acceptance of this License to do so, and all its terms and conditions for copying, distributing or modifying the Program or works based on it. 7. Each time you redistribute the Program (or any work based on the Program), the recipient automat- ically receives a license from the original licen- sor to copy, distribute or modify the Program sub- ject to these terms and conditions. You may not impose any further restrictions on the recipients' exercise of the rights granted herein. You are not responsible for enforcing compliance by third parties to this License. Using GNU CC 7 8. If, as a consequence of a court judgment or alle- gation of patent infringement or for any other reason (not limited to patent issues), conditions are imposed on you (whether by court order, agree- ment or otherwise) that contradict the conditions of this License, they do not excuse you from the conditions of this License. If you cannot distri- bute so as to satisfy simultaneously your obliga- tions under this License and any other pertinent obligations, then as a consequence you may not distribute the Program at all. For example, if a patent license would not permit royalty-free redistribution of the Program by all those who re- ceive copies directly or indirectly through you, then the only way you could satisfy both it and this License would be to refrain entirely from distribution of the Program. If any portion of this section is held invalid or unenforceable under any particular circumstance, the balance of the section is intended to apply and the section as a whole is intended to apply in other circumstances. It is not the purpose of this section to induce you to infringe any patents or other property right claims or to contest validity of any such claims; this section has the sole purpose of pro- tecting the integrity of the free software distri- bution system, which is implemented by public license practices. Many people have made generous contributions to the wide range of software dis- tributed through that system in reliance on con- sistent application of that system; it is up to the author/donor to decide if he or she is willing to distribute software through any other system and a licensee cannot impose that choice. This section is intended to make thoroughly clear what is believed to be a consequence of the rest of this License. 9. If the distribution and/or use of the Program is restricted in certain countries either by patents or by copyrighted interfaces, the original copy- right holder who places the Program under this License may add an explicit geographical distribu- tion limitation excluding those countries, so that distribution is permitted only in or among coun- tries not thus excluded. In such case, this License incorporates the limitation as if written in the body of this License. 8 Using GNU CC 10. The Free Software Foundation may publish revised and/or new versions of the General Public License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or con- cerns. Each version is given a distinguishing version number. If the Program specifies a version number of this License which applies to it and ``any later version'', you have the option of following the terms and conditions either of that version or of any later version published by the Free Software Foundation. If the Program does not specify a version number of this License, you may choose any version ever published by the Free Software Foundation. 11. If you wish to incorporate parts of the Program into other free programs whose distribution condi- tions are different, write to the author to ask for permission. For software which is copyrighted by the Free Software Foundation, write to the Free Software Foundation; we sometimes make exceptions for this. Our decision will be guided by the two goals of preserving the free status of all deriva- tives of our free software and of promoting the sharing and reuse of software generally. NO WARRANTY 12. BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EX- TENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM ``AS IS'' WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IM- PLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU. SHOULD THE PROGRAM PROVE DEFECTIVE, YOU AS- SUME THE COST OF ALL NECESSARY SERVICING, REPAIR OR CORRECTION. 13. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, IN- CIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING REN- Using GNU CC 9 DERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POS- SIBILITY OF SUCH DAMAGES. END OF TERMS AND CONDITIONS Appendix: How to Apply These Terms to Your New Programs If you develop a new program, and you want it to be of the greatest possible use to the public, the best way to achieve this is to make it free software which everyone can redistribute and change under these terms. To do so, attach the following notices to the pro- gram. It is safest to attach them to the start of each source file to most effectively convey the exclusion of war- ranty; and each file should have at least the ``copyright'' line and a pointer to where the full notice is found. one line to give the program's name and a brief idea of what it does. Copyright (C) 19yy name of author This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. Also add information on how to contact you by elec- tronic and paper mail. If the program is interactive, make it output a short notice like this when it starts in an interactive mode: Gnomovision version 69, Copyright (C) 19yy name of author Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type `show w'. This is free software, and you are welcome to redistribute it under certain conditions; type `show c' for details. 10 Using GNU CC The hypothetical commands `show w' and `show c' should show the appropriate parts of the General Public License. Of course, the commands you use may be called something other than `show w' and `show c'; they could even be mouse- clicks or menu items---whatever suits your program. You should also get your employer (if you work as a programmer) or your school, if any, to sign a ``copyright disclaimer'' for the program, if necessary. Here is a sam- ple; alter the names: Yoyodyne, Inc., hereby disclaims all copyright interest in the program `Gnomovision' (which makes passes at compilers) written by James Hacker. signature of Ty Coon, 1 April 1989 Ty Coon, President of Vice This General Public License does not permit incorporat- ing your program into proprietary programs. If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library. If this is what you want to do, use the GNU Library General Public License instead of this License. Contributors to GNU CC In addition to Richard Stallman, several people have written parts of GNU CC. o+ The idea of using RTL and some of the optimization ideas came from the U. of Arizona Portable Optim- izer, written by Jack Davidson and Christopher Fraser. See ``Register Allocation and Exhaustive Peephole Optimization'', Software Practice and Ex- perience 14 (9), Sept. 1984, 857-866. o+ Paul Rubin wrote most of the preprocessor. o+ Leonard Tower wrote parts of the parser, RTL gen- erator, and RTL definitions, and of the Vax machine description. o+ Ted Lemon wrote parts of the RTL reader and printer. o+ Jim Wilson implemented loop strength reduction and some other loop optimizations. Using GNU CC 11 o+ Nobuyuki Hikichi of Software Research Associates, Tokyo, contributed the support for the Sony NEWS machine. o+ Charles LaBrec contributed the support for the In- tegrated Solutions 68020 system. o+ Michael Tiemann of Cygnus Support wrote the front end for C++, as well as the support for inline functions and instruction scheduling. Also the descriptions of the National Semiconductor 32000 series cpu, the SPARC cpu and part of the Motorola 88000 cpu. o+ Jan Stein of the Chalmers Computer Society provid- ed support for Genix, as well as part of the 32000 machine description. o+ Randy Smith finished the Sun FPA support. o+ Robert Brown implemented the support for Encore 32000 systems. o+ David Kashtan of SRI adapted GNU CC to the Vomit- Making System (VMS). o+ Alex Crain provided changes for the 3b1. o+ Greg Satz and Chris Hanson assisted in making GNU CC work on HP-UX for the 9000 series 300. o+ William Schelter did most of the work on the Intel 80386 support. o+ Christopher Smith did the port for Convex machines. o+ Paul Petersen wrote the machine description for the Alliant FX/8. o+ Alain Lichnewsky ported GNU CC to the Mips cpu. o+ Devon Bowen, Dale Wiles and Kevin Zachmann ported GNU CC to the Tahoe. o+ Jonathan Stone wrote the machine description for the Pyramid computer. o+ Richard Kenner of New York University wrote the machine descriptions for the AMD 29000, the IBM RT PC, and the IBM RS/6000 as well as the support for instruction attributes. He also made changes to better support RISC processors including changes 12 Using GNU CC to common subexpression elimination, strength reduction, function calling sequence handling, and condition code support, in addition to generaliz- ing the code for frame pointer elimination. o+ Richard Kenner and Michael Tiemann jointly developed reorg.c, the delay slot scheduler. o+ Mike Meissner and Tom Wood of Data General fin- ished the port to the Motorola 88000. o+ Masanobu Yuhara of Fujitsu Laboratories implement- ed the machine description for the Tron architec- ture (specifically, the Gmicro). o+ NeXT, Inc. donated the front end that supports the Objective C language. o+ James van Artsdalen wrote the code that makes ef- ficient use of the Intel 80387 register stack. o+ Mike Meissner at the Open Software Foundation fin- ished the port to the MIPS cpu, including adding ECOFF debug support. o+ Ron Guilmette implemented the protoize and unpro- toize tools, the support for Dwarf symbolic debug- ging information, and much of the support for Sys- tem V Release 4. He has also worked heavily on the Intel 386 and 860 support. 1. Protect Your Freedom---Fight ``Look And Feel'' This section is a political message from the League for Programming Freedom to the users of GNU CC. It is included here as an expression of support for the League on the part of the Free Software Foundation. Apple, Lotus and Xerox are trying to create a new form of legal monopoly: a copyright on a class of user inter- faces. These monopolies would cause serious problems for users and developers of computer software and systems. Until a few years ago, the law seemed clear: no one could restrict others from using a user interface; program- mers were free to implement any interface they chose. Imi- tating interfaces, sometimes with changes, was standard practice in the computer field. The interfaces we know evolved gradually in this way; for example, the Macintosh user interface drew ideas from the Xerox interface, which in turn drew on work done at Stanford and SRI. 1-2-3 imitated Using GNU CC 13 VisiCalc, and dBase imitated a database program from JPL. Most computer companies, and nearly all computer users, were happy with this state of affairs. The companies that are suing say it does not offer ``enough incentive'' to develop their products, but they must have considered it ``enough'' when they made their decision to do so. It seems they are not satisfied with the opportunity to continue to compete in the marketplace---not even with a head start. If Xerox, Lotus, and Apple are permitted to make law through the courts, the precedent will hobble the software industry: o+ Gratuitous incompatibilities will burden users. Imagine if each car manufacturer had to arrange the pedals in a different order. o+ Software will become and remain more expensive. Users will be ``locked in'' to proprietary interfaces, for which there is no real competition. o+ Large companies have an unfair advantage wherever lawsuits become commonplace. Since they can easily afford to sue, they can intimidate small companies with threats even when they don't really have a case. o+ User interface improvements will come slower, since incremental evolution through creative imitation will no longer be permitted. o+ Even Apple, etc., will find it harder to make improvements if they can no longer adapt the good ideas that others introduce, for fear of weakening their own legal positions. Some users suggest that this stagnation may already have started. o+ If you use GNU software, you might find it of some concern that user interface copyright will make it hard for the Free Software Foundation to develop programs compatible with the interfaces that you already know. To protect our freedom from lawsuits like these, a group of programmers and users have formed a new grass-roots political organization, the League for Programming Freedom. The purpose of the League is to oppose new monopolistic practices such as user-interface copyright and software patents; it calls for a return to the legal policies of the 14 Using GNU CC recent past, in which these practices were not allowed. The League is not concerned with free software as an issue, and not affiliated with the Free Software Foundation. The League's membership rolls include John McCarthy, inventor of Lisp, Marvin Minsky, founder of the Artificial Intelligence lab, Guy L. Steele, Jr., author of well-known books on Lisp and C, as well as Richard Stallman, the developer of GNU CC. Please join and add your name to the list. Membership dues in the League are $42 per year for programmers, managers and professionals; $10.50 for stu- dents; $21 for others. The League needs both activist members and members who only pay their dues. To join, or for more information, phone (617) 492-0023 or write to: League for Programming Freedom 1 Kendall Square #143 P.O. Box 9171 Cambridge, MA 02139 You can also send electronic mail to league@prep.ai.mit.edu. Here are some suggestions from the League for things you can do to protect your freedom to write programs: o+ Don't buy from Xerox, Lotus or Apple. Buy from their competitors or from the defendants they are suing. o+ Don't develop software to work with the systems made by these companies. o+ Port your existing software to competing systems, so that you encourage users to switch. o+ Write letters to company presidents to let them know their conduct is unacceptable. o+ Tell your friends and colleagues about this issue and how it threatens to ruin the computer industry. o+ Above all, don't work for the look-and-feel plaintiffs, and don't accept contracts from them. o+ Write to Congress to explain the importance of this issue. Using GNU CC 15 House Subcommittee on Intellectual Property 2137 Rayburn Bldg Washington, DC 20515 Senate Subcommittee on Patents, Trademarks and Copyrights United States Senate Washington, DC 20510 (These committees have received lots of mail already; let's give them even more.) Express your opinion! You can make a difference. 2. GNU CC Command Options When you invoke GNU CC, it normally does preprocessing, compilation, assembly and linking. The ``overall options'' allow you to stop this process at an intermediate stage. For example, the `-c' option says not to run the linker. Then the output consists of object files output by the assembler. Other options are passed on to one stage of processing. Some options control the preprocessor and others the com- piler itself. Yet other options control the assembler and linker; most of these are not documented here, since you rarely need to use any of them. The GNU C compiler uses a command syntax much like the Unix C compiler. The gcc program accepts options and file names as operands. Multiple single-letter options may not be grouped: `-dr' is very different from `-d -r' You can mix options and other arguments. For the most part, the order you use doesn't matter; gcc reorders the command-line options so that the choices specified by option flags are applied to all input files. Order does matter when you use several options of the same kind; for example, if you specify `-L' more than once, the directories are searched in the order specified. Many options have long names starting with `-f' or with `-W'---for example, `-fforce-mem', `-fstrength-reduce', `- Wformat' and so on. Most of these have both positive and negative forms; the negative form of `-ffoo' would be `- fno-foo'. This manual documents only one of these two forms, whichever one is not the default. Here is a summary of all the options, grouped by type. Explanations are in the following sections. 16 Using GNU CC Overall Options See section Overall Options,,Options Controlling the Kind of Output. -c -S -E -o file -pipe -v -x language Language Options See section Dialect Options,,Options Controlling Dialect. -ansi -fbuiltin -fcond-mismatch -fno-asm -fsigned-bitfields -fsigned-char -funsigned-bitfields -funsigned-char -fwritable-strings -traditional -traditional-cpp -trigraphs Warning Options See section Warning Options,,Options to Request or Suppress Warnings. -fsyntax-only -pedantic -pedantic-errors -w -W -Wall -Waggregate-return -Wcast-align -Wcast-qual -Wcomment -Wconversion -Werror -Wformat -Wid-clash-len -Wimplicit -Wmissing-prototypes -Wno-parentheses -Wpointer-arith -Wreturn-type -Wshadow -Wstrict-prototypes -Wswitch -Wtraditional -Wtrigraphs -Wuninitialized -Wunused -Wwrite-strings -Wchar-subscripts Debugging Options See section Debugging Options,,Options for Debugging Your Program or GCC. -a -dletters -fpretend-float -g -ggdb -gdwarf -gstabs -gstabs+ -gcoff -p -pg -save-temps Optimization Options See section Optimize Options,,Options that Control Optimization. -fcaller-saves -fcse-follow-jumps -fdelayed-branch Using GNU CC 17 -fexpensive-optimizations -ffloat-store -fforce-addr -fforce-mem -finline -finline-functions -fkeep-inline-functions -fno-defer-pop -fno-function-cse -fomit-frame-pointer -frerun-cse-after-loop -fschedule-insns -fschedule-insns2 -fstrength-reduce -fthread-jumps -funroll-all-loops -funroll-loops -O -O2 Preprocessor Options See section Preprocessor Options,,Options Controlling the Preprocessor. -C -dD -dM -dN -Dmacro[=defn] -E -H -include file -imacros file -M -MD -MM -MMD -nostdinc -P -trigraphs -Umacro Linker Options See section Link Options,,Options for Linking. object-file-name -llibrary -nostdlib -static Directory Options See section Directory Options,,Options for Directory Search. -Bprefix -Idir -I- -Ldir Target Options See section Target Options,,Target Machine and Compiler Version. -b machine -V version Machine Dependent Options See section Submodel Options,,Hardware Models and Configurations. 18 Using GNU CC M680x0 Options -m68000 -m68020 -m68881 -mbitfield -mc68000 -mc68020 -mfpa -mnobitfield -mrtd -mshort -msoft-float VAX Options -mg -mgnu -munix SPARC Options -mfpu -mno-epilogue Convex Options -margcount -mc1 -mc2 -mnoargcount AMD29K Options -m29000 -m29050 -mbw -mdw -mkernel-registers -mlarge -mnbw -mnodw -msmall -mstack-check -muser-registers M88K Options -m88000 -m88100 -m88110 -mbig-pic -mcheck-zero-division -mhandle-large-shift -midentify-revision -mno-check-zero-division -mno-ocs-debug-info -mno-ocs-frame-position -mno-optimize-arg-area -mno-underscores -mocs-debug-info -mocs-frame-position -moptimize-arg-area -mshort-data-num -msvr3 -msvr4 -mtrap-large-shift -muse-div-instruction -mversion-03.00 -mwarn-passed-structs RS/6000 Options -mfp-in-toc -mno-fop-in-toc RT Options -mcall-lib-mul -mfp-arg-in-fpregs -mfp-arg-in-gregs -mfull-fp-blocks -mhc-struct-return -min-line-mul -mminimum-fp-blocks -mnohc-struct-return MIPS Options -mcpu=cpu type -mips2 -mips3 -mint64 -mlong64 -mlonglong128 -mmips-as -mgas -mrnames -mno-rnames -mgpopt -mno-gpopt -mstats -mno-stats -mmemcpy -mno-memcpy -mno-mips-tfile -mmips-tfile -msoft-float -mhard-float -mabicalls -mno-abicalls -mhalf-pic -mno-half-pic -G num Code Generation Options See section Code Gen Options,,Options for Code Generation Conventions. -fcall-saved-reg -fcall-used-reg -ffixed-reg -fno-common -fpcc-struct-return -fpic -fPIC -fshared-data -fshort-enums -fshort-double -fvolatile Using GNU CC 19 2.1. Options Controlling the Kind of Output Compilation can involve up to four stages: preprocess- ing, compilation proper, assembly and linking, always in that order. The first three stages apply to an individual source file, and end by producing an object file; linking combines all the object files (those newly compiled, and those specified as input) into an executable file. For any given input file, the file name suffix deter- mines what kind of compilation is done: file.c C source code which must be preprocessed. file.i C source code which should not be preprocessed. file.m Objective-C source code file.h C header file (not to be compiled or linked). file.cc file.cxx file.C C++ source code which must be preprocessed. file.s Assembler code. file.S Assembler code which must be preprocessed. other An object file to be fed straight into linking. Any file name with no recognized suffix is treated this way. You can specify the input language explicitly with the `-x' option: -x language Specify explicitly the language for the following input files (rather than choosing a default based on the file name suffix). This option applies to all following input files until the next `-x' 20 Using GNU CC option. Possible values of language are `c', `objective-c', `c-header', `c++', `cpp-output', `assembler', and `assembler-with-cpp'. -x none Turn off any specification of a language, so that subsequent files are handled according to their file name suffixes (as they are if `-x' has not been used at all). If you only want some of the stages of compilation, you can use `-x' (or filename suffixes) to tell gcc where to start, and one of the options `-c', `-S', or `-E' to say where gcc is to stop. Note that some combinations (for example, `-x cpp-output -E' instruct gcc to do nothing at all. -c Compile or assemble the source files, but do not link. The linking stage simply is not done. The ultimate output is in the form of an object file for each source file. By default, the object file name for a source file is made by replacing the suffix `.c', `.i', `.s', etc., with `.o'. Unrecognized input files, not requiring compilation or assembly, are ignored. -S Stop after the stage of compilation proper; do not assemble. The output is in the form of an assembler code file for each non-assembler input file specified. By default, the assembler file name for a source file is made by replacing the suffix `.c', `.i', etc., with `.s'. Input files that don't require compilation are ignored. -E Stop after the preprocessing stage; do not run the compiler proper. The output is in the form of preprocessed source code, which is sent to the standard output. Input files which don't require preprocessing are ignored. -o file Place output in file file. This applies regardless to whatever sort of output is being Using GNU CC 21 produced, whether it be an executable file, an object file, an assembler file or preprocessed C code. Since only one output file can be specified, it does not make sense to use `-o' when compiling more than one input file, unless you are producing an executable file as output. If `-o' is not specified, the default is to put an executable file in `a.out', the object file for `source.suffix' in `source.o', its assembler file in `source.s', and all preprocessed C source on standard output. -v Print (on standard error output) the commands executed to run the stages of compilation. Also print the version number of the compiler driver program and of the preprocessor and the compiler proper. -pipe Use pipes rather than temporary files for communication between the various stages of compilation. This fails to work on some systems where the assembler is unable to read from a pipe; but the GNU assembler has no trouble. 2.2. Options Controlling Dialect The following options control the dialect of C that the compiler accepts: -ansi Support all ANSI standard C programs. This turns off certain features of GNU C that are incompatible with ANSI C, such as the asm, inline and typeof keywords, and predefined macros such as unix and vax that identify the type of system you are using. It also enables the undesirable and rarely used ANSI trigraph feature, and disallows `$' as part of identifiers. The alternate keywords __asm__, __extension__, __inline__ and __typeof__ continue to work despite `-ansi'. You would not want to use them in an ANSI C program, of course, but it useful to put them in header files that might be included in compilations done with `-ansi'. Alternate predefined macros such as __unix__ and __vax__ are also available, with or without `-ansi'. 22 Using GNU CC The `-ansi' option does not cause non-ANSI programs to be rejected gratuitously. For that, `-pedantic' is required in addition to `-ansi'. See section Warning Options. The macro __STRICT_ANSI__ is predefined when the `-ansi' option is used. Some header files may notice this macro and refrain from declaring certain functions or defining certain macros that the ANSI standard doesn't call for; this is to avoid interfering with any programs that might use these names for other things. -fno-asm Do not recognize asm, inline or typeof as a keyword. These words may then be used as identifiers. You can use __asm__, __inline__ and __typeof__ instead. `-ansi' implies `-fno-asm'. -fno-builtin Don't recognize non-ANSI built-in functions. `- ansi' also has this effect. Currently, the only function affected is alloca. -trigraphs Support ANSI C trigraphs. You don't want to know about this brain-damage. The `-ansi' option implies `-trigraphs'. -traditional Attempt to support some aspects of traditional C compilers. Specifically: o+ All extern declarations take effect globally even if they are written inside of a function definition. This includes implicit declarations of functions. o+ The keywords typeof, inline, signed, const and volatile are not recognized. (You can still use the alternative keywords such as __typeof__, __inline__, and so on.) o+ Comparisons between pointers and integers are always allowed. o+ Integer types unsigned short and unsigned char promote to unsigned int. o+ Out-of-range floating point literals are not an error. Using GNU CC 23 o+ String ``constants'' are not necessarily constant; they are stored in writable space, and identical looking constants are allocated separately. (This is the same as the effect of `-fwritable-strings'.) o+ All automatic variables not declared register are preserved by longjmp. Ordinarily, GNU C follows ANSI C: automatic variables not declared volatile may be clobbered. o+ In the preprocessor, comments convert to nothing at all, rather than to a space. This allows traditional token concatenation. o+ In the preprocessor, macro arguments are recognized within string constants in a macro definition (and their values are stringified, though without additional quote marks, when they appear in such a context). The preprocessor always considers a string constant to end at a newline. o+ The predefined macro __STDC__ is not defined when you use `-traditional', but __GNUC__ is (since the GNU extensions which __GNUC__ indicates are not affected by `- traditional'). If you need to write header files that work differently depending on whether `-traditional' is in use, by testing both of these predefined macros you can distinguish four situations: GNU C, traditional GNU C, other ANSI C compilers, and other old C compilers. -traditional-cpp Attempt to support some aspects of traditional C preprocessors. This includes the last three items in the table immediately above, but none of the other effects of `-traditional'. -fcond-mismatch Allow conditional expressions with mismatched types in the second and third arguments. The value of such an expression is void. -funsigned-char Let the type char be unsigned, like unsigned char. Each kind of machine has a default for what char should be. It is either like unsigned char by default or like signed char by default. 24 Using GNU CC Ideally, a portable program should always use signed char or unsigned char when it depends on the signedness of an object. But many programs have been written to use plain char and expect it to be signed, or expect it to be unsigned, depending on the machines they were written for. This option, and its inverse, let you make such a program work with the opposite default. The type char is always a distinct type from each of signed char or unsigned char, even though its behavior is always just like one of those two. -fsigned-char Let the type char be signed, like signed char. Note that this is equivalent to `-fno-unsigned- char', which is the negative form of `-funsigned- char'. Likewise, `-fno-signed-char' is equivalent to `-funsigned-char'. -fsigned-bitfields -funsigned-bitfields -fno-signed-bitfields -fno-unsigned-bitfields These options control whether a bitfield is signed or unsigned, when the declaration does not use either signed or unsigned. By default, such a bitfield is signed, because this is consistent: the basic integer types such as int are signed types. However, when `-traditional' is used, bitfields are all unsigned no matter what. -fwritable-strings Store string constants in the writable data segment and don't uniquize them. This is for compatibility with old programs which assume they can write into string constants. `-traditional' also has this effect. Writing into string constants is a very bad idea; ``constants'' should be constant. 2.3. Options to Request or Suppress Warnings Warnings are diagnostic messages that report construc- tions which are not inherently erroneous but which are risky Using GNU CC 25 or suggest there may have been an error. You can request many specific warnings with options beginning `-W', for example `-Wimplicit' to request warnings on implicit declarations. Each of these specific warning options also has a negative form beginning `-Wno-' to turn off warnings; for example, `-Wno-implicit'. This manual lists only one of the two forms, whichever is not the default. These options control the amount and kinds of warnings produced by GNU CC: -fsyntax-only Check the code for syntax errors, but don't emit any output. -w Inhibit all warning messages. -pedantic Issue all the warnings demanded by strict ANSI standard C; reject all programs that use forbidden extensions. Valid ANSI standard C programs should compile properly with or without this option (though a rare few will require `-ansi'). However, without this option, certain GNU extensions and traditional C features are supported as well. With this option, they are rejected. `-pedantic' does not cause warning messages for use of the alternate keywords whose names begin and end with `__'. Pedantic warnings are also disabled in the expression that follows __extension__. However, only system header files should use these escape routes; application programs should avoid them. See section Alternate Keywords. This option is not intended to be useful; it exists only to satisfy pedants who would otherwise claim that GNU CC fails to support the ANSI standard. Some users try to use `-pedantic' to check programs for strict ANSI C conformance. They soon find that it does not do quite what they want: it finds some non-ANSI practices, but not all---only those for which ANSI C requires a diagnostic. A feature to report any failure to conform to ANSI C might be useful in some instances, but would 26 Using GNU CC require considerable additional work and would be quite different from `-pedantic'. We recommend, rather, that users take advantage of the extensions of GNU C and disregard the limitations of other compilers. Aside from certain supercomputers and obsolete small machines, there is less and less reason ever to use any other C compiler other than for bootstrapping GNU CC. -pedantic-errors Like `-pedantic', except that errors are produced rather than warnings. -W Print extra warning messages for these events: o+ A nonvolatile automatic variable might be changed by a call to longjmp. These warnings as well are possible only in optimizing compilation. The compiler sees only the calls to setjmp. It cannot know where longjmp will be called; in fact, a signal handler could call it at any point in the code. As a result, you may get a warning even when there is in fact no problem because longjmp cannot in fact be called at the place which would cause a problem. o+ A function can return either with or without a value. (Falling off the end of the function body is considered returning without a value.) For example, this function would evoke such a warning: foo (a) { if (a > 0) return a; } o+ An expression-statement contains no side effects. o+ An unsigned value is compared against zero with `>' or `<='. -Wimplicit Warn whenever a function or parameter is Using GNU CC 27 implicitly declared. -Wreturn-type Warn whenever a function is defined with a return-type that defaults to int. Also warn about any return statement with no return- value in a function whose return-type is not void. -Wunused Warn whenever a local variable is unused aside from its declaration, whenever a function is declared static but never defined, and whenever a statement computes a result that is explicitly not used. -Wswitch Warn whenever a switch statement has an index of enumeral type and lacks a case for one or more of the named codes of that enumeration. (The presence of a default label prevents this warning.) case labels outside the enumeration range also provoke warnings when this option is used. -Wcomment Warn whenever a comment-start sequence `/*' appears in a comment. -Wtrigraphs Warn if any trigraphs are encountered (assuming they are enabled). -Wformat Check calls to printf and scanf, etc., to make sure that the arguments supplied have types appropriate to the format string specified. -Wchar-subscripts Warn if an array subscript has type char. This is a common cause of error, as programmers often forget that this type is signed on some machines. -Wuninitialized An automatic variable is used without first being initialized. These warnings are possible only in optimizing compilation, because they require data flow information that is computed only when optimizing. If you don't specify `-O', you simply won't get these warnings. 28 Using GNU CC These warnings occur only for variables that are candidates for register allocation. Therefore, they do not occur for a variable that is declared volatile, or whose address is taken, or whose size is other than 1, 2, 4 or 8 bytes. Also, they do not occur for structures, unions or arrays, even when they are in registers. Note that there may be no warning about a variable that is used only to compute a value that itself is never used, because such computations may be deleted by data flow analysis before the warnings are printed. These warnings are made optional because GNU CC is not smart enough to see all the reasons why the code might be correct despite appearing to have an error. Here is one example of how this can happen: { int x; switch (y) { case 1: x = 1; break; case 2: x = 4; break; case 3: x = 5; } foo (x); } If the value of y is always 1, 2 or 3, then x is always initialized, but GNU CC doesn't know this. Here is another common case: { int save_y; if (change_y) save_y = y, y = new_y; ... if (change_y) y = save_y; } This has no bug because save_y is used only if it is set. Using GNU CC 29 Some spurious warnings can be avoided if you declare as volatile all the functions you use that never return. See section Function Attributes. -Wall All of the above `-W' options combined. These are all the options which pertain to usage that we recommend avoiding and that we believe is easy to avoid, even in conjunction with macros. The remaining `-W...' options are not implied by `- Wall' because they warn about constructions that we consider reasonable to use, on occasion, in clean programs. -Wtraditional Warn about certain constructs that behave differently in traditional and ANSI C. o+ Macro arguments occurring within string constants in the macro body. These would substitute the argument in traditional C, but are part of the constant in ANSI C. o+ A function declared external in one block and then used after the end of the block. o+ A switch statement has an operand of type long. -Wshadow Warn whenever a local variable shadows another local variable. -Wid-clash-len Warn whenever two distinct identifiers match in the first len characters. This may help you prepare a program that will compile with certain obsolete, brain-damaged compilers. -Wpointer-arith Warn about anything that depends on the ``size of'' a function type or of void. GNU C assigns these types a size of 1, for convenience in calculations with void * pointers and pointers to functions. -Wcast-qual Warn whenever a pointer is cast so as to remove a type qualifier from the target type. For example, 30 Using GNU CC warn if a const char * is cast to an ordinary char *. -Wcast-align Warn whenever a pointer is cast such that the required alignment of the target is increased. For example, warn if a char * is cast to an int * on machines where integers can only be accessed at two- or four-byte boundaries. -Wwrite-strings Give string constants the type const char[length] so that copying the address of one into a non- const char * pointer will get a warning. These warnings will help you find at compile time code that can try to write into a string constant, but only if you have been very careful about using const in declarations and prototypes. Otherwise, it will just be a nuisance; this is why we did not make `-Wall' request these warnings. -Wconversion Warn if a prototype causes a type conversion that is different from what would happen to the same argument in the absence of a prototype. This includes conversions of fixed point to floating and vice versa, and conversions changing the width or signedness of a fixed point argument except when the same as the default promotion. -Waggregate-return Warn if any functions that return structures or unions are defined or called. (In languages where you can return an array, this also elicits a warning.) -Wstrict-prototypes Warn if a function is declared or defined without specifying the argument types. (An old-style function definition is permitted without a warning if preceded by a declaration which specifies the argument types.) -Wmissing-prototypes Warn if a global function is defined without a previous prototype declaration. This warning is issued even if the definition itself provides a prototype. The aim is to detect global functions that fail to be declared in header files. -Wredundant-decls Warn if anything is declared more than once in the same scope, even in cases where multiple Using GNU CC 31 declaration is valid and changes nothing. -Wnested-externs Warn if an extern declaration is encountered within an function. -Wno-parentheses Disable warnings that parentheses are suggested around an expression. -Werror Make all warnings into errors. 2.4. Options for Debugging Your Program or GNU CC GNU CC has various special options that are used for debugging either your program or GCC: -g Produce debugging information in the operating system's native format (stabs or COFF or DWARF). GDB can work with this debugging information. On most systems that use stabs format, `-g' enables use of extra debugging information that only GDB can use; this extra information makes debugging work better in GDB but will probably make DBX crash or refuse to read the program. If you want to control for certain whether to generate the extra information, use `-gstabs+' or `-gstabs' (see below). Unlike most other C compilers, GNU CC allows you to use `-g' with `-O'. The shortcuts taken by optimized code may occasionally produce surprising results: some variables you declared may not exist at all; flow of control may briefly move where you did not expect it; some statements may not be executed because they compute constant results or their values were already at hand; some statements may execute in different places because they were moved out of loops. Nevertheless it proves possible to debug optimized output. This makes it reasonable to use the optimizer for programs that might have bugs. The following options are useful when GNU CC is generated with the capability for more than one debugging format. -ggdb Produce debugging information in the native format 32 Using GNU CC (if that is supported), including GDB extensions if at all possible. -gstabs Produce debugging information in stabs format (if that is supported), without GDB extensions. This is the format used by DBX on most BSD systems. -gstabs+ Produce debugging information in stabs format (if that is supported), using GDB extensions. The use of these extensions is likely to make DBX crash or refuse to read the program. -gcoff Produce debugging information in COFF format (if that is supported). This is the format used by SDB on COFF systems. -gdwarf Produce debugging information in DWARF format (if that is supported). This is the format used by SDB on systems that use DWARF. -glevel -ggdblevel -gstabslevel -gcofflevel -gdwarflevel Request debugging information and also use level to specify how much information. The default level is 2. Level 1 produces minimal information, enough for making backtraces in parts of the program that you don't plan to debug. This includes descriptions of functions and external variables, but no information about local variables and no line numbers. Level 3 includes extra information, such as all the macro definitions present in the program. Some debuggers support macro expansion when you use `-g3'. -p Generate extra code to write profile information suitable for the analysis program prof. Using GNU CC 33 -pg Generate extra code to write profile information suitable for the analysis program gprof. -a Generate extra code to write profile information for basic blocks, which will record the number of times each basic block is executed. This data could be analyzed by a program like tcov. Note, however, that the format of the data is not what tcov expects. Eventually GNU gprof should be extended to process this data. -dletters Says to make debugging dumps during compilation at times specified by letters. This is used for debugging the compiler. The file names for most of the dumps are made by appending a word to the source file name (e.g. `foo.c.rtl' or `foo.c.jump'). Here are the possible letters for use in letters, and their meanings: `M' Dump all macro definitions, at the end of preprocessing, and write no output. `N' Dump all macro names, at the end of preprocessing. `D' Dump all macro definitions, at the end of preprocessing, in addition to normal output. `y' Dump debugging information during parsing, to standard error. `r' Dump after RTL generation, to `file.rtl'. `x' Just generate RTL for a function instead of compiling it. Usually used with `r'. `j' Dump after first jump optimization, to `file.jump'. `s' Dump after CSE (including the jump optimization that sometimes follows CSE), to `file.cse'. `L' Dump after loop optimization, to `file.loop'. `t' Dump after the second CSE pass (including the jump optimization that sometimes follows CSE), to `file.cse2'. `f' Dump after flow analysis, to `file.flow'. 34 Using GNU CC `c' Dump after instruction combination, to `file.combine'. `S' Dump after the first instruction scheduling pass, to `file.sched'. `l' Dump after local register allocation, to `file.lreg'. `g' Dump after global register allocation, to `file.greg'. `R' Dump after the second instruction scheduling pass, to `file.sched2'. `J' Dump after last jump optimization, to `file.jump2'. `d' Dump after delayed branch scheduling, to `file.dbr'. `k' Dump after conversion from registers to stack, to `file.stack'. `a' Produce all the dumps listed above. `m' Print statistics on memory usage, at the end of the run, to standard error. `p' Annotate the assembler output with a comment indicating which pattern and alternative was used. -fpretend-float When running a cross-compiler, pretend that the target machine uses the same floating point format as the host machine. This causes incorrect output of the actual floating constants, but the actual instruction sequence will probably be the same as GNU CC would make when running on the target machine. -save-temps Store the usual ``temporary'' intermediate files permanently; place them in the current directory and name them based on the source file. Thus, compiling `foo.c' with `-c -save-temps' would produce files `foo.cpp' and `foo.s', as well as `foo.o'. Using GNU CC 35 2.5. Options That Control Optimization These options control various sorts of optimizations: -O Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large function. Without `-O', the compiler's goal is to reduce the cost of compilation and to make debugging produce the expected results. Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the program counter to any other statement in the function and get exactly the results you would expect from the source code. Without `-O', only variables declared register are allocated in registers. The resulting compiled code is a little worse than produced by PCC without `-O'. With `-O', the compiler tries to reduce code size and execution time. When `-O' is specified, `-fthread-jumps' and `- fdelayed-branch' are turned on. On some machines other flags may also be turned on. -O2 Highly optimize. All supported optimizations that do not involve a space-speed tradeoff are performed. As compared to `-O', this option will increase both compilation time and the performance of the generated code. All `-fflag' options that control optimization are turned on when `-O2' is specified, except for `- funroll-loops' and `-funroll-all-loops'. Options of the form `-fflag' specify machine- independent flags. Most flags have both positive and nega- tive forms; the negative form of `-ffoo' would be `-fno- foo'. In the table below, only one of the forms is listed- --the one which is not the default. You can figure out the other form by either removing `no-' or adding it. -ffloat-store Do not store floating point variables in registers. This prevents undesirable excess precision on machines such as the 68000 where the floating registers (of the 68881) keep more precision than a double is supposed to have. 36 Using GNU CC For most programs, the excess precision does only good, but a few programs rely on the precise definition of IEEE floating point. Use `-ffloat- store' for such programs. -fno-defer-pop Always pop the arguments to each function call as soon as that function returns. For machines which must pop arguments after a function call, the compiler normally lets arguments accumulate on the stack for several function calls and pops them all at once. -fforce-mem Force memory operands to be copied into registers before doing arithmetic on them. This may produce better code by making all memory references potential common subexpressions. When they are not common subexpressions, instruction combination should eliminate the separate register-load. I am interested in hearing about the difference this makes. -fforce-addr Force memory address constants to be copied into registers before doing arithmetic on them. This may produce better code just as `-fforce-mem' may. I am interested in hearing about the difference this makes. -fomit-frame-pointer Don't keep the frame pointer in a register for functions that don't need one. This avoids the instructions to save, set up and restore frame pointers; it also makes an extra register available in many functions. It also makes debugging impossible on some machines. INTERNALS On some machines, such as the Vax, this flag has no effect, because the standard calling sequence automatically handles the frame pointer and nothing is saved by pretending it doesn't exist. The machine-description macro FRAME_POINTER_REQUIRED controls whether a target machine supports this flag. See section Registers. INTERNALS On some machines, such as the Vax, this flag has no effect, because the standard calling sequence automatically handles the frame pointer and nothing is saved by pretending it doesn't exist. The machine-description macro FRAME_POINTER_REQUIRED controls whether a target machine supports this flag. See section Using GNU CC 37 Registers,,Register Usage, gcc.info, Using and Porting GCC. -finline Pay attention to the inline keyword. Normally the negation of this option `-fno-inline' is used to keep the compiler from expanding any functions inline. However, the opposite effect may be desirable when compiling without optimization, since inline expansion is turned off in that case. -finline-functions Integrate all simple functions into their callers. The compiler heuristically decides which functions are simple enough to be worth integrating in this way. If all calls to a given function are integrated, and the function is declared static, then the function is normally not output as assembler code in its own right. -fcaller-saves Enable values to be allocated in registers that will be clobbered by function calls, by emitting extra instructions to save and restore the registers around such calls. Such allocation is done only when it seems to result in better code than would otherwise be produced. This option is enabled by default on certain machines, usually those which have no call- preserved registers to use instead. -fkeep-inline-functions Even if all calls to a given function are integrated, and the function is declared static, nevertheless output a separate run-time callable version of the function. -fno-function-cse Do not put function addresses in registers; make each instruction that calls a constant function contain the function's address explicitly. This option results in less efficient code, but some strange hacks that alter the assembler output may be confused by the optimizations performed when this option is not used. The following options control specific optimizations. The `-O2' option turns on all of these optimizations except 38 Using GNU CC `-funroll-loops' and `-funroll-all-loops'. The `-O' option usually turns on the `-fthread-jumps' and `-fdelayed-branch' options, but specific machines may change the default optim- izations. You can use the following flags in the rare cases when ``fine-tuning'' of optimizations to be performed is desired. -fstrength-reduce Perform the optimizations of loop strength reduction and elimination of iteration variables. -fthread-jumps Perform optimizations where we check to see if a jump branches to a location where another comparison subsumed by the first is found. If so, the first branch is redirected to either the destination of the second branch or a point immediately following it, depending on whether the condition is known to be true or false. -fcse-follow-jumps In common subexpression elimination, scan through jump instructions in certain cases. This is not as powerful as completely global CSE, but not as slow either. -frerun-cse-after-loop Re-run common subexpression elimination after loop optimizations has been performed. -fexpensive-optimizations Perform a number of minor optimizations that are relatively expensive. -fdelayed-branch If supported for the target machine, attempt to reorder instructions to exploit instruction slots available after delayed branch instructions. -fschedule-insns If supported for the target machine, attempt to reorder instructions to eliminate execution stalls due to required data being unavailable. This helps machines that have slow floating point or memory load instructions by allowing other instructions to be issued until the result of the load or floating point instruction is required. -fschedule-insns2 Similar to `-fschedule-insns', but requests an additional pass of instruction scheduling after register allocation has been done. This is Using GNU CC 39 especially useful on machines with a relatively small number of registers and where memory load instructions take more than one cycle. -funroll-loops Perform the optimization of loop unrolling. This is only done for loops whose number of iterations can be determined at compile time or run time. `-funroll-loop' implies `-fstrength-reduce' and `-frerun-cse-after-loop'. -funroll-all-loops Perform the optimization of loop unrolling. This is done for all loops and usually makes programs run more slowly. `-funroll-all-loops' implies `- fstrength-reduce' and `-frerun-cse-after-loop'. -fno-peephole Disable any machine-specific peephole optimizations. 2.6. Options Controlling the Preprocessor These options control the C preprocessor, which is run on each C source file before actual compilation. If you use the `-E' option, nothing is done except preprocessing. Some of these options make sense only together with `-E' because they cause the preprocessor out- put to be unsuitable for actual compilation. -include file Process file as input before processing the regular input file. In effect, the contents of file are compiled first. Any `-D' and `-U' options on the command line are always processed before `-include file', regardless of the order in which they are written. All the `-include' and `-imacros' options are processed in the order in which they are written. -imacros file Process file as input, discarding the resulting output, before processing the regular input file. Because the output generated from file is discarded, the only effect of `-imacros file' is to make the macros defined in file available for use in the main input. Any `-D' and `-U' options on the command line are always processed before `-imacros file', regardless of the order in which they are written. 40 Using GNU CC All the `-include' and `-imacros' options are processed in the order in which they are written. -nostdinc Do not search the standard system directories for header files. Only the directories you have specified with `-I' options (and the current directory, if appropriate) are searched. See section Directory Options, for information on `- I'. By using both `-nostdinc' and `-I-', you can limit the include-file search path to only those directories you specify explicitly. -undef Do not predefine any nonstandard macros. (Including architecture flags). -E Run only the C preprocessor. Preprocess all the C source files specified and output the results to standard output or to the specified output file. -C Tell the preprocessor not to discard comments. Used with the `-E' option. -P Tell the preprocessor not to generate `#line' commands. Used with the `-E' option. -M Tell the preprocessor to output a rule suitable for make describing the dependencies of each object file. For each source file, the preprocessor outputs one make-rule whose target is the object file name for that source file and whose dependencies are all the files `#include'd in it. This rule may be a single line or may be continued with `\'-newline if it is long. The list of rules is printed on standard output instead of the preprocessed C program. `-M' implies `-E'. Another way to specify output of a make rule is by setting the environment variable DEPENDENCIES_OUTPUT (see section Environment Variables). -MM Like `-M' but the output mentions only the user header files included with `#include "file"'. System header files included with `#include ' are omitted. Using GNU CC 41 -MD Like `-M' but the dependency information is written to files with names made by replacing `.c' with `.d' at the end of the input file names. This is in addition to compiling the file as specified---`-MD' does not inhibit ordinary compilation the way `-M' does. The Mach utility `md' can be used to merge the `.d' files into a single dependency file suitable for using with the `make' command. -MMD Like `-MD' except mention only user header files, not system header files. -H Print the name of each header file used, in addition to other normal activities. -Dmacro Define macro macro with the string `1' as its definition. -Dmacro=defn Define macro macro as defn. All instances of `-D' on the command line are processed before any `-U' options. -Umacro Undefine macro macro. `-U' options are evaluated after all `-D' options, but before any `-include' and `-imacros' options. -dM Tell the preprocessor to output only a list of the macro definitions that are in effect at the end of preprocessing. Used with the `-E' option. -dD Tell the preprocessing to pass all macro definitions into the output, in their proper sequence in the rest of the output. -dN Like `-dD' except that the macro arguments and contents are omitted. Only `#define name' is included in the output. -trigraphs Support ANSI C trigraphs. You don't want to know about this brain-damage. The `-ansi' option also has this effect. 42 Using GNU CC 2.7. Options for Linking These options come into play when the compiler links object files into an executable output file. They are mean- ingless if the compiler is not doing a link step. object-file-name A file name that does not end in a special recognized suffix is considered to name an object file or library. (Object files are distinguished from libraries by the linker according to the file contents.) If linking is done, these object files are used as input to the linker. -c -S -E If any of these options is used, then the linker is not run, and object file names should not be used as arguments. See section Overall Options. -llibrary Search the library named library when linking. It makes a difference where in the command you write this option; the linker searches processes libraries and object files in the order they are specified. Thus, `foo.o -lz bar.o' seaches library `z' after file `foo.o' but before `bar.o'. If `bar.o' refers to functions in `z', those functions may not be loaded. The linker searches a standard list of directories for the library, which is actually a file named `liblibrary.a'. The linker then uses this file as if it had been specified precisely by name. The directories searched include several standard system directories plus any that you specify with `-L'. Normally the files found this way are library files---archive files whose members are object files. The linker handles an archive file by scanning through it for members which define symbols that have so far been referenced but not defined. But if the file that is found is an ordinary object file, it is linked in the usual fashion. The only difference between using an `- l' option and specifying a file name is that `-l' surrounds library with `lib' and `.a' and searches several directories. Using GNU CC 43 -nostdlib Don't use the standard system libraries and startup files when linking. Only the files you specify will be passed to the linker. -static On systems that support dynamic linking, this prevents linking with the shared libraries. On other systems, this option has no effect. -dynamic On systems that support dynamic linking, you can use this option to request it explicitly. -shared Produce a shared object which can then be linked with other objects to form an executable. Only a few systems support this option. -symbolic Bind references to global symbols when building a shared object. Warn about any unresolved references (unless overridden by the link editor option `-Xlinker -z -Xlinker defs'). Only a few systems support this option. -Xlinker option Pass option as an option to the linker. You can use this to supply system-specific linker options which GNU CC does not know how to recognize. If you want to pass an option that takes an argument, you must use `-Xlinker' twice, once for the option and once for the argument. For example, to pass `-assert definitions', you must write `-Xlinker -assert -Xlinker definitions'. It does not work to write `-Xlinker "-assert definitions"', because this passes the entire string as a single argument, which is not what the linker expects. 2.8. Options for Directory Search These options specify directories to search for header files, for libraries and for parts of the compiler: -Idir Append directory dir to the list of directories searched for include files. -I- Any directories you specify with `-I' options before the `-I-' option are searched only for the 44 Using GNU CC case of `#include "file"'; they are not searched for `#include '. If additional directories are specified with `-I' options after the `-I-', these directories are searched for all `#include' directives. (Ordinarily all `-I' directories are used this way.) In addition, the `-I-' option inhibits the use of the current directory (where the current input file came from) as the first search directory for `#include "file"'. There is no way to override this effect of `-I-'. With `-I.' you can specify searching the directory which was current when the compiler was invoked. That is not exactly the same as what the preprocessor does by default, but it is often satisfactory. `-I-' does not inhibit the use of the standard system directories for header files. Thus, `-I-' and `-nostdinc' are independent. -Ldir Add directory dir to the list of directories to be searched for `-l'. -Bprefix This option specifies where to find the executables, libraries and data files of the compiler itself. The compiler driver program runs one or more of the subprograms `cpp', `cc1', `as' and `ld'. It tries prefix as a prefix for each program it tries to run, both with and without `machine/version/' (see section Target Options). For each subprogram to be run, the compiler driver first tries the `-B' prefix, if any. If that name is not found, or if `-B' was not specified, the driver tries two standard prefixes, which are `/usr/lib/gcc/' and `/usr/local/lib/gcc/'. If neither of those results in a file name that is found, the unmodified program name is searched for using the directories specified in your `PATH' environment variable. `-B' prefixes that effectively specify directory names also apply to libraries in the linker, because the compiler translates these options into `-L' options for the linker. Using GNU CC 45 The run-time support file `libgcc.a' can also be searched for using the `-B' prefix, if needed. If it is not found there, the two standard prefixes above are tried, and that is all. The file is left out of the link if it is not found by those means. Another way to specify a prefix much like the `-B' prefix is to use the environment variable GCC_EXEC_PREFIX. See section Environment Variables. 2.9. Specifying Target Machine and Compiler Version By default, GNU CC compiles code for the same type of machine that you are using. However, it can also be installed as a cross-compiler, to compile for some other type of machine. In fact, several different configurations of GNU CC, for different target machines, can be installed side by side. Then you specify which one to use with the `-b' option. In addition, older and newer versions of GNU CC can be installed side by side. One of them (probably the newest) will be the default, but you may sometimes wish to use another. -b machine The argument machine specifies the target machine for compilation. This is useful when you have installed GNU CC as a cross-compiler. The value to use for machine is the same as was specified as the machine type when configuring GNU CC as a cross-compiler. For example, if a cross- compiler was configured with `configure i386v', meaning to compile for an 80386 running System V, then you would specify `-b i386v' to run that cross compiler. When you do not specify `-b', it normally means to compile for the same type of machine that you are using. -V version The argument version specifies which version of GNU CC to run. This is useful when multiple versions are installed. For example, version might be `2.0', meaning to run GNU CC version 2.0. The default version, when you do not specify `-V', is controlled by the way GNU CC is installed. 46 Using GNU CC Normally, it will be a version that is recommended for general use. The `-b' and `-V' options actually work by controlling part of the file name used for the executable files and libraries used for compilation. A given version of GNU CC, for a given target machine, is normally kept in the direc- tory `/usr/local/lib/gcc/machine/version'. It follows that sites can customize the effect of `-b' or `-V' either by changing the names of these directories or adding alternate names (or symbolic links). Thus, if `/usr/local/lib/gcc/80386' is a link to `/usr/local/lib/gcc/i386v', then `-b 80386' will be an alias for `-b i386v'. In one respect, the `-b' or `-V' do not completely change to a different compiler: the top-level driver program gcc that you originally invoked continues to run and invoke the other executables (preprocessor, compiler per se, assem- bler and linker) that do the real work. However, since no real work is done in the driver program, it usually does not matter that the driver program in use is not the one for the specified target and version. The only way that the driver program depends on the target machine is in the parsing and handling of special machine-specific options. However, this is controlled by a file which is found, along with the other executables, in the directory for the specified version and target machine. As a result, a single installed driver program adapts to any specified target machine and compiler version. The driver program executable does control one signifi- cant thing, however: the default version and target machine. Therefore, you can install different instances of the driver program, compiled for different targets or versions, under different names. For example, if the driver for version 2.0 is installed as ogcc and that for version 2.1 is installed as gcc, then the command gcc will use version 2.1 by default, while ogcc will use 2.0 by default. However, you can choose either version with either command with the `-V' option. 2.10. Specifying Hardware Models and Configurations Earlier we discussed the standard option `-b' which chooses among different installed compilers for completely different target machines, such as Vax vs. 68000 vs. 80386. Using GNU CC 47 In addition, each of these target machine types can have its own special options, starting with `-m', to choose among various hardware models or configurations---for exam- ple, 68010 vs 68020, floating coprocessor or none. A single installed version of the compiler can compile for any model or configuration, according to the options specified. INTERNALS These options are defined by the macro TARGET_SWITCHES in the machine description. The default for the options is also defined by that macro, which enables you to change the defaults. 2.10.1. M680x0 Options These are the `-m' options defined for the 68000 series. The default values for these options depends on which style of 68000 was selected when the compiler was con- figured; the defaults for the most common choices are given below. -m68020 -mc68020 Generate output for a 68020 (rather than a 68000). This is the default when the compiler is configured for 68020-based systems. -m68000 -mc68000 Generate output for a 68000 (rather than a 68020). This is the default when the compiler is configured for a 68000-based systems. -m68881 Generate output containing 68881 instructions for floating point. This is the default for most 68020 systems unless `-nfp' was specified when the compiler was configured. -mfpa Generate output containing Sun FPA instructions for floating point. -msoft-float Generate output containing library calls for floating point. Warning: the requisite libraries are not part of GNU CC. Normally the facilities of the machine's usual C compiler are used, but this can't be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. 48 Using GNU CC -mshort Consider type int to be 16 bits wide, like short int. -mnobitfield Do not use the bit-field instructions. `-m68000' implies `-mnobitfield'. -mbitfield Do use the bit-field instructions. `-m68020' implies `-mbitfield'. This is the default if you use the unmodified sources configured for a 68020. -mrtd Use a different function-calling convention, in which functions that take a fixed number of arguments return with the rtd instruction, which pops their arguments while returning. This saves one instruction in the caller since there is no need to pop the arguments there. This calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler. Also, you must provide function prototypes for all functions that take variable numbers of arguments (including printf); otherwise incorrect code will be generated for calls to those functions. In addition, seriously incorrect code will result if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.) The rtd instruction is supported by the 68010 and 68020 processors, but not by the 68000. 2.10.2. VAX Options These `-m' options are defined for the Vax: -munix Do not output certain jump instructions (aobleq and so on) that the Unix assembler for the Vax cannot handle across long ranges. -mgnu Do output those jump instructions, on the assumption that you will assemble with the GNU assembler. Using GNU CC 49 -mg Output code for g-format floating point numbers instead of d-format. 2.10.3. SPARC Options These `-m' switches are supported on the Sparc: -mno-epilogue Generate separate return instructions for return statements. This has both advantages and disadvantages; I don't recall what they are. 2.10.4. Convex Options These `-m' options are defined for the Convex: -mc1 Generate output for a C1. This is the default when the compiler is configured for a C1. -mc2 Generate output for a C2. This is the default when the compiler is configured for a C2. -margcount Generate code which puts an argument count in the word preceding each argument list. Some nonportable Convex and Vax programs need this word. (Debuggers don't, except for functions with variable-length argument lists; this info is in the symbol table.) -mnoargcount Omit the argument count word. This is the default if you use the unmodified sources. 2.10.5. AMD29K Options These `-m' options are defined for the AMD Am29000: -mdw Generate code that assumes the DW bit is set, i.e., that byte and halfword operations are directly supported by the hardware. This is the default. -mnodw Generate code that assumes the DW bit is not set. 50 Using GNU CC -mbw Generate code that assumes the system supports byte and halfword write operations. This is the default. -mnbw Generate code that assumes the systems does not support byte and halfword write operations. `- mnbw' implies `-mnodw'. -msmall Use a small memory model that assumes that all function addresses are either within a single 256 KB segment or at an absolute address of less than 256K. This allows the call instruction to be used instead of a const, consth, calli sequence. -mlarge Do not assume that the call instruction can be used; this is the default. -m29050 Generate code for the Am29050. -m29000 Generate code for the Am29000. This is the default. -mkernel-registers Generate references to registers gr64-gr95 instead of gr96-gr127. This option can be used when compiling kernel code that wants a set of global registers disjoint from that used by user-mode code. Note that when this option is used, register names in `-f' flags must use the normal, user-mode, names. -muser-registers Use the normal set of global registers, gr96- gr127. This is the default. -mstack-check Insert a call to __msp_check after each stack adjustment. This is often used for kernel code. 2.10.6. M88K Options These `-m' options are defined for Motorola 88K archi- tectures: Using GNU CC 51 -m88000 Generate code that works well on both the m88100 and the m88110. -m88100 Generate code tha Generate code that works best for the m88100, but that also runs on the m88110. -m88110 Generate code that works best for the m88110, and may not run on the m88100. -midentify-revision Include an ident directive in the assembler output recording the source file name, compiler name and version, timestamp, and compilation flags used. -mno-underscores In assembler output, emit symbol names without adding an underscore character at the beginning of each name. The default is to use an underscore as prefix on each name. -mocs-debug-info -mno-ocs-debug-info Include (or omit) additional debugging information (about registers used in each stack frame) as specified in the 88open Object Compatibility Standard, ``OCS''. This extra information allows debugging of code that has had the frame pointer eliminated. The default for DG/UX, SVr4, and Delta 88 SVr3.2 is to include this information; other 88k configurations omit this information by default. -mocs-frame-position When emitting COFF debugging information for automatic variables and parameters stored on the stack, use the offset from the canonical frame address, which is the stack pointer (register 31) on entry to the function. The DG/UX, SVr4, Delta88 SVr3.2, and BCS configurations use `- mocs-frame-position'; other 88k configurations have the default `-mno-ocs-frame-position'. -mno-ocs-frame-position When emitting COFF debugging information for automatic variables and parameters stored on the stack, use the offset from the frame pointer register (register 30). When this option is in effect, the frame pointer is not eliminated when debugging information is selected by the -g 52 Using GNU CC switch. -moptimize-arg-area -mno-optimize-arg-area Control how to store function arguments in stack frames. `-moptimize-arg-area' saves space, but was ruled illegal by 88open. `-mno-optimize-arg- area' conforms to the 88open standards. By default GNU CC does not optimize the argument area. -mshort-data-num Generate smaller data references by making them relative to r0, which allows loading a value using a single instruction (rather than the usual two). You control which data references are affected by specifying num with this option. For example, if you specify `-mshort-data-512', then the data references affected are those involving displacements of less than 512 bytes. `-mshort- data-num' is not effective for num greater than 64K. -msvr4 -msvr3 Turn on (`-msvr4') or off (`-msvr3') compiler extensions related to System V release 4 (SVr4). This controls the following: 1. Which variant of the assembler syntax to emit (which you can select independently using `- mversion-03.00'). 2. `-msvr4' makes the C preprocessor recognize `#pragma weak' that is used on System V release 4. 3. `-msvr4' makes GNU CC issue additional declaration directives used in SVr4. `-msvr3' is the default for all m88K configurations except the SVr4 configuration. -mversion-03.00 In the DG/UX configuration, there are two flavors of SVr4. This option modifies `-msvr4' to select whether the hybrid-COFF or real-ELF flavor is used. All other configurations ignore this option. Using GNU CC 53 -mno-check-zero-division -mcheck-zero-division Early models of the 88K architecture had problems with division by zero; in particular, many of them didn't trap. Use these options to avoid including (or to include explicitly) additional code to detect division by zero and signal an exception. All GNU CC configurations for the 88K use `- mcheck-zero-division' by default. -muse-div-instruction Do not emit code to check both the divisor and dividend when doing signed integer division to see if either is negative, and adjust the signs so the divide is done using non-negative numbers. Instead, rely on the operating system to calculate the correct value when the div instruction traps. This results in different behavior when the most negative number is divided by -1, but is useful when most or all signed integer divisions are done with positive numbers. -mtrap-large-shift -mhandle-large-shift Include code to detect bit-shifts of more than 31 bits; respectively, trap such shifts or emit code to handle them properly. By default GNU CC makes no special provision for large bit shifts. -mwarn-passed-structs Warn when a function passes a struct as an argument or result. Structure-passing conventions have changed during the evolution of the C language, and are often the source of portability problems. By default, GNU CC issues no such warning. 2.10.7. IBM RS/6000 Options Only one pair of `-m' options is defined for the IBM RS/6000: -mfp-in-toc -mno-fp-in-toc Control whether or not floating-point constants go in the Table of Contents (TOC), a table of all global variable and function addresses. By default GNU CC puts floating-point constants there; if the TOC overflows, `-mno-fp-in-toc' will 54 Using GNU CC reduce the size of the TOC, which may avoid the overflow. 2.10.8. IBM RT Options These `-m' options are defined for the IBM RT PC: -min-line-mul Use an in-line code sequence for integer multiplies. This is the default. -mcall-lib-mul Call lmul$$ for integer multiples. -mfull-fp-blocks Generate full-size floating point data blocks, including the minimum amount of scratch space recommended by IBM. This is the default. -mminimum-fp-blocks Do not include extra scratch space in floating point data blocks. This results in smaller code, but slower execution, since scratch space must be allocated dynamically. -mfp-arg-in-fpregs Use a calling sequence incompatible with the IBM calling convention in which floating point arguments are passed in floating point registers. Note that varargs.h and stdargs.h will not work with floating point operands if this option is specified. -mfp-arg-in-gregs Use the normal calling convention for floating point arguments. This is the default. -mhc-struct-return Return structures of more than one word in memory, rather than in a register. This provides compatibility with the MetaWare HighC (hc) compiler. Use `-fpcc-struct-return' for compatibility with the Portable C Compiler (pcc). -mnohc-struct-return Return some structures of more than one word in registers, when convenient. This is the default. For compatibility with the IBM-supplied compilers, use either `-fpcc-struct-return' or `-mhc-struct- return'. Using GNU CC 55 2.10.9. MIPS Options These `-m' options are defined for the MIPS family of computers: -mcpu=cpu type Assume the defaults for the machine type cpu type when scheduling insturctions. The default cpu type is `default', which picks the longest cycles times for any of the machines, in order that the code run at reasonable rates on all MIPS cpu's. Other choices for cpu type are `r2000', `r3000', `r4000', and `r6000'. While picking a specific cpu type will schedule things appropriately for that particular chip, the compiler will not generate any code that does not meet level 1 of the MIPS ISA (instruction set architecture) without the `-mips2' or `-mips3' switches being used. -mips2 Issue instructions from level 2 of the MIPS ISA (branch likely, square root instructions). The `-mcpu=r4000' or `-mcpu=r6000' switch must be used in conjuction with `-mips2'. -mips3 Issue instructions from level 3 of the MIPS ISA (64 bit instructions). You must use the `- mcpu=r4000' switch along with `-mips3'. -mint64 -mlong64 -mlonglong128 These options don't work at present. -mmips-as Generate code for the MIPS assembler, and invoke `mips-tfile' to add normal debug information. This is the default for all platforms except for the OSF/1 reference platform, using the OSF/rose object format. If the either of the `-gstabs' or `-gstabs+' switches are used, the `mips-tfile' program will encapsulate the stabs within MIPS ECOFF. -mgas Generate code for the GNU assembler. This is the default on the OSF/1 reference platform, using the OSF/rose object format. 56 Using GNU CC -mrnames -mno-rnames The `-mrnames' switch says to output code using the MIPS software names for the registers, instead of the hardware names (ie, a0 instead of $4). The GNU assembler does not support the `-mrnames' switch, and the MIPS assembler will be instructed to run the MIPS C preprocessor over the source file. The `-mno-rnames' switch is default. -mgpopt -mno-gpopt The `-mgpopt' switch says to write all of the data declarations before the instructions in the text section, to all the MIPS assembler to generate one word memory references instead of using two words for short global or static data items. This is on by default if optimization is selected. -mstats -mno-stats For each non-inline function processed, the `- mstats' switch causes the compiler to emit one line to the standard error file to print statistics about the program (number of registers saved, stack size, etc.). -mmemcpy -mno-memcpy The `-mmemcpy' switch makes all block moves call the appropriate string function (`memcpy' or `bcopy') instead of possibly generating inline code. -mmips-tfile -mno-mips-tfile The `-mno-mips-tfile' switch causes the compiler not postprocess the object file with the `mips- tfile' program, after the MIPS assembler has generated it to add debug support. If `mips- tfile' is not run, then no local variables will be available to the debugger. In addition, `stage2' and `stage3' objects will have the temporary file names passed to the assembler embedded in the object file, which means the objects will not compare the same. Using GNU CC 57 -msoft-float Generate output containing library calls for floating point. Warning: the requisite libraries are not part of GNU CC. Normally the facilities of the machine's usual C compiler are used, but this can't be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. -mhard-float Generate output containing floating point instructions. This is the default if you use the unmodified sources. -mfp64 Assume that the FR bit in the status word is on, and that there are 32 64-bit floating point registers, instead of 32 32-bit floating point registers. You must also specify the `- mcpu=r4000' and `-mips3' switches. -mfp32 Assume that there are 32 32-bit floating point registers. This is the default. -mabicalls -mno-abicalls Emit the `.abicalls', `.cpload', and `.cprestore' pseudo operations that some System V.4 ports use for position independent code. -mhalf-pic -mno-half-pic Put pointers to extern references into the data section and load them up, rather than put the references in the text section. These options do not work at present. -G num Put global and static items less than or equal to num bytes into the small data or bss sections instead of the normal data or bss section. This allows the assembler to emit one word memory reference instructions based on the global pointer (gp or $28), instead of the normal two words used. By default, num is 8 when the MIPS assembler is used, and 0 when the GNU assembler is used. The `-G num' switch is also passed to the assembler and linker. All modules should be compiled with the same `-G num' value. 58 Using GNU CC INTERNALS These options are defined by the macro TARGET_SWITCHES in the machine description. The default for the options is also defined by that macro, which enables you to change the defaults. 2.11. Options for Code Generation Conventions These machine-independent options control the interface conventions used in code generation. Most of them have both positive and negative forms; the negative form of `-ffoo' would be `-fno-foo'. In the table below, only one of the forms is listed---the one which is not the default. You can figure out the other form by either removing `no-' or adding it. -fpcc-struct-return Use the same convention for returning struct and union values that is used by the usual C compiler on your system. This convention is less efficient for small structures, and on many machines it fails to be reentrant; but it has the advantage of allowing intercallability between GNU CC-compiled code and PCC-compiled code. -fshort-enums Allocate to an enum type only as many bytes as it needs for the declared range of possible values. Specifically, the enum type will be equivalent to the smallest integer type which has enough room. -fshort-double Use the same size for double as for float. -fshared-data Requests that the data and non-const variables of this compilation be shared data rather than private data. The distinction makes sense only on certain operating systems, where shared data is shared between processes running the same program, while private data exists in one copy per process. -fno-common Allocate even uninitialized global variables in the bss section of the object file, rather than generating them as common blocks. This has the effect that if the same variable is declared (without extern) in two different compilations, you will get an error when you link them. The only reason this might be useful is if you wish to verify that the program will work on other systems which always work this way. Using GNU CC 59 -fno-ident Ignore the `#ident' directive. -fno-gnu-linker Don't output global initializations such as C++ constructors and destructors in the form used by the GNU linker (on systems where the GNU linker is the standard method of handling them). Use this option when you want to use a ``collect'' program and a non-GNU linker. -finhibit-size-directive Don't output a .size assembler directive, or anything else that would cause trouble if the function is split in the middle, and the two halves are placed at locations far apart in memory. This option is used when compiling `crtstuff.c'; you should not need to use it for anything else. -fvolatile Consider all memory references through pointers to be volatile. -fpic If supported for the target machines, generate position-independent code, suitable for use in a shared library. All addresses will be accessed through a global offset table (GOT). If the GOT size for the linked executable exceeds a machine- specific maximum size, you will get an error message from the linker indicating that `-fpic' does not work; recompile with `-fPIC' instead. (These maximums are 16k on the m88k, 8k on the Sparc, and 32k on the m68k and RS/6000. The 386 has no such limit.) Position-independent code requires special support, and therefore works only on certain machines. Code generated for the IBM RS/6000 is always position-independent. -fPIC If supported for the target machine, emit position-independent code, suitable for dynamic linking and avoiding any limit on the size of the global offset table. This option makes a difference on the m68k, m88k and the Sparc. Position-independent code requires special support, and therefore works only on certain machines. 60 Using GNU CC -ffixed-reg Treat the register named reg as a fixed register; generated code should never refer to it (except perhaps as a stack pointer, frame pointer or in some other fixed role). reg must be the name of a register. The register names accepted are machine-specific and are defined in the REGISTER_NAMES macro in the machine description macro file. This flag does not have a negative form, because it specifies a three-way choice. -fcall-used-reg Treat the register named reg as an allocatable register that is clobbered by function calls. It may be allocated for temporaries or variables that do not live across a call. Functions compiled this way will not save and restore the register reg. Use of this flag for a register that has a fixed pervasive role in the machine's execution model, such as the stack pointer or frame pointer, will produce disastrous results. This flag does not have a negative form, because it specifies a three-way choice. -fcall-saved-reg Treat the register named reg as an allocatable register saved by functions. It may be allocated even for temporaries or variables that live across a call. Functions compiled this way will save and restore the register reg if they use it. Use of this flag for a register that has a fixed pervasive role in the machine's execution model, such as the stack pointer or frame pointer, will produce disastrous results. A different sort of disaster will result from the use of this flag for a register in which function values may be returned. This flag does not have a negative form, because it specifies a three-way choice. Using GNU CC 61 2.12. Environment Variables Affecting GNU CC This section describes several environment variables that affect how GNU CC operates. They work by specifying directories or prefixes to use when searching for various kinds of files. INTERNALS Note that you can also specify places to search using options such as `-B', `-I' and `-L' (see sec- tion Directory Options). These take precedence over places specified using environment variables, which in turn take precedence over those specified by the configuration of GNU CC. INTERNALS Note that you can also specify places to search using options such as `-B', `-I' and `-L' (see section Directory Options). These take precedence over places specified using environment variables, which in turn take precedence over those specified by the configuration of GNU CC. See section Driver. TMPDIR If TMPDIR is set, it specifies the directory to use for temporary files. GNU CC uses temporary files to hold the output of one stage of compilation which is to be used as input to the next stage: for example, the output of the preprocessor, which is the input to the compiler proper. GCC_EXEC_PREFIX If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the names of the subprograms executed by the compiler. No slash is added when this prefix is combined with the name of a subprogram, but you can specify a prefix that ends with a slash if you wish. If GNU CC cannot find the subprogram using the specified prefix, it tries looking in the usual places for the subprogram. Other prefixes specified with `-B' take precedence over this prefix. This prefix is also used for finding files such as `crt0.o' that are used for linking. In addition, the prefix is used in an unusual way in finding the directories to search for header files. For each of the standard directories whose name normally begins with `/usr/local/lib/gcc' (more precisely, with the value of GCC_INCLUDE_DIR), GNU CC tries replacing that 62 Using GNU CC beginning with the specified prefix to produce an alternate directory name. Thus, with `-Bfoo/', GNU CC will search `foo/bar' where it would normally search `/usr/local/lib/bar'. These alternate directories are searched first; the standard directories come next. COMPILER_PATH The value of COMPILER_PATH is a colon-separated list of directories, much like PATH. GNU CC tries the directories thus specified when searching for subprograms, if it can't find the subprograms using GCC_EXEC_PREFIX. LIBRARY_PATH The value of LIBRARY_PATH is a colon-separated list of directories, much like PATH. GNU CC tries the directories thus specified when searching for special linker files, if it can't find them using GCC_EXEC_PREFIX. Linking using GNU CC also uses these directories when searching for ordinary libraries for the `-l' option (but directories specified with `-L' come first). C_INCLUDE_PATH C++_INCLUDE_PATH OBJC_INCLUDE_PATH variable's value is a colon-separated list of directories, much like PATH. When GNU CC searches for header files, it tries the directories listed in the variable for the language you are using, after the directories specified with `-I' but before the standard header file directories. DEPENDENCIES_OUTPUT If this variable is set, its value specifies how to output dependencies for Make based on the header files processed by the compiler. This output looks much like the output from the `-M' option (see section Preprocessor Options), but it goes to a separate file, and is in addition to the usual results of compilation. The value of DEPENDENCIES_OUTPUT can be just a file name, in which case the Make rules are written to that file, guessing the target name from the source file name. Or the value can have the form `file target', in which case the rules are written to file file using target as the target name. Using GNU CC 63 3. Installing GNU CC Here is the procedure for installing GNU CC on a Unix system. See below for VMS systems, and modified procedures needed on other systems including Sun, 3B1, SCO Unix and Unos. The following section says how to compile in a separate directory on Unix; here we assume you compile in the same directory that contains the source files. 1. If you have built GNU CC previously in the same directory for a different target machine, do `make cleanconfig' to delete all files that might be invalid. 2. On a Sequent system, go to the Berkeley universe. 3. On a System V release 4 system, make sure `/usr/bin' precedes `/usr/ucb' in PATH. The cc command in `/usr/ucb' uses libraries which have bugs. 4. Specify the host and target machine configurations. You do this by running the file `configure' with appropriate arguments. If you are building a compiler to produce code for the machine it runs on, specify just one machine type. To build a cross-compiler, specify two configurations, one for the host machine (which the compiler runs on), and one for the target machine (which the compiler produces code for). The command looks like this: configure --host=sun3-sunos3 --target=sparc-sun-sunos4.1 A configuration name may be canonical or it may be more or less abbreviated. A canonical configuration name has three parts, separated by dashes. It looks like this: `cpu-company-system'. (The three parts may themselves contain dashes; `configure' can figure out which dashes serve which purpose.) For example, `m68k-sun-sunos4.1' specifies a Sun 3. You can also replace parts of the configuration by nicknames or aliases. For 64 Using GNU CC example, `sun3' stands for `m68k-sun', so `sun3-sunos4.1' is another way to specify a Sun 3. You can also use simply `sun3-sunos', since the version of Sunos is assumed by default to be version 4. `sun3-bsd' also works, since `configure' knows that the only BSD variant on a Sun 3 is Sunos. You can specify a version number after any of the system types, and some of the CPU types. In most cases, the version is irrelevant, and will be ignored. So you might as well specify the version if you know it. Here are the possible CPU types: a29k, arm, cn, hppa, i386, i860, m68000, m68k, m88k, mips, ns32k, romp, rs6000, sparc, vax. Note that the type hppa currently works only with Berkeley systems, not with HP/UX. Here are the recognized company names. As you can see, customary abbreviations are used rather than the longer official names. alliant, altos, apollo, att, convergent, convex, crds, dec, dg, encore, harris, hp, ibm, mips, motorola, ncr, next, ns, omron, sequent, sgi, sony, sun, tti, unicom. The company name is meaningful only to disambiguate when the rest of the information supplied is insufficient. You can omit it, writing just `cpu-system', if it is not needed. For example, `vax-ultrix4.2' is equivalent to `vax-dec-ultrix4.2'. Here is a list of system types: bsd, sysv, mach, minix, genix, ultrix, vms, sco, esix, isc, aix, sunos, hpux, unos, luna, dgux, newsos, osfrose, osf, dynix, aos, ctix. You can omit the system type; then `configure' guesses the operating system from the CPU and company. Using GNU CC 65 Often a particular model of machine has a name. Many of these names are recognized as an alias for a CPU/company combination. The alias `sun3', mentioned above, is an example of this: it stands for `m68k-sun'. Sometimes we accept a company name as a machine name, when the name is popularly used for a particular machine. Here is a table of the known machine names: 3300, 3b1, 7300, altos3068, altos, apollo68, att-7300, balance, convex-cn, crds, decstation-3100, decstation-dec, decstation, delta, encore, gmicro, hp7nn, hp8nn, hp9k2nn, hp9k3nn, hp9k7nn, hp9k8nn, iris4d, iris, isi68, m3230, magnum, merlin, miniframe, mmax, news-3600, news800, news, next, pbd, pc532, pmax, ps2, risc-news, rtpc, sun2, sun386i, sun386, sun3, sun4, symmetry, tower-32, tower. If you specify an impossible combination such as `i860-dg-vms', then you may get an error message from `configure', or it may ignore part of the information and do the best it can with the rest. `configure' always prints the canonical name for the alternative that it used. On certain systems, you must specify whether you want GNU CC to work with the usual compilation tools or with the GNU compilation tools (including GAS). Use the `--gas' argument when you run `configure', if you want to use the GNU tools. The systems were this makes a difference are `i386-anything-sysv', `i860-anything-bsd', `m68k-hp-hpux', `m68k- sony-bsd', `m68k-altos-sysv', `m68000-hp- hpux', and `m68000-att-sysv'. On any other system, `--gas' has no effect. On certain systems, you must specify whether the machine has a floating point unit. These systems are `m68k-sun-sunosn' and `m68k-isi- bsd'. On any other system, `--nfp' currently has no effect, though perhaps there are other systems where it could usefully make a difference. If you want to install your own homemade configuration files, you can use `local' as 66 Using GNU CC the company name to access them. If you use configuration `cpu-local', the entire configuration name is used to form the configuration file names. Thus, if you specify `m68k-local', then the files used are `m68k-local.md', `m68k- local.h', `m68k-local.c', `xm-m68k-local.h', `t-m68k-local', and `x-m68k-local'. Here is a list of configurations that have special treatment: `m68000-att' AT&T 3b1, a.k.a. 7300 PC. Special procedures are needed to compile GNU CC with this machine's standard C compiler, due to bugs in that compiler. See section 3b1 Install. You can bootstrap it more easily with previous versions of GNU CC if you have them. `m68000-hp-bsd' HP 9000 series 200 running BSD. Note that the C compiler that comes with this system cannot compile GNU CC; contact law@super.org to get binaries of GNU CC for bootstrapping. `m68k-altos' Altos 3068. You must use the GNU assembler, linker and debugger, with COFF-encapsulation. Also, you must fix a kernel bug. Details in the file `ALTOS-README'. `m68k-hp-hpux' HP 9000 series 200 or 300 running HPUX. GNU CC does not support the special symbol table used by HP's debugger, but you can debug programs with GDB if you specify `--gas' to use the GNU tools instead. In order to use the GNU tools, you must install a library conversion program called hpxt. `m68k-sun' Sun 3. We do not provide a configuration file to use the Sun FPA by default, because programs that establish signal handlers for floating point traps inherently cannot work Using GNU CC 67 with the FPA. `m88k-dgux' Motorola m88k running DG/UX. To build native or cross compilers on DG/UX, you must first change to the 88open BCS software development environment. This is done by issuing this command: eval `sde-target m88kbcs` `ns32k-encore' Encore ns32000 system. Encore systems are supported only under BSD. `ns32k-*-genix' National Semiconductor ns32000 system. Genix has bugs in alloca and malloc; you must get the compiled versions of these from GNU Emacs. `ns32k-utek' UTEK ns32000 system (``merlin''). The C compiler that comes with this system cannot compile GNU CC; contact `tektronix!reed!mason' to get binaries of GNU CC for bootstrapping. `rs6000-ibm' IBM PowerStation/6000 machines. Due to the nonstandard debugging information required for this machine, `-g' is not available in this configuration. `vax-dec-ultrix' Don't try compiling with Vax C (vcc). It produces incorrect code in some cases (for example, when alloca is used). Meanwhile, compiling `cp-parse.c' with pcc does not work because of an internal table size limitation in that compiler. To avoid this problem, compile just the GNU C compiler first, and use it to recompile building all the languages that you want to run. Here we spell out what files will be set up by configure. Normally you need not be concerned 68 Using GNU CC with these files. o+ INTERNALS A symbolic link named `config.h' is made to the top-level config file for the machine you will run the compiler on (see section Config). This file is responsible for defining information about the host machine. It includes `tm.h'. INTERNALS A symbolic link named `config.h' is made to the top-level config file for the machine you plan to run the compiler on (see section Config,,The Configuration File, gcc.info, Using and Porting GCC). This file is responsible for defining information about the host machine. It includes `tm.h'. The top-level config file is located in the subdirectory `config'. Its name is always `xm-something.h'; usually `xm-machine.h', but there are some exceptions. If your system does not support symbolic links, you might want to set up `config.h' to contain a `#include' command which refers to the appropriate file. o+ A symbolic link named `tconfig.h' is made to the top-level config file for your target machine. This is used for compiling certain programs to run on that machine. o+ A symbolic link named `tm.h' is made to the machine-description macro file for your target machine. It should be in the subdirectory `config' and its name is often `machine.h'. o+ A symbolic link named `md' will be made to the machine description pattern file. It should be in the `config' subdirectory and its name should be `machine.md'; but machine is often not the same as the name used in the `tm.h' file because the `md' files are more general. o+ A symbolic link named `aux-output.c' will be made to the output subroutine file for your machine. It should be in the `config' subdirectory and its name should be `machine.c'. o+ The command file `configure' also constructs `Makefile' by adding some text to the Using GNU CC 69 template file `Makefile.in'. The additional text comes from files in the `config' directory, named `t-target' and `h-host'. If these files do not exist, it means nothing needs to be added for a given target or host. 5. Make sure the Bison parser generator is installed. (This is unnecessary if the Bison output files `c-parse.c' and `cexp.c' are more recent than `c- parse.y' and `cexp.y' and you do not plan to change the `.y' files.) Bison versions older than Sept 8, 1988 will produce incorrect output for `c-parse.c'. 6. Build the compiler. Just type `make LANGUAGES=c' in the compiler directory. `LANGUAGES=c' specifies that only the C compiler should be compiled. The makefile normally builds compilers for all the supported languages; currently, C, C++ and Objective C. However, C is the only language that is sure to work when you build with other non-GNU C compilers. In addition, building anything but C at this stage is a waste of time. In general, you can specify the languages to build by typing the argument `LANGUAGES="list"', where list is one or more words from the list `c', `c++', and `objective-c'. Ignore any warnings you may see about ``statement not reached'' in `insn-emit.c'; they are normal. Any other compilation errors may represent bugs in the port to your machine or operating system, and should be investigated and reported (see section Bugs). Some commercial compilers fail to compile GNU CC because they have bugs or limitations. For example, the Microsoft compiler is said to run out of macro space. Some Ultrix compilers run out of expression space; then you need to break up the statement where the problem happens. 7. If you are using COFF-encapsulation, you must convert `libgcc.a' to a GNU-format library at this point. See the file `README-ENCAP' in the directory containing the GNU binary file utilities, for directions. 70 Using GNU CC 8. Move the first-stage object files and executables into a subdirectory with this command: make stage1 The files are moved into a subdirectory named `stage1'. Once installation is complete, you may wish to delete these files with rm -r stage1. 9. Recompile the compiler with itself, with this command: make CC=stage1/gcc CFLAGS="-g -O -Bstage1/" This is called making the stage 2 compiler. The command shown above builds compilers for all the supported languages. If you don't want them all, you can specify the languages to build by typing the argument `LANGUAGES="list"'. list should contain one or more words from the list `c', `c++', and `objective-c', separated by spaces. On a 68000 or 68020 system lacking floating point hardware, unless you have selected a `tm.h' file that expects by default that there is no such hardware, do this instead: make CC=stage1/gcc CFLAGS="-g -O -Bstage1/ -msoft-float" 10. If you wish to test the compiler by compiling it with itself one more time, do this: make stage2 make CC=stage2/gcc CFLAGS="-g -O -Bstage2/" This is called making the stage 3 compiler. Aside from the `-B' option, the options should be the same as when you made the stage 2 compiler. Using GNU CC 71 Then compare the latest object files with the stage 2 object files---they ought to be identical, unless they contain time stamps. On systems where object files do not contain time stamps, you can do this (in Bourne shell): for file in *.o; do cmp $file stage2/$file done This will mention any object files that differ between stage 2 and stage 3. Any difference, no matter how innocuous, indicates that the stage 2 compiler has compiled GNU CC incorrectly, and is therefore a potentially serious bug which you should investigate and report (see section Bugs). On systems that use COFF object files, bytes 5 to 8 will always be different, since it is a timestamp. On these systems, you can do the comparison as follows (in Bourne shell): for file in *.o; do tail +10c $file > foo1 tail +10c stage2/$file > foo2 cmp foo1 foo2 || echo $file done On MIPS machines, you need to use the shell script `ecoff-cmp' to compare two object files if you have built the compiler with the `- mno-mips-tfile' option. Thus, do this: for file in *.o; do ecoff-cmp $file stage2/$file done 11. Install the compiler driver, the compiler's passes and run-time support. You can use the following command: 72 Using GNU CC make CC=stage2/gcc install (Use the same value for CC that you used when compiling the files that are being installed.) This copies the files `cc1', `cpp' and `libgcc.a' to files `cc1', `cpp' and `libgcc.a' in directory `/usr/local/lib/gcc/target/version', which is where the compiler driver program looks for them. Here target is the target machine type specified when you ran `configure', and version is the version number of GNU CC. This naming scheme permits various versions and/or cross-compilers to coexist. It also copies the driver program `gcc' into the directory `/usr/local/bin', so that it appears in typical execution search paths. Warning: there is a bug in alloca in the Sun library. To avoid this bug, install the binaries of GNU CC that were compiled by GNU CC. They use alloca as a built-in function and never the one in the library. 12. If you will be using C++ or Objective C, and your operating system does not handle constructors, then you must build and install the program collect2. Do this with the following command: make CC="stage2/gcc -O" install-collect2 The systems that do handle constructors on their own include system V release 4, and system V release 3 on the Intel 386. Berkeley systems that use the ``a.out'' object file format handle constructors without collect2 if you use the GNU linker. But if you don't use the GNU linker, then you need collect2 on these systems. 13. Build and install protoize if you want it. Type Using GNU CC 73 make CC="stage2/gcc -O" install-proto There is as yet no documentation for protoize. Sorry. 14. Correct errors in the header files on your machine. Various system header files often contain constructs which are incompatible with ANSI C, and they will not work when you compile programs with GNU CC. This behavior consists of substituting for macro argument names when they appear inside of character constants. The most common offender is `ioctl.h'. You can overcome this problem when you compile by specifying the `-traditional' option. Alternatively, on Sun systems and 4.3BSD at least, you can correct the include files by running the shell script `fixincludes'. This installs modified, corrected copies of the files `ioctl.h', `ttychars.h' and many others, in a special directory where only GNU CC will normally look for them. This script will work on various systems because it chooses the files by searching all the system headers for the problem cases that we know about. Use the following command to do this: make install-fixincludes If you selected a different directory for GNU CC installation when you installed it, by specifying the Make variable prefix or libdir, specify it the same way in this command. Note that some systems are starting to come with ANSI C system header files. On these systems, don't run `fixincludes'; it may not work, and is certainly not necessary. If you cannot install the compiler's passes and run- time support in `/usr/local/lib', you can alternatively use 74 Using GNU CC the `-B' option to specify a prefix by which they may be found. The compiler concatenates the prefix with the names `cpp', `cc1' and `libgcc.a'. Thus, you can put the files in a directory `/usr/foo/gcc' and specify `-B/usr/foo/gcc/' when you run GNU CC. Also, you can specify an alternative default directory for these files by setting the Make variable libdir when you make GNU CC. 3.1. Compilation in a Separate Directory If you wish to build the object files and executables in a directory other than the one containing the source files, here is what you must do differently: 1. Make sure you have a version of Make that supports the VPATH feature. (GNU Make supports it, as do Make versions on most BSD systems.) 2. Go to that directory before running `configure': mkdir gcc-sun3 cd gcc-sun3 On systems that do not support symbolic links, this directory must be on the same file system as the source code directory. 3. Specify where to find `configure' when you run it: ../gcc-2.00/configure ... This also tells configure where to find the compiler sources; configure takes the directory from the file name that was used to invoke it. But if you want to be sure, you can specify the source directory with the `-- srcdir' option, like this: ../gcc-2.00/configure --srcdir=../gcc-2.00 sun3 Using GNU CC 75 The directory you specify with `--srcdir' need not be the same as the one that configure is found in. Now, you can run make in that directory. You need not repeat the configuration steps shown above, when ordinary source files change. You must, however, run configure again when the configuration files change, if your system does not support symbolic links. 3.2. Installing GNU CC on the Sun Make sure the environment variable FLOAT_OPTION is not set when you compile `libgcc.a'. If this option were set to f68881 when `libgcc.a' is compiled, the resulting code would demand to be linked with a special startup file and would not link properly without special pains. There is a bug in alloca in certain versions of the Sun library. To avoid this bug, install the binaries of GNU CC that were compiled by GNU CC. They use alloca as a built-in function and never the one in the library. Some versions of the Sun compiler crash when compiling GNU CC. The problem is a segmentation fault in cpp. This problem seems to be due to the bulk of data in the environ- ment variables. You may be able to avoid it by using the following command to compile GNU CC with Sun CC: make CC="TERMCAP=x OBJS=x LIBFUNCS=x STAGESTUFF=x cc" 3.3. Installing GNU CC on the 3b1 Installing GNU CC on the 3b1 is difficult if you do not already have GNU CC running, due to bugs in the installed C compiler. However, the following procedure might work. We are unable to test it. 1. Comment out the `#include "config.h"' line on line 37 of `cccp.c' and do `make cpp'. This makes a preliminary version of GNU cpp. 2. Save the old `/lib/cpp' and copy the preliminary GNU cpp to that file name. 3. Undo your change in `cccp.c', or reinstall the original version, and do `make cpp' again. 76 Using GNU CC 4. Copy this final version of GNU cpp into `/lib/cpp'. 5. Replace every occurrence of obstack_free in the file `tree.c' with _obstack_free. 6. Run make to get the first-stage GNU CC. 7. Reinstall the original version of `/lib/cpp'. 8. Now you can compile GNU CC with itself and install it in the normal fashion. 3.4. Installing GNU CC on SCO System V 3.2 The compiler that comes with this system does not work properly with `-O'. Therefore, you should redefine the Make variable CCLIBFLAGS not to use `-O'. In addition, the compiler produces incorrect output when compiling parts of GNU CC; the resulting executable `cc1' does not work properly when it is used with `-O'. Therefore, what you must do after building the first stage is use GNU CC to compile itself without optimization. Here is how: make -k cc1 CC="./gcc -B./" You can think of this as ``stage 1.1'' of the installa- tion process. However, using this command has the effect of discarding the faulty stage 1 executable for `cc1' and replacing it with stage 1.1. You can then proceed with `make stage1' and the rest of installation. On Xenix, the same thing is necessary; in addition, you may have to remove `-g' from the options used with cc, and you may have to simplify complicated statements in the sources of GNU CC to get them to compile. 3.5. Installing GNU CC on Unos Use `configure unos' for building on Unos. The Unos assembler is named casm instead of as. For some strange reason linking `/bin/as' to `/bin/casm' changes the behavior, and does not work. So, when installing GNU CC, you should install the following script as `as' in the subdirectory where the passes of GCC are installed: Using GNU CC 77 #!/bin/sh casm $* The default Unos library is named `libunos.a' instead of `libc.a'. To allow GNU CC to function, either change all references to `-lc' in `gcc.c' to `-lunos' or link `/lib/libc.a' to `/lib/libunos.a'. When compiling GNU CC with the standard compiler, to overcome bugs in the support of alloca, do not use `-O' when making stage 2. Then use the stage 2 compiler with `-O' to make the stage 3 compiler. This compiler will have the same characteristics as the usual stage 2 compiler on other sys- tems. Use it to make a stage 4 compiler and compare that with stage 3 to verify proper compilation. Unos uses memory segmentation instead of demand paging, so you will need a lot of memory. 5 Mb is barely enough if no other tasks are running. If linking `cc1' fails, try putting the object files into a library and linking from that library. 3.6. Installing GNU CC on VMS The VMS version of GNU CC is distributed in a backup saveset containing both source code and precompiled binaries. To install the `gcc' command so you can use the com- piler easily, in the same manner as you use the VMS C com- piler, you must install the VMS CLD file for GNU CC as fol- lows: 1. Define the VMS logical names `GNU_CC' and `GNU_CC_INCLUDE' to point to the directories where the GNU CC executables (`gcc-cpp', `gcc-cc1', etc.) and the C include files are kept. This should be done with the commands: $ assign /super /system disk:[gcc.] gnu_cc $ assign /super /system disk:[gcc.include.] gnu_cc_include with the appropriate disk and directory names. These commands can be placed in your system startup file so they will be executed whenever the machine is rebooted. You may, if you choose, do this via the `GCC_INSTALL.COM' script in the `[GCC]' 78 Using GNU CC directory. 2. Install the `GCC' command with the command line: $ set command /table=sys$library:dcltables gnu_cc:[000000]gcc 3. To install the help file, do the following: $ lib/help sys$library:helplib.hlb gcc.hlp Now you can invoke the compiler with a command like `gcc /verbose file.c', which is equivalent to the command `gcc -v -c file.c' in Unix. If you wish to use GNU C++ you must first install GNU CC, and then perform the following steps: 1. Define the VMS logical name `GNU_GXX_INCLUDE' to point to the directory where the preprocessor will search for the C++ header files. This can be done with the command: $ assign /super /system disk:[gcc.gxx_include.] gnu_gxx_include with the appropriate disk and directory name. If you are going to be using libg++, you should place the libg++ header files in the directory that this logical name points to. 2. Obtain the file `gcc-cc1plus.exe', and place this in the same directory that `gcc-cc1.exe' is kept. 3. You will need several library functions which are used to call the constructors and destructors for global objects. These functions are part of the libg++ distribution, and you will automatically get them if you install libg++. If you are not planning to install libg++, you will need to obtain the files `gxx-startup- Using GNU CC 79 1.mar' and `gstart.cc' from the libg++ distribution, compile them, and supply them to the linker whenever you link a C++ program. The GNU C++ compiler can be invoked with a command like `gcc /plus /verbose file.cc', which is equivalent to the command `g++ -v -c file.cc' in Unix. We try to put corresponding binaries and sources on the VMS distribution tape. But sometimes the binaries will be from an older version that the sources, because we don't always have time to update them. (Use the `/version' option to determine the version number of the binaries and compare it with the source file `version.c' to tell whether this is so.) In this case, you should use the binaries you get to recompile the sources. If you must recompile, here is how: 1. Copy the file `vms.h' to `tm.h', `xm-vms.h' to `config.h', `vax.md' to `md.' and `vax.c' to `aux-output.c'. The files to be copied are found in the subdirectory named `config'; they should be copied to the main directory of GNU CC. If you wish, you may use the command file `config- gcc.com' to perform these steps for you. 2. Setup the logical names and command tables as defined above. In addition, define the VMS logical name `GNU_BISON' to point at the to the directories where the Bison executable is kept. This should be done with the command: $ assign /super /system disk:[bison.] gnu_bison You may, if you choose, use the `INSTALL_BISON.COM' script in the `[BISON]' directory. 3. Install the `BISON' command with the command line: $ set command /table=sys$library:dcltables gnu_bison:[000000]bison 4. Type `@make-gcc' to recompile everything (alternatively, you may submit the file `make-gcc.com' to a batch queue). If you wish 80 Using GNU CC to build the GNU C++ compiler as well as the GNU CC compiler, you must first edit `make- gcc.com' and follow the instructions that appear in the comments. If you are building GNU CC with a previous version of GNU CC, you also should check to see that you have the newest version of the assembler. In particular, GNU CC version 2 treats global constant variables slightly differently from GNU CC version 1, and GAS version 1.38.1 does not have the patches required to work with GCC version 2. If you use GAS 1.38.1, then extern const variables will not have the read-only bit set, and the linker will generate warning messages about mismatched psect attributes for these variables. These warning messages are merely a nuisance, and can safely be ignored. If you are compiling with a version of GNU CC older than 1.33, specify `/DEFINE=("inline=")' as an option in all the compilations. This requires editing all the gcc commands in `make-cc1.com'. (The older versions had problems supporting inline.) Once you have a working 1.33 or newer GNU CC, you can change this file back. Under previous versions of GNU CC, the generated code would occasionally give strange results when linked to the sharable `VAXCRTL' library. Now this should work. Even with this version, however, GNU CC itself should not be linked to the sharable `VAXCRTL'. The qsort routine supplied with `VAXCRTL' has a bug which can cause a compiler crash. Similarly, the preprocessor should not be linked to the sharable `VAXCRTL'. The strncat routine supplied with `VAXCRTL' has a bug which can cause the preprocessor to go into an infinite loop. If you attempt to link to the sharable `VAXCRTL', the VMS linker will strongly resist any effort to force it to use the qsort and strncat routines from `gcclib'. Until the bugs in `VAXCRTL' have been fixed, linking any of the com- piler components to the sharable VAXCRTL is not recommended. (These routines can be bypassed by placing duplicate copies of qsort and strncat in `gcclib' under different names, and patching the compiler sources to use these routines). Both of the bugs in `VAXCRTL' are still present in VMS version Using GNU CC 81 5.4-1, which is the most recent version as of this writing. The executables that are generated by `make-cc1.com' and `make-cccp.com' use the nonshared version of `VAXCRTL' (and thus use the qsort and strncat routines from `gcclib.olb'). 4. Known Causes of Trouble with GNU CC Here are some of the things that have caused trouble for people installing or using GNU CC. o+ On certain systems, defining certain environment variables such as CC can interfere with the functioning of make. o+ Cross compilation can run into trouble for certain machines because some target machines' assemblers require floating point numbers to be written as integer constants in certain contexts. The compiler writes these integer constants by examining the floating point value as an integer and printing that integer, because this is simple to write and independent of the details of the floating point representation. But this does not work if the compiler is running on a different machine with an incompatible floating point format, or even a different byte-ordering. In addition, correct constant folding of floating point values requires representing them in the target machine's format. (The C standard does not quite require this, but in practice it is the only way to win.) INTERNALS It is now possible to overcome these problems by defining macros such as REAL_VALUE_TYPE. But doing so is a substantial amount of work for each target machine. See section Cross-compilation. INTERNALS It is now possible to overcome these problems by defining macros such as REAL_VALUE_TYPE. But doing so is a substantial amount of work for each target machine. See section Cross-compilation,,Cross Compilation and Floating Point Format, gcc.info, Using and Porting GCC. o+ Users often think it is a bug when GNU CC reports an error for code like this: 82 Using GNU CC int foo (short); int foo (x) short x; {...} The error message is correct: this code really is erroneous, because the old-style non- prototype definition passes subword integers in their promoted types. In other words, the argument is really an int, not a short. The correct prototype is this: int foo (int); o+ Users often think it is a bug when GNU CC reports an error for code like this: int foo (struct mumble *); struct mumble { ... }; int foo (struct mumble *x) { ... } This code really is erroneous, because the scope of struct mumble the prototype is limited to the argument list containing it. It does not refer to the struct mumble defined with file scope immediately below---they are two unrelated types with similar names in different scopes. But in the definition of foo, the file-scope type is used because that is available to be inherited. Thus, the definition and the prototype do not match, and you get an error. This behavior may seem silly, but it's what the ANSI standard specifies. It is easy enough for you to make your code work by moving the definition of struct mumble above the prototype. It's not worth being incompatible with ANSI C just to avoid an Using GNU CC 83 error for the example shown above. o+ Certain local variables aren't recognized by debuggers when you compile with optimization. This occurs because sometimes GNU CC optimizes the variable out of existence. There is no way to tell the debugger how to compute the value such a variable ``would have had'', and it is not clear that would be desirable anyway. So GNU CC simply does not mention the eliminated variable when it writes debugging information. You have to expect a certain amount of disagreement between the executable and your source code, when you use optimization. o+ -2147483648 is positive. This is because 2147483648 cannot fit in the type int, so (following the ANSI C rules) its data type is unsigned long int. Negating this value yields 2147483648 again. o+ Sometimes on a Sun 4 you may observe a crash in the program genflags while building GCC. This is said to be due to a bug in sh. You can probably get around it by running genflags manually and then retrying the make. o+ On some versions of Ultrix, the system supplied compiler cannot compile `cp-parse.c' because it cannot handle so many cases in a switch statement. You can work around this problem by compiling with GNU CC. o+ On some BSD systems including some versions of Ultrix, use of profiling causes static variable destructors (currently used only in C++) not to be run. o+ On the IBM RS/6000, compiling code of the form extern int foo; ... foo ... 84 Using GNU CC static int foo; will cause the linker to report an undefined symbol foo. Although this behavior differs from most other systems, it is not a bug because redefining an extern variable as static is undefined in ANSI C. For additional common problems, see `Incompatibili- ties'. 5. How To Get Help with GNU CC If you need help installing, using or changing GNU CC, there are two ways to find it: o+ Send a message to a suitable network mailing list. First try bug-gcc@prep.ai.mit.edu, and if that brings no response, try help-gcc@prep.ai.mit.edu. o+ Look in the service directory for someone who might help you for a fee. The service directory is found in the file named `SERVICE' in the GNU CC distribution. 6. Incompatibilities of GNU CC There are several noteworthy incompatibilities between GNU C and most existing (non-ANSI) versions of C. The `- traditional' option eliminates most of these incompatibili- ties, but not all, by telling GNU C to behave like the other C compilers. o+ GNU CC normally makes string constants read-only. If several identical-looking string constants are used, GNU CC stores only one copy of the string. One consequence is that you cannot call mktemp with a string constant argument. The function mktemp always alters the string its argument points to. Another consequence is that sscanf does not work on some systems when passed a string constant as its format control string or input. This is because sscanf incorrectly tries to write into the string constant. Likewise fscanf and scanf. Using GNU CC 85 The best solution to these problems is to change the program to use char-array variables with initialization strings for these purposes instead of string constants. But if this is not possible, you can use the `-fwritable-strings' flag, which directs GNU CC to handle string constants the same way most C compilers do. `-traditional' also has this effect, among others. o+ GNU CC does not substitute macro arguments when they appear inside of string constants. For example, the following macro in GNU CC #define foo(a) "a" will produce output "a" regardless of what the argument a is. The `-traditional' option directs GNU CC to handle such cases (among others) in the old- fashioned (non-ANSI) fashion. o+ When you use setjmp and longjmp, the only automatic variables guaranteed to remain valid are those declared volatile. This is a consequence of automatic register allocation. Consider this function: jmp_buf j; foo () { int a, b; a = fun1 (); if (setjmp (j)) return a; a = fun2 (); /* longjmp (j) may occur in fun3. */ return a + fun3 (); } Here a may or may not be restored to its first value when the longjmp occurs. If a is allocated in a register, then its first value is restored; otherwise, it keeps the last 86 Using GNU CC value stored in it. If you use the `-W' option with the `-O' option, you will get a warning when GNU CC thinks such a problem might be possible. The `-traditional' option directs GNU C to put variables in the stack by default, rather than in registers, in functions that call setjmp. This results in the behavior found in traditional C compilers. o+ Declarations of external variables and functions within a block apply only to the block containing the declaration. In other words, they have the same scope as any other declaration in the same place. In some other C compilers, a extern declaration affects all the rest of the file even if it happens within a block. The `-traditional' option directs GNU C to treat all extern declarations as global, like traditional compilers. o+ In traditional C, you can combine long, etc., with a typedef name, as shown here: typedef int foo; typedef long foo bar; In ANSI C, this is not allowed: long and other type modifiers require an explicit int. Because this criterion is expressed by Bison grammar rules rather than C code, the `- traditional' flag cannot alter it. o+ PCC allows typedef names to be used as function parameters. The difficulty described immediately above applies here too. o+ PCC allows whitespace in the middle of compound assignment operators such as `+='. GNU CC, following the ANSI standard, does not allow this. The difficulty described immediately above applies here too. o+ GNU CC will flag unterminated character constants inside of preprocessor conditionals Using GNU CC 87 that fail. Some programs have English comments enclosed in conditionals that are guaranteed to fail; if these comments contain apostrophes, GNU CC will probably report an error. For example, this code would produce an error: #if 0 You can't expect this to work. #endif The best solution to such a problem is to put the text into an actual C comment delimited by `/*...*/'. However, `-traditional' suppresses these error messages. o+ When compiling functions that return float, PCC converts it to a double. GNU CC actually returns a float. If you are concerned with PCC compatibility, you should declare your functions to return double; you might as well say what you mean. o+ When compiling functions that return structures or unions, GNU CC output code normally uses a method different from that used on most versions of Unix. As a result, code compiled with GNU CC cannot call a structure-returning function compiled with PCC, and vice versa. The method used by GNU CC is as follows: a structure or union which is 1, 2, 4 or 8 bytes long is returned like a scalar. A structure or union with any other size is stored into an address supplied by the caller (usually in a special, fixed register, but on some machines it is passed on the stack). The machine- description macros STRUCT_VALUE and STRUCT_INCOMING_VALUE tell GNU CC where to pass this address. By contrast, PCC on most target machines returns structures and unions of any size by copying the data into an area of static storage, and then returning the address of that storage as if it were a pointer value. The caller must copy the data from that memory area to the place where the value is wanted. GNU CC does not use this method because it is 88 Using GNU CC slower and nonreentrant. On some newer machines, PCC uses a reentrant convention for all structure and union returning. GNU CC on most of these machines uses a compatible convention when returning structures and unions in memory, but still returns small structures and unions in registers. You can tell GNU CC to use a compatible convention for all structure and union returning with the option `-fpcc-struct- return'. There are also system-specific incompatibilities. o+ On the Alliant, the system's own convention for returning structures and unions is unusual, and is not compatible with GNU CC no matter what options are used. o+ On the IBM RT PC, the MetaWare HighC compiler (hc) uses yet another convention for structure and union returning. Use `-mhc-struct-return' to tell GNU CC to use a convention compatible with it. o+ On Ultrix, the Fortran compiler expects registers 2 through 5 to be saved by function calls. However, the C compiler uses conventions compatible with BSD Unix: registers 2 through 5 may be clobbered by function calls. GNU CC uses the same convention as the Ultrix C compiler. You can use these options to produce code compatible with the Fortran compiler: -fcall-saved-r2 -fcall-saved-r3 -fcall-saved-r4 -fcall-saved-r5 Using GNU CC 89 o+ DBX rejects some files produced by GNU CC, though it accepts similar constructs in output from PCC. Until someone can supply a coherent description of what is valid DBX input and what is not, there is nothing I can do about these problems. You are on your own. 7. GNU Extensions to the C Language GNU C provides several language features not found in ANSI standard C. (The `-pedantic' option directs GNU CC to print a warning message if any of these features is used.) To test for the availability of these features in condi- tional compilation, check for a predefined macro __GNUC__, which is always defined under GNU CC. 7.1. Statements and Declarations within Expressions A compound statement in parentheses may appear inside an expression in GNU C. This allows you to declare vari- ables within an expression. For example: ({ int y = foo (); int z; if (y > 0) z = y; else z = - y; z; }) is a valid (though slightly more complex than necessary) expression for the absolute value of foo (). This feature is especially useful in making macro definitions ``safe'' (so that they evaluate each operand exactly once). For example, the ``maximum'' function is commonly defined as a macro in standard C as follows: #define max(a,b) ((a) > (b) ? (a) : (b)) But this definition computes either a or b twice, with bad results if the operand has side effects. In GNU C, if you know the type of the operands (here let's assume int), you can define the macro safely as follows: #define maxint(a,b) \ 90 Using GNU CC ({int _a = (a), _b = (b); _a > _b ? _a : _b; }) Embedded statements are not allowed in constant expres- sions, such as the value of an enumeration constant, the width of a bit field, or the initial value of a static vari- able. If you don't know the type of the operand, you can still do this, but you must use typeof (see section Typeof) or type naming (see section Naming Types). 7.2. Locally Declared Labels Each statement expression is a scope in which local labels can be declared. A local label is simply an identif- ier; you can jump to it with an ordinary goto statement, but only from within the statement expression it belongs to. A local label declaration looks like this: __label__ label; or __label__ label1, label2, ...; Local label declarations must come at the beginning of the statement expression, right after the `({', before any ordinary declarations. The label declaration defines the label name, but does not define the label itself. You must do this in the usual way, with label:, within the statements of the statement expression. The local label feature is useful because statement expressions are often used in macros. If the macro contains nested loops, a goto can be useful for breaking out of them. However, an ordinary label whose scope is the whole function cannot be used: if the macro can be expanded several times in one function, the label will be multiply defined in that function. A local label avoids this problem. For example: #define SEARCH(array, target) \ Using GNU CC 91 ({ \ __label__ found; \ typeof (target) _SEARCH_target = (target); \ typeof (*(array)) *_SEARCH_array = (array); \ int i, j; \ int value; \ for (i = 0; i < max; i++) \ for (j = 0; j < max; j++) \ if (_SEARCH_array[i][j] == _SEARCH_target) \ { value = i; goto found; } \ value = -1; \ found: \ value; \ }) 7.3. Labels as Values You can get the address of a label defined in the current function (or a containing function) with the unary operator `&&'. The value has type void *. This value is a constant and can be used wherever a constant of that type is valid. For example: void *ptr; ... ptr = &&foo; To use these values, you need to be able to jump to one. This is done with the computed goto statementThe , goto *exp;. For example, goto *ptr; Any expression of type void * is allowed. One way of using these constants is in initializing a static array that will serve as a jump table: ____________________ The analogous feature in Fortran is called an assigned goto, but that name seems inappropriate in C, where one can do more than simply store label addresses in label vari- ables. 92 Using GNU CC static void *array[] = { &&foo, &&bar, &&hack }; Then you can select a label with indexing, like this: goto *array[i]; Note that this does not check whether the subscript is in bounds---array indexing in C never does that. Such an array of label values serves a purpose much like that of the switch statement. The switch statement is cleaner, so use that rather than an array unless the problem does not fit a switch statement very well. Another use of label values is in an interpreter for threaded code. The labels within the interpreter function can be stored in the threaded code for super-fast dispatch- ing. 7.4. Nested Functions A nested function is a function defined inside another function. The nested function's name is local to the block where it is defined. For example, here we define a nested function named square, and call it twice: foo (double a, double b) { double square (double z) { return z * z; } return square (a) + square (b); } The nested function can access all the variables of the containing function that are visible at the point of its definition. This is called lexical scoping. For example, here we show a nested function which uses an inherited vari- able named offset: bar (int *array, int offset, int size) { int access (int *array, int index) { return array[index + offset]; } Using GNU CC 93 int i; ... for (i = 0; i < size; i++) ... access (array, i) ... } It is possible to call the nested function from outside the scope of its name by storing its address or passing the address to another function: hack (int *array, int size) { void store (int index, int value) { array[index] = value; } intermediate (store, size); } Here, the function intermediate receives the address of store as an argument. If intermediate calls store, the arguments given to store are used to store into array. But this technique works only so long as the containing function (hack, in this example) does not exit. If you try to call the nested function through its address after the containing function has exited, all hell will break loose. A nested function can jump to a label inherited from a containing function, provided the label was explicitly declared in the containing function (see section Local Labels). Such a jump returns instantly to the containing function, exiting the nested function which did the goto and any intermediate functions as well. Here is an example: bar (int *array, int offset, int size) { __label__ failure; int access (int *array, int index) { if (index > size) goto failure; return array[index + offset]; } int i; ... for (i = 0; i < size; i++) ... access (array, i) ... ... 94 Using GNU CC return 0; /* Control comes here from access if it detects an error. */ failure: return -1; } A nested function always has internal linkage. Declar- ing one with extern is erroneous. If you need to declare the nested function before its definition, use auto (which is otherwise meaningless for function declarations). bar (int *array, int offset, int size) { __label__ failure; auto int access (int *, int); ... int access (int *array, int index) { if (index > size) goto failure; return array[index + offset]; } ... } 7.5. Naming an Expression's Type You can give a name to the type of an expression using a typedef declaration with an initializer. Here is how to define name as a type name for the type of exp: typedef name = exp; This is useful in conjunction with the statements- within-expressions feature. Here is how the two together can be used to define a safe ``maximum'' macro that operates on any arithmetic type: #define max(a,b) \ ({typedef _ta = (a), _tb = (b); \ _ta _a = (a); _tb _b = (b); \ _a > _b ? _a : _b; }) Using GNU CC 95 The reason for using names that start with underscores for the local variables is to avoid conflicts with variable names that occur within the expressions that are substituted for a and b. Eventually we hope to design a new form of declaration syntax that allows you to declare variables whose scopes start only after their initializers; this will be a more reliable way to prevent such conflicts. 7.6. Referring to a Type with typeof Another way to refer to the type of an expression is with typeof. The syntax of using of this keyword looks like sizeof, but the construct acts semantically like a type name defined with typedef. There are two ways of writing the argument to typeof: with an expression or with a type. Here is an example with an expression: typeof (x[0](1)) This assumes that x is an array of functions; the type described is that of the values of the functions. Here is an example with a typename as the argument: typeof (int *) Here the type described is that of pointers to int. If you are writing a header file that must work when included in ANSI C programs, write __typeof__ instead of typeof. See section Alternate Keywords. A typeof-construct can be used anywhere a typedef name could be used. For example, you can use it in a declara- tion, in a cast, or inside of sizeof or typeof. o+ This declares y with the type of what x points to. typeof (*x) y; 96 Using GNU CC o+ This declares y as an array of such values. typeof (*x) y[4]; o+ This declares y as an array of pointers to characters: typeof (typeof (char *)[4]) y; It is equivalent to the following traditional C declaration: char *y[4]; To see the meaning of the declaration using typeof, and why it might be a useful way to write, let's rewrite it with these macros: #define pointer(T) typeof(T *) #define array(T, N) typeof(T [N]) Now the declaration can be rewritten this way: array (pointer (char), 4) y; Thus, array (pointer (char), 4) is the type of arrays of 4 pointers to char. 7.7. Generalized Lvalues Compound expressions, condi- tional expressions and casts are allowed as lvalues provided their operands are lvalues. This means that you can take their addresses or store values into them. For example, a compound expression can be assigned, provided the last expression in the sequence is an lvalue. These two expressions are equivalent: Using GNU CC 97 (a, b) += 5 a, (b += 5) Similarly, the address of the compound expression can be taken. These two expressions are equivalent: &(a, b) a, &b A conditional expression is a valid lvalue if its type is not void and the true and false branches are both valid lvalues. For example, these two expressions are equivalent: (a ? b : c) = 5 (a ? b = 5 : (c = 5)) A cast is a valid lvalue if its operand is an lvalue. A simple assignment whose left-hand side is a cast works by converting the right-hand side first to the specified type, then to the type of the inner left-hand side expression. After this is stored, the value is converted back to the specified type to become the value of the assignment. Thus, if a has type char *, the following two expressions are equivalent: (int)a = 5 (int)(a = (char *)(int)5) An assignment-with-arithmetic operation such as `+=' applied to a cast performs the arithmetic using the type resulting from the cast, and then continues as in the previ- ous case. Therefore, these two expressions are equivalent: (int)a += 5 (int)(a = (char *)(int) ((int)a + 5)) You cannot take the address of an lvalue cast, because the use of its address would not work out coherently. 98 Using GNU CC Suppose that &(int)f were permitted, where f has type float. Then the following statement would try to store an integer bit-pattern where a floating point number belongs: *&(int)f = 1; This is quite different from what (int)f = 1 would do- --that would convert 1 to floating point and store it. Rather than cause this inconsistancy, we think it is better to prohibit use of `&' on a cast. If you really do want an int * pointer with the address of f, you can simply write (int *)&f. 7.8. Conditional Expressions with Omitted Operands The middle operand in a conditional expression may be omitted. Then if the first operand is nonzero, its value is the value of the conditional expression. Therefore, the expression x ? : y has the value of x if that is nonzero; otherwise, the value of y. This example is perfectly equivalent to x ? x : y In this simple case, the ability to omit the middle operand is not especially useful. When it becomes useful is when the first operand does, or may (if it is a macro argument), contain a side effect. Then repeating the operand in the middle would perform the side effect twice. Omitting the middle operand uses the value already computed without the undesirable effects of recomputing it. 7.9. Double-Word Integers GNU C supports data types for integers that are twice as long as long int. Simply write long long int for a signed integer, or unsigned long long int for an unsigned Using GNU CC 99 integer. You can use these types in arithmetic like any other integer types. Addition, subtraction, and bitwise boolean operations on these types are open-coded on all types of machines. Multiplication is open-coded if the machine sup- ports fullword-to-doubleword a widening multiply instruc- tion. Division and shifts are open-coded only on machines that provide special support. The operations that are not open-coded use special library routines that come with GNU CC. There may be pitfalls when you use long long types for function arguments, unless you declare function prototypes. If a function expects type int for its argument, and you pass a value of type long long int, confusion will result because the caller and the subroutine will disagree about the number of bytes for the argument. Likewise, if the function expects long long int and you pass int. The best way to avoid such problems is to use prototypes. 7.10. Arrays of Length Zero Zero-length arrays are allowed in GNU C. They are very useful as the last element of a structure which is really a header for a variable-length object: struct line { int length; char contents[0]; }; { struct line *thisline = (struct line *) malloc (sizeof (struct line) + this_length); thisline->length = this_length; } In standard C, you would have to give contents a length of 1, which means either you waste space or complicate the argument to malloc. 7.11. Arrays of Variable Length Variable-length automatic arrays are allowed in GNU C. These arrays are declared like any other automatic arrays, but with a length that is not a constant expression. The storage is allocated at the point of declaration and deallo- cated when the brace-level is exited. For example: 100 Using GNU CC FILE * concat_fopen (char *s1, char *s2, char *mode) { char str[strlen (s1) + strlen (s2) + 1]; strcpy (str, s1); strcat (str, s2); return fopen (str, mode); } Jumping or breaking out of the scope of the array name deallocates the storage. Jumping into the scope is not allowed; you get an error message for it. You can use the function alloca to get an effect much like variable-length arrays. The function alloca is avail- able in many other C implementations (but not in all). On the other hand, variable-length arrays are more elegant. There are other differences between these two methods. Space allocated with alloca exists until the containing function returns. The space for a variable-length array is deallocated as soon as the array name's scope ends. (If you use both variable-length arrays and alloca in the same func- tion, deallocation of a variable-length array will also deallocate anything more recently allocated with alloca.) You can also use variable-length arrays as arguments to functions: struct entry tester (int len, char data[len][len]) { ... } The length of an array is computed once when the storage is allocated and is remembered for the scope of the array in case you access it with sizeof. If you want to pass the array first and the length afterward, you can use a forward declaration in the parame- ter list---another GNU extension. struct entry tester (int len; char data[len][len], int len) { Using GNU CC 101 ... } The `int len' before the semicolon is a parameter for- ward declaration, and it serves the purpose of making the name len known when the declaration of data is parsed. You can write any number of such parameter forward declarations in the parameter list. They can be separated by commas or semicolons, but the last one must end with a semicolon, which is followed by the ``real'' parameter declarations. Each forward declaration must match a ``real'' declaration in parameter name and data type. 7.12. Non-Lvalue Arrays May Have Subscripts Subscripting is allowed on arrays that are not lvalues, even though the unary `&' operator is not. For example, this is valid in GNU C though not valid in other C dialects: struct foo {int a[4];}; struct foo f(); bar (int index) { return f().a[index]; } 7.13. Arithmetic on void- and Function-Pointers In GNU C, addition and subtraction operations are sup- ported on pointers to void and on pointers to functions. This is done by treating the size of a void or of a function as 1. A consequence of this is that sizeof is also allowed on void and on function types, and returns 1. The option `-Wpointer-arith' requests a warning if these extensions are used. 7.14. Non-Constant Initializers The elements of an aggregate initializer for an automatic variable are not required to be constant expres- sions in GNU C. Here is an example of an initializer with run-time varying elements: 102 Using GNU CC foo (float f, float g) { float beat_freqs[2] = { f-g, f+g }; ... } 7.15. Constructor Expressions GNU C supports constructor expressions. A constructor looks like a cast containing an initializer. Its value is an object of the type specified in the cast, containing the elements specified in the initializer. Usually, the specified type is a structure. Assume that struct foo and structure are declared as shown: struct foo {int a; char b[2];} structure; Here is an example of constructing a struct foo with a con- structor: structure = ((struct foo) {x + y, 'a', 0}); This is equivalent to writing the following: { struct foo temp = {x + y, 'a', 0}; structure = temp; } You can also construct an array. If all the elements of the constructor are (made up of) simple constant expres- sions, suitable for use in initializers, then the construc- tor is an lvalue and can be coerced to a pointer to its first element, as shown here: char **foo = (char *[]) { "x", "y", "z" }; Using GNU CC 103 Array constructors whose elements are not simple con- stants are not very useful, because the constructor is not an lvalue. There are only two valid ways to use it: to sub- script it, or initialize an array variable with it. The former is probably slower than a switch statement, while the latter does the same thing an ordinary C initializer would do. Here is an example of subscripting an array construc- tor: output = ((int[]) { 2, x, 28 }) [input]; Constructor expressions for scalar types and union types are is also allowed, but then the constructor expres- sion is equivalent to a cast. 7.16. Labeled Elements in Initializers Standard C requires the elements of an initializer to appear in a fixed order, the same as the order of the ele- ments in the array or structure being initialized. In GNU C you can give the elements in any order, speci- fying the array indices or structure field names they apply to. To specify an array index, write `[index]' before the element value. For example, int a[6] = { [4] 29, [2] 15 }; is equivalent to int a[6] = { 0, 0, 15, 0, 29, 0 }; The index values must be constant expressions, even if the array being initialized is automatic. In a structure initializer, specify the name of a field to initialize with `fieldname:' before the element value. For example, given the following structure, struct point { int x, y; }; 104 Using GNU CC the following initialization struct point p = { y: yvalue, x: xvalue }; is equivalent to struct point p = { xvalue, yvalue }; You can also use an element label when initializing a union, to specify which element of the union should be used. For example, union foo { int i; double d; }; union foo f = { d: 4 }; will convert 4 to a double to store it in the union using the second element. By contrast, casting 4 to type union foo would store it into the union as the integer i, since it is an integer. (See section Cast to Union.) You can combine this technique of naming elements with ordinary C initialization of successive elements. Each ini- tializer element that does not have a label applies to the next consecutive element of the array or structure. For example, int a[6] = { [1] v1, v2, [4] v4 }; is equivalent to int a[6] = { 0, v1, v2, 0, v4, 0 }; Labeling the elements of an array initializer is espe- cially useful when the indices are characters or belong to an enum type. For example: Using GNU CC 105 int whitespace[256] = { [' '] 1, ['\t'] 1, ['\h'] 1, ['\f'] 1, ['\n'] 1, ['\r'] 1 }; 7.17. Case Ranges You can specify a range of consecutive values in a sin- gle case label, like this: case low ... high: This has the same effect as the proper number of individual case labels, one for each integer value from low to high, inclusive. This feature is especially useful for ranges of ASCII character codes: case 'A' ... 'Z': Be careful: Write spaces around the ..., for otherwise it may be parsed wrong when you use it with integer values. For example, write this: case 1 ... 5: rather than this: case 1...5: 7.18. Cast to a Union Type A cast to union type is like any other cast, except that the type specified is a union type. You can specify the type either with union tag or with a typedef name. The types that may be cast to the union type are those of the members of the union. Thus, given the following 106 Using GNU CC union and variables: union foo { int i; double d; }; int x; double y; both x and y can be cast to type union foo. Using the cast as the right-hand side of an assignment to a variable of union type is equivalent to storing in a member of the union: union foo u; ... u = (union foo) x =_ u.i = x u = (union foo) y =_ u.d = y You can also use the union cast as a function argument: void hack (union foo); ... hack ((union foo) x); 7.19. Declaring Attributes of Functions In GNU C, you declare certain things about functions called in your program which help the compiler optimize function calls. A few standard library functions, such as abort and exit, cannot return. GNU CC knows this automatically. Some programs define their own functions that never return. You can declare them volatile to tell the compiler this fact. For example, extern void volatile fatal (); void fatal (...) { ... /* Print error message. */ ... exit (1); } Using GNU CC 107 The volatile keyword tells the compiler to assume that fatal cannot return. This makes slightly better code, but more importantly it helps avoid spurious warnings of unini- tialized variables. It does not make sense for a volatile function to have a return type other than void. Many functions do not examine any values except their arguments, and have no effects except the return value. Such a function can be subject to common subexpression elim- ination and loop optimization just as an arithmetic operator would be. These functions should be declared const. For example, extern int const square (); says that the hypothetical function square is safe to call fewer times than the program says. Note that a function that has pointer arguments and examines the data pointed to must not be declared const. Likewise, a function that calls a non-const function usually must not be const. It does not make sense for a const func- tion to return void. We recommend placing the keyword const after the function's return type. It makes no difference in the exam- ple above, but when the return type is a pointer, it is the only way to make the function itself const. For example, const char *mincp (int); says that mincp returns const char *---a pointer to a const object. To declare mincp const, you must write this: char * const mincp (int); Some people object to this feature, suggesting that ANSI C's #pragma should be used instead. There are two rea- sons for not doing this. 1. It is impossible to generate #pragma commands from a macro. 108 Using GNU CC 2. The #pragma command is just as likely as these keywords to mean something else in another compiler. These two reasons apply to almost any application that might be proposed for #pragma. It is basically a mistake to use #pragma for anything. 7.20. Dollar Signs in Identifier Names In GNU C, you may use dollar signs in identifier names. This is because many traditional C implementations allow such identifiers. Dollar signs are allowed on certain machines if you specify `-traditional'. On a few systems they are allowed by default, even if `-traditional' is not used. But they are never allowed if you specify `-ansi'. There are certain ANSI C programs (obscure, to be sure) that would compile incorrectly if dollar signs were permit- ted in identifiers. For example: #define foo(a) #a #define lose(b) foo (b) #define test$ lose (test) 7.21. The Character ESC in Constants You can use the sequence `\e' in a string or character constant to stand for the ASCII character ESC. 7.22. Inquiring on Alignment of Types or Variables The keyword __alignof__ allows you to inquire about how an object is aligned, or the minimum alignment usually required by a type. Its syntax is just like sizeof. For example, if the target machine requires a double value to be aligned on an 8-byte boundary, then __alignof__ (double) is 8. This is true on many RISC machines. On more traditional machine designs, __alignof__ (double) is 4 or even 2. Some machines never actually require alignment; they allow reference to any data type even at an odd addresses. For these machines, __alignof__ reports the recommended alignment of a type. Using GNU CC 109 When the operand of __alignof__ is an lvalue rather than a type, the value is the largest alignment that the lvalue is known to have. It may have this alignment as a result of its data type, or because it is part of a struc- ture and inherits alignment from that structure. For exam- ple, after this declaration: struct foo { int x; char y; } foo1; the value of __alignof__ (foo1.y) is probably 2 or 4, the same as __alignof__ (int), even though the data type of foo1.y does not itself demand any alignment. 7.23. Specifying Attributes of Variables The keyword __attribute__ allows you to specify special attributes of variables or structure fields. The only attributes currently defined are the aligned and format attributes. The aligned attribute specifies the alignment of the variable or structure field. For example, the declaration: int x __attribute__ ((aligned (16))) = 0; causes the compiler to allocate the global variable x on a 16-byte boundary. On a 68000, this could be used in con- junction with an asm expression to access the move16 instruction which requires 16-byte aligned operands. You can also specify the alignment of structure fields. For example, to create a double-word aligned int pair, you could write: struct foo { int x[2] __attribute__ ((aligned (8))); }; This is an alternative to creating a union with a double member that forces the union to be double-word aligned. It is not possible to specify the alignment of func- tions; the alignment of functions is determined by the machine's requirements and cannot be changed. 110 Using GNU CC The format attribute specifies that a function takes printf or scanf style arguments which should be type-checked against a format string. For example, the declaration: extern int my_printf (void *my_object, const char *my_format, ...) __attribute__ ((format (printf, 2, 3))); causes the compiler to check the arguments in calls to my_printf for consistency with the printf style format string argument my_format. The first parameter of the format attribute determines how the format string is interpreted, and should be either printf or scanf. The second parameter specifies the number of the format string argument (starting from 1). The third parameter specifies the number of the first argument which should be checked against the format string. For functions where the arguments are not available to be checked (such as vprintf), specify the third parameter as zero. In this case the compiler only checks the format string for consistency. In the example above, the format string (my_format) is the second argument to my_print and the arguments to check start with the third argument, so the correct parameters for the format attribute are 2 and 3. The format attribute allows you to identify your own functions which take format strings as arguments, so that GNU CC can check the calls to these functions for errors. The compiler always checks formats for the ANSI library functions printf, fprintf, sprintf, scanf, fscanf, sscanf, vprintf, vfprintf and vsprintf whenever such warnings are requested (using `-Wformat'), so there is no need to modify the header file `stdio.h'. 7.24. An Inline Function is As Fast As a Macro By declaring a function inline, you can direct GNU CC to integrate that function's code into the code for its callers. This makes execution faster by eliminating the function-call overhead; in addition, if any of the actual argument values are constant, their known values may permit simplifications at compile time so that not all of the inline function's code needs to be included. To declare a function inline, use the inline keyword in its declaration, like this: Using GNU CC 111 inline int inc (int *a) { (*a)++; } (If you are writing a header file to be included in ANSI C programs, write __inline__ instead of inline. See section Alternate Keywords.) You can also make all ``simple enough'' functions inline with the option `-finline-functions'. Note that cer- tain usages in a function definition can make it unsuitable for inline substitution. When a function is both inline and static, if all calls to the function are integrated into the caller, and the function's address is never used, then the function's own assembler code is never referenced. In this case, GNU CC does not actually output assembler code for the function, unless you specify the option `-fkeep-inline-functions'. Some calls cannot be integrated for various reasons (in par- ticular, calls that precede the function's definition cannot be integrated, and neither can recursive calls within the definition). If there is a nonintegrated call, then the function is compiled to assembler code as usual. The func- tion must also be compiled as usual if the program refers to its address, because that can't be inlined. When an inline function is not static, then the com- piler must assume that there may be calls from other source files; since a global symbol can be defined only once in any program, the function must not be defined in the other source files, so the calls therein cannot be integrated. Therefore, a non-static inline function is always compiled on its own in the usual fashion. If you specify both inline and extern in the function definition, then the definition is used only for inlining. In no case is the function compiled on its own, not even if you refer to its address explicitly. Such an address becomes an external reference, as if you had only declared the function, and had not defined it. This combination of inline and extern has almost the effect of a macro. The way to use it is to put a function definition in a header file with these keywords, and put another copy of the definition (lacking inline and extern) in a library file. The definition in the header file will cause most calls to the function to be inlined. If any uses 112 Using GNU CC of the function remain, they will refer to the single copy in the library. 7.25. Assembler Instructions with C Expression Operands In an assembler instruction using asm, you can now specify the operands of the instruction using C expressions. This means no more guessing which registers or memory loca- tions will contain the data you want to use. You must specify an assembler instruction template much like what appears in a machine description, plus an operand constraint string for each operand. For example, here is how to use the 68881's fsinx instruction: asm ("fsinx %1,%0" : "=f" (result) : "f" (angle)); INTERNALS Here angle is the C expression for the input operand while result is that of the output operand. Each has `"f"' as its operand constraint, saying that a floating point register is required. The `=' in `=f' indicates that the operand is an output; all output operands' constraints must use `='. The constraints use the same language used in the machine description (see section Constraints). INTERNALS Here angle is the C expression for the input operand while result is that of the output operand. Each has `"f"' as its operand constraint, saying that a floating point register is required. The `=' in `=f' indicates that the operand is an output; all output operands' constraints must use `='. The constraints use the same language used in the machine description (see section Constraints,,Operand Constraints, gcc.info, Using and Porting GCC). Each operand is described by an operand-constraint string followed by the C expression in parentheses. A colon separates the assembler template from the first output operand, and another separates the last output operand from the first input, if any. Commas separate output operands and separate inputs. The total number of operands is lim- ited to ten or to the maximum number of operands in any instruction pattern in the machine description, whichever is greater. If there are no output operands, and there are input operands, then there must be two consecutive colons sur- rounding the place where the output operands would go. Using GNU CC 113 Output operand expressions must be lvalues; the com- piler can check this. The input operands need not be lvalues. The compiler cannot check whether the operands have data types that are reasonable for the instruction being executed. It does not parse the assembler instruction template and does not know what it means, or whether it is valid assembler input. The extended asm feature is most often used for machine instructions that the compiler itself does not know exist. The output operands must be write-only; GNU CC will assume that the values in these operands before the instruc- tion are dead and need not be generated. Extended asm does not support input-output or read-write operands. For this reason, the constraint character `+', which indicates such an operand, may not be used. When the assembler instruction has a read-write operand, or an operand in which only some of the bits are to be changed, you must logically split its function into two separate operands, one input operand and one write-only out- put operand. The connection between them is expressed by constraints which say they need to be in the same location when the instruction executes. You can use the same C expression for both operands, or different expressions. For example, here we write the (fictitious) `combine' instruc- tion with bar as its read-only source operand and foo as its read-write destination: asm ("combine %2,%0" : "=r" (foo) : "0" (foo), "g" (bar)); The constraint `"0"' for operand 1 says that it must occupy the same location as operand 0. A digit in constraint is allowed only in an input operand, and it must refer to an output operand. Only a digit in the constraint can guarantee that one operand will be in the same place as another. The mere fact that foo is the value of both operands is not enough to guarantee that they will be in the same place in the gen- erated assembler code. The following would not work: asm ("combine %2,%0" : "=r" (foo) : "r" (foo), "g" (bar)); Various optimizations or reloading could cause operands 0 and 1 to be in different registers; GNU CC knows no reason not to do so. For example, the compiler might find a copy 114 Using GNU CC of the value of foo in one register and use it for operand 1, but generate the output operand 0 in a different register (copying it afterward to foo's own address). Of course, since the register for operand 1 is not even mentioned in the assembler code, the result will not work, but GNU CC can't tell that. Some instructions clobber specific hard registers. To describe this, write a third colon after the input operands, followed by the names of the clobbered hard registers (given as strings). Here is a realistic example for the Vax: asm volatile ("movc3 %0,%1,%2" : /* no outputs */ : "g" (from), "g" (to), "g" (count) : "r0", "r1", "r2", "r3", "r4", "r5"); If you refer to a particular hardware register from the assembler code, then you will probably have to list the register after the third colon to tell the compiler that the register's value is modified. In many assemblers, the register names begin with `%'; to produce one `%' in the assembler code, you must write `%%' in the input. You can put multiple assembler instructions together in a single asm template, separated either with newlines (writ- ten as `\n') or with semicolons if the assembler allows such semicolons. The GNU assembler allows semicolons and all Unix assemblers seem to do so. The input operands are guaranteed not to use any of the clobbered registers, and neither will the output operands' addresses, so you can read and write the clobbered registers as many times as you like. Here is an example of multiple instructions in a template; it assumes that the subroutine _foo accepts arguments in registers 9 and 10: asm ("movl %0,r9;movl %1,r10;call _foo" : /* no outputs */ : "g" (from), "g" (to) : "r9", "r10"); INTERNALS Unless an output operand has the `&' con- straint modifier, GNU CC may allocate it in the same regis- ter as an unrelated input operand, on the assumption that the inputs are consumed before the outputs are produced. This assumption may be false if the assembler code actually consists of more than one instruction. In such a case, use Using GNU CC 115 `&' for each output operand that may not overlap an input. See section Modifiers. INTERNALS Unless an output operand has the `&' constraint modifier, GNU CC may allocate it in the same register as an unrelated input operand, on the assumption that the inputs are consumed before the outputs are produced. This assump- tion may be false if the assembler code actually consists of more than one instruction. In such a case, use `&' for each output operand that may not overlap an input. See section Modifiers,,Constraint Modifier Characters,gcc.info,Using and Porting GCC. If you want to test the condition code produced by an assembler instruction, you must include a branch and a label in the asm construct, as follows: asm ("clr %0;frob %1;beq 0f;mov #1,%0;0:" : "g" (result) : "g" (input)); This assumes your assembler supports local labels, as the GNU assembler and most Unix assemblers do. Usually the most convenient way to use these asm instructions is to encapsulate them in macros that look like functions. For example, #define sin(x) \ ({ double __value, __arg = (x); \ asm ("fsinx %1,%0": "=f" (__value): "f" (__arg)); \ __value; }) Here the variable __arg is used to make sure that the instruction operates on a proper double value, and to accept only those arguments x which can convert automatically to a double. Another way to make sure the instruction operates on the correct data type is to use a cast in the asm. This is different from using a variable __arg in that it converts more different types. For example, if the desired type were int, casting the argument to int would accept a pointer with no complaint, while assigning the argument to an int vari- able named __arg would warn about using a pointer unless the caller explicitly casts it. 116 Using GNU CC If an asm has output operands, GNU CC assumes for optimization purposes that the instruction has no side effects except to change the output operands. This does not mean that instructions with a side effect cannot be used, but you must be careful, because the compiler may eliminate them if the output operands aren't used, or move them out of loops, or replace two with one if they constitute a common subexpression. Also, if your instruction does have a side effect on a variable that otherwise appears not to change, the old value of the variable may be reused later if it hap- pens to be found in a register. You can prevent an asm instruction from being deleted, moved significantly, or combined, by writing the keyword volatile after the asm. For example: #define set_priority(x) \ asm volatile ("set_priority %0": /* no outputs */ : "g" (x)) An instruction without output operands will not be deleted or moved significantly, regardless, unless it is unreach- able. Note that even a volatile asm instruction can be moved in ways that appear insignificant to the compiler, such as across jump instructions. You can't expect a sequence of volatile asm instructions to remain perfectly consecutive. If you want consecutive output, use a single asm. It is a natural idea to look for a way to give access to the condition code left by the assembler instruction. However, when we attempted to implement this, we found no way to make it work reliably. The problem is that output operands might need reloading, which would result in addi- tional following ``store'' instructions. On most machines, these instructions would alter the condition code before there was time to test it. This problem doesn't arise for ordinary ``test'' and ``compare'' instructions because they don't have any output operands. If you are writing a header file that should be includ- able in ANSI C programs, write __asm__ instead of asm. See section Alternate Keywords. 7.26. Controlling Names Used in Assembler Code You can specify the name to be used in the assembler code for a C function or variable by writing the asm (or __asm__) keyword after the declarator as follows: Using GNU CC 117 int foo asm ("myfoo") = 2; This specifies that the name to be used for the variable foo in the assembler code should be `myfoo' rather than the usual `_foo'. On systems where an underscore is normally prepended to the name of a C function or variable, this feature allows you to define names for the linker that do not start with an underscore. You cannot use asm in this way in a function defini- tion; but you can get the same effect by writing a declara- tion for the function before its definition and putting asm there, like this: extern func () asm ("FUNC"); func (x, y) int x, y; ... It is up to you to make sure that the assembler names you choose do not conflict with any other assembler symbols. Also, you must not use a register name; that would produce completely invalid assembler code. GNU CC does not as yet have the ability to store static variables in registers. Perhaps that will be added. 7.27. Variables in Specified Registers GNU C allows you to put a few global variables into specified hardware registers. You can also specify the register in which an ordinary register variable should be allocated. o+ Global register variables reserve registers throughout the program. This may be useful in programs such as programming language interpreters which have a couple of global variables that are accessed very often. o+ Local register variables in specific registers do not reserve the registers. The compiler's data flow analysis is capable of determining where the specified registers contain live values, and where they are available for other uses. 118 Using GNU CC These local variables are sometimes convenient for use with the extended asm feature (see section Extended Asm), if you want to write one output of the assembler instruction directly into a particular register. (This will work provided the register you specify fits the constraints specified for that operand in the asm.) 7.27.1. Defining Global Register Variables You can define a global register variable in GNU C like this: register int *foo asm ("a5"); Here a5 is the name of the register which should be used. Choose a register which is normally saved and restored by function calls on your machine, so that library routines will not clobber it. Naturally the register name is cpu-dependent, so you would need to conditionalize your program according to cpu type. The register a5 would be a good choice on a 68000 for a variable of pointer type. On machines with register win- dows, be sure to choose a ``global'' register that is not affected magically by the function call mechanism. In addition, operating systems on one type of cpu may differ in how they name the registers; then you would need additional conditionals. For example, some 68000 operating systems call this register %a5. Eventually there may be a way of asking the compiler to choose a register automatically, but first we need to figure out how it should choose and how to enable you to guide the choice. No solution is evident. Defining a global register variable in a certain regis- ter reserves that register entirely for this use, at least within the current compilation. The register will not be allocated for any other purpose in the functions in the current compilation. The register will not be saved and restored by these functions. Stores into this register are never deleted even if they would appear to be dead, but references may be deleted or moved or simplified. It is not safe to access the global register variables from signal handlers, or from more than one thread of con- trol, because the system library routines may temporarily Using GNU CC 119 use the register for other things (unless you recompile them specially for the task at hand). It is not safe for one function that uses a global register variable to call another such function foo by way of a third function lose that was compiled without knowledge of this variable (i.e. in a different source file in which the variable wasn't declared). This is because lose might save the register and put some other value there. For exam- ple, you can't expect a global register variable to be available in the comparison-function that you pass to qsort, since qsort might have put something else in that register. (If you are prepared to recompile qsort with the same global register variable, you can solve this problem.) If you want to recompile qsort or other source files which do not actually use your global register variable, so that they will not use that register for any other purpose, then it suffices to specify the compiler option `-ffixed- reg'. You need not actually add a global register declara- tion to their source code. A function which can alter the value of a global regis- ter variable cannot safely be called from a function com- piled without this variable, because it could clobber the value the caller expects to find there on return. There- fore, the function which is the entry point into the part of the program that uses the global register variable must explicitly save and restore the value which belongs to its caller. On most machines, longjmp will restore to each global register variable the value it had at the time of the setjmp. On some machines, however, longjmp will not change the value of global register variables. To be portable, the function that called setjmp should make other arrangements to save the values of the global register variables, and to restore them in a longjmp. This way, the same thing will happen regardless of what longjmp does. All global register variable declarations must precede all function definitions. If such a declaration could appear after function definitions, the declaration would be too late to prevent the register from being used for other purposes in the preceding functions. Global register variables may not have initial values, because an executable file has no means to supply initial contents for a register. On the Sparc, there are reports that g3 ... g7 are suitable registers, but certain library functions, such as getwd, as well as the subroutines for division and 120 Using GNU CC remainder, modify g3 and g4. g1 and g2 are local tem- poraries. On the 68000, a2 ... a5 should be suitable, as should d2 ... d7. Of course, it will not do to use more than a few of those. 7.27.2. Specifying Registers for Local Variables You can define a local register variable with a speci- fied register like this: register int *foo asm ("a5"); Here a5 is the name of the register which should be used. Note that this is the same syntax used for defining global register variables, but for a local variable it would appear within a function. Naturally the register name is cpu-dependent, but this is not a problem, since specific registers are most often useful with explicit assembler instructions (see section Extended Asm). Both of these things generally require that you conditionalize your program according to cpu type. In addition, operating systems on one type of cpu may differ in how they name the registers; then you would need additional conditionals. For example, some 68000 operating systems call this register %a5. Eventually there may be a way of asking the compiler to choose a register automatically, but first we need to figure out how it should choose and how to enable you to guide the choice. No solution is evident. Defining such a register variable does not reserve the register; it remains available for other uses in places where flow control determines the variable's value is not live. However, these registers are made unavailable for use in the reload pass. I would not be surprised if excessive use of this feature leaves the compiler too few available registers to compile certain functions. 7.28. Alternate Keywords The option `-traditional' disables certain keywords; `-ansi' disables certain others. This causes trouble when you want to use GNU C extensions, or ANSI C features, in a general-purpose header file that should be usable by all programs, including ANSI C programs and traditional ones. Using GNU CC 121 The keywords asm, typeof and inline cannot be used since they won't work in a program compiled with `-ansi', while the keywords const, volatile, signed, typeof and inline won't work in a program compiled with `-traditional'. The way to solve these problems is to put `__' at the beginning and end of each problematical keyword. For exam- ple, use __asm__ instead of asm, __const__ instead of const, and __inline__ instead of inline. Other C compilers won't accept these alternative key- words; if you want to compile with another compiler, you can define the alternate keywords as macros to replace them with the customary keywords. It looks like this: #ifndef __GNUC__ #define __asm__ asm #endif `-pedantic' causes warnings for many GNU C extensions. You can prevent such warnings within one expression by writ- ing __extension__ before the expression. __extension__ has no effect aside from this. 7.29. Incomplete enum Types You can define an enum tag without specifying its pos- sible values. This results in an incomplete type, much like what you get if you write struct foo without describing the elements. A later declaration which does specify the possi- ble values completes the type. You can't allocate variables or storage using the type while it is incomplete. However, you can work with pointers to that type. This extension may not be very useful, but it makes the handling of enum more consistent with the way struct and union are handled. 8. Reporting Bugs Your bug reports play an essential role in making GNU CC reliable. When you encounter a problem, the first thing to do is to see if it is already known. See section Trouble. Also look in `Incompatibilities'. If it isn't known, then you should report the problem. 122 Using GNU CC Reporting a bug may help you by bringing a solution to your problem, or it may not. (If it does not, look in the service directory; see `Service'.) In any case, the princi- pal function of a bug report is to help the entire community by making the next version of GNU CC work better. Bug reports are your contribution to the maintenance of GNU CC. In order for a bug report to serve its purpose, you must include the information that makes for fixing the bug. 8.1. Have You Found a Bug? If you are not sure whether you have found a bug, here are some guidelines: o+ If the compiler gets a fatal signal, for any input whatever, that is a compiler bug. Reliable compilers never crash. o+ If the compiler produces invalid assembly code, for any input whatever (except an asm statement), that is a compiler bug, unless the compiler reports errors (not just warnings) which would ordinarily prevent the assembler from being run. o+ If the compiler produces valid assembly code that does not correctly execute the input source code, that is a compiler bug. However, you must double-check to make sure, because you may have run into an incompatibility between GNU C and traditional C (see section Incompatibilities). These incompatibilities might be considered bugs, but they are inescapable consequences of valuable features. Or you may have a program whose behavior is undefined, which happened by chance to give the desired results with another C compiler. For example, in many nonoptimizing compilers, you can write `x;' at the end of a function instead of `return x;', with the same results. But the value of the function is undefined if return is omitted; it is not a bug when GNU CC produces different results. Problems often result from expressions with two increment operators, as in f (*p++, *p++). Your previous compiler might have interpreted that expression the way you intended; GNU CC might interpret it another way. Neither compiler is wrong. The bug is in your code. Using GNU CC 123 After you have localized the error to a single source line, it should be easy to check for these things. If your program is correct and well defined, you have found a compiler bug. o+ If the compiler produces an error message for valid input, that is a compiler bug. Note that the following is not valid input, and the error message for it is not a bug: int foo (char); int foo (x) char x; { ... } The prototype says to pass a char, while the definition says to pass an int and treat the value as a char. This is what the ANSI standard says, and it makes sense. o+ If the compiler does not produce an error message for invalid input, that is a compiler bug. However, you should note that your idea of ``invalid input'' might be my idea of ``an extension'' or ``support for traditional practice''. o+ If you are an experienced user of C compilers, your suggestions for improvement of GNU CC are welcome in any case. 8.2. How to Report Bugs Send bug reports for GNU C to one of these addresses: bug-gcc@prep.ai.mit.edu {ucbvax|mit-eddie|uunet}!prep.ai.mit.edu!bug-gcc Do not send bug reports to `help-gcc', or to the news- group `gnu.gcc.help'. Most users of GNU CC do not want to receive bug reports. Those that do, have asked to be on `bug-gcc'. 124 Using GNU CC The mailing list `bug-gcc' has a newsgroup which serves as a repeater. The mailing list and the newsgroup carry exactly the same messages. Often people think of posting bug reports to the newsgroup instead of mailing them. This appears to work, but it has one problem which can be cru- cial: a newsgroup posting does not contain a mail path back to the sender. Thus, if I need to ask for more information, I may be unable to reach you. For this reason, it is better to send bug reports to the mailing list. As a last resort, send bug reports on paper to: GNU Compiler Bugs Free Software Foundation 675 Mass Ave Cambridge, MA 02139 The fundamental principle of reporting bugs usefully is this: report all the facts. If you are not sure whether to state a fact or leave it out, state it! Often people omit facts because they think they know what causes the problem and they conclude that some details don't matter. Thus, you might assume that the name of the variable you use in an example does not matter. Well, prob- ably it doesn't, but one cannot be sure. Perhaps the bug is a stray memory reference which happens to fetch from the location where that name is stored in memory; perhaps, if the name were different, the contents of that location would fool the compiler into doing the right thing despite the bug. Play it safe and give a specific, complete example. That is the easiest thing for you to do, and the most help- ful. Keep in mind that the purpose of a bug report is to enable me to fix the bug if it is not known. It isn't very important what happens if the bug is already known. There- fore, always write your bug reports on the assumption that the bug is not known. Sometimes people give a few sketchy facts and ask, ``Does this ring a bell?'' Those bug reports are useless, and I urge everyone to refuse to respond to them except to chide the sender to report bugs properly. To enable me to fix the bug, you should include all these things: o+ The version of GNU CC. You can get this by running it with the `-v' option. Using GNU CC 125 Without this, I won't know whether there is any point in looking for the bug in the current version of GNU CC. o+ A complete input file that will reproduce the bug. If the bug is in the C preprocessor, send me a source file and any header files that it requires. If the bug is in the compiler proper (`cc1'), run your source file through the C preprocessor by doing `gcc -E sourcefile > outfile', then include the contents of outfile in the bug report. (Any `-I', `-D' or `-U' options that you used in actual compilation should also be used when doing this.) A single statement is not enough of an example. In order to compile it, it must be embedded in a function definition; and the bug might depend on the details of how this is done. Without a real example I can compile, all I can do about your bug report is wish you luck. It would be futile to try to guess how to provoke the bug. For example, bugs in register allocation and reloading frequently depend on every little detail of the function they happen in. o+ The command arguments you gave GNU CC to compile that example and observe the bug. For example, did you use `-O'? To guarantee you won't omit something important, list them all. If I were to try to guess the arguments, I would probably guess wrong and then I would not encounter the bug. o+ The type of machine you are using, and the operating system name and version number. o+ The operands you gave to the configure command when you installed the compiler. o+ A description of what behavior you observe that you believe is incorrect. For example, ``It gets a fatal signal,'' or, ``There is an incorrect assembler instruction in the output.'' Of course, if the bug is that the compiler gets a fatal signal, then I will certainly notice it. But if the bug is incorrect output, I might not notice unless it is glaringly wrong. I won't study all the assembler code from a 50-line C program just on the off chance that it might be wrong. 126 Using GNU CC Even if the problem you experience is a fatal signal, you should still say so explicitly. Suppose something strange is going on, such as, your copy of the compiler is out of synch, or you have encountered a bug in the C library on your system. (This has happened!) Your copy might crash and mine would not. If you told me to expect a crash, then when mine fails to crash, I would know that the bug was not happening for me. If you had not told me to expect a crash, then I would not be able to draw any conclusion from my observations. Often the observed symptom is incorrect output when your program is run. Sad to say, this is not enough information for me unless the program is short and simple. If you send me a large program, I don't have time to figure out how it would work if compiled correctly, much less which line of it was compiled wrong. So you will have to do that. Tell me which source line it is, and what incorrect result happens when that line is executed. A person who understands the program can find this as easily as a bug in the program itself. o+ If you send me examples of output from GNU CC, please use `-g' when you make them. The debugging information includes source line numbers which are essential for correlating the output with the input. o+ If you wish to suggest changes to the GNU CC source, send me context diffs. If you even discuss something in the GNU CC source, refer to it by context, not by line number. The line numbers in my development sources don't match those in your sources. Your line numbers would convey no useful information to me. o+ Additional information from a debugger might enable me to find a problem on a machine which I do not have available myself. However, you need to think when you collect this information if you want it to have any chance of being useful. For example, many people send just a backtrace, but that is never useful by itself. A simple backtrace with arguments conveys little about GNU CC because the compiler is largely data-driven; the same functions are called over and over for different RTL insns, doing different things Using GNU CC 127 depending on the details of the insn. Most of the arguments listed in the backtrace are useless because they are pointers to RTL list structure. The numeric values of the pointers, which the debugger prints in the backtrace, have no significance whatever; all that matters is the contents of the objects they point to (and most of the contents are other such pointers). In addition, most compiler passes consist of one or more loops that scan the RTL insn sequence. The most vital piece of information about such a loop---which insn it has reached---is usually in a local variable, not in an argument. What you need to provide in addition to a backtrace are the values of the local variables for several stack frames up. When a local variable or an argument is an RTX, first print its value and then use the GDB command pr to print the RTL expression that it points to. (If GDB doesn't run on your machine, use your debugger to call the function debug_rtx with the RTX as an argument.) In general, whenever a variable is a pointer, its value is no use without the data it points to. In addition, include a debugging dump from just before the pass in which the crash happens. Most bugs involve a series of insns, not just one. Here are some things that are not necessary: o+ A description of the envelope of the bug. Often people who encounter a bug spend a lot of time investigating which changes to the input file will make the bug go away and which changes will not affect it. This is often time consuming and not very useful, because the way I will find the bug is by running a single example under the debugger with breakpoints, not by pure deduction from a series of examples. I recommend that you save your time for something else. Of course, if you can find a simpler example to report instead of the original one, that is a convenience for me. Errors in the output will be easier to spot, running under the debugger will take less time, etc. Most GNU CC bugs involve 128 Using GNU CC just one function, so the most straightforward way to simplify an example is to delete all the function definitions except the one where the bug occurs. Those earlier in the file may be replaced by external declarations if the crucial function depends on them. (Exception: inline functions may affect compilation of functions defined later in the file.) However, simplification is not vital; if you don't want to do this, report the bug anyway and send me the entire test case you used. o+ A patch for the bug. A patch for the bug does help me if it is a good one. But don't omit the necessary information, such as the test case, on the assumption that a patch is all I need. I might see problems with your patch and decide to fix the problem another way, or I might not understand it at all. Sometimes with a program as complicated as GNU CC it is very hard to construct an example that will make the program follow a certain path through the code. If you don't send me the example, I won't be able to construct one, so I won't be able to verify that the bug is fixed. And if I can't understand what bug you are trying to fix, or why your patch should be an improvement, I won't install it. A test case will help me to understand. o+ A guess about what the bug is or what it depends on. Such guesses are usually wrong. Even I can't guess right about such things without first using the debugger to find the facts. 8.3. Certain Changes We Don't Want to Make This section lists changes that people frequently request, but which we do not make because we think GNU CC is better without them. o+ Checking the number and type of arguments to a function which has an old-fashioned definition and no prototype. Using GNU CC 129 Such a feature would work only occasionally---only for calls that appear in the same file as the called function, following the definition. The only way to check all calls reliably is to add a prototype for the function. But adding a prototype will eliminate the need for this feature. So the feature is not worthwhile. o+ Warning about using an expression whose type is signed as a shift count. Shift count operands are probably signed more often than unsigned. Warning about this would cause far more annoyance than good. o+ Warning about assigning a signed value to an unsigned variable. Such assignments must be very common; warning about them would cause more annoyance than good. o+ Making bitfields unsigned by default on particular machines where ``the ABI standard'' says to do so. The ANSI C standard leaves it up to the implementation whether a bitfield declared plain int is signed or not. This in effect creates two alternative dialects of C. The GNU C compiler supports both dialects; you can specify the dialect you want with the option `- fsigned-bitfields' or `-funsigned-bitfields'. However, this leaves open the question of which dialect to use by default. Currently, the preferred dialect makes plain bitfields signed, because this is simplest. Since int is the same as signed int in every other context, it is cleanest for them to be the same in bitfields as well. Some computer manufacturers have published Application Binary Interface standards which specify that plain bitfields should be unsigned. It is a mistake, however, to say anything about this issue in an ABI. This is because the handling of plain bitfields distinguishes two dialects of C. Both dialects are meaningful on every type of machine. Whether a particular object file was compiled using signed bitfields or unsigned is of no concern to functions in any other object file, even if they access the same bitfields in the same data structures. 130 Using GNU CC A given program is written in one or the other of these two dialects. The program stands a chance to work on most any machine if it is compiled with the proper dialect. It is unlikely to work at all if compiled with the wrong dialect. Many users appreciate the GNU C compiler because it provides an environment that is uniform across machines. These users would be inconvenienced if the compiler treated plain bitfields differently on certain machines. Occasionally users write programs intended only for a particular machine type. On these occasions, the users would benefit if the GNU C compiler were to support by default the same dialect as the other compilers on that machine. But such applications are rare. And users writing a program to run on more than one type of machine cannot possibly benefit from this kind of compatibility. This is why GNU CC does and will treat plain bitfields in the same fashion on all types of machines (by default). (Of course, users strongly concerned about portability should indicate explicitly in each bitfield whether it is signed or not.) o+ Undefining __STDC__ when `-ansi' is not used. Currently, GNU CC defines __STDC__ as long as you don't use `-traditional'. This provides good results in practice. Programmers normally use conditionals on __STDC__ to ask whether it is safe to use certain features of ANSI C, such as function prototypes or ANSI token concatenation. Since plain `gcc' supports all the features of ANSI C, the correct answer to these questions is ``yes''. Some users try to use __STDC__ to check for the availability of certain library facilities. This is actually incorrect usage in an ANSI C program, because the ANSI C standard says that a conforming freestanding implementation should define __STDC__ even though it does not have the library facilities. `gcc -ansi -pedantic' is a conforming freestanding implementation, and it is therefore required to define __STDC__, even though it does not come with an ANSI C library. Using GNU CC 131 Sometimes people say that defining __STDC__ in a compiler that does not completely conform to the ANSI C standard somehow violates the standard. This is illogical. The standard is a standard for compilers that are supposed to conform. It says nothing about what any other compilers should do. Whatever the ANSI C standard says is relevant to the design of plain `gcc' without `-ansi' only for pragmatic reasons, not as a requirement. o+ Undefining __STDC__ in C++. Programs written to compile with C++-to-C translators get the value of __STDC__ that goes with the C compiler that is subsequently used. These programs must test __STDC__ to determine what kind of C preprocessor that compiler uses: whether they should concatenate tokens in the ANSI C fashion or in the traditional fashion. These programs work properly with GNU C++ if __STDC__ is defined. They would not work otherwise. In addition, many header files are written to provide prototypes in ANSI C but not in traditional C. Many of these header files can work without change in C++ provided __STDC__ is defined. If __STDC__ is not defined, they will all fail, and will all need to be changed to test explicitly for C++ as well. INTERNALS 132 Using GNU CC 9. Using GNU CC on VMS INTERNALS Using GNU CC 133 10. Using GNU CC on VMS 10.1. Include Files and VMS Due to the differences between the filesystems of Unix and VMS, GNU CC attempts to translate file names in `#include' into names that VMS will understand. The basic strategy is to prepend a prefix to the specification of the include file, convert the whole filename to a VMS filename, and then try to open the file. GNU CC tries various pre- fixes one by one until one of them succeeds: 1. The first prefix is the `GNU_CC_INCLUDE:' logical name: this is where GNU C header files are traditionally stored. If you wish to store header files in non-standard locations, then you can assign the logical `GNU_CC_INCLUDE' to be a search list, where each element of the list is suitable for use with a rooted logical. 2. The next prefix tried is `SYS$SYSROOT:[SYSLIB.]'. This is where VAX-C header files are traditionally stored. 3. If the include file specification by itself is a valid VMS filename, the preprocessor then uses this name with no prefix in an attempt to open the include file. 4. If the file specification is not a valid VMS filename (i.e. does not contain a device or a directory specifier, and contains a `/' character), the preprocessor tries to convert it from Unix syntax to VMS syntax. Conversion works like this: the first directory name becomes a device, and the rest of the directories are converted into VMS-format directory names. For example, `X11/foobar.h' is translated to `X11:[000000]foobar.h' or `X11:foobar.h', whichever one can be opened. This strategy allows you to assign a logical name to point to the actual location of the header files. 5. If none of these strategies succeeds, the `#include' fails. Include directives of the form: #include foobar 134 Using GNU CC are a common source of incompatibility between VAX-C and GNU CC. VAX-C treats this much like a standard #include directive. That is incompatible with the ANSI C behavior implemented by GNU CC: to expand the name foobar as a macro. Macro expansion should eventually yield one of the two standard formats for #include: #include "file" #include If you have this problem, the best solution is to modify the source to convert the #include directives to one of the two standard forms. That will work with either com- piler. If you want a quick and dirty fix, define the file names as macros with the proper expansion, like this: #define stdio This will work, as long as the name doesn't conflict with anything else in the program. Another source of incompatibility is that VAX-C assumes that: #include "foobar" is actually asking for the file `foobar.h'. GNU CC does not make this assumption, and instead takes what you ask for literally; it tries to read the file `foobar'. The best way to avoid this problem is to always specify the desired file extension in your include directives. GNU CC for VMS is distributed with a set of include files that is sufficient to compile most general purpose programs. Even though the GNU CC distribution does not con- tain header files to define constants and structures for some VMS system-specific functions, there is no reason why you cannot use GNU CC with any of these functions. You first may have to generate or create header files, either by using the public domain utility UNSDL (which can be found on a DECUS tape), or by extracting the relevant modules from one of the system macro libraries, and using an editor to Using GNU CC 135 construct a C header file. 10.2. Global Declarations and VMS GNU CC does not provide the globalref, globaldef and globalvalue keywords of VAX-C. You can get the same effect with an obscure feature of GAS, the GNU assembler. (This requires GAS version 1.39 or later.) The following macros allow you to use this feature in a fairly natural way: #ifdef __GNUC__ #define GLOBALREF(NAME) \ NAME asm("_$$PsectAttributes_GLOBALSYMBOL$$" #NAME ) #define GLOBALDEF(NAME,VALUE) \ NAME asm("_$$PsectAttributes_GLOBALSYMBOL$$" #NAME ) = VALUE #define GLOBALVALUEREF(NAME) \ const NAME [1] asm("_$$PsectAttributes_GLOBALVALUE$$" #NAME ) #define GLOBALVALUEDEF(NAME,VALUE) \ const NAME [1] asm("_$$PsectAttributes_GLOBALVALUE$$" #NAME ) = {VALUE} #else #define GLOBALREF(NAME) globalref NAME #define GLOBALDEF(NAME,VALUE) globaldef NAME = VALUE #define GLOBALVALUEDEF(NAME,VALUE) globalvalue NAME = VALUE #define GLOBALVALUEREF(NAME) globalvalue NAME #endif (The _$$PsectAttributes_GLOBALSYMBOL prefix at the start of the name is removed by the assembler, after it has modified the attributes of the symbol). These macros are provided in the VMS binaries distribution in a header file `GNU_HACKS.H'. An example of the usage is: int GLOBALREF (ijk); int GLOBALDEF (jkl, 0); The macros GLOBALREF and GLOBALDEF cannot be used straightforwardly for arrays, since there is no way to insert the array dimension into the declaration at the right place. However, you can declare an array with these macros if you first define a typedef for the array type, like this: typedef int intvector[10]; intvector GLOBALREF (foo); 136 Using GNU CC Array and structure initializers will also break the macros; you can define the initializer to be a macro of its own, or you can expand the GLOBALDEF macro by hand. You may find a case where you wish to use the GLOBALDEF macro with a large array, but you are not interested in explicitly ini- tializing each element of the array. In such cases you can use an initializer like: {0,}, which will initialize the entire array to 0. A shortcoming of this implementation is that a variable declared with GLOBALVALUEREF or GLOBALVALUEDEF is always an array. For example, the declaration: int GLOBALVALUEREF(ijk); declares the variable ijk as an array of type int [1]. This is done because a globalvalue is actually a constant; its ``value'' is what the linker would normally consider an address. That is not how an integer value works in C, but it is how an array works. So treating the symbol as an array name gives consistent results---with the exception that the value seems to have the wrong type. Don't try to access an element of the array. It doesn't have any ele- ments. The array ``address'' may not be the address of actual storage. The fact that the symbol is an array may lead to warn- ings where the variable is used. Insert type casts to avoid the warnings. Here is an example; it takes advantage of the ANSI C feature allowing macros that expand to use the same name as the macro itself. int GLOBALVALUEREF (ss$_normal); int GLOBALVALUEDEF (xyzzy,123); #ifdef __GNUC__ #define ss$_normal ((int) ss$_normal) #define xyzzy ((int) xyzzy) #endif Don't use globaldef or globalref with a variable whose type is an enumeration type; this is not implemented. Instead, make the variable an integer, and use a global- valuedef for each of the enumeration values. An example of this would be: #ifdef __GNUC__ Using GNU CC 137 int GLOBALDEF (color, 0); int GLOBALVALUEDEF (RED, 0); int GLOBALVALUEDEF (BLUE, 1); int GLOBALVALUEDEF (GREEN, 3); #else enum globaldef color {RED, BLUE, GREEN = 3}; #endif 10.3. Other VMS Issues GNU CC automatically arranges for main to return 1 by default if you fail to specify an explicit return value. This will be interpreted by VMS as a status code indicating a normal successful completion. Version 1 of GNU CC did not provide this default. GNU CC on VMS works only with the GNU assembler, GAS. You need version 1.37 or later of GAS in order to produce value debugging information for the VMS debugger. Use the ordinary VMS linker with the object files produced by GAS. Under previous versions of GNU CC, the generated code would occasionally give strange results when linked to the sharable `VAXCRTL' library. Now this should work. A caveat for use of const global variables: the const modifier must be specified in every external declaration of the variable in all of the source files that use that vari- able. Otherwise the linker will issue warnings about con- flicting attributes for the variable. Your program will still work despite the warnings, but the variable will be placed in writable storage. The VMS linker does not distinguish between upper and lower case letters in function and variable names. However, usual practice in C is to distinguish case. Normally GNU CC (by means of the assembler GAS) implements usual C behavior by augmenting each name that is not all lower-case. A name is augmented by truncating it to at most 23 characters and then adding more characters at the end which encode the case pattern the rest. Name augmentation yields bad results for programs that use precompiled libraries (such as Xlib) which were gen- erated by another compiler. You can use the compiler option `/NOCASE_HACK' to inhibit augmentation; it makes external C functions and variables case-independent as is usual on VMS. Alternatively, you could write all references to the func- tions and variables in such libraries using lower case; this will work on VMS, but is not portable to other systems. 138 Using GNU CC Function and variable names are handled somewhat dif- ferently with GNU C++. The GNU C++ compiler performs name mangling on function names, which means that it adds infor- mation to the function name to describe the data types of the arguments that the function takes. One result of this is that the name of a function can become very long. Since the VMS linker only recognizes the first 31 characters in a name, special action is taken to ensure that each function and variable has a unique name that can be represented in 31 characters. If the name (plus a name augmentation, if required) is less than 32 characters in length, then no special action is performed. If the name is longer than 31 characters, the assembler (GAS) will generate a hash string based upon the function name, truncate the function name to 23 characters, and append the hash string to the truncated name. If the `/VERBOSE' compiler option is used, the assembler will print both the full and truncated names of each symbol that is truncated. The `/NOCASE_HACK' compiler option should not be used when you are compiling programs that use libg++. libg++ has several instances of objects (i.e. Filebuf and filebuf) which become indistinguishable in a case-insensitive environment. This leads to cases where you need to inhibit augmentation selectively (if you were using libg++ and Xlib in the same program, for example). There is no special feature for doing this, but you can get the result by defin- ing a macro for each mixed case symbol for which you wish to inhibit augmentation. The macro should expand into the lower case equivalent of itself. For example: #define StuDlyCapS studlycaps These macro definitions can be placed in a header file to minimize the number of changes to your source code. INTERNALS Using GNU CC 139 11. GNU CC and Portability The main goal of GNU CC was to make a good, fast com- piler for machines in the class that the GNU system aims to run on: 32-bit machines that address 8-bit bytes and have several general registers. Elegance, theoretical power and simplicity are only secondary. GNU CC gets most of the information about the target machine from a machine description which gives an algebraic formula for each of the machine's instructions. This is a very clean way to describe the target. But when the com- piler needs information that is difficult to express in this fashion, I have not hesitated to define an ad-hoc parameter to the machine description. The purpose of portability is to reduce the total work needed on the compiler; it was not of interest for its own sake. GNU CC does not contain machine dependent code, but it does contain code that depends on machine parameters such as endianness (whether the most significant byte has the highest or lowest address of the bytes in a word) and the availability of autoincrement addressing. In the RTL- generation pass, it is often necessary to have multiple strategies for generating code for a particular kind of syn- tax tree, strategies that are usable for different combina- tions of parameters. Often I have not tried to address all possible cases, but only the common ones or only the ones that I have encountered. As a result, a new target may require additional strategies. You will know if this hap- pens because the compiler will call abort. Fortunately, the new strategies can be added in a machine-independent fashion, and will affect only the target machines that need them. INTERNALS 140 Using GNU CC 12. Interfacing to GNU CC Output GNU CC is normally configured to use the same function calling convention normally in use on the target system. This is done with the machine-description macros described (see section Machine Macros). However, returning of structure and union values is done differently on some target machines. As a result, functions compiled with PCC returning such types cannot be called from code compiled with GNU CC, and vice versa. This does not cause trouble often because few Unix library rou- tines return structures or unions. GNU CC code returns structures and unions that are 1, 2, 4 or 8 bytes long in the same registers used for int or double return values. (GNU CC typically allocates variables of such types in registers also.) Structures and unions of other sizes are returned by storing them into an address passed by the caller (usually in a register). The machine- description macros STRUCT_VALUE and STRUCT_INCOMING_VALUE tell GNU CC where to pass this address. By contrast, PCC on most target machines returns struc- tures and unions of any size by copying the data into an area of static storage, and then returning the address of that storage as if it were a pointer value. The caller must copy the data from that memory area to the place where the value is wanted. This is slower than the method used by GNU CC, and fails to be reentrant. On some target machines, such as RISC machines and the 80386, the standard system convention is to pass to the sub- routine the address of where to return the value. On these machines, GNU CC has been configured to be compatible with the standard compiler, when this method is used. It may not be compatible for structures of 1, 2, 4 or 8 bytes. GNU CC uses the system's standard convention for pass- ing arguments. On some machines, the first few arguments are passed in registers; in others, all are passed on the stack. It would be possible to use registers for argument passing on any machine, and this would probably result in a significant speedup. But the result would be complete incompatibility with code that follows the standard conven- tion. So this change is practical only if you are switching to GNU CC as the sole C compiler for the system. We may implement register argument passing on certain machines once we have a complete GNU system so that we can compile the libraries with GNU CC. On some machines (particularly the Sparc), certain types of arguments are passed ``by invisible reference''. Using GNU CC 141 This means that the value is stored in memory, and the address of the memory location is passed to the subroutine. If you use longjmp, beware of automatic variables. ANSI C says that automatic variables that are not declared volatile have undefined values after a longjmp. And this is all GNU CC promises to do, because it is very difficult to restore register variables correctly, and one of GNU CC's features is that it can put variables in registers without your asking it to. If you want a variable to be unaltered by longjmp, and you don't want to write volatile because old C compilers don't accept it, just take the address of the variable. If a variable's address is ever taken, even if just to compute it and ignore it, then the variable cannot go in a register: { int careful; &careful; ... } Code compiled with GNU CC may call certain library rou- tines. Most of them handle arithmetic for which there are no instructions. This includes multiply and divide on some machines, and floating point operations on any machine for which floating point support is disabled with `-msoft- float'. Some standard parts of the C library, such as bcopy or memcpy, are also called automatically. The usual func- tion call interface is used for calling the library rou- tines. These library routines should be defined in the library `libgcc.a', which GNU CC automatically searches whenever it links a program. On machines that have multiply and divide instructions, if hardware floating point is in use, normally `libgcc.a' is not needed, but it is searched just in case. Each arithmetic function is defined in `libgcc1.c' to use the corresponding C arithmetic operator. As long as the file is compiled with another C compiler, which supports all the C arithmetic operators, this file will work portably. However, `libgcc1.c' does not work if compiled with GNU CC, because each arithmetic function would compile into a call to itself! INTERNALS 142 Using GNU CC 13. Passes and Files of the Compiler The overall control structure of the compiler is in `toplev.c'. This file is responsible for initialization, decoding arguments, opening and closing files, and sequenc- ing the passes. The parsing pass is invoked only once, to parse the entire input. The RTL intermediate code for a function is generated as the function is parsed, a statement at a time. Each statement is read in as a syntax tree and then con- verted to RTL; then the storage for the tree for the state- ment is reclaimed. Storage for types (and the expressions for their sizes), declarations, and a representation of the binding contours and how they nest, remain until the func- tion is finished being compiled; these are all needed to output the debugging information. Each time the parsing pass reads a complete function definition or top-level declaration, it calls the function rest_of_compilation or rest_of_decl_compilation in `toplev.c', which are responsible for all further processing necessary, ending with output of the assembler language. All other compiler passes run, in sequence, within rest_of_compilation. When that function returns from com- piling a function definition, the storage used for that function definition's compilation is entirely freed, unless it is an inline function (see section Inline). Here is a list of all the passes of the compiler and their source files. Also included is a description of where debugging dumps can be requested with `-d' options. o+ Parsing. This pass reads the entire text of a function definition, constructing partial syntax trees. This and RTL generation are no longer truly separate passes (formerly they were), but it is easier to think of them as separate. The tree representation does not entirely follow C syntax, because it is intended to support other languages as well. Language-specific data type analysis is also done in this pass, and every tree node that represents an expression has a data type attached. Variables are represented as declaration nodes. Constant folding and some arithmetic simplifications are also done during this pass. The language-independent source files for parsing are `stor-layout.c', `fold-const.c', and `tree.c'. Using GNU CC 143 There are also header files `tree.h' and `tree.def' which define the format of the tree representation. The source files for parsing C are `c-parse.y', `c-decl.c', `c-typeck.c', `c-convert.c', `c- lang.c', and `c-aux-info.c' along with header files `c-lex.h', and `c-tree.h'. The source files for parsing C++ are `cp-parse.y', `cp-class.c', `cp-cvt.c', `cp-decl.c', `cp-decl.c', `cp-decl2.c', `cp- dem.c', `cp-except.c', `cp-expr.c', `cp-init.c', `cp-lex.c', `cp- method.c', `cp-ptree.c', `cp-search.c', `cp-tree.c', `cp-type2.c', and `cp-typeck.c', along with header files `cp- tree.def', `cp-tree.h', and `cp-decl.h'. The special source files for parsing Objective C are `objc-parse.y', `objc-actions.c', `objc- tree.def', and `objc-actions.h'. Certain C- specific files are used for this as well. The file `c-common.c' is also used for all of the above languages. o+ RTL generation. This is the conversion of syntax tree into RTL code. It is actually done statement-by-statement during parsing, but for most purposes it can be thought of as a separate pass. This is where the bulk of target-parameter- dependent code is found, since often it is necessary for strategies to apply only when certain standard kinds of instructions are available. The purpose of named instruction patterns is to provide this information to the RTL generation pass. Optimization is done in this pass for if- conditions that are comparisons, boolean operations or conditional expressions. Tail recursion is detected at this time also. Decisions are made about how best to arrange loops and how to output switch statements. The source files for RTL generation include `stmt.c', `function.c', `expr.c', `calls.c', `explow.c', `expmed.c', `optabs.c' and `emit- rtl.c'. Also, the file `insn-emit.c', generated from the machine description by the program 144 Using GNU CC genemit, is used in this pass. The header file `expr.h' is used for communication within this pass. The header files `insn-flags.h' and `insn- codes.h', generated from the machine description by the programs genflags and gencodes, tell this pass which standard names are available for use and which patterns correspond to them. Aside from debugging information output, none of the following passes refers to the tree structure representation of the function (only part of which is saved). The decision of whether the function can and should be expanded inline in its subsequent callers is made at the end of rtl generation. The function must meet certain criteria, currently related to the size of the function and the types and number of parameters it has. Note that this function may contain loops, recursive calls to itself (tail-recursive functions can be inlined!), gotos, in short, all constructs supported by GNU CC. The file `integrate.c' contains the code to save a function's rtl for later inlining and to inline that rtl when the function is called. The header file `integrate.h' is also used for this purpose. The option `-dr' causes a debugging dump of the RTL code after this pass. This dump file's name is made by appending `.rtl' to the input file name. o+ Jump optimization. This pass simplifies jumps to the following instruction, jumps across jumps, and jumps to jumps. It deletes unreferenced labels and unreachable code, except that unreachable code that contains a loop is not recognized as unreachable in this pass. (Such loops are deleted later in the basic block analysis.) It also converts some code originally written with jumps into sequences of instructions that directly set values from the results of comparisons, if the machine has such instructions. Jump optimization is performed two or three times. The first time is immediately following RTL generation. The second time is after CSE, but only if CSE says repeated jump optimization is needed. The last time is right before the final pass. That time, cross-jumping and deletion of Using GNU CC 145 no-op move instructions are done together with the optimizations described above. The source file of this pass is `jump.c'. The option `-dj' causes a debugging dump of the RTL code after this pass is run for the first time. This dump file's name is made by appending `.jump' to the input file name. o+ Register scan. This pass finds the first and last use of each register, as a guide for common subexpression elimination. Its source is in `regclass.c'. o+ Jump threading. This pass detects a condition jump that branches to an identical or inverse test. Such jumps can be `threaded' through the second conditional test. The source code for this pass is in `jump.c'. This optimization is only performed if `-fthread-jumps' is enabled. o+ Common subexpression elimination. This pass also does constant propagation. Its source file is `cse.c'. If constant propagation causes conditional jumps to become unconditional or to become no-ops, jump optimization is run again when CSE is finished. The option `-ds' causes a debugging dump of the RTL code after this pass. This dump file's name is made by appending `.cse' to the input file name. o+ Loop optimization. This pass moves constant expressions out of loops, and optionally does strength-reduction and loop unrolling as well. Its source files are `loop.c' and `unroll.c', plus the header `loop.h' used for communication between them. Loop unrolling uses some functions in `integrate.c' and the header `integrate.h'. The option `-dL' causes a debugging dump of the RTL code after this pass. This dump file's name is made by appending `.loop' to the input file name. o+ If `-frerun-cse-after-loop' was enabled, a second common subexpression elimination pass is performed after the loop optimization pass. Jump threading is also done again at this time if it was specified. 146 Using GNU CC The option `-dt' causes a debugging dump of the RTL code after this pass. This dump file's name is made by appending `.cse2' to the input file name. o+ Stupid register allocation is performed at this point in a nonoptimizing compilation. It does a little data flow analysis as well. When stupid register allocation is in use, the next pass executed is the reloading pass; the others in between are skipped. The source file is `stupid.c'. o+ Data flow analysis (`flow.c'). This pass divides the program into basic blocks (and in the process deletes unreachable loops); then it computes which pseudo-registers are live at each point in the program, and makes the first instruction that uses a value point at the instruction that computed the value. This pass also deletes computations whose results are never used, and combines memory references with add or subtract instructions to make autoincrement or autodecrement addressing. The option `-df' causes a debugging dump of the RTL code after this pass. This dump file's name is made by appending `.flow' to the input file name. If stupid register allocation is in use, this dump file reflects the full results of such allocation. o+ Instruction combination (`combine.c'). This pass attempts to combine groups of two or three instructions that are related by data flow into single instructions. It combines the RTL expressions for the instructions by substitution, simplifies the result using algebra, and then attempts to match the result against the machine description. The option `-dc' causes a debugging dump of the RTL code after this pass. This dump file's name is made by appending `.combine' to the input file name. o+ Instruction scheduling (`sched.c'). This pass looks for instructions whose output will not be available by the time that it is used in subsequent instructions. (Memory loads and floating point instructions often have this behavior on RISC machines). It re-orders Using GNU CC 147 instructions within a basic block to try to separate the definition and use of items that otherwise would cause pipeline stalls. Instruction scheduling is performed twice. The first time is immediately after instruction combination and the second is immediately after reload. The option `-dS' causes a debugging dump of the RTL code after this pass is run for the first time. The dump file's name is made by appending `.sched' to the input file name. o+ Register class preferencing. The RTL code is scanned to find out which register class is best for each pseudo register. The source file is `regclass.c'. o+ Local register allocation (`local-alloc.c'). This pass allocates hard registers to pseudo registers that are used only within one basic block. Because the basic block is linear, it can use fast and powerful techniques to do a very good job. The option `-dl' causes a debugging dump of the RTL code after this pass. This dump file's name is made by appending `.lreg' to the input file name. o+ Global register allocation (`global-alloc.c'). This pass allocates hard registers for the remaining pseudo registers (those whose life spans are not contained in one basic block). o+ Reloading. This pass renumbers pseudo registers with the hardware registers numbers they were allocated. Pseudo registers that did not get hard registers are replaced with stack slots. Then it finds instructions that are invalid because a value has failed to end up in a register, or has ended up in a register of the wrong kind. It fixes up these instructions by reloading the problematical values temporarily into registers. Additional instructions are generated to do the copying. The reload pass also optionally eliminates the frame pointer and inserts instructions to save and restore call-clobbered registers around calls. Source files are `reload.c' and `reload1.c', plus the header `reload.h' used for communication 148 Using GNU CC between them. The option `-dg' causes a debugging dump of the RTL code after this pass. This dump file's name is made by appending `.greg' to the input file name. o+ Instruction scheduling is repeated here to try to avoid pipeline stalls due to memory loads generated for spilled pseudo registers. The option `-dR' causes a debugging dump of the RTL code after this pass. This dump file's name is made by appending `.sched2' to the input file name. o+ Jump optimization is repeated, this time including cross-jumping and deletion of no-op move instructions. The option `-dJ' causes a debugging dump of the RTL code after this pass. This dump file's name is made by appending `.jump2' to the input file name. o+ Delayed branch scheduling. This optional pass attempts to find instructions that can go into the delay slots of other instructions, usually jumps and calls. The source file name is `reorg.c'. The option `-dd' causes a debugging dump of the RTL code after this pass. This dump file's name is made by appending `.dbr' to the input file name. o+ Conversion from usage of some hard registers to usage of a register stack may be done at this point. Currently, this is supported only for the floating-point registers of the Intel 80387 coprocessor. The source file name is `reg- stack.c'. The options `-dk' causes a debugging dump of the RTL code after this pass. This dump file's name is made by appending `.stack' to the input file name. o+ Final. This pass outputs the assembler code for the function. It is also responsible for identifying spurious test and compare instructions. Machine-specific peephole optimizations are performed at the same time. The function entry and exit sequences are generated Using GNU CC 149 directly as assembler code in this pass; they never exist as RTL. The source files are `final.c' plus `insn- output.c'; the latter is generated automatically from the machine description by the tool `genoutput'. The header file `conditions.h' is used for communication between these files. o+ Debugging information output. This is run after final because it must output the stack slot offsets for pseudo registers that did not get hard registers. Source files are `dbxout.c' for DBX symbol table format, `sdbout.c' for SDB symbol table format, and `dwarfout.c' for DWARF symbol table format. Some additional files are used by all or many passes: o+ Every pass uses `machmode.def' and `machmode.h' which define the machine modes. o+ Several passes use `real.h', which defines the default representation of floating point constants and how to operate on them. o+ All the passes that work with RTL use the header files `rtl.h' and `rtl.def', and subroutines in file `rtl.c'. The tools gen* also use these files to read and work with the machine description RTL. o+ Several passes refer to the header file `insn- config.h' which contains a few parameters (C macro definitions) generated automatically from the machine description RTL by the tool genconfig. o+ Several passes use the instruction recognizer, which consists of `recog.c' and `recog.h', plus the files `insn-recog.c' and `insn-extract.c' that are generated automatically from the machine description by the tools `genrecog' and `genextract'. o+ Several passes use the header files `regs.h' which defines the information recorded about pseudo register usage, and `basic-block.h' which defines the information recorded about basic blocks. o+ `hard-reg-set.h' defines the type HARD_REG_SET, a bit-vector with a bit for each hard register, and some macros to manipulate it. This type is just int if the machine has few enough hard registers; 150 Using GNU CC otherwise it is an array of int and some of the macros expand into loops. o+ Several passes use instruction attributes. A definition of the attributes defined for a particular machine is in file `insn-attr.h', which is generated from the machine description by the program `genattr'. The file `insn-attrtab.c' contains subroutines to obtain the attribute values for insns. It is generated from the machine description by the program `genattrtab'. INTERNALS Using GNU CC 151 14. RTL Representation Most of the work of the compiler is done on an inter- mediate representation called register transfer language. In this language, the instructions to be output are described, pretty much one by one, in an algebraic form that describes what the instruction does. RTL is inspired by Lisp lists. It has both an internal form, made up of structures that point at other structures, and a textual form that is used in the machine description and in printed debugging dumps. The textual form uses nested parentheses to indicate the pointers in the internal form. 14.1. RTL Object Types RTL uses four kinds of objects: expressions, integers, strings and vectors. Expressions are the most important ones. An RTL expression (``RTX'', for short) is a C struc- ture, but it is usually referred to with a pointer; a type that is given the typedef name rtx. An integer is simply an int; their written form uses decimal digits. A string is a sequence of characters. In core it is represented as a char * in usual C fashion, and it is writ- ten in C syntax as well. However, strings in RTL may never be null. If you write an empty string in a machine descrip- tion, it is represented in core as a null pointer rather than as a pointer to a null character. In certain contexts, these null pointers instead of strings are valid. Within RTL code, strings are most commonly found inside symbol_ref expressions, but they appear in other contexts in the RTL expressions that make up machine descriptions. A vector contains an arbitrary number of pointers to expressions. The number of elements in the vector is expli- citly present in the vector. The written form of a vector consists of square brackets (`[...]') surrounding the ele- ments, in sequence and with whitespace separating them. Vectors of length zero are not created; null pointers are used instead. Expressions are classified by expression codes (also called RTX codes). The expression code is a name defined in `rtl.def', which is also (in upper case) a C enumeration constant. The possible expression codes and their meanings are machine-independent. The code of an RTX can be extracted with the macro GET_CODE (x) and altered with 152 Using GNU CC PUT_CODE (x, newcode). The expression code determines how many operands the expression contains, and what kinds of objects they are. In RTL, unlike Lisp, you cannot tell by looking at an operand what kind of object it is. Instead, you must know from its context---from the expression code of the containing expres- sion. For example, in an expression of code subreg, the first operand is to be regarded as an expression and the second operand as an integer. In an expression of code plus, there are two operands, both of which are to be regarded as expressions. In a symbol_ref expression, there is one operand, which is to be regarded as a string. Expressions are written as parentheses containing the name of the expression type, its flags and machine mode if any, and then the operands of the expression (separated by spaces). Expression code names in the `md' file are written in lower case, but when they appear in C code they are written in upper case. In this manual, they are shown as follows: const_int. In a few contexts a null pointer is valid where an expression is normally wanted. The written form of this is (nil). 14.2. Access to Operands For each expression type `rtl.def' specifies the number of contained objects and their kinds, with four possibili- ties: `e' for expression (actually a pointer to an expres- sion), `i' for integer, `s' for string, and `E' for vector of expressions. The sequence of letters for an expression code is called its format. Thus, the format of subreg is `ei'. A few other format characters are used occasionally: u `u' is equivalent to `e' except that it is printed differently in debugging dumps. It is used for pointers to insns. n `n' is equivalent to `i' except that it is printed differently in debugging dumps. It is used for the line number or code number of a note insn. S `S' indicates a string which is optional. In the RTL objects in core, `S' is equivalent to `s', but when the object is read, from an `md' file, the string value of this operand may be omitted. An omitted string is taken to be the null string. Using GNU CC 153 V `V' indicates a vector which is optional. In the RTL objects in core, `V' is equivalent to `E', but when the object is read from an `md' file, the vector value of this operand may be omitted. An omitted vector is effectively the same as a vector of no elements. 0 `0' means a slot whose contents do not fit any normal category. `0' slots are not printed at all in dumps, and are often used in special ways by small parts of the compiler. There are macros to get the number of operands, the format, and the class of an expression code: GET_RTX_LENGTH (code) Number of operands of an RTX of code code. GET_RTX_FORMAT (code) The format of an RTX of code code, as a C string. GET_RTX_CLASS (code) A single character representing the type of RTX operation that code code performs. The following classes are defined: o An RTX code that represents an actual object, such as reg or mem. subreg is not in this class. < An RTX code for a comparison. The codes in this class are NE, EQ, LE, LT, GE, GT, LEU, LTU, GEU, GTU. 1 An RTX code for a unary arithmetic operation, such as neg. c An RTX code for a commutative binary operation, other than NE and EQ (which have class `<'). 2 An RTX code for a noncommutative binary operation, such as MINUS. b An RTX code for a bitfield operation (ZERO_EXTRACT and SIGN_EXTRACT). 3 An RTX code for other three input operations, such as IF_THEN_ELSE. 154 Using GNU CC i An RTX code for a machine insn (INSN, JUMP_INSN, and CALL_INSN). m An RTX code for something that matches in insns, such as MATCH_DUP. x All other RTX codes. Operands of expressions are accessed using the macros XEXP, XINT and XSTR. Each of these macros takes two argu- ments: an expression-pointer (RTX) and an operand number (counting from zero). Thus, XEXP (x, 2) accesses operand 2 of expression x, as an expression. XINT (x, 2) accesses the same operand as an integer. XSTR, used in the same fashion, would access it as a string. Any operand can be accessed as an integer, as an expression or as a string. You must choose the correct method of access for the kind of value actually stored in the operand. You would do this based on the expression code of the containing expression. That is also how you would know how many operands there are. For example, if x is a subreg expression, you know that it has two operands which can be correctly accessed as XEXP (x, 0) and XINT (x, 1). If you did XINT (x, 0), you would get the address of the expression operand but cast as an integer; that might occasionally be useful, but it would be cleaner to write (int) XEXP (x, 0). XEXP (x, 1) would also compile without error, and would return the second, integer operand cast as an expression pointer, which would probably result in a crash when accessed. Nothing stops you from writing XEXP (x, 28) either, but this will access memory past the end of the expression with unpredictable results. Access to operands which are vectors is more compli- cated. You can use the macro XVEC to get the vector-pointer itself, or the macros XVECEXP and XVECLEN to access the ele- ments and length of a vector. Using GNU CC 155 XVEC (exp, idx) Access the vector-pointer which is operand number idx in exp. XVECLEN (exp, idx) Access the length (number of elements) in the vector which is in operand number idx in exp. This value is an int. XVECEXP (exp, idx, eltnum) Access element number eltnum in the vector which is in operand number idx in exp. This value is an RTX. It is up to you to make sure that eltnum is not negative and is less than XVECLEN (exp, idx). All the macros defined in this section expand into lvalues and therefore can be used to assign the operands, lengths and vector elements as well as to access them. 14.3. Flags in an RTL Expression RTL expressions contain several flags (one-bit bit- fields) that are used in certain types of expression. Most often they are accessed with the following macros: MEM_VOLATILE_P (x) In mem expressions, nonzero for volatile memory references. Stored in the volatil field and printed as `/v'. MEM_IN_STRUCT_P (x) In mem expressions, nonzero for reference to an entire structure, union or array, or to a component of one. Zero for references to a scalar variable or through a pointer to a scalar. Stored in the in_struct field and printed as `/s'. REG_LOOP_TEST_P In reg expressions, nonzero if this register's entire life is contained in the exit test code for some loop. Stored in the in_struct field and printed as `/s'. REG_USERVAR_P (x) In a reg, nonzero if it corresponds to a variable present in the user's source code. Zero for temporaries generated internally by the compiler. Stored in the volatil field and printed as `/v'. 156 Using GNU CC REG_FUNCTION_VALUE_P (x) Nonzero in a reg if it is the place in which this function's value is going to be returned. (This happens only in a hard register.) Stored in the integrated field and printed as `/i'. The same hard register may be used also for collecting the values of functions called by this one, but REG_FUNCTION_VALUE_P is zero in this kind of use. RTX_UNCHANGING_P (x) Nonzero in a reg or mem if the value is not changed. (This flag is not set for memory references via pointers to constants. Such pointers only guarantee that the object will not be changed explicitly by the current function. The object might be changed by other functions or by aliasing.) Stored in the unchanging field and printed as `/u'. RTX_INTEGRATED_P (insn) Nonzero in an insn if it resulted from an in-line function call. Stored in the integrated field and printed as `/i'. This may be deleted; nothing currently depends on it. SYMBOL_REF_USED (x) In a symbol_ref, indicates that x has been used. This is normally only used to ensure that x is only declared external once. Stored in the used field. SYMBOL_REF_FLAG (x) In a symbol_ref, this is used as a flag for machine-specific purposes. Stored in the volatil field and printed as `/v'. LABEL_OUTSIDE_LOOP_P In label_ref expressions, nonzero if this is a reference to a label that is outside the innermost loop containing the reference to the label. Stored in the in_struct field and printed as `/s'. INSN_DELETED_P (insn) In an insn, nonzero if the insn has been deleted. Stored in the volatil field and printed as `/v'. INSN_ANNULLED_BRANCH_P (insn) In an insn in the delay slot of a branch insn, indicates that an annulling branch should be used. See the discussion under sequence below. Stored in the unchanging field and printed as `/u'. Using GNU CC 157 INSN_FROM_TARGET_P (insn) In an insn in a delay slot of a branch, indicates that the insn is from the target of the branch. If the branch insn has INSN_ANNULLED_BRANCH_P set, this insn should only be executed if the branch is taken. For annulled branches with this bit clear, the insn should be executed only if the branch is not taken. Stored in the in_struct field and printed as `/s'. CONSTANT_POOL_ADDRESS_P (x) Nonzero in a symbol_ref if it refers to part of the current function's ``constants pool''. These are addresses close to the beginning of the function, and GNU CC assumes they can be addressed directly (perhaps with the help of base registers). Stored in the unchanging field and printed as `/u'. CONST_CALL_P (x) In a call_insn, indicates that the insn represents a call to a const function. Stored in the unchanging field and printed as `/u'. LABEL_PRESERVE_P (x) In a code_label, indicates that the label can never be deleted. Labels referenced by a a non- local goto will have this bit set. Stored in the in_struct field and printed as `/s'. SCHED_GROUP_P (insn) During instruction scheduling, in an insn, indicates that the previous insn must be scheduled together with this insn. This is used to ensure that certain groups of instructions will not be split up by the instruction scheduling pass, for example, use insns before a call_insn may not be separated from the call_insn. Stored in the in_struct field and printed as `/s'. These are the fields which the above macros refer to: used Normally, this flag is used only momentarily, at the end of RTL generation for a function, to count the number of times an expression appears in insns. Expressions that appear more than once are copied, according to the rules for shared structure (see section Sharing). In a symbol_ref, it indicates that an external declaration for the symbol has already been 158 Using GNU CC written. In a reg, it is used by the leaf register renumbering code to ensure that each register is only renumbered once. volatil This flag is used in mem,symbol_ref and reg expressions and in insns. In RTL dump files, it is printed as `/v'. In a mem expression, it is 1 if the memory reference is volatile. Volatile memory references may not be deleted, reordered or combined. In a symbol_ref expression, it is used for machine-specific purposes. In a reg expression, it is 1 if the value is a user-level variable. 0 indicates an internal compiler temporary. In an insn, 1 means the insn has been deleted. in_struct In mem expressions, it is 1 if the memory datum referred to is all or part of a structure or array; 0 if it is (or might be) a scalar variable. A reference through a C pointer has 0 because the pointer might point to a scalar variable. This information allows the compiler to determine something about possible cases of aliasing. In an insn in the delay slot of a branch, 1 means that this insn is from the target of the branch. During instruction scheduling, in an insn, 1 means that this insn must be scheduled as part of a group together with the previous insn. In reg expressions, it is 1 if the register has its entire life contained within the test expression of some loopl. In label_ref expressions, 1 means that the referenced label is outside the innermost loop containing the insn in which the label_ref was found. In code_label expressions, it is 1 if the label may never be deleted. This is used for labels which are the target of non-local gotos. Using GNU CC 159 In an RTL dump, this flag is represented as `/s'. unchanging In reg and mem expressions, 1 means that the value of the expression never changes. In an insn, 1 means that this is an annulling branch. In a symbol_ref expression, 1 means that this symbol addresses something in the per-function constants pool. In a call_insn, 1 means that this instruction is a call to a const function. In an RTL dump, this flag is represented as `/u'. integrated In some kinds of expressions, including insns, this flag means the rtl was produced by procedure integration. In a reg expression, this flag indicates the register containing the value to be returned by the current function. On machines that pass parameters in registers, the same register number may be used for parameters as well, but this flag is not set on such uses. 14.4. Machine Modes A machine mode describes a size of data object and the representation used for it. In the C code, machine modes are represented by an enumeration type, enum machine_mode, defined in `machmode.def'. Each RTL expression has room for a machine mode and so do certain kinds of tree expressions (declarations and types, to be precise). In debugging dumps and machine descriptions, the machine mode of an RTL expression is written after the expression code with a colon to separate them. The letters `mode' which appear at the end of each machine mode name are omitted. For example, (reg:SI 38) is a reg expression with machine mode SImode. If the mode is VOIDmode, it is not written at all. Here is a table of machine modes. The term ``byte'' below refers to an object of BITS_PER_UNIT bits (see section Storage Layout). 160 Using GNU CC QImode ``Quarter-Integer'' mode represents a single byte treated as an integer. HImode ``Half-Integer'' mode represents a two-byte integer. PSImode ``Partial Single Integer'' mode represents an integer which occupies four bytes but which doesn't really use all four. On some machines, this is the right mode to use for pointers. SImode ``Single Integer'' mode represents a four-byte integer. PDImode ``Partial Double Integer'' mode represents an integer which occupies eight bytes but which doesn't really use all eight. On some machines, this is the right mode to use for certain pointers. DImode ``Double Integer'' mode represents an eight-byte integer. TImode ``Tetra Integer'' (?) mode represents a sixteen- byte integer. SFmode ``Single Floating'' mode represents a single- precision (four byte) floating point number. DFmode ``Double Floating'' mode represents a double- precision (eight byte) floating point number. XFmode ``Extended Floating'' mode represents a triple- precision (twelve byte) floating point number. This mode is used for IEEE extended floating point. TFmode ``Tetra Floating'' mode represents a quadruple- precision (sixteen byte) floating point number. CCmode ``Condition Code'' mode represents the value of a Using GNU CC 161 condition code, which is a machine-specific set of bits used to represent the result of a comparison operation. Other machine-specific modes may also be used for the condition code. These modes are not used on machines that use cc0 (see see section Condition Code). BLKmode ``Block'' mode represents values that are aggregates to which none of the other modes apply. In RTL, only memory references can have this mode, and only if they appear in string-move or vector instructions. On machines which have no such instructions, BLKmode will not appear in RTL. VOIDmode Void mode means the absence of a mode or an unspecified mode. For example, RTL expressions of code const_int have mode VOIDmode because they can be taken to have whatever mode the context requires. In debugging dumps of RTL, VOIDmode is expressed by the absence of any mode. SCmode, DCmode, XCmode, TCmode These modes stand for a complex number represented as a pair of floating point values. The values are in SFmode, DFmode, XFmode, and TFmode, respectively. Since C does not support complex numbers, these machine modes are only partially implemented. The machine description defines Pmode as a C macro which expands into the machine mode used for addresses. Normally this is the mode whose size is BITS_PER_WORD, SImode on 32-bit machines. The only modes which a machine description must support are QImode, and the modes corresponding to BITS_PER_WORD, FLOAT_TYPE_SIZE and DOUBLE_TYPE_SIZE. The compiler will attempt to use DImode for 8-byte structures and unions, but this can be prevented by overriding the definition of MAX_FIXED_MODE_SIZE. Alternatively, you can have the com- piler use TImode for 16-byte structures and unions. Like- wise, you can arrange for the C type short int to avoid using HImode. Very few explicit references to machine modes remain in the compiler and these few references will soon be removed. Instead, the machine modes are divided into mode classes. These are represented by the enumeration type enum mode_class defined in `machmode.h'. The possible mode classes are: 162 Using GNU CC MODE_INT Integer modes. By default these are QImode, HImode, SImode, DImode, and TImode. MODE_PARTIAL_INT The ``partial integer'' modes, PSImode and PDImode. MODE_FLOAT floating point modes. By default these are SFmode, DFmode, XFmode and TFmode. MODE_COMPLEX_INT Complex integer modes. (These are not currently implemented). MODE_COMPLEX_FLOAT Complex floating point modes. By default these are SCmode, DCmode, XCmode, and TCmode. MODE_FUNCTION Algol or Pascal function variables including a static chain. (These are not currently implemented). MODE_CC Modes representing condition code values. These are CCmode plus any modes listed in the EXTRA_CC_MODES macro. See section Jump Patterns, also see `Condition Code'. MODE_RANDOM This is a catchall mode class for modes which don't fit into the above classes. Currently VOIDmode and BLKmode are in MODE_RANDOM. Here are some C macros that relate to machine modes: GET_MODE (x) Returns the machine mode of the RTX x. PUT_MODE (x, newmode) Alters the machine mode of the RTX x to be newmode. NUM_MACHINE_MODES Stands for the number of machine modes available on the target machine. This is one greater than the largest numeric value of any machine mode. GET_MODE_NAME (m) Returns the name of mode m as a string. Using GNU CC 163 GET_MODE_CLASS (m) Returns the mode class of mode m. GET_MODE_WIDER_MODE (m) Returns the next wider natural mode. E.g., GET_WIDER_MODE(QImode) returns HImode. GET_MODE_SIZE (m) Returns the size in bytes of a datum of mode m. GET_MODE_BITSIZE (m) Returns the size in bits of a datum of mode m. GET_MODE_MASK (m) Returns a bitmask containing 1 for all bits in a word that fit within mode m. This macro can only be used for modes whose bitsize is less than or equal to HOST_BITS_PER_INT. GET_MODE_ALIGNMENT (m)) Return the required alignment, in bits, for an object of mode m. GET_MODE_UNIT_SIZE (m) Returns the size in bytes of the subunits of a datum of mode m. This is the same as GET_MODE_SIZE except in the case of complex modes. For them, the unit size is the size of the real or imaginary part. GET_MODE_NUNITS (m) Returns the number of units contained in a mode, i.e., GET_MODE_SIZE divided by GET_MODE_UNIT_SIZE. GET_CLASS_NARROWEST_MODE (c) Returns the narrowest mode in mode class c. The global variables byte_mode and word_mode contain modes whose classes are MODE_INT and whose bitsizes are BITS_PER_UNIT or BITS_PER_WORD, respectively. On 32-bit machines, these are QImode and SImode, respectively. 14.5. Constant Expression Types The simplest RTL expressions are those that represent constant values. (const_int i) This type of expression represents the integer value i. i is customarily accessed with the macro INTVAL as in INTVAL (exp), which is equivalent to XINT (exp, 0). 164 Using GNU CC Keep in mind that the result of INTVAL is an integer on the host machine. If the host machine has more bits in an int than the target machine has in the mode in which the constant will be used, then some of the bits you get from INTVAL will be superfluous. In many cases, for proper results, you must carefully disregard the values of those bits. There is only one expression object for the integer value zero; it is the value of the variable const0_rtx. Likewise, the only expression for integer value one is found in const1_rtx, the only expression for integer value two is found in const2_rtx, and the only expression for integer value negative one is found in constm1_rtx. Any attempt to create an expression of code const_int and value zero, one, two or negative one will return const0_rtx, const1_rtx, const2_rtx or constm1_rtx as appropriate. Similarly, there is only one object for the integer whose value is STORE_FLAG_VALUE. It is found in const_true_rtx. If STORE_FLAG_VALUE is one, const_true_rtx and const1_rtx will point to the same object. If STORE_FLAG_VALUE is -1, const_true_rtx and constm1_rtx will point to the same object. (const_double:m addr i0 i1 ...) Represents either a floating-point constant of mode m or an integer constant that is too large to fit into HOST_BITS_PER_INT bits but small enough to fit within twice that number of bits (GNU CC does not provide a mechanism to represent even larger constants). In the latter case, m will be VOIDmode. addr is used to contain the mem expression that corresponds to the location in memory that at which the constant can be found. If it has not been allocated a memory location, but is on the chain of all const_double expressions in this compilation (maintained using an undisplayed field), addr contains const0_rtx. If it is not on the chain, addr contains cc0_rtx. addr is customarily accessed with the macro CONST_DOUBLE_MEM and the chain field via CONST_DOUBLE_CHAIN. If m is VOIDmode, the bit of the value are stored in i0 and i1. i0 is customarily accessed with the Using GNU CC 165 macro CONST_DOUBLE_LOW and i1 with CONST_DOUBLE_HIGH. If the constant is floating point (either single or double precision), then the number of integers used to store the value depends on the size of REAL_VALUE_TYPE (see section Cross-compilation). The integers represent a double. To convert them to a double, do union real_extract u; bcopy (&CONST_DOUBLE_LOW (x), &u, sizeof u); and then refer to u.d. The macro CONST0_RTX (mode) refers to an expression with value 0 in mode mode. If mode mode is of mode class MODE_INT, it returns const0_rtx. Otherwise, it returns a CONST_DOUBLE expression in mode mode. Similarly, the macro CONST1_RTX (mode) refers to an expression with value 1 in mode mode and similarly for CONST2_RTX. (const_string str) Represents a constant string with value str. Currently this is used only for insn attributes (see section Insn Attributes) since constant strings in C are placed in memory. (symbol_ref symbol) Represents the value of an assembler label for data. symbol is a string that describes the name of the assembler label. If it starts with a `*', the label is the rest of symbol not including the `*'. Otherwise, the label is symbol, usually prefixed with `_'. (label_ref label) Represents the value of an assembler label for code. It contains one operand, an expression, which must be a code_label that appears in the instruction sequence to identify the place where the label should go. The reason for using a distinct expression type for code label references is so that jump optimization can distinguish them. 166 Using GNU CC (const:m exp) Represents a constant that is the result of an assembly-time arithmetic computation. The operand, exp, is an expression that contains only constants (const_int, symbol_ref and label_ref expressions) combined with plus and minus. However, not all combinations are valid, since the assembler cannot do arbitrary arithmetic on relocatable symbols. m should be Pmode. (high:m exp) Represents the high-order bits of exp, usually a symbol_ref. The number of bits is machine- dependent and is normally the number of bits specified in an instruction that initializes the high order bits of a register. It is used with lo_sum to represent the typical two- instruction sequence used in RISC machines to reference a global memory location. m should be Pmode. 14.6. Registers and Memory Here are the RTL expression types for describing access to machine registers and to main memory. (reg:m n) For small values of the integer n (less than FIRST_PSEUDO_REGISTER), this stands for a reference to machine register number n: a hard register. For larger values of n, it stands for a temporary value or pseudo register. The compiler's strategy is to generate code assuming an unlimited number of such pseudo registers, and later convert them into hard registers or into memory references. m is the machine mode of the reference. It is necessary because machines can generally refer to each register in more than one mode. For example, a register may contain a full word but there may be instructions to refer to it as a half word or as a single byte, as well as instructions to refer to it as a floating point number of various precisions. Even for a register that the machine can access in only one mode, the mode must always be specified. Using GNU CC 167 The symbol FIRST_PSEUDO_REGISTER is defined by the machine description, since the number of hard registers on the machine is an invariant characteristic of the machine. Note, however, that not all of the machine registers must be general registers. All the machine registers that can be used for storage of data are given hard register numbers, even those that can be used only in certain instructions or can hold only certain types of data. A hard register may be accessed in various modes throughout one function, but each pseudo register is given a natural mode and is accessed only in that mode. When it is necessary to describe an access to a pseudo register using a nonnatural mode, a subreg expression is used. A reg expression with a machine mode that specifies more than one word of data may actually stand for several consecutive registers. If in addition the register number specifies a hardware register, then it actually represents several consecutive hardware registers starting with the specified one. Each pseudo register number used in a function's RTL code is represented by a unique reg expression. Some pseudo register numbers, those within the range of FIRST_VIRTUAL_REGISTER to LAST_VIRTUAL_REGISTER only appear during the RTL generation phase and are eliminated before the optimization phases. These represent locations in the stack frame that cannot be determined until RTL generation for the function has been completed. The following virtual register numbers are defined: VIRTUAL_INCOMING_ARGS_REGNUM This points to the first word of the incoming arguments passed on the stack. Normally these arguments are placed there by the caller, but the callee may have pushed some arguments that were previously passed in registers. When RTL generation is complete, this virtual register is replaced by the sum of the register given by ARG_POINTER_REGNUM and the value of FIRST_PARM_OFFSET. 168 Using GNU CC VIRTUAL_STACK_VARS_REGNUM If FRAME_GROWS_DOWNWARDS is defined, this points to immediately above the first variable on the stack. Otherwise, it points to the first variable on the stack. It is replaced with the sum of the register given by FRAME_POINTER_REGNUM and the value STARTING_FRAME_OFFSET. VIRTUAL_STACK_DYNAMIC_REGNUM This points to the location of dynamically allocated memory on the stack immediately after the stack pointer has been adjusted by the amount of memory desired. It is replaced by the sum of the register given by STACK_POINTER_REGNUM and the value STACK_DYNAMIC_OFFSET. VIRTUAL_OUTGOING_ARGS_REGNUM This points to the location in the stack at which outgoing arguments should be written when the stack is pre-pushed (arguments pushed using push insns should always use STACK_POINTER_REGNUM). It is replaced by the sum of the register given by STACK_POINTER_REGNUM and the value STACK_POINTER_OFFSET. (subreg:m reg wordnum) subreg expressions are used to refer to a register in a machine mode other than its natural one, or to refer to one register of a multi-word reg that actually refers to several registers. Each pseudo-register has a natural mode. If it is necessary to operate on it in a different mode--- for example, to perform a fullword move instruction on a pseudo-register that contains a single byte---the pseudo-register must be enclosed in a subreg. In such a case, wordnum is zero. Usually m is at least as narrow as the mode of reg, in which case it is restricting consideration to only the bits of reg that are in m. However, sometimes m is wider than the mode of reg. These subreg expressions are often called paradoxical. They are used in cases where we want to refer to an object in a wider mode but do not care what value the additional bits have. The reload pass Using GNU CC 169 ensures that paradoxical references are only made to hard registers. The other use of subreg is to extract the individual registers of a multi-register value. Machine modes such as DImode and TImode can indicate values longer than a word, values which usually require two or more consecutive registers. To access one of the registers, use a subreg with mode SImode and a wordnum that says which register. The compilation parameter WORDS_BIG_ENDIAN, if set to 1, says that word number zero is the most significant part; otherwise, it is the least significant part. Between the combiner pass and the reload pass, it is possible to have a paradoxical subreg which contains a mem instead of a reg as its first operand. After the reload pass, it is also possible to have a non-paradoxical subreg which contains a mem; this usually occurs when the mem is a stack slot which replaced a pseudo register. Note that it is not valid to access a DFmode value in SFmode using a subreg. On some machines the most significant part of a DFmode value does not have the same format as a single-precision floating value. It is also not valid to access a single word of a multi-word value in a hard register when less registers can hold the value than would be expected from its size. For example, some 32-bit machines have floating-point registers that can hold an entire DFmode value. If register 10 were such a register (subreg:SI (reg:DF 10) 1) would be invalid because there is no way to convert that reference to a single machine register. The reload pass prevents subreg expressions such as these from being formed. The first operand of a subreg expression is customarily accessed with the SUBREG_REG macro and the second operand is customarily accessed with the SUBREG_WORD macro. (scratch:m) This represents a scratch register that will be required for the execution of a single instruction and not used subsequently. It is converted into a reg by either the local register allocator or the 170 Using GNU CC reload pass. scratch is usually present inside a clobber operation (see section Side Effects). (cc0) This refers to the machine's condition code register. It has no operands and may not have a machine mode. There are two ways to use it: o+ To stand for a complete set of condition code flags. This is best on most machines, where each comparison sets the entire series of flags. With this technique, (cc0) may be validly used in only two contexts: as the destination of an assignment (in test and compare instructions) and in comparison operators comparing against zero (const_int with value zero; that is to say, const0_rtx). o+ To stand for a single flag that is the result of a single condition. This is useful on machines that have only a single flag bit, and in which comparison instructions must specify the condition to test. With this technique, (cc0) may be validly used in only two contexts: as the destination of an assignment (in test and compare instructions) where the source is a comparison operator, and as the first operand of if_then_else (in a conditional branch). There is only one expression object of code cc0; it is the value of the variable cc0_rtx. Any attempt to create an expression of code cc0 will return cc0_rtx. Instructions can set the condition code implicitly. On many machines, nearly all instructions set the condition code based on the value that they compute or store. It is not necessary to record these actions explicitly in the RTL because the machine description includes a prescription for recognizing the instructions that do so (by means of the macro NOTICE_UPDATE_CC). See section Condition Code. Only instructions whose sole purpose is to set the condition code, and instructions that use the condition code, need mention (cc0). Using GNU CC 171 On some machines, the condition code register is given a register number and a reg is used instead of (cc0). This is usually the preferable approach if only a small subset of instructions modify the condition code. Other machines store condition codes in general registers; in such cases a pseudo register should be used. Some machines, such as the Sparc and RS/6000, have two sets of arithmetic instructions, one that sets and one that does not set the condition code. This is best handled by normally generating the instruction that does not set the condition code, and making a pattern that both performs the arithmetic and sets the condition code register (which would not be (cc0) in this case). For examples, search for `addcc' and `andcc' in `sparc.md'. (pc) This represents the machine's program counter. It has no operands and may not have a machine mode. (pc) may be validly used only in certain specific contexts in jump instructions. There is only one expression object of code pc; it is the value of the variable pc_rtx. Any attempt to create an expression of code pc will return pc_rtx. All instructions that do not jump alter the program counter implicitly by incrementing it, but there is no need to mention this in the RTL. (mem:m addr) This RTX represents a reference to main memory at an address represented by the expression addr. m specifies how large a unit of memory is accessed. 14.7. RTL Expressions for Arithmetic Unless otherwise specified, all the operands of arith- metic expressions must be valid for mode m. An operand is valid for mode m if it has mode m, or if it is a const_int or const_double and m is a mode of class MODE_INT. For commutative binary operations, constants should be placed in the second operand. (plus:m x y) Represents the sum of the values represented by x and y carried out in machine mode m. 172 Using GNU CC (lo_sum:m x y) Like plus, except that it represents that sum of x and the low-order bits of y. The number of low order bits is machine-dependent but is normally the number of bits in a Pmode item minus the number of bits set by the high code (see section Constants). m should be Pmode. (minus:m x y) Like plus but represents subtraction. (compare:m x y) Represents the result of subtracting y from x for purposes of comparison. The result is computed without overflow, as if with infinite precision. Of course, machines can't really subtract with infinite precision. However, they can pretend to do so when only the sign of the result will be used, which is the case when the result is stored in the condition code. And that is the only way this kind of expression may validly be used: as a value to be stored in the condition codes. The mode m is not related to the modes of x and y, but instead is the mode of the condition code value. If (cc0) is used, it is VOIDmode. Otherwise it is some mode in class MODE_CC, often CCmode. See section Condition Code. Normally, x and y must have the same mode. Otherwise, compare is valid only if the mode of x is in class MODE_INT and y is a const_int or const_double with mode VOIDmode. The mode of x determines what mode the comparison is to be done in; thus it must not be VOIDmode. If one of the operands is a constant, it should be placed in the second operand and the comparison code adjusted as appropriate. A compare specifying two VOIDmode constants is not valid since there is no way to know in what mode the comparison is to be performed; the comparison must either be folded during the compilation or the first operand must be loaded into a register while its mode is still known. (neg:m x) Represents the negation (subtraction from zero) of the value represented by x, carried out in mode m. Using GNU CC 173 (mult:m x y) Represents the signed product of the values represented by x and y carried out in machine mode m. Some machines support a multiplication that generates a product wider than the operands. Write the pattern for this as (mult:m (sign_extend:m x) (sign_extend:m y)) where m is wider than the modes of x and y, which need not be the same. Write patterns for unsigned widening multiplication similarly using zero_extend. (div:m x y) Represents the quotient in signed division of x by y, carried out in machine mode m. If m is a floating point mode, it represents the exact quotient; otherwise, the integerized quotient. Some machines have division instructions in which the operands and quotient widths are not all the same; you should represent such instructions using truncate and sign_extend as in, (truncate:m1 (div:m2 x (sign_extend:m2 y))) (udiv:m x y) Like div but represents unsigned division. (mod:m x y) (umod:m x y) Like div and udiv but represent the remainder instead of the quotient. (smin:m x y) (smax:m x y) Represents the smaller (for smin) or larger (for smax) of x and y, interpreted as signed integers in mode m. 174 Using GNU CC (umin:m x y) (umax:m x y) Like smin and smax, but the values are interpreted as unsigned integers. (not:m x) Represents the bitwise complement of the value represented by x, carried out in mode m, which must be a fixed-point machine mode. (and:m x y) Represents the bitwise logical-and of the values represented by x and y, carried out in machine mode m, which must be a fixed-point machine mode. (ior:m x y) Represents the bitwise inclusive-or of the values represented by x and y, carried out in machine mode m, which must be a fixed-point mode. (xor:m x y) Represents the bitwise exclusive-or of the values represented by x and y, carried out in machine mode m, which must be a fixed-point mode. (ashift:m x c) Represents the result of arithmetically shifting x left by c places. x have mode m, a fixed-point machine mode. c be a fixed-point mode or be a constant with mode VOIDmode; which mode is determined by the mode called for in the machine description entry for the left-shift instruction. For example, on the Vax, the mode of c is QImode regardless of m. (lshift:m x c) Like lshift but for arithmetic left shift. ashift and lshift are identical operations; we customarily use ashift for both. (lshiftrt:m x c) (ashiftrt:m x c) Like lshift and ashift but for right shift. Unlike the case for left shift, these two operations are distinct. (rotate:m x c) Using GNU CC 175 (rotatert:m x c) Similar but represent left and right rotate. If c is a constant, use rotate. (abs:m x) Represents the absolute value of x, computed in mode m. (sqrt:m x) Represents the square root of x, computed in mode m. Most often m will be a floating point mode. (ffs:m x) Represents one plus the index of the least significant 1-bit in x, represented as an integer of mode m. (The value is zero if x is zero.) The mode of x need not be m; depending on the target machine, various mode combinations may be valid. 14.8. Comparison Operations Comparison operators test a relation on two operands and are considered to represent a machine-dependent nonzero value described by, but not necessarily equal to, STORE_FLAG_VALUE (see section Misc) if the relation holds, or zero if it does not. The mode of the comparison opera- tion is independent of the mode of the data being compared. If the comparison operation is being tested (e.g., the first operand of an if_then_else), the mode must be VOIDmode. If the comparison operation is producing data to be stored in some variable, the mode must be in class MODE_INT. All com- parison operations producing data must use the same mode, which is machine-specific. There are two ways that comparison operations may be used. The comparison operators may be used to compare the condition codes (cc0) against zero, as in (eq (cc0) (const_int 0)). Such a construct actually refers to the result of the preceding instruction in which the condition codes were set. The instructing setting the condition code must be adjacent to the instruction using the condition code; only note insns may separate them. Alternatively, a comparison operation may directly com- pare two data objects. The mode of the comparison is deter- mined by the operands; they must both be valid for a common machine mode. A comparison with both operands constant would be invalid as the machine mode could not be deduced from it, but such a comparison should never exist in RTL due to constant folding. 176 Using GNU CC In the example above, if (cc0) were last set to (com- pare x y), the comparison operation is identical to (eq x y). Usually only one style of comparisons is supported on a particular machine, but the combine pass will try to merge the operations to produce the eq shown in case it exists in the context of the particular insn involved. Inequality comparisons come in two flavors, signed and unsigned. Thus, there are distinct expression codes gt and gtu for signed and unsigned greater-than. These can produce different results for the same pair of integer values: for example, 1 is signed greater-than -1 but not unsigned greater-than, because -1 when regarded as unsigned is actu- ally 0xffffffff which is greater than 1. The signed comparisons are also used for floating point values. Floating point comparisons are distinguished by the machine modes of the operands. (eq:m x y) 1 if the values represented by x and y are equal, otherwise 0. (ne:m x y) 1 if the values represented by x and y are not equal, otherwise 0. (gt:m x y) 1 if the x is greater than y. If they are fixed- point, the comparison is done in a signed sense. (gtu:m x y) Like gt but does unsigned comparison, on fixed- point numbers only. (lt:m x y) (ltu:m x y) Like gt and gtu but test for ``less than''. (ge:m x y) (geu:m x y) Like gt and gtu but test for ``greater than or equal''. (le:m x y) (leu:m x y) Like gt and gtu but test for ``less than or equal''. Using GNU CC 177 (if_then_else cond then else) This is not a comparison operation but is listed here because it is always used in conjunction with a comparison operation. To be precise, cond is a comparison expression. This expression represents a choice, according to cond, between the value represented by then and the one represented by else. On most machines, if_then_else expressions are valid only to express conditional jumps. (cond [test1 value1 test2 value2 ...] default) Similar to if_then_else, but more general. Each of test1, test2, ... is performed in turn. The result of this expression is the value corresponding to the first non-zero test, or default if none of the tests are non-zero expressions. This is currently not valid for instruction patterns and is supported only for insn attributes. See section Insn Attributes. 14.9. Bit Fields Special expression codes exist to represent bit-field instructions. These types of expressions are lvalues in RTL; they may appear on the left side of an assignment, indicating insertion of a value into the specified bit field. (sign_extract:m loc size pos) This represents a reference to a sign-extended bit field contained or starting in loc (a memory or register reference). The bit field is size bits wide and starts at bit pos. The compilation option BITS_BIG_ENDIAN says which end of the memory unit pos counts from. If loc is in memory, its mode must be a single- byte integer mode. If loc is in a register, the mode to use is specified by the operand of the insv or extv pattern (see section Standard Names) and is usually a full-word integer mode. The mode of pos is machine-specific and is also specified in the insv or extv pattern. The mode m is the same as the mode that would be used for loc if it were a register. 178 Using GNU CC (zero_extract:m loc size pos) Like sign_extract but refers to an unsigned or zero-extended bit field. The same sequence of bits are extracted, but they are filled to an entire word with zeros instead of by sign- extension. 14.10. Conversions All conversions between machine modes must be represented by explicit conversion operations. For example, an expression which is the sum of a byte and a full word cannot be written as (plus:SI (reg:QI 34) (reg:SI 80)) because the plus operation requires two operands of the same machine mode. Therefore, the byte-sized operand is enclosed in a conversion operation, as in (plus:SI (sign_extend:SI (reg:QI 34)) (reg:SI 80)) The conversion operation is not a mere placeholder, because there may be more than one way of converting from a given starting mode to the desired final mode. The conver- sion operation code says how to do it. For all conversion operations, x must not be VOIDmode because the mode in which to do the conversion would not be known. The conversion must either be done at compile-time or x must be placed into a register. (sign_extend:m x) Represents the result of sign-extending the value x to machine mode m. m must be a fixed-point mode and x a fixed-point value of a mode narrower than m. (zero_extend:m x) Represents the result of zero-extending the value x to machine mode m. m must be a fixed-point mode and x a fixed-point value of a mode narrower than m. (float_extend:m x) Represents the result of extending the value x to machine mode m. m must be a floating point mode and x a floating point value of a mode narrower than m. (truncate:m x) Represents the result of truncating the value x to Using GNU CC 179 machine mode m. m must be a fixed-point mode and x a fixed-point value of a mode wider than m. (float_truncate:m x) Represents the result of truncating the value x to machine mode m. m must be a floating point mode and x a floating point value of a mode wider than m. (float:m x) Represents the result of converting fixed point value x, regarded as signed, to floating point mode m. (unsigned_float:m x) Represents the result of converting fixed point value x, regarded as unsigned, to floating point mode m. (fix:m x) When m is a fixed point mode, represents the result of converting floating point value x to mode m, regarded as signed. How rounding is done is not specified, so this operation may be used validly in compiling C code only for integer- valued operands. (unsigned_fix:m x) Represents the result of converting floating point value x to fixed point mode m, regarded as unsigned. How rounding is done is not specified. (fix:m x) When m is a floating point mode, represents the result of converting floating point value x (valid for mode m) to an integer, still represented in floating point mode m, by rounding towards zero. 14.11. Declarations Declaration expression codes do not represent arith- metic operations but rather state assertions about their operands. (strict_low_part (subreg:m (reg:n r) 0)) This expression code is used in only one context: operand 0 of a set expression. In addition, the operand of this expression must be a non- paradoxical subreg expression. The presence of strict_low_part says that the part of the register which is meaningful in mode n, but 180 Using GNU CC is not part of mode m, is not to be altered. Normally, an assignment to such a subreg is allowed to have undefined effects on the rest of the register when m is less than a word. 14.12. Side Effect Expressions The expression codes described so far represent values, not actions. But machine instructions never produce values; they are meaningful only for their side effects on the state of the machine. Special expression codes are used to represent side effects. The body of an instruction is always one of these side effect codes; the codes described above, which represent values, appear only as the operands of these. (set lval x) Represents the action of storing the value of x into the place represented by lval. lval must be an expression representing a place that can be stored in: reg (or subreg or strict_low_part), mem, pc or cc0. If lval is a reg, subreg or mem, it has a machine mode; then x must be valid for that mode. If lval is a reg whose machine mode is less than the full width of the register, then it means that the part of the register specified by the machine mode is given the specified value and the rest of the register receives an undefined value. Likewise, if lval is a subreg whose machine mode is narrower than the mode of the register, the rest of the register can be changed in an undefined way. If lval is a strict_low_part of a subreg, then the part of the register specified by the machine mode of the subreg is given the value x and the rest of the register is not changed. If lval is (cc0), it has no machine mode, and x may be either a compare expression or a value that may have any mode. The latter case represents a ``test'' instruction. The expression (set (cc0) (reg:m n)) is equivalent to (set (cc0) (compare (reg:m n) (const_int 0))). Use the former expression to save space during the compilation. If lval is (pc), we have a jump instruction, and the possibilities for x are very limited. It may Using GNU CC 181 be a label_ref expression (unconditional jump). It may be an if_then_else (conditional jump), in which case either the second or the third operand must be (pc) (for the case which does not jump) and the other of the two must be a label_ref (for the case which does jump). x may also be a mem or (plus:SI (pc) y), where y may be a reg or a mem; these unusual patterns are used to represent jumps through branch tables. If lval is neither (cc0) nor (pc), the mode of lval must not be VOIDmode and the mode of x must be valid for the mode of lval. lval is customarily accessed with the SET_DEST macro and x with the SET_SRC macro. (return) As the sole expression in a pattern, represents a return from the current function, on machines where this can be done with one instruction, such as Vaxes. On machines where a multi-instruction ``epilogue'' must be executed in order to return from the function, returning is done by jumping to a label which precedes the epilogue, and the return expression code is never used. Inside an if_then_else expression, represents the value to be placed in pc to return to the caller. Note that an insn pattern of (return) is logically equivalent to (set (pc) (return)), but the latter form is never used. (call function nargs) Represents a function call. function is a mem expression whose address is the address of the function to be called. nargs is an expression which can be used for two purposes: on some machines it represents the number of bytes of stack argument; on others, it represents the number of argument registers. Each machine has a standard machine mode which function must have. The machine description defines macro FUNCTION_MODE to expand into the requisite mode name. The purpose of this mode is to specify what kind of addressing is allowed, on machines where the allowed kinds of addressing depend on the machine mode being addressed. (clobber x) Represents the storing or possible storing of an 182 Using GNU CC unpredictable, undescribed value into x, which must be a reg, scratch or mem expression. One place this is used is in string instructions that store standard values into particular hard registers. It may not be worth the trouble to describe the values that are stored, but it is essential to inform the compiler that the registers will be altered, lest it attempt to keep data in them across the string instruction. If x is (mem:BLK (const_int 0)), it means that all memory locations must be presumed clobbered. Note that the machine description classifies certain hard registers as ``call-clobbered''. All function call instructions are assumed by default to clobber these registers, so there is no need to use clobber expressions to indicate this fact. Also, each function call is assumed to have the potential to alter any memory location, unless the function is declared const. If the last group of expressions in a parallel are each a clobber expression whose arguments are reg or match_scratch (see section RTL Template) expressions, the combiner phase can add the appropriate clobber expressions to an insn it has constructed when doing so will cause a pattern to be matched. This feature can be used, for example, on a machine that whose multiply and add instructions don't use an MQ register but which has an add- accumulate instruction that does clobber the MQ register. Similarly, a combined instruction might require a temporary register while the constituent instructions might not. When a clobber expression for a register appears inside a parallel with other side effects, the register allocator guarantees that the register is unoccupied both before and after that insn. However, the reload phase may allocate a register used for one of the inputs unless the `&' constraint is specified for the selected alternative (see section Modifiers). You can clobber either a specific hard register, a pseudo register, or a scratch expression; in the latter two cases, GNU CC will allocate a hard register that is available there for use as a temporary. Using GNU CC 183 For instructions that require a temporary register, you should use scratch instead of a pseudo-register because this will allow the combiner phase to add the clobber when required. You do this by coding (clobber (match_scratch ...)). If you do clobber a pseudo register, use one which appears nowhere else---generate a new one each time. Otherwise, you may confuse CSE. There is one other known use for clobbering a pseudo register in a parallel: when one of the input operands of the insn is also clobbered by the insn. In this case, using the same pseudo register in the clobber and elsewhere in the insn produces the expected results. (use x) Represents the use of the value of x. It indicates that the value in x at this point in the program is needed, even though it may not be apparent why this is so. Therefore, the compiler will not attempt to delete previous instructions whose only effect is to store a value in x. x must be a reg expression. During the delayed branch scheduling phase, x may be an insn. This indicates that x previously was located at this place in the code and its data dependencies need to be taken into account. These use insns will be deleted before the delayed branch scheduling phase exits. (parallel [x0 x1 ...]) Represents several side effects performed in parallel. The square brackets stand for a vector; the operand of parallel is a vector of expressions. x0, x1 and so on are individual side effect expressions---expressions of code set, call, return, clobber or use. ``In parallel'' means that first all the values used in the individual side-effects are computed, and second all the actual side-effects are performed. For example, (parallel [(set (reg:SI 1) (mem:SI (reg:SI 1))) (set (mem:SI (reg:SI 1)) (reg:SI 1))]) says unambiguously that the values of hard register 1 and the memory location addressed by it are 184 Using GNU CC interchanged. In both places where (reg:SI 1) appears as a memory address it refers to the value in register 1 before the execution of the insn. It follows that it is incorrect to use parallel and expect the result of one set to be available for the next one. For example, people sometimes attempt to represent a jump- if-zero instruction this way: (parallel [(set (cc0) (reg:SI 34)) (set (pc) (if_then_else (eq (cc0) (const_int 0)) (label_ref ...) (pc)))]) But this is incorrect, because it says that the jump condition depends on the condition code value before this instruction, not on the new value that is set by this instruction. Peephole optimization, which takes place together with final assembly code output, can produce insns whose patterns consist of a parallel whose elements are the operands needed to output the resulting assembler code---often reg, mem or constant expressions. This would not be well-formed RTL at any other stage in compilation, but it is ok then because no further optimization remains to be done. However, the definition of the macro NOTICE_UPDATE_CC, if any, must deal with such insns if you define any peephole optimizations. (sequence [insns ...]) Represents a sequence of insns. Each of the insns that appears in the vector is suitable for appearing in the chain of insns, so it must be an insn, jump_insn, call_insn, code_label, barrier or note. A sequence RTX is never placed in an actual insn during RTL generation. It represents the sequence of insns that result from a define_expand before those insns are passed to emit_insn to insert them in the chain of insns. When actually inserted, the individual sub-insns are separated out and the sequence is forgotten. Using GNU CC 185 After delay-slot scheduling is completed, an insn and all the insns that reside in its delay slots are grouped together into a sequence. The insn requiring the delay slot is the first insn in the vector; subsequent insns are to be placed in the delay slot. INSN_ANNULLED_BRANCH_P is set on an insn in a delay slot to indicate that a branch insn should be used that will conditionally annul the effect of the insns in the delay slots. In such a case, INSN_FROM_TARGET_P indicates that the insn is from the target of the branch and should be executed only if the branch is taken; otherwise the insn should be executed only if the branch is not taken. See section Delay Slots. These expression codes appear in place of a side effect, as the body of an insn, though strictly speaking they do not always describe side effects as such: (asm_input s) Represents literal assembler code as described by the string s. (unspec [operands ...] index) (unspec [operands ...] index) Represents a machine-specific operation on operands. index selects between multiple macine- specific operations. unspec_volatile is used for volatile operations and operations that may trap; unspec is used for other operations. These codes may appear themselves inside a pattern of an insn, inside a parallel, or inside an expression. (addr_vec:m [lr0 lr1 ...]) Represents a table of jump addresses. The vector elements lr0, etc., are label_ref expressions. The mode m specifies how much space is given to each address; normally m would be Pmode. (addr_diff_vec:m base [lr0 lr1 ...]) Represents a table of jump addresses expressed as offsets from base. The vector elements lr0, etc., are label_ref expressions and so is base. The mode m specifies how much space is given to each address-difference. 186 Using GNU CC 14.13. Embedded Side-Effects on Addresses Four special side-effect expression codes appear as memory addresses. (pre_dec:m x) Represents the side effect of decrementing x by a standard amount and represents also the value that x has after being decremented. x must be a reg or mem, but most machines allow only a reg. m must be the machine mode for pointers on the machine in use. The amount x is decremented by is the length in bytes of the machine mode of the containing memory reference of which this expression serves as the address. Here is an example of its use: (mem:DF (pre_dec:SI (reg:SI 39))) This says to decrement pseudo register 39 by the length of a DFmode value and use the result to address a DFmode value. (pre_inc:m x) Similar, but specifies incrementing x instead of decrementing it. (post_dec:m x) Represents the same side effect as pre_dec but a different value. The value represented here is the value x has before being decremented. (post_inc:m x) Similar, but specifies incrementing x instead of decrementing it. These embedded side effect expressions must be used with care. Instruction patterns may not use them. Until the `flow' pass of the compiler, they may occur only to represent pushes onto the stack. The `flow' pass finds cases where registers are incremented or decremented in one instruction and used as an address shortly before or after; these cases are then transformed to use pre- or post- increment or -decrement. If a register used as the operand of these expressions is used in another address in an insn, the original value of the register is used. Uses of the register outside of an address are not permitted within the same insn as a use in an embedded side effect expression because such insns behave Using GNU CC 187 differently on different machines and hence must be treated as ambiguous and disallowed. An instruction that can be represented with an embedded side effect could also be represented using parallel con- taining an additional set to describe how the address regis- ter is altered. This is not done because machines that allow these operations at all typically allow them wherever a memory address is called for. Describing them as addi- tional parallel stores would require doubling the number of entries in the machine description. 14.14. Assembler Instructions as Expressions The RTX code asm_operands represents a value produced by a user-specified assembler instruction. It is used to represent an asm statement with arguments. An asm statement with a single output operand, like this: asm ("foo %1,%2,%0" : "=a" (outputvar) : "g" (x + y), "di" (*z)); is represented using a single asm_operands RTX which represents the value that is stored in outputvar: (set rtx-for-outputvar (asm_operands "foo %1,%2,%0" "a" 0 [rtx-for-addition-result rtx-for-*z] [(asm_input:m1 "g") (asm_input:m2 "di")])) Here the operands of the asm_operands RTX are the assembler template string, the output-operand's constraint, the index-number of the output operand among the output operands specified, a vector of input operand RTX's, and a vector of input-operand modes and constraints. The mode m1 is the mode of the sum x+y; m2 is that of *z. When an asm statement has multiple output values, its insn has several such set RTX's inside of a parallel. Each set contains a asm_operands; all of these share the same assembler template and vectors, but each contains the con- straint for the respective output operand. They are also distinguished by the output-operand index number, which is 0, 1, ... for successive output operands. 188 Using GNU CC 14.15. Insns The RTL representation of the code for a function is a doubly-linked chain of objects called insns. Insns are expressions with special codes that are used for no other purpose. Some insns are actual instructions; others represent dispatch tables for switch statements; others represent labels to jump to or various sorts of declarative information. In addition to its own specific data, each insn must have a unique id-number that distinguishes it from all other insns in the current function (after delayed branch schedul- ing, copies of an insn with the same id-number may be present in multiple places in a function, but these copies will always be identical and will only appear inside a sequence), and chain pointers to the preceding and following insns. These three fields occupy the same position in every insn, independent of the expression code of the insn. They could be accessed with XEXP and XINT, but instead three spe- cial macros are always used: INSN_UID (i) Accesses the unique id of insn i. PREV_INSN (i) Accesses the chain pointer to the insn preceding i. If i is the first insn, this is a null pointer. NEXT_INSN (i) Accesses the chain pointer to the insn following i. If i is the last insn, this is a null pointer. The first insn in the chain is obtained by calling get_insns; the last insn is the result of calling get_last_insn. Within the chain delimited by these insns, the NEXT_INSN and PREV_INSN pointers must always correspond: if insn is not the first insn, NEXT_INSN (PREV_INSN (insn)) == insn is always true and if insn is not the last insn, PREV_INSN (NEXT_INSN (insn)) == insn Using GNU CC 189 is always true. After delay slot scheduling, some of the insns in the chain might be sequence expressions, which contain a vector of insns. The value of NEXT_INSN in all but the last of these insns is the next insn in the vector; the value of NEXT_INSN of the last insn in the vector is the same as the value of NEXT_INSN for the sequence in which it is con- tained. Similar rules apply for PREV_INSN. This means that the above invariants are not neces- sarily true for insns inside sequence expressions. Specifi- cally, if insn is the first insn in a sequence, NEXT_INSN (PREV_INSN (insn)) is the insn containing the sequence expression, as is the value of PREV_INSN (NEXT_INSN (insn)) is insn is the last insn in the sequence expression. You can use these expressions to find the containing sequence expression. Every insn has one of the following six expression codes: insn The expression code insn is used for instructions that do not jump and do not do function calls. sequence expressions are always contained in insns with code insn even if one of those insns should jump or do function calls. Insns with code insn have four additional fields beyond the three mandatory ones listed above. These four are described in a table below. jump_insn The expression code jump_insn is used for instructions that may jump (or, more generally, may contain label_ref expressions). If there is an instruction to return from the current function, it is recorded as a jump_insn. jump_insn insns have the same extra fields as insn insns, accessed in the same way and in addition contains a field JUMP_LABEL which is defined once jump optimization has completed. For simple conditional and unconditional jumps, this field contains the code_label to which this insn will (possibly conditionally) branch. In a more complex jump, JUMP_LABEL records one of the labels that the insn refers to; the only way to find the others is to scan the entire body of the insn. 190 Using GNU CC Return insns count as jumps, but since they do not refer to any labels, they have zero in the JUMP_LABEL field. call_insn The expression code call_insn is used for instructions that may do function calls. It is important to distinguish these instructions because they imply that certain registers and memory locations may be altered unpredictably. A call_insn insn may be preceeded by insns that contain a single use expression and be followed by insns the contain a single clobber expression. If so, these use and clobber expressions are treated as being part of the function call. There must not even be a note between the call_insn and the use or clobber insns for this special treatment to take place. This is somewhat of a kludge and will be removed in a later version of GNU CC. call_insn insns have the same extra fields as insn insns, accessed in the same way. code_label A code_label insn represents a label that a jump insn can jump to. It contains two special fields of data in addition to the three standard ones. CODE_LABEL_NUMBER is used to hold the label number, a number that identifies this label uniquely among all the labels in the compilation (not just in the current function). Ultimately, the label is represented in the assembler output as an assembler label, usually of the form `Ln' where n is the label number. When a code_label appears in an RTL expression, it normally appears within a label_ref which represents the address of the label, as a number. The field LABEL_NUSES is only defined once the jump optimization phase is completed and contains the number of times this label is referenced in the current function. barrier Barriers are placed in the instruction stream when control cannot flow past them. They are placed after unconditional jump instructions to indicate that the jumps are unconditional and after calls to volatile functions, which do not return (e.g., exit). They contain no information beyond the three standard fields. Using GNU CC 191 note note insns are used to represent additional debugging and declarative information. They contain two nonstandard fields, an integer which is accessed with the macro NOTE_LINE_NUMBER and a string accessed with NOTE_SOURCE_FILE. If NOTE_LINE_NUMBER is positive, the note represents the position of a source line and NOTE_SOURCE_FILE is the source file name that the line came from. These notes control generation of line number data in the assembler output. Otherwise, NOTE_LINE_NUMBER is not really a line number but a code with one of the following values (and NOTE_SOURCE_FILE must contain a null pointer): NOTE_INSN_DELETED Such a note is completely ignorable. Some passes of the compiler delete insns by altering them into notes of this kind. NOTE_INSN_BLOCK_BEG NOTE_INSN_BLOCK_END These types of notes indicate the position of the beginning and end of a level of scoping of variable names. They control the output of debugging information. NOTE_INSN_LOOP_BEG NOTE_INSN_LOOP_END These types of notes indicate the position of the beginning and end of a while or for loop. They enable the loop optimizer to find loops quickly. NOTE_INSN_LOOP_CONT Appears at the place in a loop that continue statements jump to. NOTE_INSN_LOOP_VTOP This note indicates the place in a loop where the exit test begins for those loops in which the exit test has been duplicated. This position becomes another virtual start of the loop when considering loop invariants. NOTE_INSN_FUNCTION_END Appears near the end of the function body, just before the label that return statements 192 Using GNU CC jump to (on machine where a single instruction does not suffice for returning). This note may be deleted by jump optimization. NOTE_INSN_SETJMP Appears following each call to setjmp or a related function. These codes are printed symbolically when they appear in debugging dumps. The machine mode of an insn is normally VOIDmode, but some phases use the mode for various purposes; for example, the reload pass sets it to HImode if the insn needs reload- ing but not register elimination and QImode if both are required. The common subexpression elimination pass sets the mode of an insn to QImode when it is the first insn in a block that has already been processed. Here is a table of the extra fields of insn, jump_insn and call_insn insns: PATTERN (i) An expression for the side effect performed by this insn. This must be one of the following codes: set, call, use, clobber, return, asm_input, asm_output, addr_vec, addr_diff_vec, trap_if, unspec, unspec_volatile, or parallel. If it is a parallel, each element of the parallel must be one these codes, except that parallel expressions cannot be nested and addr_vec and addr_diff_vec are not permitted inside a parallel expression. INSN_CODE (i) An integer that says which pattern in the machine description matches this insn, or -1 if the matching has not yet been attempted. Such matching is never attempted and this field remains -1 on an insn whose pattern consists of a single use, clobber, asm_input, addr_vec or addr_diff_vec expression. Matching is also never attempted on insns that result from an asm statement. These contain at least one asm_operands expression. The function asm_noperands returns a non-negative value for such insns. Using GNU CC 193 In the debugging output, this field is printed as a number followed by a symbolic representation that locates the pattern in the `md' file as some small positive or negative offset from a named pattern. LOG_LINKS (i) A list (chain of insn_list expressions) giving information about dependencies between instructions within a basic block. Neither a jump nor a label may come between the related insns. REG_NOTES (i) A list (chain of expr_list and insn_list expressions) giving miscellaneous information about the insn. It is often information pertaining to the registers used in this insn. The LOG_LINKS field of an insn is a chain of insn_list expressions. Each of these has two operands: the first is an insn, and the second is another insn_list expression (the next one in the chain). The last insn_list in the chain has a null pointer as second operand. The significant thing about the chain is which insns appear in it (as first operands of insn_list expressions). Their order is not sig- nificant. This list is originally set up by the flow analysis pass; it is a null pointer until then. Flow only adds links for those data dependencies which can be used for instruc- tion combination. For each insn, the flow analysis pass adds a link to insns which store into registers values that are used for the first time in this insn. The instruction scheduling pass adds extra links so that every dependence will be represented. Links represent data dependencies, antidependencies and output dependencies; the machine mode of the link distinguishes these three types: antidependen- cies have mode REG_DEP_ANTI, output dependencies have mode REG_DEP_OUTPUT, and data dependencies have mode VOIDmode. The REG_NOTES field of an insn is a chain similar to the LOG_LINKS field but it includes expr_list expressions in addition to insn_list expressions. There are several kinds of register notes, which are distinguished by the machine mode, which in a register note is really understood as being an enum reg_note. The first operand op of the note is data whose meaning depends on the kind of note. The macro REG_NOTE_KIND (x) returns the the kind of register note. Its counterpart, the macro PUT_REG_NOTE_KIND (x, newkind) sets the register note type of x to be newkind. 194 Using GNU CC Register notes are of three classes: They may say some- thing about an input to an insn, they may say something about an output of an insn, or they may create a linkage between two insns. There are also a set of values that are only used in LOG_LINKS. These register notes annotate inputs to an insn: REG_DEAD The value in op dies in this insn; that is to say, altering the value immediately after this insn would not affect the future behavior of the program. This does not necessarily mean that the register op has no useful value after this insn since it may also be an output of the insn. In such a case, however, a REG_DEAD note would be redundant and is usually not present until after the reload pass, but no code relies on this fact. REG_INC The register op is incremented (or decremented; at this level there is no distinction) by an embedded side effect inside this insn. This means it appears in a post_inc, pre_inc, post_dec or pre_dec expression. REG_NONNEG The register op is known to have a nonnegative value when this insn is reached. This is used so that decrement and branch until zero instructions, such as the m68k dbra, can be matched. The REG_NONNEG note is added to insns only if the machine description contains a pattern named `decrement_and_branch_until_zero'. REG_NO_CONFLICT This insn does not cause a conflict between op and the item being set by this insn even though it might appear that it does. In other words, if the destination register and op could otherwise be assigned the same register, this insn does not prevent that assignment. Insns with this note are usually part of a block that begins with a clobber insn specifying a multi-word pseudo register (which will be the output of the block), a group of insns that each set one word of the value and have the REG_NO_CONFLICT note attached, and a final insn that copies the output to itself with an attached Using GNU CC 195 REG_EQUAL note giving the expression being computed. This block is encapsulated with REG_LIBCALL and REG_RETVAL notes on the first and last insns, respectively. REG_LABEL This insn uses op, a code_label, but is not a jump_insn. The presence of this note allows jump optimization to be aware that op is, in fact, being used. The following notes describe attributes of outputs of an insn: REG_EQUIV REG_EQUAL This note is only valid on an insn that sets only one register and indicates that that register will be equal to op at run time; the scope of this equivalence differs between the two types of notes. The value which the insn explicitly copies into the register may look different from op, but they will be equal at run time. If the output of the single set is a strict_low_part expression, the note refers to the register that is contained in SUBREG_REG of the subreg expression. For REG_EQUIV, the register is equivalent to op throughout the entire function, and could validly be replaced in all its occurrences by op. (``Validly'' here refers to the data flow of the program; simple replacement may make some insns invalid.) For example, when a constant is loaded into a register that is never assigned any other value, this kind of note is used. When a parameter is copied into a pseudo-register at entry to a function, a note of this kind records that the register is equivalent to the stack slot where the parameter was passed. Although in this case the register may be set by other insns, it is still valid to replace the register by the stack slot throughout the function. In the case of REG_EQUAL, the register that is set by this insn will be equal to op at run time at the end of this insn but not necessarily elsewhere in the function. In this case, op is typically an arithmetic expression. For example, when a sequence of insns such as a library call is used 196 Using GNU CC to perform an arithmetic operation, this kind of note is attached to the insn that produces or copies the final value. These two notes are used in different ways by the compiler passes. REG_EQUAL is used by passes prior to register allocation (such as common subexpression elimination and loop optimization) to tell them how to think of that value. REG_EQUIV notes are used by register allocation to indicate that there is an available substitute expression (either a constant or a mem expression for the location of a parameter on the stack) that may be used in place of a register if insufficient registers are available. Except for stack homes for parameters, which are indicated by a REG_EQUIV note and are not useful to the early optimization passes and pseudo registers that are equivalent to a memory location throughout there entire life, which is not detected until later in the compilation, all equivalences are initially indicated by an attached REG_EQUAL note. In the early stages of register allocation, a REG_EQUAL note is changed into a REG_EQUIV note if op is a constant and the insn represents the only set of its destination register. Thus, compiler passes prior to register allocation need only check for REG_EQUAL notes and passes subsequent to register allocation need only check for REG_EQUIV notes. REG_UNUSED The register op being set by this insn will not be used in a subsequent insn. This differs from a REG_DEAD note, which indicates that the value in an input will not be used subsequently. These two notes are independent; both may be present for the same register. REG_WAS_0 The single output of this insn contained zero before this insn. op is the insn that set it to zero. You can rely on this note if it is present and op has not been deleted or turned into a note; its absence implies nothing. These notes describe linkages between insns. They occur in pairs: one insn has one of a pair of notes that points to a second insn, which has the inverse note pointing Using GNU CC 197 back to the first insn. REG_RETVAL This insn copies the value of a multi-insn sequence (for example, a library call), and op is the first insn of the sequence (for a library call, the first insn that was generated to set up the arguments for the library call). Loop optimization uses this note to treat such a sequence as a single operation for code motion purposes and flow analysis uses this note to delete such sequences whose results are dead. A REG_EQUAL note will also usually be attached to this insn to provide the expression being computed by the sequence. REG_LIBCALL This is the inverse of REG_RETVAL: it is placed on the first insn of a multi-insn sequence, and it points to the last one. REG_CC_SETTER REG_CC_USER On machines that use cc0, the insns which set and use cc0 set and use cc0 are adjacent. However, when branch delay slot filling is done, this may no longer be true. In this case a REG_CC_USER note will be placed on the insn setting cc0 to point to the insn using cc0 and a REG_CC_SETTER note will be placed on the insn using cc0 to point to the insn setting cc0. These values are only used in the LOG_LINKS field, and indicate the type of dependency that each link represents. Links which indicate a data dependence (a read after write dependence) do not use any code, they simply have mode VOID- mode, and are printed without any descriptive text. REG_DEP_ANTI This indicates an anti dependence (a write after read dependence). REG_DEP_OUTPUT This indicates an output dependence (a write after write dependence). For convenience, the machine mode in an insn_list or expr_list is printed using these symbolic codes in debugging 198 Using GNU CC dumps. The only difference between the expression codes insn_list and expr_list is that the first operand of an insn_list is assumed to be an insn and is printed in debug- ging dumps as the insn's unique id; the first operand of an expr_list is printed in the ordinary way as an expression. 14.16. RTL Representation of Function-Call Insns Insns that call subroutines have the RTL expression code call_insn. These insns must satisfy special rules, and their bodies must use a special RTL expression code, call. A call expression has two operands, as follows: (call (mem:fm addr) nbytes) Here nbytes is an operand that represents the number of bytes of argument data being passed to the subroutine, fm is a machine mode (which must equal as the definition of the FUNCTION_MODE macro in the machine description) and addr represents the address of the subroutine. For a subroutine that returns no value, the call expression as shown above is the entire body of the insn, except that the insn might also contain use or clobber expressions. For a subroutine that returns a value whose mode is not BLKmode, the value is returned in a hard register. If this register's number is r, then the body of the call insn looks like this: (set (reg:m r) (call (mem:fm addr) nbytes)) This RTL expression makes it clear (to the optimizer passes) that the appropriate register receives a useful value in this insn. When a subroutine returns a BLKmode value, it is han- dled by passing to the subroutine the address of a place to store the value. So the call insn itself does not ``return'' any value, and it has the same RTL form as a call that returns nothing. Using GNU CC 199 On some machines, the call instruction itself clobbers some register, for example to contain the return address. call_insn insns on these machines should have a body which is a parallel that contains both the call expression and clobber expressions that indicate which registers are des- troyed. Similarly, if the call instruction requires some register other than the stack pointer that is not explicitly mentioned it its RTL, a use subexpression should mention that register. Functions that are called are assumed to modify all registers listed in the configuration macro CALL_USED_REGISTERS (see section Register Basics) and, with the exception of const functions and library calls, to modify all of memory. Insns containing just use expressions directly precede the call_insn insn to indicate which registers contain inputs to the function. Similarly, if registers other than those in CALL_USED_REGISTERS are clobbered by the called function, insns containing a single clobber follow immedi- ately after the call to indicate which registers. 14.17. Structure Sharing Assumptions The compiler assumes that certain kinds of RTL expres- sions are unique; there do not exist two distinct objects representing the same value. In other cases, it makes an opposite assumption: that no RTL expression object of a cer- tain kind appears in more than one place in the containing structure. These assumptions refer to a single function; except for the RTL objects that describe global variables and external functions, and a few standard objects such as small integer constants, no RTL objects are common to two func- tions. o+ Each pseudo-register has only a single reg object to represent it, and therefore only a single machine mode. o+ For any symbolic label, there is only one symbol_ref object referring to it. o+ There is only one const_int expression with value 0, only one with value 1, and only one with value -1. Some other integer values are also stored uniquely. o+ There is only one pc expression. 200 Using GNU CC o+ There is only one cc0 expression. o+ There is only one const_double expression with value 0 for each floating point mode. Likewise for values 1 and 2. o+ No label_ref or scratch appears in more than one place in the RTL structure; in other words, it is safe to do a tree-walk of all the insns in the function and assume that each time a label_ref or scratch is seen it is distinct from all others that are seen. o+ Only one mem object is normally created for each static variable or stack slot, so these objects are frequently shared in all the places they appear. However, separate but equal objects for these variables are occasionally made. o+ When a single asm statement has multiple output operands, a distinct asm_operands expression is made for each output operand. However, these all share the vector which contains the sequence of input operands. This sharing is used later on to test whether two asm_operands expressions come from the same statement, so all optimizations must carefully preserve the sharing if they copy the vector at all. o+ No RTL object appears in more than one place in the RTL structure except as described above. Many passes of the compiler rely on this by assuming that they can modify RTL objects in place without unwanted side-effects on other insns. o+ During initial RTL generation, shared structure is freely introduced. After all the RTL for a function has been generated, all shared structure is copied by unshare_all_rtl in `emit-rtl.c', after which the above rules are guaranteed to be followed. o+ During the combiner pass, shared structure within an insn can exist temporarily. However, the shared structure is copied before the combiner is finished with the insn. This is done by calling copy_rtx_if_shared, which is a subroutine of unshare_all_rtl. INTERNALS Using GNU CC 201 15. Machine Descriptions A machine description has two parts: a file of instruc- tion patterns (`.md' file) and a C header file of macro definitions. The `.md' file for a target machine contains a pattern for each instruction that the target machine supports (or at least each instruction that is worth telling the compiler about). It may also contain comments. A semicolon causes the rest of the line to be a comment, unless the semicolon is inside a quoted string. See the next chapter for information on the C header file. 15.1. Everything about Instruction Patterns Each instruction pattern contains an incomplete RTL expression, with pieces to be filled in later, operand con- straints that restrict how the pieces can be filled in, and an output pattern or C code to generate the assembler out- put, all wrapped up in a define_insn expression. A define_insn is an RTL expression containing four or five operands: 1. An optional name. The presence of a name indicate that this instruction pattern can perform a certain standard job for the RTL-generation pass of the compiler. This pass knows certain names and will use the instruction patterns with those names, if the names are defined in the machine description. The absence of a name is indicated by writing an empty string where the name should go. Nameless instruction patterns are never used for generating RTL code, but they may permit several simpler insns to be combined later on. Names that are not thus known and used in RTL- generation have no effect; they are equivalent to no name at all. 2. The RTL template (see section RTL Template) is a vector of incomplete RTL expressions which show what the instruction should look like. It is incomplete because it may contain match_operand, match_operator, and match_dup expressions that stand for operands of the instruction. 202 Using GNU CC If the vector has only one element, that element is the template for the instruction pattern. If the vector has multiple elements, then the instruction pattern is a parallel expression containing the elements described. 3. A condition. This is a string which contains a C expression that is the final test to decide whether an insn body matches this pattern. For a named pattern, the condition (if present) may not depend on the data in the insn being matched, but only the target-machine-type flags. The compiler needs to test these conditions during initialization in order to learn exactly which named instructions are available in a particular run. For nameless patterns, the condition is applied only when matching an individual insn, and only after the insn has matched the pattern's recognition template. The insn's operands may be found in the vector operands. 4. The output template: a string that says how to output matching insns as assembler code. `%' in this string specifies where to substitute the value of an operand. See section Output Template. When simple substitution isn't general enough, you can specify a piece of C code to compute the output. See section Output Statement. 5. Optionally, a vector containing the values of attributes for insns matching this pattern. See section Insn Attributes. 15.2. Example of define_insn Here is an actual example of an instruction pattern, for the 68000/68020. (define_insn "tstsi" [(set (cc0) (match_operand:SI 0 "general_operand" "rm"))] "" "* { if (TARGET_68020 || ! ADDRESS_REG_P (operands[0])) return \"tstl %0\"; return \"cmpl #0,%0\"; }") Using GNU CC 203 This is an instruction that sets the condition codes based on the value of a general operand. It has no condi- tion, so any insn whose RTL description has the form shown may be handled according to this pattern. The name `tstsi' means ``test a SImode value'' and tells the RTL generation pass that, when it is necessary to test such a value, an insn to do so can be constructed using this pattern. The output control string is a piece of C code which chooses which output template to return based on the kind of operand and the specific type of CPU for which code is being generated. `"rm"' is an operand constraint. Its meaning is explained below. 15.3. RTL Template for Generating and Recognizing Insns The RTL template is used to define which insns match the particular pattern and how to find their operands. For named patterns, the RTL template also says how to construct an insn from specified operands. Construction involves substituting specified operands into a copy of the template. Matching involves determining the values that serve as the operands in the insn being matched. Both of these activities are controlled by special expression types that direct matching and substitution of the operands. (match_operand:m n predicate constraint) This expression is a placeholder for operand number n of the insn. When constructing an insn, operand number n will be substituted at this point. When matching an insn, whatever appears at this position in the insn will be taken as operand number n; but it must satisfy predicate or this instruction pattern will not match at all. Operand numbers must be chosen consecutively counting from zero in each instruction pattern. There may be only one match_operand expression in the pattern for each operand number. Usually operands are numbered in the order of appearance in match_operand expressions. predicate is a string that is the name of a C function that accepts two arguments, an expression and a machine mode. During matching, the function will be called with the putative operand as the expression and m as the mode argument (if m is not 204 Using GNU CC specified, VOIDmode will be used, which normally causes predicate to accept any mode). If it returns zero, this instruction pattern fails to match. predicate may be an empty string; then it means no test is to be done on the operand, so anything which occurs in this position is valid. Most of the time, predicate will reject modes other than m---but not always. For example, the predicate address_operand uses m as the mode of memory ref that the address should be valid for. Many predicates accept const_int nodes even though their mode is VOIDmode. constraint controls reloading and the choice of the best register class to use for a value, as explained later (see section Constraints). People are often unclear on the difference between the constraint and the predicate. The predicate helps decide whether a given insn matches the pattern. The constraint plays no role in this decision; instead, it controls various decisions in the case of an insn which does match. On CISC machines, predicate is most often "general_operand". This function checks that the putative operand is either a constant, a register or a memory reference, and that it is valid for mode m. For an operand that must be a register, predicate should be "register_operand". It would be valid to use "general_operand", since the reload pass would copy any non-register operands through registers, but this would make GNU CC do extra work, it would prevent invariant operands (such as constant) from being removed from loops, and it would prevent the register allocator from doing the best possible job. On RISC machines, it is usually most efficient to allow predicate to accept only objects that the constraints allow. For an operand that must be a constant, either use "immediate_operand" for predicate, or make the instruction pattern's extra condition require a constant, or both. You cannot expect the constraints to do this work! If the constraints allow only constants, but the predicate allows something else, the compiler will crash when that case arises. Using GNU CC 205 (match_scratch:m n constraint) This expression is also a placeholder for operand number n and indicates that operand must be a scratch or reg expression. When matching patterns, this is completely equivalent to (match_operand:m n "scratch_operand" pred) but, when generating RTL, it produces a (scratch:m) expression. If the last few expressions in a parallel are clobber expressions whose operands are either a hard register or match_scratch, the combiner can add them when necessary. See section Side Effects. (match_dup n) This expression is also a placeholder for operand number n. It is used when the operand needs to appear more than once in the insn. In construction, match_dup behaves exactly like match_operand: the operand is substituted into the insn being constructed. But in matching, match_dup behaves differently. It assumes that operand number n has already been determined by a match_operand appearing earlier in the recognition template, and it matches only an identical-looking expression. (match_operator:m n predicate [operands...]) This pattern is a kind of placeholder for a variable RTL expression code. When constructing an insn, it stands for an RTL expression whose expression code is taken from that of operand n, and whose operands are constructed from the patterns operands. When matching an expression, it matches an expression if the function predicate returns nonzero on that expression and the patterns operands match the operands of the expression. Suppose that the function commutative_operator is defined as follows, to match any expression whose operator is one of the commutative 206 Using GNU CC arithmetic operators of RTL and whose mode is mode: int commutative_operator (x, mode) rtx x; enum machine_mode mode; { enum rtx_code code = GET_CODE (x); if (GET_MODE (x) != mode) return 0; return GET_RTX_CLASS (code) == 'c' || code == EQ || code == NE; } Then the following pattern will match any RTL expression consisting of a commutative operator applied to two general operands: (match_operator:SI 3 "commutative_operator" [(match_operand:SI 1 "general_operand" "g") (match_operand:SI 2 "general_operand" "g")]) Here the vector [operands...] contains two patterns because the expressions to be matched all contain two operands. When this pattern does match, the two operands of the commutative operator are recorded as operands 1 and 2 of the insn. (This is done by the two instances of match_operand.) Operand 3 of the insn will be the entire commutative expression: use GET_CODE (operands[3]) to see which commutative operator was used. The machine mode m of match_operator works like that of match_operand: it is passed as the second argument to the predicate function, and that function is solely responsible for deciding whether the expression to be matched ``has'' that mode. When constructing an insn, argument 3 of the gen-function will specify the operation (i.e. the expression code) for the expression to be made. It should be an RTL expression, whose expression code is copied into a new Using GNU CC 207 expression whose operands are arguments 1 and 2 of the gen-function. The subexpressions of argument 3 are not used; only its expression code matters. When match_operator is used in a pattern for matching an insn, it usually best if the operand number of the match_operator is higher than that of the actual operands of the insn. This improves register allocation because the register allocator often looks at operands 1 and 2 of insns to see if it can do register tying. There is no way to specify constraints in match_operator. The operand of the insn which corresponds to the match_operator never has any constraints because it is never reloaded as a whole. However, if parts of its operands are matched by match_operand patterns, those parts may have constraints of their own. (address (match_operand:m n "address_operand" "")) This complex of expressions is a placeholder for an operand number n in a ``load address'' instruction: an operand which specifies a memory location in the usual way, but for which the actual operand value used is the address of the location, not the contents of the location. address expressions never appear in RTL code, only in machine descriptions. And they are used only in machine descriptions that do not use the operand constraint feature. When operand constraints are in use, the letter `p' in the constraint serves this purpose. m is the machine mode of the memory location being addressed, not the machine mode of the address itself. That mode is always the same on a given target machine (it is Pmode, which normally is SImode), so there is no point in mentioning it; thus, no machine mode is written in the address expression. If some day support is added for machines in which addresses of different kinds of objects appear differently or are used differently (such as the PDP-10), different formats would perhaps need different machine modes and these modes might be written in the address expression. 208 Using GNU CC 15.4. Output Templates and Operand Substitution The output template is a string which specifies how to output the assembler code for an instruction pattern. Most of the template is a fixed string which is output literally. The character `%' is used to specify where to substitute an operand; it can also be used to identify places where dif- ferent variants of the assembler require different syntax. In the simplest case, a `%' followed by a digit n says to output operand n at that point in the string. `%' followed by a letter and a digit says to output an operand in an alternate fashion. Four letters have stan- dard, built-in meanings described below. The machine description macro PRINT_OPERAND can define additional letters with nonstandard meanings. `%cdigit' can be used to substitute an operand that is a constant value without the syntax that normally indicates an immediate operand. `%ndigit' is like `%cdigit' except that the value of the constant is negated before printing. `%adigit' can be used to substitute an operand as if it were a memory reference, with the actual operand treated as the address. This may be useful when outputting a ``load address'' instruction, because often the assembler syntax for such an instruction requires you to write the operand as if it were a memory reference. `%ldigit' is used to substitute a label_ref into a jump instruction. `%' followed by a punctuation character specifies a substitution that does not use an operand. Only one case is standard: `%%' outputs a `%' into the assembler code. Other nonstandard cases can be defined in the PRINT_OPERAND macro. You must also define which punctuation characters are valid with the PRINT_OPERAND_PUNCT_VALID_P macro. The template may generate multiple assembler instruc- tions. Write the text for the instructions, with `\;' between them. When the RTL contains two operands which are required by constraint to match each other, the output template must refer only to the lower-numbered operand. Matching operands are not always identical, and the rest of the compiler arranges to put the proper RTL expression for printing into the lower-numbered operand. Using GNU CC 209 One use of nonstandard letters or punctuation following `%' is to distinguish between different assembler languages for the same machine; for example, Motorola syntax versus MIT syntax for the 68000. Motorola syntax requires periods in most opcode names, while MIT syntax does not. For exam- ple, the opcode `movel' in MIT syntax is `move.l' in Motorola syntax. The same file of patterns is used for both kinds of output syntax, but the character sequence `%.' is used in each place where Motorola syntax wants a period. The PRINT_OPERAND macro for Motorola syntax defines the sequence to output a period; the macro for MIT syntax defines it to do nothing. 15.5. C Statements for Generating Assembler Output Often a single fixed template string cannot produce correct and efficient assembler code for all the cases that are recognized by a single instruction pattern. For exam- ple, the opcodes may depend on the kinds of operands; or some unfortunate combinations of operands may require extra machine instructions. If the output control string starts with a `@', then it is actually a series of templates, each on a separate line. (Blank lines and leading spaces and tabs are ignored.) The templates correspond to the pattern's constraint alterna- tives (see section Multi-Alternative). For example, if a target machine has a two-address add instruction `addr' to add into a register and another `addm' to add a register to memory, you might write this pattern: (define_insn "addsi3" [(set (match_operand:SI 0 "general_operand" "r,m") (plus:SI (match_operand:SI 1 "general_operand" "0,0") (match_operand:SI 2 "general_operand" "g,r")))] "" "@ addr %1,%0 addm %1,%0") If the output control string starts with a `*', then it is not an output template but rather a piece of C program that should compute a template. It should execute a return statement to return the template-string you want. Most such templates use C string literals, which require doublequote characters to delimit them. To include these doublequote characters in the string, prefix each one with `\'. The operands may be found in the array operands, whose C data type is rtx []. 210 Using GNU CC It is very common to select different ways of generat- ing assembler code based on whether an immediate operand is within a certain range. Be careful when doing this, because the result of INTVAL is an integer on the host machine. If the host machine has more bits in an int than the target machine has in the mode in which the constant will be used, then some of the bits you get from INTVAL will be superflu- ous. For proper results, you must carefully disregard the values of those bits. It is possible to output an assembler instruction and then go on to output or compute more of them, using the sub- routine output_asm_insn. This receives two arguments: a template-string and a vector of operands. The vector may be operands, or it may be another array of rtx that you declare locally and initialize yourself. When an insn pattern has multiple alternatives in its constraints, often the appearance of the assembler code is determined mostly by which alternative was matched. When this is so, the C code can test the variable which_alternative, which is the ordinal number of the alter- native that was actually satisfied (0 for the first, 1 for the second alternative, etc.). For example, suppose there are two opcodes for storing zero, `clrreg' for registers and `clrmem' for memory loca- tions. Here is how a pattern could use which_alternative to choose between them: (define_insn "" [(set (match_operand:SI 0 "general_operand" "r,m") (const_int 0))] "" "* return (which_alternative == 0 ? \"clrreg %0\" : \"clrmem %0\"); ") The example above, where the assembler code to generate was solely determined by the alternative, could also have been specified as follows, having the output control string start with a `@': (define_insn "" [(set (match_operand:SI 0 "general_operand" "r,m") (const_int 0))] "" "@ Using GNU CC 211 clrreg %0 clrmem %0") 15.6. Operand Constraints Each match_operand in an instruction pattern can specify a constraint for the type of operands allowed. Con- straints can say whether an operand may be in a register, and which kinds of register; whether the operand can be a memory reference, and which kinds of address; whether the operand may be an immediate constant, and which possible values it may have. Constraints can also require two operands to match. 15.6.1. Simple Constraints The simplest kind of constraint is a string full of letters, each of which describes one kind of operand that is permitted. Here are the letters that are allowed: `m' A memory operand is allowed, with any kind of address that the machine supports in general. `o' A memory operand is allowed, but only if the address is offsettable. This means that adding a small integer (actually, the width in bytes of the operand, as determined by its machine mode) may be added to the address and the result is also a valid memory address. For example, an address which is constant is offsettable; so is an address that is the sum of a register and a constant (as long as a slightly larger constant is also within the range of address-offsets supported by the machine); but an autoincrement or autodecrement address is not offsettable. More complicated indirect/indexed addresses may or may not be offsettable depending on the other addressing modes that the machine supports. Note that in an output operand which can be matched by another operand, the constraint letter `o' is valid only when accompanied by both `<' (if the target machine has predecrement addressing) and `>' (if the target machine has preincrement addressing). `V' A memory operand that is not offsettable. In other words, anything that would fit the `m' constraint but not the `o' constraint. 212 Using GNU CC `<' A memory operand with autodecrement addressing (either predecrement or postdecrement) is allowed. `>' A memory operand with autoincrement addressing (either preincrement or postincrement) is allowed. `r' A register operand is allowed provided that it is in a general register. `d', `a', `f', ... Other letters can be defined in machine-dependent fashion to stand for particular classes of registers. `d', `a' and `f' are defined on the 68000/68020 to stand for data, address and floating point registers. `i' An immediate integer operand (one with constant value) is allowed. This includes symbolic constants whose values will be known only at assembly time. `n' An immediate integer operand with a known numeric value is allowed. Many systems cannot support assembly-time constants for operands less than a word wide. Constraints for these operands should use `n' rather than `i'. `I', `J', `K', ... `P' Other letters in the range `I' through `P' may be defined in a machine-dependent fashion to permit immediate integer operands with explicit integer values in specified ranges. For example, on the 68000, `I' is defined to stand for the range of values 1 to 8. This is the range permitted as a shift count in the shift instructions. `E' An immediate floating operand (expression code const_double) is allowed, but only if the target floating point format is the same as that of the host machine (on which the compiler is running). `F' An immediate floating operand (expression code const_double) is allowed. `G', `H' `G' and `H' may be defined in a machine-dependent fashion to permit immediate floating operands in particular ranges of values. `s' An immediate integer operand whose value is not an explicit integer is allowed. Using GNU CC 213 This might appear strange; if an insn allows a constant operand with a value not known at compile time, it certainly must allow any known value. So why use `s' instead of `i'? Sometimes it allows better code to be generated. For example, on the 68000 in a fullword instruction it is possible to use an immediate operand; but if the immediate value is between -128 and 127, better code results from loading the value into a register and using the register. This is because the load into the register can be done with a `moveq' instruction. We arrange for this to happen by defining the letter `K' to mean ``any integer outside the range -128 to 127'', and then specifying `Ks' in the operand constraints. `g' Any register, memory or immediate integer operand is allowed, except for registers that are not general registers. `X' Any operand whatsoever is allowed, even if it does not satisfy general_operand. This is normally used in the constraint of a match_scratch when certain alternatives will not actually require a scratch register. `0', `1', `2', ... `9' An operand that matches the specified operand number is allowed. If a digit is used together with letters within the same alternative, the digit should come last. This is called a matching constraint and what it really means is that the assembler has only a single operand that fills two roles considered separate in the RTL insn. For example, an add insn has two input operands and one output operand in the RTL, but on most machines an add instruction really has only two operands, one of them an input-output operand. Matching constraints work only in circumstances like that add insn. More precisely, the two operands that match must include one input-only operand and one output-only operand. Moreover, the digit must be a smaller number than the number of the operand that uses it in the constraint. For operands to match in a particular case usually means that they are identical-looking RTL expressions. But in a few special cases specific kinds of dissimilarity are allowed. For example, 214 Using GNU CC *x as an input operand will match *x++ as an output operand. For proper results in such cases, the output template should always use the output- operand's number when printing the operand. `p' An operand that is a valid memory address is allowed. This is for ``load address'' and ``push address'' instructions. `p' in the constraint must be accompanied by address_operand as the predicate in the match_operand. This predicate interprets the mode specified in the match_operand as the mode of the memory reference for which the address would be valid. `Q', `R', `S', ... `U' Letters in the range `Q' through `U' may be defined in a machine-dependent fashion to stand for arbitrary operand types. The machine description macro EXTRA_CONSTRAINT is passed the operand as its first argument and the constraint letter as its second operand. A typical use for this would be to distinguish certain types of memory references that affect other insn operands. Do not define these constraint letters to accept register references (reg); the reload pass does not expect this and would not handle it properly. In order to have valid assembler code, each operand must satisfy its constraint. But a failure to do so does not prevent the pattern from applying to an insn. Instead, it directs the compiler to modify the code so that the con- straint will be satisfied. Usually this is done by copying an operand into a register. Contrast, therefore, the two instruction patterns that follow: (define_insn "" [(set (match_operand:SI 0 "general_operand" "r") (plus:SI (match_dup 0) (match_operand:SI 1 "general_operand" "r")))] "" "...") Using GNU CC 215 which has two operands, one of which must appear in two places, and (define_insn "" [(set (match_operand:SI 0 "general_operand" "r") (plus:SI (match_operand:SI 1 "general_operand" "0") (match_operand:SI 2 "general_operand" "r")))] "" "...") which has three operands, two of which are required by a constraint to be identical. If we are considering an insn of the form (insn n prev next (set (reg:SI 3) (plus:SI (reg:SI 6) (reg:SI 109))) ...) the first pattern would not apply at all, because this insn does not contain two identical subexpressions in the right place. The pattern would say, ``That does not look like an add instruction; try other patterns.'' The second pattern would say, ``Yes, that's an add instruction, but there is something wrong with it.'' It would direct the reload pass of the compiler to generate additional insns to make the constraint true. The results might look like this: (insn n2 prev n (set (reg:SI 3) (reg:SI 6)) ...) (insn n n2 next (set (reg:SI 3) (plus:SI (reg:SI 3) (reg:SI 109))) ...) It is up to you to make sure that each operand, in each pattern, has constraints that can handle any RTL expression that could be present for that operand. (When multiple alternatives are in use, each pattern must, for each possi- ble combination of operand expressions, have at least one alternative which can handle that combination of operands.) The constraints don't need to allow any possible operand--- 216 Using GNU CC when this is the case, they do not constrain---but they must at least point the way to reloading any possible operand so that it will fit. o+ If the constraint accepts whatever operands the predicate permits, there is no problem: reloading is never necessary for this operand. For example, an operand whose constraints permit everything except registers is safe provided its predicate rejects registers. An operand whose predicate accepts only constant values is safe provided its constraints include the letter `i'. If any possible constant value is accepted, then nothing less than `i' will do; if the predicate is more selective, then the constraints may also be more selective. o+ Any operand expression can be reloaded by copying it into a register. So if an operand's constraints allow some kind of register, it is certain to be safe. It need not permit all classes of registers; the compiler knows how to copy a register into another register of the proper class in order to make an instruction valid. o+ A nonoffsettable memory reference can be reloaded by copying the address into a register. So if the constraint uses the letter `o', all memory references are taken care of. o+ A constant operand can be reloaded by allocating space in memory to hold it as preinitialized data. Then the memory reference can be used in place of the constant. So if the constraint uses the letters `o' or `m', constant operands are not a problem. o+ If the constraint permits a constant and a pseudo register used in an insn was not allocated to a hard register and is equivalent to a constant, the register will be replaced with the constant. If the predicate does not permit a constant and the insn is re-recognized for some reason, the compiler will crash. Thus the predicate must always recognize any objects allowed by the constraint. If the operand's predicate can recognize registers, but the constraint does not permit them, it can make the Using GNU CC 217 compiler crash. When this operand happens to be a register, the reload pass will be stymied, because it does not know how to copy a register temporarily into memory. 15.6.2. Multiple Alternative Constraints Sometimes a single instruction has multiple alternative sets of possible operands. For example, on the 68000, a logical-or instruction can combine register or an immediate value into memory, or it can combine any kind of operand into a register; but it cannot combine one memory location into another. These constraints are represented as multiple alterna- tives. An alternative can be described by a series of letters for each operand. The overall constraint for an operand is made from the letters for this operand from the first alternative, a comma, the letters for this operand from the second alternative, a comma, and so on until the last alternative. Here is how it is done for fullword logical-or on the 68000: (define_insn "iorsi3" [(set (match_operand:SI 0 "general_operand" "=m,d") (ior:SI (match_operand:SI 1 "general_operand" "%0,0") (match_operand:SI 2 "general_operand" "dKs,dmKs")))] ...) The first alternative has `m' (memory) for operand 0, `0' for operand 1 (meaning it must match operand 0), and `dKs' for operand 2. The second alternative has `d' (data register) for operand 0, `0' for operand 1, and `dmKs' for operand 2. The `=' and `%' in the constraints apply to all the alternatives; their meaning is explained in the next section (see section Class Preferences). If all the operands fit any one alternative, the instruction is valid. Otherwise, for each alternative, the compiler counts how many instructions must be added to copy the operands so that that alternative applies. The alterna- tive requiring the least copying is chosen. If two alterna- tives need the same amount of copying, the one that comes first is chosen. These choices can be altered with the `?' and `!' characters: ? Disparage slightly the alternative that the `?' appears in, as a choice when no alternative applies exactly. The compiler regards this alternative as one unit more costly for each `?' that appears in it. 218 Using GNU CC ! Disparage severely the alternative that the `!' appears in. This alternative can still be used if it fits without reloading, but if reloading is needed, some other alternative will be used. When an insn pattern has multiple alternatives in its constraints, often the appearance of the assembler code is determined mostly by which alternative was matched. When this is so, the C code for writing the assembler code can use the variable which_alternative, which is the ordinal number of the alternative that was actually satisfied (0 for the first, 1 for the second alternative, etc.). See section Output Statement. 15.6.3. Register Class Preferences The operand constraints have another function: they enable the compiler to decide which kind of hardware regis- ter a pseudo register is best allocated to. The compiler examines the constraints that apply to the insns that use the pseudo register, looking for the machine-dependent letters such as `d' and `a' that specify classes of regis- ters. The pseudo register is put in whichever class gets the most ``votes''. The constraint letters `g' and `r' also vote: they vote in favor of a general register. The machine description says which registers are considered general. Of course, on some machines all registers are equivalent, and no register classes are defined. Then none of this complexity is relevant. 15.6.4. Constraint Modifier Characters `=' Means that this operand is write-only for this instruction: the previous value is discarded and replaced by output data. `+' Means that this operand is both read and written by the instruction. When the compiler fixes up the operands to satisfy the constraints, it needs to know which operands are inputs to the instruction and which are outputs from it. `=' identifies an output; `+' identifies an operand that is both input and output; all other operands are assumed to be input only. `&' Means (in a particular alternative) that this operand is written before the instruction is finished using the input operands. Therefore, this operand may not lie in a register that is Using GNU CC 219 used as an input operand or as part of any memory address. `&' applies only to the alternative in which it is written. In constraints with multiple alternatives, sometimes one alternative requires `&' while others do not. See, for example, the `movdf' insn of the 68000. `&' does not obviate the need to write `='. `%' Declares the instruction to be commutative for this operand and the following operand. This means that the compiler may interchange the two operands if that is the cheapest way to make all operands fit the constraints. This is often used in patterns for addition instructions that really have only two operands: the result must go in one of the arguments. Here for example, is how the 68000 halfword-add instruction is defined: (define_insn "addhi3" [(set (match_operand:HI 0 "general_operand" "=m,r") (plus:HI (match_operand:HI 1 "general_operand" "%0,0") (match_operand:HI 2 "general_operand" "di,g")))] ...) `#' Says that all following characters, up to the next comma, are to be ignored as a constraint. They are significant only for choosing register preferences. `*' Says that the following character should be ignored when choosing register preferences. `*' has no effect on the meaning of the constraint as a constraint, and no effect on reloading. Here is an example: the 68000 has an instruction to sign-extend a halfword in a data register, and can also sign-extend a value by copying it into an address register. While either kind of register is acceptable, the constraints on an address-register destination are less strict, so it is best if register allocation makes an address register its goal. Therefore, `*' is used so that the `d' constraint letter (for data register) is ignored when computing register preferences. 220 Using GNU CC (define_insn "extendhisi2" [(set (match_operand:SI 0 "general_operand" "=*d,a") (sign_extend:SI (match_operand:HI 1 "general_operand" "0,g")))] ...) 15.6.5. Not Using Constraints Some machines are so clean that operand constraints are not required. For example, on the Vax, an operand valid in one context is valid in any other context. On such a machine, every operand constraint would be `g', excepting only operands of ``load address'' instructions which are written as if they referred to a memory location's contents but actual refer to its address. They would have constraint `p'. For such machines, instead of writing `g' and `p' for all the constraints, you can choose to write a description with empty constraints. Then you write `""' for the con- straint in every match_operand. Address operands are iden- tified by writing an address expression around the match_operand, not by their constraints. When the machine description has just empty con- straints, certain parts of compilation are skipped, making the compiler faster. However, few machines actually do not need constraints; all machine descriptions now in existence use constraints. 15.7. Standard Names for Patterns Used in Generation Here is a table of the instruction names that are mean- ingful in the RTL generation pass of the compiler. Giving one of these names to an instruction pattern tells the RTL generation pass that it can use the pattern in to accomplish a certain task. `movm' Here m stands for a two-letter machine mode name, in lower case. This instruction pattern moves data with that machine mode from operand 1 to operand 0. For example, `movsi' moves full-word data. If operand 0 is a subreg with mode m of a register whose own mode is wider than m, the effect of this instruction is to store the specified value in the part of the register that corresponds to mode m. Using GNU CC 221 The effect on the rest of the register is undefined. This class of patterns is special in several ways. First of all, each of these names must be defined, because there is no other way to copy a datum from one place to another. Second, these patterns are not used solely in the RTL generation pass. Even the reload pass can generate move insns to copy values from stack slots into temporary registers. When it does so, one of the operands is a hard register and the other is an operand that can need to be reloaded into a register. Therefore, when given such a pair of operands, the pattern must generate RTL which needs no reloading and needs no temporary registers---no registers other than the operands. For example, if you support the pattern with a define_expand, then in such a case the define_expand mustn't call force_reg or any other such function which might generate new pseudo registers. This requirement exists even for subword modes on a RISC machine where fetching those modes from memory normally requires several insns and some temporary registers. Look in `spur.md' to see how the requirement can be satisfied. During reload a memory reference with an invalid address may be passed as an operand. Such an address will be replaced with a valid address later in the reload pass. In this case, nothing may be done with the address except to use it as it stands. If it is copied, it will not be replaced with a valid address. No attempt should be made to make such an address into a valid address and no routine (such as change_address) that will do so may be called. Note that general_operand will fail when applied to such an address. The global variable reload_in_progress (which must be explicitly declared if required) can be used to determine whether such special handling is required. The variety of operands that have reloads depends on the rest of the machine description, but typically on a RISC machine these can only be pseudo registers that did not get hard registers, 222 Using GNU CC while on other machines explicit memory references will get optional reloads. If a scratch register is required to move an object to or from memory, it can be allocated using gen_reg_rtx prior to reload. But this is impossible during and after reload. If there are cases needing scratch registers after reload, you must define SECONDARY_INPUT_RELOAD_CLASS and/or SECONDARY_OUTPUT_RELOAD_CLASS to detect them, and provide patterns `reload_inm' or `reload_outm' to handle them. See section Register Classes. The constraints on a `movem' must permit moving any hard register to any other hard register provided that HARD_REGNO_MODE_OK permits mode m in both registers and REGISTER_MOVE_COST applied to their classes returns a value of 2. It is obligatory to support floating point `movem' instructions into and out of any registers that can hold fixed point values, because unions and structures (which have modes SImode or DImode) can be in those registers and they may have floating point members. There may also be a need to support fixed point `movem' instructions in and out of floating point registers. Unfortunately, I have forgotten why this was so, and I don't know whether it is still true. If HARD_REGNO_MODE_OK rejects fixed point values in floating point registers, then the constraints of the fixed point `movem' instructions must be designed to avoid ever trying to reload into a floating point register. `reload_inm' `reload_outm' Like `movm', but used when a scratch register is required to move between operand 0 and operand 1. Operand 2 describes the scratch register. See the discussion of the SECONDARY_RELOAD_CLASS macro in see section Register Classes. `movstrictm' Like `movm' except that if operand 0 is a subreg with mode m of a register whose natural mode is wider, the `movstrictm' instruction is guaranteed not to alter any of the register except the part which belongs to mode m. Using GNU CC 223 `addm3' Add operand 2 and operand 1, storing the result in operand 0. All operands must have mode m. This can be used even on two-address machines, by means of constraints requiring operands 1 and 0 to be the same location. `subm3', `mulm3' `divm3', `udivm3', `modm3', `umodm3' `sminm3', `smaxm3', `uminm3', `umaxm3' `andm3', `iorm3', `xorm3' Similar, for other arithmetic operations. `mulhisi3' Multiply operands 1 and 2, which have mode HImode, and store a SImode product in operand 0. `mulqihi3', `mulsidi3' Similar widening-multiplication instructions of other widths. `umulqihi3', `umulhisi3', `umulsidi3' Similar widening-multiplication instructions that do unsigned multiplication. `divmodm4' Signed division that produces both a quotient and a remainder. Operand 1 is divided by operand 2 to produce a quotient stored in operand 0 and a remainder stored in operand 3. For machines with an instruction that produces both a quotient and a remainder, provide a pattern for `divmodm4' but do not provide patterns for `divm3' and `modm3'. This allows optimization in the relatively common case when both the quotient and remainder are computed. If an instruction that just produces a quotient or just a remainder exists and is more efficient than the instruction that produces both, write the output routine of `divmodm4' to call find_reg_note and look for a REG_UNUSED note on the quotient or remainder and generate the appropriate instruction. `udivmodm4' Similar, but does unsigned division. 224 Using GNU CC `ashlm3' Arithmetic-shift operand 1 left by a number of bits specified by operand 2, and store the result in operand 0. Operand 2 has mode SImode, not mode m. `ashrm3', `lshlm3', `lshrm3', `rotlm3', `rotrm3' Other shift and rotate instructions. Logical and arithmetic left shift are the same. Machines that do not allow negative shift counts often have only one instruction for shifting left. On such machines, you should define a pattern named `ashlm3' and leave `lshlm3' undefined. `negm2' Negate operand 1 and store the result in operand 0. `absm2' Store the absolute value of operand 1 into operand 0. `sqrtm2' Store the square root of operand 1 into operand 0. `ffsm2' Store into operand 0 one plus the index of the least significant 1-bit of operand 1. If operand 1 is zero, store zero. m is the mode of operand 0; operand 1's mode is specified by the instruction pattern, and the compiler will convert the operand to that mode before generating the instruction. `one_cmplm2' Store the bitwise-complement of operand 1 into operand 0. `cmpm' Compare operand 0 and operand 1, and set the condition codes. The RTL pattern should look like this: (set (cc0) (compare (match_operand:m 0 ...) (match_operand:m 1 ...))) `tstm' Compare operand 0 against zero, and set the condition codes. The RTL pattern should look Using GNU CC 225 like this: (set (cc0) (match_operand:m 0 ...)) `tstm' patterns should not be defined for machines that do not use (cc0). Doing so would confuse the optimizer since it would no longer be clear which set operations were comparisons. The `cmpm' patterns should be used instead. `movstrm' Block move instruction. The addresses of the destination and source strings are the first two operands, and both are in mode Pmode. The number of bytes to move is the third operand, in mode m. The fourth operand is the known shared alignment of the source and destination, in the form of a const_int rtx. Thus, if the compiler knows that both source and destination are word-aligned, it may provide the value 4 for this operand. These patterns need not give special consideration to the possibility that the source and destination strings might overlap. `cmpstrm' Block compare instruction, with five operands. Operand 0 is the output; it has mode m. The remaining four operands are like the operands of `movstrm'. The two memory blocks specified are compared byte by byte in lexicographic order. The effect of the instruction is to store a value in operand 0 whose sign indicates the result of the comparison. `floatmn2' Convert signed integer operand 1 (valid for fixed point mode m) to floating point mode n and store in operand 0 (which has mode n). `floatunsmn2' Convert unsigned integer operand 1 (valid for fixed point mode m) to floating point mode n and store in operand 0 (which has mode n). 226 Using GNU CC `fixmn2' Convert operand 1 (valid for floating point mode m) to fixed point mode n as a signed number and store in operand 0 (which has mode n). This instruction's result is defined only when the value of operand 1 is an integer. `fixunsmn2' Convert operand 1 (valid for floating point mode m) to fixed point mode n as an unsigned number and store in operand 0 (which has mode n). This instruction's result is defined only when the value of operand 1 is an integer. `ftruncm2' Convert operand 1 (valid for floating point mode m) to an integer value, still represented in floating point mode m, and store it in operand 0 (valid for floating point mode m). `fix_truncmn2' Like `fixmn2' but works for any floating point value of mode m by converting the value to an integer. `fixuns_truncmn2' Like `fixunsmn2' but works for any floating point value of mode m by converting the value to an integer. `truncmn' Truncate operand 1 (valid for mode m) to mode n and store in operand 0 (which has mode n). Both modes must be fixed point or both floating point. `extendmn' Sign-extend operand 1 (valid for mode m) to mode n and store in operand 0 (which has mode n). Both modes must be fixed point or both floating point. `zero_extendmn' Zero-extend operand 1 (valid for mode m) to mode n and store in operand 0 (which has mode n). Both modes must be fixed point. `extv' Extract a bit field from operand 1 (a register or memory operand), where operand 2 specifies the width in bits and operand 3 the starting bit, and store it in operand 0. Operand 0 must have mode word_mode. Operand 1 may have Using GNU CC 227 mode byte_mode or word_mode; often word_mode is allowed only for registers. Operands 2 and 3 must be valid for word_mode. The RTL generation pass generates this instruction only with constants for operands 2 and 3. The bit-field value is sign-extended to a full word integer before it is stored in operand 0. `extzv' Like `extv' except that the bit-field value is zero-extended. `insv' Store operand 3 (which must be valid for word_mode) into a bit field in operand 0, where operand 1 specifies the width in bits and operand 2 the starting bit. Operand 0 may have mode byte_mode or word_mode; often word_mode is allowed only for registers. Operands 1 and 2 must be valid for word_mode. The RTL generation pass generates this instruction only with constants for operands 1 and 2. `scond' Store zero or nonzero in the operand according to the condition codes. Value stored is nonzero iff the condition cond is true. cond is the name of a comparison operation expression code, such as eq, lt or leu. You specify the mode that the operand must have when you write the match_operand expression. The compiler automatically sees which mode you have used and supplies an operand of that mode. The value stored for a true condition must have 1 as its low bit, or else must be negative. Otherwise the instruction is not suitable and you should omit it from the machine description. You describe to the compiler exactly which value is stored by defining the macro STORE_FLAG_VALUE (see section Misc). If a description cannot be found that can be used for all the `scond' patterns, you should omit those operations from the machine description. 228 Using GNU CC These operations may fail, but should do so only in relatively uncommon cases; if they would fail for common cases involving integer comparisons, it is best to omit these patterns. If these operations are omitted, the compiler will usually generate code that copies the constant one to the target and branches around an assignment of zero to the target. If this code is more efficient than the potential instructions used for the `scond' pattern followed by those required to convert the result into a 1 or a zero in SImode, you should omit the `scond' operations from the machine description. `bcond' Conditional branch instruction. Operand 0 is a label_ref that refers to the label to jump to. Jump if the condition codes meet condition cond. Some machines do not follow the model assumed here where a comparison instruction is followed by a conditional branch instruction. In that case, the `cmpm' (and `tstm') patterns should simply store the operands away and generate all the required insns in a define_expand (see section Expander Definitions) for the conditional branch operations. All calls to expand `vcond' patterns are immediately preceded by calls to expand either a `cmpm' pattern or a `tstm' pattern. Machines that use a pseudo register for the condition code value, or where the mode used for the comparison depends on the condition being tested, should also use the above mechanism. See section Jump Patterns The above discussion also applies to `scond' patterns. `call' Subroutine call instruction returning no value. Operand 0 is the function to call; operand 1 is the number of bytes of arguments pushed (in mode SImode, except it is normally a const_int); operand 2 is the number of registers used as operands. Using GNU CC 229 On most machines, operand 2 is not actually stored into the RTL pattern. It is supplied for the sake of some RISC machines which need to put this information into the assembler code; they can put it in the RTL instead of operand 1. Operand 0 should be a mem RTX whose address is the address of the function. Note, however, that this address can be a symbol_ref expression even if it would not be a legitimate memory address on the target machine. If it is also not a valid argument for a call instruction, the pattern for this operation should be a define_expand (see section Expander Definitions) that places the address into a register and uses that register in the call instruction. `call_value' Subroutine call instruction returning a value. Operand 0 is the hard register in which the value is returned. There are three more operands, the same as the three operands of the `call' instruction (but with numbers increased by one). Subroutines that return BLKmode objects use the `call' insn. `call_pop', `call_value_pop' Similar to `call' and `call_value', except used if defined and if RETURN_POPS_ARGS is non-zero. They should emit a parallel that contains both the function call and a set to indicate the adjustment made to the frame pointer. For machines where RETURN_POPS_ARGS can be non-zero, the use of these patterns increases the number of functions for which the frame pointer can be eliminated, if desired. `return' Subroutine return instruction. This instruction pattern name should be defined only if a single instruction can do all the work of returning from a function. Like the `movm' patterns, this pattern is also used after the RTL generation phase. In this case it is to support machines where multiple instructions are usually needed to return from 230 Using GNU CC a function, but some class of functions only requires one instruction to implement a return. Normally, the applicable functions are those which do not need to save any registers or allocate stack space. For such machines, the condition specified in this pattern should only be true when reload_completed is non-zero and the function's epilogue would only be a single instruction. For machines with register windows, the routine leaf_function_p may be used to determine if a register window push is required. Machines that have conditional return instructions should define patterns such as (define_insn "" [(set (pc) (if_then_else (match_operator 0 "comparison_operator" [(cc0) (const_int 0)]) (return) (pc)))] "condition" "...") where condition would normally be the same condition specified on the named `return' pattern. `nop' No-op instruction. This instruction pattern name should always be defined to output a no- op in assembler code. (const_int 0) will do as an RTL pattern. `indirect_jump' An instruction to jump to an address which is operand zero. This pattern name is mandatory on all machines. `casesi' Instruction to jump through a dispatch table, including bounds checking. This instruction takes five operands: 1. The index to dispatch on, which has mode SImode. Using GNU CC 231 2. The lower bound for indices in the table, an integer constant. 3. The total range of indices in the table---the largest index minus the smallest one (both inclusive). 4. A label that precedes the table itself. 5. A label to jump to if the index has a value outside the bounds. (If the machine- description macro CASE_DROPS_THROUGH is defined, then an out-of-bounds index drops through to the code following the jump table instead of jumping to this label. In that case, this label is not actually used by the `casesi' instruction, but it is always provided as an operand.) The table is a addr_vec or addr_diff_vec inside of a jump_insn. The number of elements in the table is one plus the difference between the upper bound and the lower bound. `tablejump' Instruction to jump to a variable address. This is a low-level capability which can be used to implement a dispatch table when there is no `casesi' pattern. This pattern requires two operands: the address or offset, and a label which should immediately precede the jump table. If the macro CASE_VECTOR_PC_RELATIVE is defined then the first operand is an offset which counts from the address of the table; otherwise, it is an absolute address to jump to. The `tablejump' insn is always the last insn before the jump table it uses. Its assembler code normally has no need to use the second operand, but you should incorporate it in the RTL pattern so that the jump optimizer will not delete the table as unreachable code. 15.8. When the Order of Patterns Matters Sometimes an insn can match more than one instruction pattern. Then the pattern that appears first in the machine description is the one used. Therefore, more specific pat- terns (patterns that will match fewer things) and faster 232 Using GNU CC instructions (those that will produce better code when they do match) should usually go first in the description. In some cases the effect of ordering the patterns can be used to hide a pattern when it is not valid. For exam- ple, the 68000 has an instruction for converting a fullword to floating point and another for converting a byte to floating point. An instruction converting an integer to floating point could match either one. We put the pattern to convert the fullword first to make sure that one will be used rather than the other. (Otherwise a large integer might be generated as a single-byte immediate quantity, which would not work.) Instead of using this pattern order- ing it would be possible to make the pattern for convert-a- byte smart enough to deal properly with any constant value. 15.9. Interdependence of Patterns Every machine description must have a named pattern for each of the conditional branch names `bcond'. The recogni- tion template must always have the form (set (pc) (if_then_else (cond (cc0) (const_int 0)) (label_ref (match_operand 0 "" "")) (pc))) In addition, every machine description must have an anonymous pattern for each of the possible reverse- conditional branches. Their templates look like (set (pc) (if_then_else (cond (cc0) (const_int 0)) (pc) (label_ref (match_operand 0 "" "")))) They are necessary because jump optimization can turn direct-conditional branches into reverse-conditional branches. It is often convenient to use the match_operator con- struct to reduce the number of patterns that must be speci- fied for branches. For example, (define_insn "" [(set (pc) Using GNU CC 233 (if_then_else (match_operator 0 "comparison_operator" [(cc0) (const_int 0)]) (pc) (label_ref (match_operand 1 "" ""))))] "condition" "...") In some cases machines support instructions identical except for the machine mode of one or more operands. For example, there may be ``sign-extend halfword'' and ``sign- extend byte'' instructions whose patterns are (set (match_operand:SI 0 ...) (extend:SI (match_operand:HI 1 ...))) (set (match_operand:SI 0 ...) (extend:SI (match_operand:QI 1 ...))) Constant integers do not specify a machine mode, so an instruction to extend a constant value could match either pattern. The pattern it actually will match is the one that appears first in the file. For correct results, this must be the one for the widest possible mode (HImode, here). If the pattern matches the QImode instruction, the results will be incorrect if the constant value does not actually fit that mode. Such instructions to extend constants are rarely gen- erated because they are optimized away, but they do occa- sionally happen in nonoptimized compilations. If a constraint in a pattern allows a constant, the reload pass may replace a register with a constant permitted by the constraint in some cases. Similarly for memory references. You must ensure that the predicate permits all objects allowed by the constraints to prevent the compiler from crashing. Because of this substitution, you should not provide separate patterns for increment and decrement instructions. Instead, they should be generated from the same pattern that supports register-register add insns by examining the operands and generating the appropriate machine instruction. 15.10. Defining Jump Instruction Patterns For most machines, GNU CC assumes that the machine has a condition code. A comparison insn sets the condition 234 Using GNU CC code, recording the results of both signed and unsigned com- parison of the given operands. A separate branch insn tests the condition code and branches or not according its value. The branch insns come in distinct signed and unsigned fla- vors. Many common machines, such as the Vax, the 68000 and the 32000, work this way. Some machines have distinct signed and unsigned compare instructions, and only one set of conditional branch instructions. The easiest way to handle these machines is to treat them just like the others until the final stage where assembly code is written. At this time, when output- ting code for the compare instruction, peek ahead at the following branch using next_cc0_user (insn). (The variable insn refers to the insn being output, in the output-writing code in an instruction pattern.) If the RTL says that is an unsigned branch, output an unsigned compare; otherwise out- put a signed compare. When the branch itself is output, you can treat signed and unsigned branches identically. The reason you can do this is that GNU CC always gen- erates a pair of consecutive RTL insns, possibly separated by note insns, one to set the condition code and one to test it, and keeps the pair inviolate until the end. To go with this technique, you must define the machine-description macro NOTICE_UPDATE_CC to do CC_STATUS_INIT; in other words, no compare instruction is superfluous. Some machines have compare-and-branch instructions and no condition code. A similar technique works for them. When it is time to ``output'' a compare instruction, record its operands in two static variables. When outputting the branch-on-condition-code instruction that follows, actually output a compare-and-branch instruction that uses the remem- bered operands. It also works to define patterns for compare-and-branch instructions. In optimizing compilation, the pair of com- pare and branch instructions will be combined according to these patterns. But this does not happen if optimization is not requested. So you must use one of the solutions above in addition to any special patterns you define. In many RISC machines, most instructions do not affect the condition code and there may not even be a separate con- dition code register. On these machines, the restriction that the definition and use of the condition code be adja- cent insns is not necessary and can prevent important optim- izations. For example, on the IBM RS/6000, there is a delay for taken branches unless the condition code register is set three instructions earlier than the conditional branch. The Using GNU CC 235 instruction scheduler cannot perform this optimization if it is not permitted to separate the definition and use of the condition code register. On these machines, do not use (cc0), but instead use a register to represent the condition code. If there is a specific condition code register in the machine, use a hard register. If the condition code or comparison result can be placed in any general register, or if there are multiple condition registers, use a pseudo register. On some machines, the type of branch instruction gen- erated may depend on the way the condition code was pro- duced; for example, on the 68k and Sparc, setting the condi- tion code directly from an add or subtract instruction does not clear the overflow bit the way that a test instruction does, so a different branch instruction must be used for some conditional branches. For machines that use (cc0), the set and use of the condition code must be adjacent (separated only by note insns) allowing flags in cc_status to be used. (See section Condition Code.) Also, the com- parison and branch insns can be located from each other by using the functions prev_cc0_setter and next_cc0_user. However, this is not true on machines that do not use (cc0). On those machines, no assumptions can be made about the adjacency of the compare and branch insns and the above methods cannot be used. Instead, we use the machine mode of the condition code register to record different formats of the condition code register. Registers used to store the condition code value should have a mode that is in class MODE_CC. Normally, it will be CCmode. If additional modes are required (as for the add example mentioned above in the Sparc), define the macro EXTRA_CC_MODES to list the additional modes required (see section Condition Code). Also define EXTRA_CC_NAMES to list the names of those modes and SELECT_CC_MODE to choose a mode given an operand of a compare. If it is known during RTL generation that a different mode will be required (for example, if the machine has separate compare instructions for signed and unsigned quan- tities, like most IBM processors), they can be specified at that time. If the cases that require different modes would be made by instruction combination, the macro SELECT_CC_MODE deter- mines which machine mode should be used for the comparison result. The patterns should be written using that mode. To support the case of the add on the Sparc discussed above, we have the pattern 236 Using GNU CC (define_insn "" [(set (reg:CC_NOOV 0) (compare:CC_NOOV (plus:SI (match_operand:SI 0 "register_operand" "%r") (match_operand:SI 1 "arith_operand" "rI")) (const_int 0)))] "" "...") The SELECT_CC_MODE macro on the Sparc returns CC_NOOVmode for comparisons whose argument is a plus. 15.11. Canonicalization of Instructions There are often cases where multiple RTL expressions could represent an operation peformed by a single machine instruction. This situation is most commonly encountered with logical, branch, and multiply-accumulate instructions. In such cases, the compiler attempts to convert these multi- ple RTL expressions into a single canonical form to reduce the number of insn patterns required. In addition to algebraic simplifications, following canonicalizations are performed: o+ For commutative and comparison operators, a constant is always made the second operand. If a machine only supports a constant as the second operand, only patterns that match a constant in the second operand need be supplied. For these operators, if only one operand is a neg, not, mult, plus, or minus expression, it will be the first operand. o+ For the compare operator, a constant is always the second operand on machines where cc0 is used (see section Jump Patterns). On other machines, there are rare cases where the compiler might want to construct a compare with a constant as the first operand. However, these cases are not common enough for it to be worthwhile to provide a pattern matching a constant as the first operand unless the machine actually has such an instruction. An operand of neg, not, mult, plus, or minus is made the first operand under the same conditions as above. Using GNU CC 237 o+ (minus x (const_int n)) is converted to (plus x (const_int -n)). o+ Within address computations (i.e., inside mem), a left shift is converted into the appropriate multiplication by a power of two. De`Morgan's Law is used to move bitwise negation inside a bitwise logical-and or logical-or operation. If this results in only one operand being a not expression, it will be the first one. A machine that has an instruction that performs a bitwise logical-and of one operand with the bitwise negation of the other should specify the pattern for that instruction as (define_insn "" [(set (match_operand:m 0 ...) (and:m (not:m (match_operand:m 1 ...)) (match_operand:m 2 ...)))] "..." "...") Similarly, a pattern for a ``NAND'' instruction should be written (define_insn "" [(set (match_operand:m 0 ...) (ior:m (not:m (match_operand:m 1 ...)) (not:m (match_operand:m 2 ...))))] "..." "...") In both cases, it is not necessary to include patterns for the many logically equivalent RTL expressions. o+ The only possible RTL expressions involving both bitwise exclusive-or and bitwise negation are (xor:m x) y) and (not:m (xor:m x y)). o+ The sum of three items, one of which is a constant, will only appear in the form (plus:m (plus:m x y) constant) 238 Using GNU CC o+ On machines that do not use cc0, (compare x (const_int 0)) will be converted to x. o+ Equality comparisons of a group of bits (usually a single bit) with zero will be written using zero_extract rather than the equivalent and or sign_extract operations. 15.12. Defining Machine-Specific Peephole Optimizers In addition to instruction patterns the `md' file may contain definitions of machine-specific peephole optimiza- tions. The combiner does not notice certain peephole optimiza- tions when the data flow in the program does not suggest that it should try them. For example, sometimes two con- secutive insns related in purpose can be combined even though the second one does not appear to use a register com- puted in the first one. A machine-specific peephole optim- izer can detect such opportunities. A definition looks like this: (define_peephole [insn-pattern-1 insn-pattern-2 ...] "condition" "template" "optional insn-attributes") The last string operand may be omitted if you are not using any machine-specific information in this machine descrip- tion. If present, it must obey the same rules as in a define_insn. In this skeleton, insn-pattern-1 and so on are patterns to match consecutive insns. The optimization applies to a sequence of insns when insn-pattern-1 matches the first one, insn-pattern-2 matches the next, and so on. Each of the insns matched by a peephole must also match a define_insn. Peepholes are checked only at the last stage just before code generation, and only optionally. There- fore, any insn which would match a peephole but no define_insn will cause a crash in code generation in an unoptimized compilation, or at various optimization stages. Using GNU CC 239 The operands of the insns are matched with match_operands, match_operator, and match_dup, as usual. What is not usual is that the operand numbers apply to all the insn patterns in the definition. So, you can check for identical operands in two insns by using match_operand in one insn and match_dup in the other. The operand constraints used in match_operand patterns do not have any direct effect on the applicability of the peephole, but they will be validated afterward, so make sure your constraints are general enough to apply whenever the peephole matches. If the peephole matches but the con- straints are not satisfied, the compiler will crash. It is safe to omit constraints in all the operands of the peephole; or you can write constraints which serve as a double-check on the criteria previously tested. Once a sequence of insns matches the patterns, the con- dition is checked. This is a C expression which makes the final decision whether to perform the optimization (we do so if the expression is nonzero). If condition is omitted (in other words, the string is empty) then the optimization is applied to every sequence of insns that matches the pat- terns. The defined peephole optimizations are applied after register allocation is complete. Therefore, the peephole definition can check which operands have ended up in which kinds of registers, just by looking at the operands. The way to refer to the operands in condition is to write operands[i] for operand number i (as matched by (match_operand i ...)). Use the variable insn to refer to the last of the insns being matched; use prev_nonnote_insn to find the preceding insns. When optimizing computations with intermediate results, you can use condition to match only when the intermediate results are not used elsewhere. Use the C expression dead_or_set_p (insn, op), where insn is the insn in which you expect the value to be used for the last time (from the value of insn, together with use of prev_nonnote_insn), and op is the intermediate value (from operands[i]). Applying the optimization means replacing the sequence of insns with one new insn. The template controls ultimate output of assembler code for this combined insn. It works exactly like the template of a define_insn. Operand numbers in this template are the same ones used in matching the ori- ginal sequence of insns. 240 Using GNU CC The result of a defined peephole optimizer does not need to match any of the insn patterns in the machine description; it does not even have an opportunity to match them. The peephole optimizer definition itself serves as the insn pattern to control how the insn is output. Defined peephole optimizers are run as assembler code is being output, so the insns they produce are never com- bined or rearranged in any way. Here is an example, taken from the 68000 machine description: (define_peephole [(set (reg:SI 15) (plus:SI (reg:SI 15) (const_int 4))) (set (match_operand:DF 0 "register_operand" "f") (match_operand:DF 1 "register_operand" "ad"))] "FP_REG_P (operands[0]) && ! FP_REG_P (operands[1])" "* { rtx xoperands[2]; xoperands[1] = gen_rtx (REG, SImode, REGNO (operands[1]) + 1); #ifdef MOTOROLA output_asm_insn (\"move.l %1,(sp)\", xoperands); output_asm_insn (\"move.l %1,-(sp)\", operands); return \"fmove.d (sp)+,%0\"; #else output_asm_insn (\"movel %1,sp@\", xoperands); output_asm_insn (\"movel %1,sp@-\", operands); return \"fmoved sp@+,%0\"; #endif } ") The effect of this optimization is to change jbsr _foobar addql #4,sp movel d1,sp@- movel d0,sp@- fmoved sp@+,fp0 into jbsr _foobar movel d1,sp@ Using GNU CC 241 movel d0,sp@- fmoved sp@+,fp0 insn-pattern-1 and so on look almost like the second operand of define_insn. There is one important difference: the second operand of define_insn consists of one or more RTX's enclosed in square brackets. Usually, there is only one: then the same action can be written as an element of a define_peephole. But when there are multiple actions in a define_insn, they are implicitly enclosed in a parallel. Then you must explicitly write the parallel, and the square brackets within it, in the define_peephole. Thus, if an insn pattern looks like this, (define_insn "divmodsi4" [(set (match_operand:SI 0 "general_operand" "=d") (div:SI (match_operand:SI 1 "general_operand" "0") (match_operand:SI 2 "general_operand" "dmsK"))) (set (match_operand:SI 3 "general_operand" "=d") (mod:SI (match_dup 1) (match_dup 2)))] "TARGET_68020" "divsl%.l %2,%3:%0") then the way to mention this insn in a peephole is as fol- lows: (define_peephole [... (parallel [(set (match_operand:SI 0 "general_operand" "=d") (div:SI (match_operand:SI 1 "general_operand" "0") (match_operand:SI 2 "general_operand" "dmsK"))) (set (match_operand:SI 3 "general_operand" "=d") (mod:SI (match_dup 1) (match_dup 2)))]) ...] ...) 15.13. Defining RTL Sequences for Code Generation On some target machines, some standard pattern names for RTL generation cannot be handled with single insn, but a sequence of RTL insns can represent them. For these target machines, you can write a define_expand to specify how to 242 Using GNU CC generate the sequence of RTL. A define_expand is an RTL expression that looks almost like a define_insn; but, unlike the latter, a define_expand is used only for RTL generation and it can produce more than one RTL insn. A define_expand RTX has four operands: o+ The name. Each define_expand must have a name, since the only use for it is to refer to it by name. o+ The RTL template. This is just like the RTL template for a define_peephole in that it is a vector of RTL expressions each being one insn. o+ The condition, a string containing a C expression. This expression is used to express how the availability of this pattern depends on subclasses of target machine, selected by command-line options when GNU CC is run. This is just like the condition of a define_insn that has a standard name. o+ The preparation statements, a string containing zero or more C statements which are to be executed before RTL code is generated from the RTL template. Usually these statements prepare temporary registers for use as internal operands in the RTL template, but they can also generate RTL insns directly by calling routines such as emit_insn, etc. Any such insns precede the ones that come from the RTL template. Every RTL insn emitted by a define_expand must match some define_insn in the machine description. Otherwise, the compiler will crash when trying to generate code for the insn or trying to optimize it. The RTL template, in addition to controlling generation of RTL insns, also describes the operands that need to be specified when this pattern is used. In particular, it gives a predicate for each operand. A true operand, which needs to be specified in order to generate RTL from the pattern, should be described with a match_operand in its first occurrence in the RTL template. This enters information on the operand's predicate into the tables that record such things. GNU CC uses the information Using GNU CC 243 to preload the operand into a register if that is required for valid RTL code. If the operand is referred to more than once, subsequent references should use match_dup. The RTL template may also refer to internal ``operands'' which are temporary registers or labels used only within the sequence made by the define_expand. Inter- nal operands are substituted into the RTL template with match_dup, never with match_operand. The values of the internal operands are not passed in as arguments by the com- piler when it requests use of this pattern. Instead, they are computed within the pattern, in the preparation state- ments. These statements compute the values and store them into the appropriate elements of operands so that match_dup can find them. There are two special macros defined for use in the preparation statements: DONE and FAIL. Use them with a fol- lowing semicolon, as a statement. DONE Use the DONE macro to end RTL generation for the pattern. The only RTL insns resulting from the pattern on this occasion will be those already emitted by explicit calls to emit_insn within the preparation statements; the RTL template will not be generated. FAIL Make the pattern fail on this occasion. When a pattern fails, it means that the pattern was not truly available. The calling routines in the compiler will try other strategies for code generation using other patterns. Failure is currently supported only for binary (addition, multiplication, shifting, etc.) and bitfield (extv, extzv, and insv) operations. Here is an example, the definition of left-shift for the SPUR chip: (define_expand "ashlsi3" [(set (match_operand:SI 0 "register_operand" "") (ashift:SI (match_operand:SI 1 "register_operand" "") (match_operand:SI 2 "nonmemory_operand" "")))] "" " { if (GET_CODE (operands[2]) != CONST_INT 244 Using GNU CC || (unsigned) INTVAL (operands[2]) > 3) FAIL; }") This example uses define_expand so that it can generate an RTL insn for shifting when the shift-count is in the sup- ported range of 0 to 3 but fail in other cases where machine insns aren't available. When it fails, the compiler tries another strategy using different patterns (such as, a library call). If the compiler were able to handle nontrivial condition-strings in patterns with names, then it would be possible to use a define_insn in that case. Here is another case (zero-extension on the 68000) which makes more use of the power of define_expand: (define_expand "zero_extendhisi2" [(set (match_operand:SI 0 "general_operand" "") (const_int 0)) (set (strict_low_part (subreg:HI (match_dup 0) 0)) (match_operand:HI 1 "general_operand" ""))] "" "operands[1] = make_safe_from (operands[1], operands[0]);") Here two RTL insns are generated, one to clear the entire output operand and the other to copy the input operand into its low half. This sequence is incorrect if the input operand refers to [the old value of] the output operand, so the preparation statement makes sure this isn't so. The function make_safe_from copies the operands[1] into a tem- porary register if it refers to operands[0]. It does this by emitting another RTL insn. Finally, a third example shows the use of an internal operand. Zero-extension on the SPUR chip is done by and-ing the result against a halfword mask. But this mask cannot be represented by a const_int because the constant value is too large to be legitimate on this machine. So it must be copied into a register with force_reg and then the register used in the and. (define_expand "zero_extendhisi2" [(set (match_operand:SI 0 "register_operand" "") Using GNU CC 245 (and:SI (subreg:SI (match_operand:HI 1 "register_operand" "") 0) (match_dup 2)))] "" "operands[2] = force_reg (SImode, gen_rtx (CONST_INT, VOIDmode, 65535)); ") Note: If the define_expand is used to serve a standard binary or unary arithmetic operation or a bitfield opera- tion, then the last insn it generates must not be a code_label, barrier or note. It must be an insn, jump_insn or call_insn. If you don't need a real insn at the end, emit an insn to copy the result of the operation into itself. Such an insn will generate no code, but it can avoid problems in the compiler. 15.14. Splitting Instructions into Multiple Instructions On machines that have instructions requiring delay slots (see section Delay Slots) or that have instructions whose output is not available for multiple cycles (see sec- tion Function Units), the compiler phases that optimize these cases need to be able to move insns into one-cycle delay slots. However, some insns may generate more than one machine instruction. These insns would be unable to be placed into a delay slot. It is often possible to write the single insn as a list of individual insns, each corresponding to one machine instruction. The disadvantage of doing so is that it will cause the compilation to be slower and require more space. If the resulting insns are too complex, it may also suppress some optimizations. The define_split definition tells the compiler how to split a complex insn into several simpler insns. This spil- ling will be performed if there is a reason to believe that it might improve instruction or delay slot scheduling. The definition looks like this: (define_split [insn-pattern] "condition" [new-insn-pattern-1 new-insn-pattern-2 ...] "preparation statements") 246 Using GNU CC insn-pattern is a pattern that needs to be split and condition is the final condition to be tested, as in a define_insn. Any insn matched by a define_split must also be matched by a define_insn in case it does not need to be split. When an insn matching insn-pattern and satisfying con- dition is found, it is replaced in the insn list with the insns given by new-insn-pattern-1, new-insn-pattern-2, etc. The preparation statements are similar to those speci- fied for define_expand (see section Expander Definitions) and are executed before the new RTL is generated to prepare for the generated code or emit some insns whose pattern is not fixed. As a simple case, consider the following example from the AMD 29000 machine description, which splits a sign_extend from HImode to SImode into a pair of shift insns: (define_split [(set (match_operand:SI 0 "gen_reg_operand" "") (sign_extend:SI (match_operand:HI 1 "gen_reg_operand" "")))] "" [(set (match_dup 0) (ashift:SI (match_dup 1) (const_int 16))) (set (match_dup 0) (ashiftrt:SI (match_dup 0) (const_int 16)))] " { operands[1] = gen_lowpart (SImode, operands[1]); }") 15.15. Instruction Attributes In addition to describing the instruction supported by the target machine, the `md' file also defines a group of attributes and a set of values for each. Every generated insn is assigned a value for each attribute. One possible attribute would be the effect that the insn has on the machine's condition code. This attribute can then be used by NOTICE_UPDATE_CC to track the condition codes. 15.15.1. Defining Attributes and their Values The define_attr expression is used to define each attribute required by the target machine. It looks like: Using GNU CC 247 (define_attr name list-of-values default) name is a string specifying the name of the attribute being defined. list-of-values is either a string that specifies a comma-separated list of values that can be assigned to the attribute, or a null string to indicate that the attribute takes numeric values. default is an attribute expression that gives the value of this attribute for insns that match patterns whose defin- ition does not include an explicit value for this attribute. See section Attr Example, for more information on the han- dling of defaults. For each defined attribute, a number of definitions are written to the `insn-attr.h' file. For cases where an explicit set of values is specified for an attribute, the following are defined: o+ A `#define' is written for the symbol `HAVE_ATTR_name'. o+ An enumeral class is defined for `attr_name' with elements of the form `upper-name_upper-value' where the attribute name and value are first converted to upper case. o+ A function `get_attr_name' is defined that is passed an insn and returns the attribute value for that insn. For example, if the following is present in the `md' file: (define_attr "type" "branch,fp,load,store,arith" ...) the following lines will be written to the file `insn- attr.h'. #define HAVE_ATTR_type enum attr_type {TYPE_BRANCH, TYPE_FP, TYPE_LOAD, TYPE_STORE, TYPE_ARITH}; extern enum attr_type get_attr_type (); 248 Using GNU CC If the attribute takes numeric values, no enum type will be defined and the function to obtain the attribute's value will return int. 15.15.2. Attribute Expressions RTL expressions used to define attributes use the codes described above plus a few specific to attribute defini- tions, to be discussed below. Attribute value expressions must have one of the following forms: (const_int i) The integer i specifies the value of a numeric attribute. i must be non-negative. The value of a numeric attribute can be specified either with a const_int or as an integer represented as a string in const_string, eq_attr (see below), and set_attr (see section Tagging Insns) expressions. (const_string value) The string value specifies a constant attribute value. If value is specified as `"*"', it means that the default value of the attribute is to be used for the insn containing this expression. `"*"' obviously cannot be used in the default expression of a define_attr. If the attribute whose value is being specified is numeric, value must be a string containing a non- negative integer (normally const_int would be used in this case). Otherwise, it must contain one of the valid values for the attribute. (if_then_else test true-value false-value) test specifies an attribute test, whose format is defined below. The value of this expression is true-value if test is true, otherwise it is false-value. (cond [test1 value1 ...] default) The first operand of this expression is a vector containing an even number of expressions and consisting of pairs of test and value expressions. The value of the cond expression is that of the value corresponding to the first true test expression. If none of the test expressions are true, the value of the cond expression is that of the default expression. Using GNU CC 249 test expressions can have one of the following forms: (const_int i) This test is true if i is non-zero and false otherwise. (not test) (ior test1 test2) (and test1 test2) These tests are true if the indicated logical function is true. (match_operand:m n pred constraints) This test is true if operand n of the insn whose attribute value is being determined has mode m (this part of the test is ignored if m is VOIDmode) and the function specified by the string pred returns a non-zero value when passed operand n and mode m (this part of the test is ignored if pred is the null string). The constraints operand is ignored and should be the null string. (le arith1 arith2) (leu arith1 arith2) (lt arith1 arith2) (ltu arith1 arith2) (gt arith1 arith2) (gtu arith1 arith2) (ge arith1 arith2) (geu arith1 arith2) (ne arith1 arith2) (eq arith1 arith2) These tests are true if the indicated comparison of the two arithmetic expressions is true. Arithmetic expressions are formed with plus, minus, mult, div, mod, abs, neg, and, ior, xor, not, lshift, ashift, lshiftrt, and ashiftrt expressions. 250 Using GNU CC const_int and symbol_ref are always valid terms (see section Insn Lengths,for additional forms). symbol_ref is a string denoting a C expression that yields an int when evaluated by the `get_attr_...' routine. It should normally be a global variable. (eq_attr name value) name is a string specifying the name of an attribute. value is a string that is either a valid value for attribute name, a comma-separated list of values, or `!' followed by a value or list. If value does not begin with a `!', this test is true if the value of the name attribute of the current insn is in the list specified by value. If value begins with a `!', this test is true if the attribute's value is not in the specified list. For example, (eq_attr "type" "load,store") is equivalent to (ior (eq_attr "type" "load") (eq_attr "type" "store")) If name specifies an attribute of `alternative', it refers to the value of the compiler variable which_alternative (see section Output Statement) and the values must be small integers. For example, (eq_attr "alternative" "2,3") is equivalent to (ior (eq (symbol_ref "which_alternative") (const_int 2)) (eq (symbol_ref "which_alternative") (const_int 3))) Using GNU CC 251 Note that, for most attributes, an eq_attr test is simplified in cases where the value of the attribute being tested is known for all insns matching a particular pattern. This is by far the most common case. 15.15.3. Assigning Attribute Values to Insns The value assigned to an attribute of an insn is pri- marily determined by which pattern is matched by that insn (or which define_peephole generated it). Every define_insn and define_peephole can have an optional last argument to specify the values of attributes for matching insns. The value of any attribute not specified in a particular insn is set to the default value for that attribute, as specified in its define_attr. Extensive use of default values for attri- butes permits the specification of the values for only one or two attributes in the definition of most insn patterns, as seen in the example in the next section. The optional last argument of define_insn and define_peephole is a vector of expressions, each of which defines the value for a single attribute. The most general way of assigning an attribute's value is to use a set expression whose first operand is an attr expression giving the name of the attribute being set. The second operand of the set is an attribute expression (see section Expres- sions) giving the value of the attribute. When the attribute value depends on the `alternative' attribute (i.e., which is the applicable alternative in the constraint of the insn), the set_attr_alternative expression can can be used. It allows the specification of a vector of attribute expressions, one for each alternative. When the generality of arbitrary attribute expressions is not required, the simpler set_attr expression can be used, which allows specifying a string giving either a sin- gle attribute value or a list of attribute values, one for each alternative. The form of each of the above specifications is shown below. In each case, name is a string specifying the attri- bute to be set. (set_attr name value-string) value-string is either a string giving the desired attribute value, or a string containing a comma- separated list giving the values for succeeding alternatives. The number of elements must match the number of alternatives in the constraint of the insn pattern. 252 Using GNU CC Note that it may be useful to specify `*' for some alternative, in which case the attribute will assume its default value for insns matching that alternative. (set_attr_alternative name [value1 value2 ...]) Depending on the alternative of the insn, the value will be one of the specified values. This is a shorthand for using a cond with tests on the `alternative' attribute. (set (attr name) value) The first operand of this set must be the special RTL expression attr, whose sole operand is a string giving the name of the attribute being set. value is the value of the attribute. The following shows three different ways of represent- ing the same attribute value specification: (set_attr "type" "load,store,arith") (set_attr_alternative "type" [(const_string "load") (const_string "store") (const_string "arith")]) (set (attr "type") (cond [(eq_attr "alternative" "1") (const_string "load") (eq_attr "alternative" "2") (const_string "store")] (const_string "arith"))) The define_asm_attributes expression provides a mechan- ism to specify the attributes assigned to insns produced from an asm statement. It has the form: (define_asm_attributes [attr-sets]) where attr-sets is specified the same as for define_insn and define_peephole expressions. These values will typically be the ``worst case'' attribute values. For example, they might indicate that the condition code will be clobbered. A specification for a length attribute is handled spe- cially. To compute the length of an asm insn, the length Using GNU CC 253 specified in the define_asm_attributes expression is multi- plied by the number of machine instructions specified in the asm statement, determined by counting the number of semi- colons and newlines in the string. Therefore, the value of the length attribute specified in a define_asm_attributes should be the maximum possible length of a single machine instruction. 15.15.4. Example of Attribute Specifications The judicious use of defaulting is important in the efficient use of insn attributes. Typically, insns are divided into types and an attribute, customarily called type, is used to represent this value. This attribute is normally used only to define the default value for other attributes. An example will clarify this usage. Assume we have a RISC machine with a condition code and in which only full-word operations are performed in regis- ters. Let us assume that we can divide all insns into loads, stores, (integer) arithmetic operations, floating point operations, and branches. Here we will concern ourselves with determining the effect of an insn on the condition code and will limit our- selves to the following possible effects: The condition code can be set unpredictably (clobbered), not be changed, be set to agree with the results of the operation, or only changed if the item previously set into the condition code has been modified. Here is part of a sample `md' file for such a machine: (define_attr "type" "load,store,arith,fp,branch" (const_string "arith")) (define_attr "cc" "clobber,unchanged,set,change0" (cond [(eq_attr "type" "load") (const_string "change0") (eq_attr "type" "store,branch") (const_string "unchanged") (eq_attr "type" "arith") (if_then_else (match_operand:SI 0 "" "") (const_string "set") (const_string "clobber"))] (const_string "clobber"))) (define_insn "" [(set (match_operand:SI 0 "general_operand" "=r,r,m") (match_operand:SI 1 "general_operand" "r,m,r"))] "" "@ move %0,%1 254 Using GNU CC load %0,%1 store %0,%1" [(set_attr "type" "arith,load,store")]) Note that we assume in the above example that arith- metic operations performed on quantities smaller than a machine word clobber the condition code since they will set the condition code to a value corresponding to the full-word result. 15.15.5. Computing the Length of an Insn For many machines, multiple types of branch instruc- tions are provided, each for different length branch dis- placements. In most cases, the assembler will choose the correct instruction to use. However, when the assembler cannot do so, GCC can when a special attribute, the `length' attribute, is defined. This attribute must be defined to have numeric values by specifying a null string in its define_attr. In the case of the `length' attribute, two additional forms of arithmetic terms are allowed in test expressions: (match_dup n) This refers to the address of operand n of the current insn, which must be a label_ref. (pc) This refers to the address of the current insn. It might have been more consistent with other usage to make this the address of the next insn but this would be confusing because the length of the current insn is to be computed. For normal insns, the length will be determined by value of the `length' attribute. In the case of addr_vec and addr_diff_vec insn patterns, the length will be computed as the number of vectors multiplied by the size of each vec- tor. The following macros can be used to refine the length computation: FIRST_INSN_ADDRESS When the length insn attribute is used, this macro specifies the value to be assigned to the address of the first insn in a function. If not specified, 0 is used. Using GNU CC 255 ADJUST_INSN_LENGTH (insn, length) If defined, modifies the length assigned to instruction insn as a function of the context in which it is used. length is an lvalue that contains the initially computed length of the insn and should be updated with the correct length of the insn. If updating is required, insn must not be a varying-length insn. This macro will normally not be required. A case in which it is required is the ROMP. On this machine, the size of an addr_vec insn must be increased by two to compensate for the fact that alignment may be required. The routine that returns the value of the length attri- bute, get_attr_value, can be used by the output routine to determine the form of the branch instruction to be written, as the example below illustrates. As an example of the specification of variable-length branches, consider the IBM 360. If we adopt the convention that a register will be set to the starting address of a function, we can jump to labels within 4K of the start using a four-byte instruction. Otherwise, we need a six-byte sequence to load the address from memory and then branch to it. On such a machine, a pattern for a branch instruction might be specified as follows: (define_insn "jump" [(set (pc) (label_ref (match_operand 0 "" "")))] "" "* { return (get_attr_length (insn) == 4 ? \"b %l0\" : \"l r15,=a(%l0); br r15\"); }" [(set (attr "length") (if_then_else (lt (match_dup 0) (const_int 4096)) (const_int 4) (const_int 6)))]) 15.15.6. Delay Slot Scheduling The insn attribute mechanism can be used to specify the requirements for delay slots, if any, on a target machine. An instruction is said to require a delay slot if some 256 Using GNU CC instructions that are physically after the instruction are executed as if they were located before it. Classic exam- ples are branch and call instructions, which often execute the following instruction before the branch or call is per- formed. On some machines, conditional branch instructions can optionally annul instructions in the delay slot. This means that the instruction will not be executed for certain branch outcomes. Both instructions that annul if the branch is true and instructions that annul if the branch is false are supported. Delay slot scheduling differs from instruction scheduling in that determining whether an instruction needs a delay slot is dependent only on the type of instruction being gen- erated, not on data flow between the instructions. See the next section for a discussion of data-dependent instruction scheduling. The requirement of an insn needing one or more delay slots is indicated via the define_delay expression. It has the following form: (define_delay test [delay-1 annul-true-1 annul-false-1 delay-2 annul-true-2 annul-false-2 ...]) test is an attribute test that indicates whether this define_delay applies to a particular insn. If so, the number of required delay slots is determined by the length of the vector specified as the second argument. An insn placed in delay slot n must satisfy attribute test delay-n. annul-true-n is an attribute test that specifies which insns may be annulled if the branch is true. Similarly, annul- false-n specifies which insns in the delay slot may be annulled if the branch is false. If annulling is not sup- ported for that delay slot, (nil) should be coded. For example, in the common case where branch and call insns require a single delay slot, which may contain any insn other than a branch or call, the following would be placed in the `md' file: (define_delay (eq_attr "type" "branch,call") [(eq_attr "type" "!branch,call") (nil) (nil)]) Using GNU CC 257 Multiple define_delay expressions may be specified. In this case, each such expression specifies different delay slot requirements and there must be no insn for which tests in two define_delay expressions are both true. For example, if we have a machine that requires one delay slot for branches but two for calls, no delay slot can contain a branch or call insn, and any valid insn in the delay slot for the branch can be annulled if the branch is true, we might represent this as follows: (define_delay (eq_attr "type" "branch") [(eq_attr "type" "!branch,call") (eq_attr "type" "!branch,call") (nil)]) (define_delay (eq_attr "type" "call") [(eq_attr "type" "!branch,call") (nil) (nil) (eq_attr "type" "!branch,call") (nil) (nil)]) 15.15.7. Specifying Function Units On most RISC machines, there are instructions whose results are not available for a specific number of cycles. Common cases are instructions that load data from memory. On many machines, a pipeline stall will result if the data is referenced too soon after the load instruction. In addition, many newer microprocessors have multiple function units, usually one for integer and one for floating point, and often will incur pipeline stalls when a result that is needed is not yet ready. The descriptions in this section allow the specifica- tion of how much time must elapse between the execution of an instruction and the time when its result is used. It also allows specification of when the execution of an instruction will delay execution of similar instructions due to function unit conflicts. For the purposes of the specifications in this section, a machine is divided into function units, each of which exe- cute a specific class of instructions. Function units that accept one instruction each cycle and allow a result to be used in the succeeding instruction (usually via forwarding) need not be specified. Classic RISC microprocessors will normally have a single function unit, which we can call `memory'. The newer ``superscalar'' processors will often have function units for floating point operations, usually at least a floating point adder and multiplier. 258 Using GNU CC Each usage of a function units by a class of insns is specified with a define_function_unit expression, which looks like this: (define_function_unit name multiplicity simultaneity test ready-delay busy-delay [conflict-list]) name is a string giving the name of the function unit. multiplicity is an integer specifying the number of identical units in the processor. If more than one unit is specified, they will be scheduled independently. Only truly independent units should be counted; a pipelined unit should be specified as a single unit. (The only common example of a machine that has multiple function units for a single instruction class that are truly independent and not pipe- lined are the two multiply and two increment units of the CDC 6600.) simultaneity specifies the maximum number of insns that can be executing in each instance of the function unit simultaneously or zero if the unit is pipelined and has no limit. All define_function_unit definitions referring to func- tion unit name must have the same name and values for multi- plicity and simultaneity. test is an attribute test that selects the insns we are describing in this definition. Note that an insn may use more than one function unit and a function unit may be specified in more than one define_function_unit. ready-delay is an integer that specifies the number of cycles after which the result of the instruction can be used without introducing any stalls. busy-delay is an integer that represents the default cost if an insn is scheduled for this unit while the unit is active with another insn. If simultaneity is zero, this specification is ignored. Otherwise, a zero value indicates that these insns execute on name in a fully pipelined fashion, even if simultaneity is non-zero. A non-zero value indicates that scheduling a new insn on this unit while another is active will incur a cost. A cost of two indi- cates a single cycle delay. For a normal non-pipelined function unit, busy-delay will be twice ready-delay. Using GNU CC 259 conflict-list is an optional list giving detailed con- flict costs for this unit. If specified, it is a list of condition test expressions which are applied to insns already executing in name. For each insn that is in the list, busy-delay will be used for the conflict cost, while a value of zero will be used for insns not in the list. Typical uses of this vector are where a floating point function unit can pipeline either single- or double- precision operations, but not both, or where a memory unit can pipeline loads, but not stores, etc. As an example, consider a classic RISC machine where the result of a load instruction is not available for two cycles (a single ``delay'' instruction is required) and where only one load instruction can be executed simultane- ously. This would be specified as: (define_function_unit "memory" 1 1 (eq_attr "type" "load") 2 4) For the case of a floating point function unit that can pipeline either single or double precision, but not both, the following could be specified: (define_function_unit "fp" 1 1 (eq_attr "type" "sp_fp") 4 8 (eq_attr "type" "dp_fp")] (define_function_unit "fp" 1 1 (eq_attr "type" "dp_fp") 4 8 (eq_attr "type" "sp_fp")] Note: No code currently exists to avoid function unit conflicts, only data conflicts. Hence multiplicity, simul- taneity, busy-cost, and conflict-list are currently ignored. When such code is written, it is possible that the specifi- cations for these values may be changed. It has recently come to our attention that these specifications may not allow modeling of some of the newer ``superscalar'' proces- sors that have insns using multiple pipelined units. These insns will cause a potential conflict for the second unit used during their execution and there is no way of representing that conflict. We welcome any examples of how function unit conflicts work in such processors and sugges- tions for their representation. INTERNALS 260 Using GNU CC 16. Machine Description Macros In addition to the file `machine.md', a machine description includes a C header file conventionally given the name `machine.h'. This header file defines numerous macros that convey the information about the target machine that does not fit into the scheme of the `.md' file. The file `tm.h' should be a link to `machine.h'. The header file `config.h' includes `tm.h' and most compiler source files include `config.h'. 16.1. Controlling the Compilation Driver, `gcc' SWITCH_TAKES_ARG (char) A C expression which determines whether the option `-char' takes arguments. The value should be the number of arguments that option takes--zero, for many options. By default, this macro is defined to handle the standard options properly. You need not define it unless you wish to add additional options which take arguments. WORD_SWITCH_TAKES_ARG (name) A C expression which determines whether the option `-name' takes arguments. The value should be the number of arguments that option takes--zero, for many options. This macro rather than SWITCH_TAKES_ARG is used for multi-character option names. By default, this macro is defined to handle the standard options properly. You need not define it unless you wish to add additional options which take arguments. SWITCHES_NEED_SPACES A string-valued C expression which is nonempty if the linker needs a space between the `-L' or `-o' option and its argument. If this macro is not defined, the default value is 0. CPP_SPEC A C string constant that tells the GNU CC driver program options to pass to CPP. It can also specify how to translate options you give to GNU CC into options for GNU CC to pass to the CPP. Using GNU CC 261 Do not define this macro if it does not need to do anything. SIGNED_CHAR_SPEC A C string constant that tells the GNU CC driver program options to pass to CPP. By default, this macro is defined to pass the option `- D__CHAR_UNSIGNED__' to CPP if char will be treated as unsigned char by cc1. Do not define this macro unless you need to override the default definition. CC1_SPEC A C string constant that tells the GNU CC driver program options to pass to cc1. It can also specify how to translate options you give to GNU CC into options for GNU CC to pass to the cc1. Do not define this macro if it does not need to do anything. CC1PLUS_SPEC A C string constant that tells the GNU CC driver program options to pass to cc1plus. It can also specify how to translate options you give to GNU CC into options for GNU CC to pass to the cc1plus. Do not define this macro if it does not need to do anything. ASM_SPEC A C string constant that tells the GNU CC driver program options to pass to the assembler. It can also specify how to translate options you give to GNU CC into options for GNU CC to pass to the assembler. See the file `sun3.h' for an example of this. Do not define this macro if it does not need to do anything. ASM_FINAL_SPEC A C string constant that tells the GNU CC driver program how to run any programs which cleanup after the normal assembler. Normally, this is not needed. See the file `mips.h' for an example of this. Do not define this macro if it does not need to do anything. 262 Using GNU CC LINK_SPEC A C string constant that tells the GNU CC driver program options to pass to the linker. It can also specify how to translate options you give to GNU CC into options for GNU CC to pass to the linker. Do not define this macro if it does not need to do anything. LIB_SPEC Another C string constant used much like LINK_SPEC. The difference between the two is that LIB_SPEC is used at the end of the command given to the linker. If this macro is not defined, a default is provided that loads the standard C library from the usual place. See `gcc.c'. STARTFILE_SPEC Another C string constant used much like LINK_SPEC. The difference between the two is that STARTFILE_SPEC is used at the very beginning of the command given to the linker. If this macro is not defined, a default is provided that loads the standard C startup file from the usual place. See `gcc.c'. ENDFILE_SPEC Another C string constant used much like LINK_SPEC. The difference between the two is that ENDFILE_SPEC is used at the very end of the command given to the linker. Do not define this macro if it does not need to do anything. LINK_LIBGCC_SPECIAL Define this macro meaning that gcc should find the library `libgcc.a' by hand, rather than passing the argument `-lgcc' to tell the linker to do the search. RELATIVE_PREFIX_NOT_LINKDIR Define this macro to tell gcc that it should only translate a `-B' prefix into a `-L' linker option if the prefix indicates an absolute file name. STANDARD_EXEC_PREFIX Define this macro as a C string constant if you wish to override the standard choice of Using GNU CC 263 `/usr/local/lib/gcc/' as the default prefix to try when searching for the executable files of the compiler. MD_EXEC_PREFIX If defined, this macro is an additional prefix to try after STANDARD_EXEC_PREFIX. MD_EXEC_PREFIX is not searched when the `-b' option is used, or the compiler is built as a cross compiler. STANDARD_STARTFILE_PREFIX Define this macro as a C string constant if you wish to override the standard choice of `/usr/local/lib/gcc/' as the default prefix to try when searching for startup files such as `crt0.o'. MD_STARTFILE_PREFIX If defined, this macro supplies an additional prefix to try after the standard prefixes. MD_EXEC_PREFIX is not searched when the `-b' option is used, or the compiler is built as a cross compiler. LOCAL_INCLUDE_DIR Define this macro as a C string constant if you wish to override the standard choice of `/usr/local/include' as the default prefix to try when searching for local header files. LOCAL_INCLUDE_DIR comes before SYSTEM_INCLUDE_DIR in the search order. Cross compilers do not use this macro and do not search either `/usr/local/include' or its replacement. SYSTEM_INCLUDE_DIR Define this macro as a C string constant if you wish to specify a system-specific directory to search for header files before the standard directory. SYSTEM_INCLUDE_DIR comes before STANDARD_INCLUDE_DIR in the search order. Cross compilers do not use this macro and do not search the directory specified. STANDARD_INCLUDE_DIR Define this macro as a C string constant if you wish to override the standard choice of `/usr/include' as the default prefix to try when searching for header files. Cross compilers do not use this macro and do not search either `/usr/include' or its replacement. 264 Using GNU CC INCLUDE_DEFAULTS Define this macro if you wish to override the entire default search path for include files. The default search path includes GPLUSPLUS_INCLUDE_DIR, GCC_INCLUDE_DIR, LOCAL_INCLUDE_DIR, SYSTEM_INCLUDE_DIR, and STANDARD_INCLUDE_DIR. In addition, the macros GPLUSPLUS_INCLUDE_DIR and GCC_INCLUDE_DIR are defined automatically by `Makefile', and specify private search areas for GCC. The directory GPLUSPLUS_INCLUDE_DIR is used only for C++ programs. The definition should be an initializer for an array of structures. Each array element should have two elements: the directory name (a string constant) and a flag for C++-only directories. Mark the end of the array with a null element. For example, here is the definition used for VMS: #define INCLUDE_DEFAULTS \ { \ { "GNU_GXX_INCLUDE:", 1}, \ { "GNU_CC_INCLUDE:", 0}, \ { "SYS$SYSROOT:[SYSLIB.]", 0}, \ { ".", 0}, \ { 0, 0} \ } Here is the order of prefixes tried for exec files: 1. Any prefixes specified by the user with `-B'. 2. The environment variable GCC_EXEC_PREFIX, if any. 3. The directories specified by the environment variable COMPILER_PATH. 4. The macro STANDARD_EXEC_PREFIX. 5. `/usr/lib/gcc/'. 6. The macro MD_EXEC_PREFIX, if any. Here is the order of prefixes tried for startfiles: 1. Any prefixes specified by the user with `-B'. Using GNU CC 265 2. The environment variable GCC_EXEC_PREFIX, if any. 3. The directories specified by the environment variable LIBRARY_PATH. 4. The macro STANDARD_EXEC_PREFIX. 5. `/usr/lib/gcc/'. 6. The macro MD_EXEC_PREFIX, if any. 7. The macro MD_STARTFILE_PREFIX, if any. 8. The macro STANDARD_STARTFILE_PREFIX. 9. `/lib/'. 10. `/usr/lib/'. 16.2. Run-time Target Specification CPP_PREDEFINES Define this to be a string constant containing `- D' options to define the predefined macros that identify this machine and system. These macros will be predefined unless the `-ansi' option is specified. In addition, a parallel set of macros are predefined, whose names are made by appending `__' at the beginning and at the end. These `__' macros are permitted by the ANSI standard, so they are predefined regardless of whether `-ansi' is specified. For example, on the Sun, one can use the following value: "-Dmc68000 -Dsun -Dunix" The result is to define the macros __mc68000__, __sun__ and __unix__ unconditionally, and the macros mc68000, sun and unix provided `-ansi' is not specified. STDC_VALUE Define the value to be assigned to the built- in macro __STDC__. The default is the value `1'. 266 Using GNU CC extern int target_flags; This declaration should be present. TARGET_... This series of macros is to allow compiler command arguments to enable or disable the use of optional features of the target machine. For example, one machine description serves both the 68000 and the 68020; a command argument tells the compiler whether it should use 68020-only instructions or not. This command argument works by means of a macro TARGET_68020 that tests a bit in target_flags. Define a macro TARGET_featurename for each such option. Its definition should test a bit in target_flags; for example: #define TARGET_68020 (target_flags & 1) One place where these macros are used is in the condition-expressions of instruction patterns. Note how TARGET_68020 appears frequently in the 68000 machine description file, `m68k.md'. Another place they are used is in the definitions of the other macros in the `machine.h' file. TARGET_SWITCHES This macro defines names of command options to set and clear bits in target_flags. Its definition is an initializer with a subgrouping for each command option. Each subgrouping contains a string constant, that defines the option name, and a number, which contains the bits to set in target_flags. A negative number says to clear bits instead; the negative of the number is which bits to clear. The actual option name is made by appending `-m' to the specified name. One of the subgroupings should have a null string. The number in this grouping is the default value for target_flags. Any target options act starting with that value. Here is an example which defines `-m68000' and `-m68020' with opposite meanings, and picks Using GNU CC 267 the latter as the default: #define TARGET_SWITCHES \ { { "68020", 1}, \ { "68000", -1}, \ { "", 1}} TARGET_OPTIONS This macro is similar to TARGET_SWITCHES but defines names of command options that have values. Its definition is an initializer with a subgrouping for each command option. Each subgrouping contains a string constant, that defines the fixed part of the option name, and the address of a variable. The variable, type char *, is set to the variable part of the given option if the fixed part matches. The actual option name is made by appending `-m' to the specified name. Here is an example which defines `-mshort- data-number'. If the given option is `- mshort-data-512', the variable m88k_short_data will be set to the string "512". extern char *m88k_short_data; #define TARGET_OPTIONS { { "short-data-", &m88k_short_data } } TARGET_VERSION This macro is a C statement to print on stderr a string describing the particular machine description choice. Every machine description should define TARGET_VERSION. For example: #ifdef MOTOROLA #define TARGET_VERSION fprintf (stderr, " (68k, Motorola syntax)"); #else #define TARGET_VERSION fprintf (stderr, " (68k, MIT syntax)"); #endif OVERRIDE_OPTIONS Sometimes certain combinations of command options do not make sense on a particular 268 Using GNU CC target machine. You can define a macro OVERRIDE_OPTIONS to take account of this. This macro, if defined, is executed once just after all the command options have been parsed. Don't use this macro to turn on various extra optimizations for `-O'. That is what OPTIMIZATION_OPTIONS is for. OPTIMIZATION_OPTIONS (level) Some machines may desire to change what optimizations are performed for various optimization levels. This macro, if defined, is executed once just after the optimization level is determined and before the remainder of the command options have been parsed. Values set in this macro are used as the default values for the other command line options. level is the optimization level specified; 2 if -O2 is specified, 1 if -O is specified, and 0 if neither is specified. Do not examine write_symbols in this macro! The debugging options are not supposed to alter the generated code. 16.3. Storage Layout Note that the definitions of the macros in this table which are sizes or alignments measured in bits do not need to be constant. They can be C expressions that refer to static variables, such as the target_flags. See section Run-time Target. BITS_BIG_ENDIAN Define this macro to be the value 1 if the most significant bit in a byte has the lowest number; otherwise define it to be the value zero. This means that bit-field instructions count from the most significant bit. If the machine has no bit- field instructions, this macro is irrelevant. This macro does not affect the way structure fields are packed into bytes or words; that is controlled by BYTES_BIG_ENDIAN. BYTES_BIG_ENDIAN Define this macro to be 1 if the most significant byte in a word has the lowest number. Using GNU CC 269 WORDS_BIG_ENDIAN Define this macro to be 1 if, in a multiword object, the most significant word has the lowest number. BITS_PER_UNIT Number of bits in an addressable storage unit (byte); normally 8. BITS_PER_WORD Number of bits in a word; normally 32. MAX_BITS_PER_WORD Maximum number of bits in a word. If this is undefined, the default is BITS_PER_WORD. Otherwise, it is the constant value that is the largest value that BITS_PER_WORD can have at run- time. UNITS_PER_WORD Number of storage units in a word; normally 4. POINTER_SIZE Width of a pointer, in bits. PARM_BOUNDARY Normal alignment required for function parameters on the stack, in bits. All stack parameters receive least this much alignment regardless of data type. On most machines, this is the same as the size of an integer. STACK_BOUNDARY Define this macro if you wish to preserve a certain alignment for the stack pointer. The definition is a C expression for the desired alignment (measured in bits). If PUSH_ROUNDING is not defined, the stack will always be aligned to the specified boundary. If PUSH_ROUNDING is defined and specifies a less strict alignment than STACK_BOUNDARY, the stack may be momentarily unaligned while pushing arguments. FUNCTION_BOUNDARY Alignment required for a function entry point, in bits. BIGGEST_ALIGNMENT Biggest alignment that any data type can require on this machine, in bits. 270 Using GNU CC BIGGEST_FIELD_ALIGNMENT Biggest alignment that any structure field can require on this machine, in bits. MAX_OFILE_ALIGNMENT Biggest alignment supported by the object file format of this machine. Use this macro to limit the alignment which can be specified using the __attribute__ ((aligned (n))) construct. If not defined, the default value is BIGGEST_ALIGNMENT. DATA_ALIGNMENT (type, basic-align) If defined, a C expression to compute the alignment for a static variable. type is the data type, and basic-align is the alignment that the object would ordinarily have. The value of this macro is used instead of that alignment to align the object. If this macro is not defined, then basic-align is used. One use of this macro is to increase alignment of medium-size data to make it all fit in fewer cache lines. Another is to cause character arrays to be word-aligned so that strcpy calls that copy constants to character arrays can be done inline. CONSTANT_ALIGNMENT (constant, basic-align) If defined, a C expression to compute the alignment given to a constant that is being placed in memory. constant is the constant and basic- align is the alignment that the object would ordinarily have. The value of this macro is used instead of that alignment to align the object. If this macro is not defined, then basic-align is used. The typical use of this macro is to increase alignment for string constants to be word aligned so that strcpy calls that copy constants can be done inline. EMPTY_FIELD_BOUNDARY Alignment in bits to be given to a structure bit field that follows an empty field such as int : 0;. STRUCTURE_SIZE_BOUNDARY Number of bits which any structure or union's size must be a multiple of. Each structure or union's size is rounded up to a multiple of this. Using GNU CC 271 If you do not define this macro, the default is the same as BITS_PER_UNIT. STRICT_ALIGNMENT Define this if instructions will fail to work if given data not on the nominal alignment. If instructions will merely go slower in that case, do not define this macro. PCC_BITFIELD_TYPE_MATTERS Define this if you wish to imitate the way many other C compilers handle alignment of bitfields and the structures that contain them. The behavior is that the type written for a bitfield (int, short, or other integer type) imposes an alignment for the entire structure, as if the structure really did contain an ordinary field of that type. In addition, the bitfield is placed within the structure so that it would fit within such a field, not crossing a boundary for it. Thus, on most machines, a bitfield whose type is written as int would not cross a four-byte boundary, and would force four-byte alignment for the whole structure. (The alignment used may not be four bytes; it is controlled by the other alignment parameters.) If the macro is defined, its definition should be a C expression; a nonzero value for the expression enables this behavior. Note that if this macro is not defined, or its value is zero, some bitfields may cross more than one alignment boundary. The compiler can support such references if there are `insv', `extv', and `extzv' insns that can directly reference memory. The other known way of making bitfields work is to define STRUCTURE_SIZE_BOUNDARY as large as BIGGEST_ALIGNMENT. Then every structure can be accessed with fullwords. Unless the machine has bitfield instructions or you define STRUCTURE_SIZE_BOUNDARY that way, you must define PCC_BITFIELD_TYPE_MATTERS to have a nonzero value. BITFIELD_NBYTES_LIMITED Like PCC_BITFIELD_TYPE_MATTERS except that its effect is limited to aligning a bitfield within 272 Using GNU CC the structure. ROUND_TYPE_SIZE (struct, size, align) Define this macro as an expression for the overall size of a structure (given by struct as a tree node) when the size computed from the fields is size and the alignment is align. The default is to round size up to a multiple of align. ROUND_TYPE_ALIGN (struct, computed, specified) Define this macro as an expression for the alignment of a structure (given by struct as a tree node) if the alignment computed in the usual way is computed and the alignment explicitly specified was specified. The default is to use specified if it is larger; otherwise, use the smaller of computed and BIGGEST_ALIGNMENT MAX_FIXED_MODE_SIZE An integer expression for the size in bits of the largest integer machine mode that should actually be used. All integer machine modes of this size or smaller can be used for structures and unions with the appropriate sizes. If this macro is undefined, GET_MODE_BITSIZE (DImode) is assumed. CHECK_FLOAT_VALUE (mode, value) A C statement to validate the value value (of type double) for mode mode. This means that you check whether value fits within the possible range of values for mode mode on this target machine. The mode mode is always SFmode or DFmode. If value is not valid, you should call error to print an error message and then assign some valid value to value. Allowing an invalid value to go through the compiler can produce incorrect assembler code which may even cause Unix assemblers to crash. This macro need not be defined if there is no work for it to do. TARGET_FLOAT_FORMAT A code distinguishing the floating point format of the target machine. There are three defined values: Using GNU CC 273 IEEE_FLOAT_FORMAT This code indicates IEEE floating point. It is the default; there is no need to define this macro when the format is IEEE. VAX_FLOAT_FORMAT This code indicates the peculiar format used on the Vax. UNKNOWN_FLOAT_FORMAT This code indicates any other format. The value of this macro is compared with HOST_FLOAT_FORMAT (see section Config) to determine whether the target machine has the same format as the host machine. If any other formats are actually in use on supported machines, new codes should be defined for them. 16.4. Layout of Source Language Data Types These macros define the sizes and other characteristics of the standard basic data types used in programs being com- piled. Unlike the macros in the previous section, these apply to specific features of C and related languages, rather than to fundamental aspects of storage layout. INT_TYPE_SIZE A C expression for the size in bits of the type int on the target machine. If you don't define this, the default is one word. SHORT_TYPE_SIZE A C expression for the size in bits of the type short on the target machine. If you don't define this, the default is half a word. (If this would be less than one storage unit, it is rounded up to one unit.) LONG_TYPE_SIZE A C expression for the size in bits of the type long on the target machine. If you don't define this, the default is one word. LONG_LONG_TYPE_SIZE A C expression for the size in bits of the type long long on the target machine. If you don't define this, the default is two words. CHAR_TYPE_SIZE A C expression for the size in bits of the type 274 Using GNU CC char on the target machine. If you don't define this, the default is one quarter of a word. (If this would be less than one storage unit, it is rounded up to one unit.) FLOAT_TYPE_SIZE A C expression for the size in bits of the type float on the target machine. If you don't define this, the default is one word. DOUBLE_TYPE_SIZE A C expression for the size in bits of the type double on the target machine. If you don't define this, the default is two words. LONG_DOUBLE_TYPE_SIZE A C expression for the size in bits of the type long double on the target machine. If you don't define this, the default is two words. DEFAULT_SIGNED_CHAR An expression whose value is 1 or 0, according to whether the type char should be signed or unsigned by default. The user can always override this default with the options `-fsigned-char' and `- funsigned-char'. DEFAULT_SHORT_ENUMS A C expression to determine whether to give an enum type only as many bytes as it takes to represent the range of possible values of that type. A nonzero value means to do that; a zero value means all enum types should be allocated like int. If you don't define the macro, the default is 0. SIZE_TYPE A C expression for a string describing the name of the data type to use for size values. The typedef name size_t is defined using the contents of the string. The string can contain more than one keyword. If so, separate them with spaces, and write first any length keyword, then unsigned if appropriate, and finally int. The string must exactly match one of the data type names defined in the function init_decl_processing in the file `c-decl.c'. You may not omit int or change the order---that would cause the compiler to crash on startup. Using GNU CC 275 If you don't define this macro, the default is "long unsigned int". PTRDIFF_TYPE A C expression for a string describing the name of the data type to use for the result of subtracting two pointers. The typedef name ptrdiff_t is defined using the contents of the string. See SIZE_TYPE above for more information. If you don't define this macro, the default is "long int". WCHAR_TYPE A C expression for a string describing the name of the data type to use for wide characters. The typedef name wchar_t is defined using the contents of the string. See SIZE_TYPE above for more information. If you don't define this macro, the default is "int". WCHAR_TYPE_SIZE A C expression for the size in bits of the data type for wide characters. This is used in cpp, which cannot make use of WCHAR_TYPE. OBJC_INT_SELECTORS Define this macro if the type of Objective C selectors should be int. If this macro is not defined, then selectors should have the type struct objc_selector *. OBJC_NONUNIQUE_SELECTORS Define this macro if Objective C selector- references will be made unique by the linker (this is the default). In this case, each selector- reference will be given a separate assembler label. Otherwise, the selector-references will be gathered into an array with a single assembler label. MULTIBYTE_CHARS Define this macro to enable support for multibyte characters in the input to GNU CC. This requires that the host system support the ANSI C library functions for converting multibyte characters to wide characters. TARGET_BELL A C constant expression for the integer value for 276 Using GNU CC escape sequence `\a'. TARGET_BS TARGET_TAB TARGET_NEWLINE C constant expressions for the integer values for escape sequences `\b', `\t' and `\n'. TARGET_VT TARGET_FF TARGET_CR C constant expressions for the integer values for escape sequences `\v', `\f' and `\r'. 16.5. Register Usage This section explains how to describe what registers the target machine has, and how (in general) they can be used. The description of which registers a specific instruc- tion can use is done with register classes; see `Register Classes'. For information on using registers to access a stack frame, see `Frame Registers'. For passing values in registers, see `Register Arguments'. For returning values in registers, see `Scalar Return'. 16.5.1. Basic Characteristics of Registers FIRST_PSEUDO_REGISTER Number of hardware registers known to the compiler. They receive numbers 0 through FIRST_PSEUDO_REGISTER-1; thus, the first pseudo register's number really is assigned the number FIRST_PSEUDO_REGISTER. FIXED_REGISTERS An initializer that says which registers are used for fixed purposes all throughout the compiled code and are therefore not available for general allocation. These would include the stack pointer, the frame pointer (except on machines where that can be used as a general register when no frame pointer is needed), the program counter on machines where that is considered one of the addressable registers, and any other numbered register with a standard use. Using GNU CC 277 This information is expressed as a sequence of numbers, separated by commas and surrounded by braces. The nth number is 1 if register n is fixed, 0 otherwise. The table initialized from this macro, and the table initialized by the following one, may be overridden at run time either automatically, by the actions of the macro CONDITIONAL_REGISTER_USAGE, or by the user with the command options `-ffixed-reg', `-fcall-used- reg' and `-fcall-saved-reg'. CALL_USED_REGISTERS Like FIXED_REGISTERS but has 1 for each register that is clobbered (in general) by function calls as well as for fixed registers. This macro therefore identifies the registers that are not available for general allocation of values that must live across function calls. If a register has 0 in CALL_USED_REGISTERS, the compiler automatically saves it on function entry and restores it on function exit, if the register is used within the function. CONDITIONAL_REGISTER_USAGE Zero or more C statements that may conditionally modify two variables fixed_regs and call_used_regs (both of type char []) after they have been initialized from the two preceding macros. This is necessary in case the fixed or call- clobbered registers depend on target flags. You need not define this macro if it has no work to do. If the usage of an entire class of registers depends on the target flags, you may indicate this to GCC by using this macro to modify fixed_regs and call_used_regs to 1 for each of the registers in the classes which should not be used by GCC. Also define the macro REG_CLASS_FROM_LETTER to return NO_REGS if it is called with a letter for a class that shouldn't be used. (However, if this class is not included in GENERAL_REGS and all of the insn patterns whose constraints permit this class are controlled by target switches, then GCC will automatically avoid using these registers when the target switches are opposed to them.) 278 Using GNU CC NON_SAVING_SETJMP If this macro is defined and has a nonzero value, it means that setjmp and related functions fail to save the registers, or that longjmp fails to restore them. To compensate, the compiler avoids putting variables in registers in functions that use setjmp. 16.5.2. Order of Allocation of Registers REG_ALLOC_ORDER If defined, an initializer for a vector of integers, containing the numbers of hard registers in the order in which GNU CC should prefer to use them (from most preferred to least). If this macro is not defined, registers are used lowest numbered first (all else being equal). One use of this macro is on machines where the highest numbered registers must always be saved and the save-multiple-registers instruction supports only sequences of consecutive registers. On such machines, define REG_ALLOC_ORDER to be an initializer that lists the highest numbered allocatable register first. ORDER_REGS_FOR_LOCAL_ALLOC A C statement (sans semicolon) to choose the order in which to allocate hard registers for pseudo- registers local to a basic block. Store the desired order of registers in the array reg_alloc_order. Element 0 should be the register to allocate first; element 1, the next register; and so on. The macro body should not assume anything about the contents of reg_alloc_order before execution of the macro. On most machines, it is not necessary to define this macro. 16.5.3. How Values Fit in Registers This section discusses the macros that describe which kinds of values (specifically, which machine modes) each register can hold, and how many consecutive registers are needed for a given mode. Using GNU CC 279 HARD_REGNO_NREGS (regno, mode) A C expression for the number of consecutive hard registers, starting at register number regno, required to hold a value of mode mode. On a machine where all registers are exactly one word, a suitable definition of this macro is #define HARD_REGNO_NREGS(REGNO, MODE) \ ((GET_MODE_SIZE (MODE) + UNITS_PER_WORD - 1) \ / UNITS_PER_WORD)) HARD_REGNO_MODE_OK (regno, mode) A C expression that is nonzero if it is permissible to store a value of mode mode in hard register number regno (or in several registers starting with that one). For a machine where all registers are equivalent, a suitable definition is #define HARD_REGNO_MODE_OK(REGNO, MODE) 1 It is not necessary for this macro to check for the numbers of fixed registers, because the allocation mechanism considers them to be always occupied. On some machines, double-precision values must be kept in even/odd register pairs. The way to implement that is to define this macro to reject odd register numbers for such modes. The minimum requirement for a mode to be OK in a register is that the `movmode' instruction pattern support moves between the register and any other hard register for which the mode is OK; and that moving a value into the register and back out not alter it. Since the same instruction used to move SImode will work for all narrower integer modes, it is not necessary on any machine for HARD_REGNO_MODE_OK to distinguish between these modes, provided you define patterns `movhi', etc., to take advantage of this. 280 Using GNU CC This is useful because of the interaction between HARD_REGNO_MODE_OK and MODES_TIEABLE_P; it is very desirable for all integer modes to be tieable. Many machines have special registers for floating point arithmetic. Often people assume that floating point machine modes are allowed only in floating point registers. This is not true. Any registers that can hold integers can safely hold a floating point machine mode, whether or not floating arithmetic can be done on it in those registers. Integer move instructions can be used to move the values. On some machines, though, the converse is true: fixed-point machine modes may not go in floating registers. This is true if the floating registers normalize any value stored in them, because storing a non-floating value there would garble it. In this case, HARD_REGNO_MODE_OK should reject fixed-point machine modes in floating registers. But if the floating registers do not automatically normalize, if you can store any bit pattern in one and retrieve it unchanged without a trap, then any machine mode may go in a floating register and this macro should say so. The primary significance of special floating registers is rather that they are the registers acceptable in floating point arithmetic instructions. However, this is of no concern to HARD_REGNO_MODE_OK. You handle it by writing the proper constraints for those instructions. On some machines, the floating registers are especially slow to access, so that it is better to store a value in a stack frame than in such a register if floating point arithmetic is not being done. As long as the floating registers are not in class GENERAL_REGS, they will not be used unless some pattern's constraint asks for one. MODES_TIEABLE_P (mode1, mode2) A C expression that is nonzero if it is desirable to choose register allocation so as to avoid move instructions between a value of mode mode1 and a value of mode mode2. Using GNU CC 281 If HARD_REGNO_MODE_OK (r, mode1) and HARD_REGNO_MODE_OK (r, mode2) are ever different for any r, then MODES_TIEABLE_P (mode1, mode2) must be zero. 16.5.4. Handling Leaf Functions On some machines, a leaf function (i.e., one which make no calls) can run more efficiently if it does not make its own register window. Often this means it is required to receive its arguments in the registers where they are passed by the caller, instead of the registers where they would normally arrive. Also, the leaf function may use only those registers for its own variables and temporaries. GNU CC assigns register numbers before it knows whether the function is suitable for leaf function treatment. So it needs to renumber the registers in order to output a leaf function. The following macros accomplish this. LEAF_REGISTERS A C initializer for a vector, indexed by hard register number, which contains 1 for a register that is allowable in a candidate for leaf function treatment. If leaf function treatment involves renumbering the registers, then the registers marked here should be the ones before renumbering---those that GNU CC would ordinarily allocate. The registers which will actually be used in the assembler code, after renumbering, should not be marked with 1 in this vector. Define this macro only if the target machine offers a way to optimize the treatment of leaf functions. LEAF_REG_REMAP (regno) A C expression whose value is the register number to which regno should be renumbered, when a function is treated as a leaf function. If regno is a register number which should not appear in a leaf function before renumbering, then the expression should yield -1, which will cause the compiler to abort. Define this macro only if the target machine offers a way to optimize the treatment of leaf functions, and registers need to be renumbered to do this. 282 Using GNU CC REG_LEAF_ALLOC_ORDER If defined, an initializer for a vector of integers, containing the numbers of hard registers in the order in which the GNU CC should prefer to use them (from most preferred to least) in a leaf function. If this macro is not defined, REG_ALLOC_ORDER is used for both non-leaf and leaf-functions. Normally, it is necessary for FUNCTION_PROLOGUE and FUNCTION_EPILOGUE to treat leaf functions specially. The C variable leaf_function is nonzero for such a function. 16.5.5. Registers That Form a Stack There are special features to handle computers where some of the ``registers'' form a stack, as in the 80387 coprocessor for the 80386. Stack registers are normally written by pushing onto the stack, and are numbered relative to the top of the stack. Currently, GNU CC can only handle one group of stack- like registers, and they must be consecutively numbered. STACK_REGS Define this if the machine has any stack-like registers. FIRST_STACK_REG The number of the first stack-like register. This one is the top of the stack. LAST_STACK_REG The number of the last stack-like register. This one is the bottom of the stack. 16.5.6. Obsolete Macros for Controlling Register Usage These features do not work very well. They exist because they used to be required to generate correct code for the 80387 coprocessor of the 80386. They are no longer used by that machine description and may be removed in a later version of the compiler. Don't use them! OVERLAPPING_REGNO_P (regno) If defined, this is a C expression whose value is nonzero if hard register number regno is an overlapping register. This means a hard register which overlaps a hard register with a different number. (Such overlap is undesirable, but occasionally it allows a machine to be supported Using GNU CC 283 which otherwise could not be.) This macro must return nonzero for all the registers which overlap each other. GNU CC can use an overlapping register only in certain limited ways. It can be used for allocation within a basic block, and may be spilled for reloading; that is all. If this macro is not defined, it means that none of the hard registers overlap each other. This is the usual situation. INSN_CLOBBERS_REGNO_P (insn, regno) If defined, this is a C expression whose value should be nonzero if the insn insn has the effect of mysteriously clobbering the contents of hard register number regno. By ``mysterious'' we mean that the insn's RTL expression doesn't describe such an effect. If this macro is not defined, it means that no insn clobbers registers mysteriously. This is the usual situation; all else being equal, it is best for the RTL expression to show all the activity. PRESERVE_DEATH_INFO_REGNO_P (regno) If defined, this is a C expression whose value is nonzero if accurate REG_DEAD notes are needed for hard register number regno at the time of outputting the assembler code. When this is so, a few optimizations that take place after register allocation and could invalidate the death notes are not done when this register is involved. You would arrange to preserve death info for a register when some of the code in the machine description which is executed to write the assembler code looks at the death notes. This is necessary only when the actual hardware feature which GNU CC thinks of as a register is not actually a register of the usual sort. (It might, for example, be a hardware stack.) If this macro is not defined, it means that no death notes need to be preserved. This is the usual situation. 16.6. Register Classes On many machines, the numbered registers are not all equivalent. For example, certain registers may not be allowed for indexed addressing; certain registers may not be allowed in some instructions. These machine restrictions 284 Using GNU CC are described to the compiler using register classes. You define a number of register classes, giving each one a name and saying which of the registers belong to it. Then you can specify register classes that are allowed as operands to particular instruction patterns. In general, each register will belong to several classes. In fact, one class must be named ALL_REGS and con- tain all the registers. Another class must be named NO_REGS and contain no registers. Often the union of two classes will be another class; however, this is not required. One of the classes must be named GENERAL_REGS. There is nothing terribly special about the name, but the operand constraint letters `r' and `g' specify this class. If GENERAL_REGS is the same as ALL_REGS, just define it as a macro which expands to ALL_REGS. Order the classes so that if class x is contained in class y then x has a lower class number than y. The way classes other than GENERAL_REGS are specified in operand constraints is through machine-dependent operand constraint letters. You can define such letters to correspond to various classes, then use them in operand con- straints. You should define a class for the union of two classes whenever some instruction allows both classes. For example, if an instruction allows either a floating point (coproces- sor) register or a general register for a certain operand, you should define a class FLOAT_OR_GENERAL_REGS which includes both of them. Otherwise you will get suboptimal code. You must also specify certain redundant information about the register classes: for each class, which classes contain it and which ones are contained in it; for each pair of classes, the largest class contained in their union. When a value occupying several consecutive registers is expected in a certain class, all the registers used must belong to that class. Therefore, register classes cannot be used to enforce a requirement for a register pair to start with an even-numbered register. The way to specify this requirement is with HARD_REGNO_MODE_OK. Register classes used for input-operands of bitwise-and or shift instructions have a special requirement: each such class must have, for each fixed-point machine mode, a sub- class whose registers can transfer that mode to or from memory. For example, on some machines, the operations for Using GNU CC 285 single-byte values (QImode) are limited to certain regis- ters. When this is so, each register class that is used in a bitwise-and or shift instruction must have a subclass con- sisting of registers from which single-byte values can be loaded or stored. This is so that PREFERRED_RELOAD_CLASS can always have a possible value to return. enum reg_class An enumeral type that must be defined with all the register class names as enumeral values. NO_REGS must be first. ALL_REGS must be the last register class, followed by one more enumeral value, LIM_REG_CLASSES, which is not a register class but rather tells how many classes there are. Each register class has a number, which is the value of casting the class name to type int. The number serves as an index in many of the tables described below. N_REG_CLASSES The number of distinct register classes, defined as follows: #define N_REG_CLASSES (int) LIM_REG_CLASSES REG_CLASS_NAMES An initializer containing the names of the register classes as C string constants. These names are used in writing some of the debugging dumps. REG_CLASS_CONTENTS An initializer containing the contents of the register classes, as integers which are bit masks. The nth integer specifies the contents of class n. The way the integer mask is interpreted is that register r is in the class if mask & (1 << r) is 1. When the machine has more than 32 registers, an integer does not suffice. Then the integers are replaced by sub-initializers, braced groupings containing several integers. Each sub-initializer must be suitable as an initializer for the type HARD_REG_SET which is defined in `hard-reg-set.h'. REGNO_REG_CLASS (regno) A C expression whose value is a register class 286 Using GNU CC containing hard register regno. In general there is more that one such class; choose a class which is minimal, meaning that no smaller class also contains the register. BASE_REG_CLASS A macro whose definition is the name of the class to which a valid base register must belong. A base register is one used in an address which is the register value plus a displacement. INDEX_REG_CLASS A macro whose definition is the name of the class to which a valid index register must belong. An index register is one used in an address where its value is either multiplied by a scale factor or added to another register (as well as added to a displacement). REG_CLASS_FROM_LETTER (char) A C expression which defines the machine- dependent operand constraint letters for register classes. If char is such a letter, the value should be the register class corresponding to it. Otherwise, the value should be NO_REGS. REGNO_OK_FOR_BASE_P (num) A C expression which is nonzero if register number num is suitable for use as a base register in operand addresses. It may be either a suitable hard register or a pseudo register that has been allocated such a hard register. REGNO_OK_FOR_INDEX_P (num) A C expression which is nonzero if register number num is suitable for use as an index register in operand addresses. It may be either a suitable hard register or a pseudo register that has been allocated such a hard register. The difference between an index register and a base register is that the index register may be scaled. If an address involves the sum of two registers, neither one of them scaled, then either one may be labeled the ``base'' and the other the ``index''; but whichever labeling is used must fit the machine's constraints of which registers may serve in each capacity. The compiler will try both Using GNU CC 287 labelings, looking for one that is valid, and will reload one or both registers only if neither labeling works. PREFERRED_RELOAD_CLASS (x, class) A C expression that places additional restrictions on the register class to use when it is necessary to copy value x into a register in class class. The value is a register class; perhaps class, or perhaps another, smaller class. On many machines, the definition #define PREFERRED_RELOAD_CLASS(X,CLASS) CLASS is safe. Sometimes returning a more restrictive class makes better code. For example, on the 68000, when x is an integer constant that is in range for a `moveq' instruction, the value of this macro is always DATA_REGS as long as class includes the data registers. Requiring a data register guarantees that a `moveq' will be used. If x is a const_double, by returning NO_REGS you can force x into a memory constant. This is useful on certain machines where immediate floating values cannot be loaded into certain kinds of registers. LIMIT_RELOAD_CLASS (mode, class) A C expression that places additional restrictions on the register class to use when it is necessary to be able to hold a value of mode mode in a reload register for which class class would ordinarily be used. Unlike PREFERRED_RELOAD_CLASS, this macro should be used when there are certain modes that simply can't go in certain reload classes. The value is a register class; perhaps class, or perhaps another, smaller class. Don't define this macro unless the target machine has limitations which require the macro to do something nontrivial. 288 Using GNU CC SECONDARY_RELOAD_CLASS (class, mode, x) SECONDARY_INPUT_RELOAD_CLASS (class, mode, x) SECONDARY_OUTPUT_RELOAD_CLASS (class, mode, x) Many machines have some registers that cannot be copied directly to or from memory or even from other types of registers. An example is the `MQ' register, which on most machines, can only be copied to or from general registers, but not memory. Some machines allow copying all registers to and from memory, but require a scratch register for stores to some memory locations (e.g., those with symbolic address on the RT, and those with certain symbolic address on the Sparc when compiling PIC). In some cases, both an intermediate and a scratch register are required. You should define these macros to indicate to the reload phase that it may need to allocate at least one register for a reload in addition to the register to contain the data. Specifically, if copying x to a register class in mode requires an intermediate register, you should define SECONDARY_INPUT_RELOAD_CLASS to return the largest register class all of whose registers can be used as intermediate registers or scratch registers. If copying a register class in mode to x requires an intermediate or scratch register, you should define SECONDARY_OUTPUT_RELOAD_CLASS to return the largest register class required. If the requirements for input and output reloads are the same, the macro SECONDARY_RELOAD_CLASS should be used instead of defining both macros identically. The values returned by these macros are often GENERAL_REGS. Return NO_REGS if no spare register is needed; i.e., if x can be directly copied to or from a register of class in mode without requiring a scratch register. Do not define this macro if it would always return NO_REGS. If a scratch register is required (either with or without an intermediate register), you should define patterns for `reload_inm' or `reload_outm', as required (see section Standard Names. These patterns, which will Using GNU CC 289 normally be implemented with a define_expand, should be similar to the `movm' patterns, except that operand 2 is the scratch register. Define constraints for the reload register and scratch register that contain a single register class. If the original reload register (whose class is class) can meet the constraint given in the pattern, the value returned by these macros is used for the class of the scratch register. Otherwise, two additional reload registers are required. Their classes are obtained from the constraints in the insn pattern. x might be a pseudo-register or a subreg of a pseudo-register, which could either be in a hard register or in memory. Use true_regnum to find out; it will return -1 if the pseudo is in memory and the hard register number if it is in a register. These macros should not be used in the case where a particular class of registers can only be copied to memory and not to another class of registers. In that case, secondary reload registers are not needed and would not be helpful. Instead, a stack location must be used to perform the copy and the movm pattern should use memory as a intermediate storage. This case often occurs between floating-point and general registers. SMALL_REGISTER_CLASSES Normally the compiler will avoid choosing spill registers from registers that have been explicitly mentioned in the rtl (these registers are normally those used to pass parameters and return values). However, some machines have so few registers of certain classes that there would not be enough registers to use as spill registers if this were done. On those machines, you should define SMALL_REGISTER_CLASSES. When it is defined, the compiler allows registers explicitly used in the rtl to be used as spill registers but prevents the compiler from extending the lifetime of these registers. Defining this macro is always safe, but unnecessarily defining this macro will reduce 290 Using GNU CC the amount of optimizations that can be performed in some cases. If this macro is not defined but needs to be, the compiler will run out of reload registers and print a fatal error message. For most machines, this macro should not be defined. CLASS_MAX_NREGS (class, mode) A C expression for the maximum number of consecutive registers of class class needed to hold a value of mode mode. This is closely related to the macro HARD_REGNO_NREGS. In fact, the value of the macro CLASS_MAX_NREGS (class, mode) should be the maximum value of HARD_REGNO_NREGS (regno, mode) for all regno values in the class class. This macro helps control the handling of multiple-word values in the reload pass. Three other special macros describe which operands fit which constraint letters. CONST_OK_FOR_LETTER_P (value, c) A C expression that defines the machine-dependent operand constraint letters that specify particular ranges of integer values. If c is one of those letters, the expression should check that value, an integer, is in the appropriate range and return 1 if so, 0 otherwise. If c is not one of those letters, the value should be 0 regardless of value. CONST_DOUBLE_OK_FOR_LETTER_P (value, c) A C expression that defines the machine-dependent operand constraint letters that specify particular ranges of const_double values. If c is one of those letters, the expression should check that value, an RTX of code const_double, is in the appropriate range and return 1 if so, 0 otherwise. If c is not one of those letters, the value should be 0 regardless of value. const_double is used for all floating-point constants and for DImode fixed-point constants. A given letter can accept either or both kinds of values. It can use GET_MODE to distinguish Using GNU CC 291 between these kinds. EXTRA_CONSTRAINT (value, c) A C expression that defines the optional machine- dependent constraint letters that can be used to segregate specific types of operands, usually memory references, for the target machine. Normally this macro will not be defined. If it is required for a particular target machine, it should return 1 if value corresponds to the operand type represented by the constraint letter c. If c is not defined as an extra constraint, the value returned should be 0 regardless of value. For example, on the ROMP, load instructions cannot have their output in r0 if the memory reference contains a symbolic address. Constraint letter `Q' is defined as representing a memory address that does not contain a symbolic address. An alternative is specified with a `Q' constraint on the input and `r' on the output. The next alternative specifies `m' on the input and a register class that does not include r0 on the output. 16.7. Describing Stack Layout and Calling Conventions 16.7.1. Basic Stack Layout STACK_GROWS_DOWNWARD Define this macro if pushing a word onto the stack moves the stack pointer to a smaller address. When we say, ``define this macro if ...,'' it means that the compiler checks this macro only with #ifdef so the precise definition used does not matter. FRAME_GROWS_DOWNWARD Define this macro if the addresses of local variable slots are at negative offsets from the frame pointer. ARGS_GROW_DOWNWARD Define this macro if successive arguments to a function occupy decreasing addresses on the stack. STARTING_FRAME_OFFSET Offset from the frame pointer to the first local variable slot to be allocated. 292 Using GNU CC If FRAME_GROWS_DOWNWARD, the next slot's offset is found by subtracting the length of the first slot from STARTING_FRAME_OFFSET. Otherwise, it is found by adding the length of the first slot to the value STARTING_FRAME_OFFSET. STACK_POINTER_OFFSET Offset from the stack pointer register to the first location at which outgoing arguments are placed. If not specified, the default value of zero is used. This is the proper value for most machines. If ARGS_GROW_DOWNWARD, this is the offset to the location above the first location at which outgoing arguments are placed. FIRST_PARM_OFFSET (fundecl) Offset from the argument pointer register to the first argument's address. On some machines it may depend on the data type of the function. If ARGS_GROW_DOWNWARD, this is the offset to the location above the first argument's address. STACK_DYNAMIC_OFFSET (fundecl) Offset from the stack pointer register to an item dynamically allocated on the stack, e.g., by alloca. The default value for this macro is STACK_POINTER_OFFSET plus the length of the outgoing arguments. The default is correct for most machines. See `function.c' for details. DYNAMIC_CHAIN_ADDRESS (frameaddr) A C expression whose value is RTL representing the address in a stack frame where the pointer to the caller's frame is stored. Assume that frameaddr is an RTL expression for the address of the stack frame itself. If you don't define this macro, the default is to return the value of frameaddr---that is, the stack frame address is also the address of the stack word that points to the previous frame. 16.7.2. Registers That Address the Stack Frame STACK_POINTER_REGNUM The register number of the stack pointer register, which must also be a fixed register according to Using GNU CC 293 FIXED_REGISTERS. On most machines, the hardware determines which register this is. FRAME_POINTER_REGNUM The register number of the frame pointer register, which is used to access automatic variables in the stack frame. On some machines, the hardware determines which register this is. On other machines, you can choose any register you wish for this purpose. ARG_POINTER_REGNUM The register number of the arg pointer register, which is used to access the function's argument list. On some machines, this is the same as the frame pointer register. On some machines, the hardware determines which register this is. On other machines, you can choose any register you wish for this purpose. If this is not the same register as the frame pointer register, then you must mark it as a fixed register according to FIXED_REGISTERS, or arrange to be able to eliminate it (see section Elimination). STATIC_CHAIN_REGNUM STATIC_CHAIN_INCOMING_REGNUM Register numbers used for passing a function's static chain pointer. If register windows are used, STATIC_CHAIN_INCOMING_REGNUM is the register number as seen by the called function, while STATIC_CHAIN_REGNUM is the register number as seen by the calling function. If these registers are the same, STATIC_CHAIN_INCOMING_REGNUM need not be defined. The static chain register need not be a fixed register. If the static chain is passed in memory, these macros should not be defined; instead, the next two macros should be defined. STATIC_CHAIN STATIC_CHAIN_INCOMING If the static chain is passed in memory, these macros provide rtx giving mem expressions that denote where they are stored. STATIC_CHAIN and STATIC_CHAIN_INCOMING give the locations as seen by the calling and called functions, respectively. Often the former will be at an offset from the stack pointer and the latter at an offset from the 294 Using GNU CC frame pointer. The variables stack_pointer_rtx, frame_pointer_rtx, and arg_pointer_rtx will have been initialized prior to the use of these macros and should be used to refer to those items. If the static chain is passed in a register, the two previous macros should be defined instead. 16.7.3. Eliminating Frame Pointer and Arg Pointer FRAME_POINTER_REQUIRED A C expression which is nonzero if a function must have and use a frame pointer. This expression is evaluated in the reload pass. If its value is nonzero the function will have a frame pointer. The expression can in principle examine the current function and decide according to the facts, but on most machines the constant 0 or the constant 1 suffices. Use 0 when the machine allows code to be generated with no frame pointer, and doing so saves some time or space. Use 1 when there is no possible advantage to avoiding a frame pointer. In certain cases, the compiler does not know how to produce valid code without a frame pointer. The compiler recognizes those cases and automatically gives the function a frame pointer regardless of what FRAME_POINTER_REQUIRED says. You don't need to worry about them. In a function that does not require a frame pointer, the frame pointer register can be allocated for ordinary usage, unless you mark it as a fixed register. See FIXED_REGISTERS for more information. This macro is ignored and need not be defined if ELIMINABLE_REGS is defined. INITIAL_FRAME_POINTER_OFFSET (depth-var) A C statement to store in the variable depth-var the difference between the frame pointer and the stack pointer values immediately after the function prologue. The value would be computed from information such as the result of get_frame_size () and the tables of registers regs_ever_live and call_used_regs. Using GNU CC 295 If ELIMINABLE_REGS is defined, this macro will be not be used and need not be defined. Otherwise, it must be defined even if FRAME_POINTER_REQUIRED is defined to always be true; in that case, you may set depth-var to anything. ELIMINABLE_REGS If defined, this macro specifies a table of register pairs used to eliminate unneeded registers that point into the stack frame. If it is not defined, the only elimination attempted by the compiler is to replace references to the frame pointer with references to the stack pointer. The definition of this macro is a list of structure initializations, each of which specifies an original and replacement register. On some machines, the position of the argument pointer is not known until the compilation is completed. In such a case, a separate hard register must be used for the argument pointer. This register can be eliminated by replacing it with either the frame pointer or the argument pointer, depending on whether or not the frame pointer has been eliminated. In this case, you might specify: #define ELIMINABLE_REGS \ {{ARG_POINTER_REGNUM, STACK_POINTER_REGNUM}, \ {ARG_POINTER_REGNUM, FRAME_POINTER_REGNUM}, \ {FRAME_POINTER_REGNUM, STACK_POINTER_REGNUM}} Note that the elimination of the argument pointer with the stack pointer is specified first since that is the preferred elimination. CAN_ELIMINATE (from-reg, to-reg) A C expression that returns non-zero if the compiler is allowed to try to replace register number from-reg with register number to-reg. This macro need only be defined if ELIMINABLE_REGS is defined, and will usually be the constant 1, since most of the cases preventing register elimination are things that the compiler already knows about. INITIAL_ELIMINATION_OFFSET (from-reg, to- reg, offset-var) 296 Using GNU CC This macro is similar to INITIAL_FRAME_POINTER_OFFSET. It specifies the initial difference between the specified pair of registers. This macro must be defined if ELIMINABLE_REGS is defined. LONGJMP_RESTORE_FROM_STACK Define this macro if the longjmp function restores registers from the stack frames, rather than from those saved specifically by setjmp. Certain quantities must not be kept in registers across a call to setjmp on such machines. 16.7.4. Passing Function Arguments on the Stack The macros in this section control how arguments are passed on the stack. See the following section for other macros that control passing certain arguments in registers. PROMOTE_PROTOTYPES Define this macro if an argument declared as char or short in a prototype should actually be passed as an int. In addition to avoiding errors in certain cases of mismatch, it also makes for better code on certain machines. PUSH_ROUNDING (npushed) A C expression that is the number of bytes actually pushed onto the stack when an instruction attempts to push npushed bytes. If the target machine does not have a push instruction, do not define this macro. That directs GNU CC to use an alternate strategy: to allocate the entire argument block and then store the arguments into it. On some machines, the definition #define PUSH_ROUNDING(BYTES) (BYTES) will suffice. But on other machines, instructions that appear to push one byte actually push two bytes in an attempt to maintain alignment. Then the definition should be #define PUSH_ROUNDING(BYTES) (((BYTES) + 1) & ~1) Using GNU CC 297 ACCUMULATE_OUTGOING_ARGS If defined, the maximum amount of space required for outgoing arguments will be computed and placed into the variable current_function_outgoing_args_size. No space will be pushed onto the stack for each call; instead, the function prologue should increase the stack frame size by this amount. It is not proper to define both PUSH_ROUNDING and ACCUMULATE_OUTGOING_ARGS. REG_PARM_STACK_SPACE Define this macro if functions should assume that stack space has been allocated for arguments even when their values are passed in registers. The value of this macro is the size, in bytes, of the area reserved for arguments passed in registers. This space can either be allocated by the caller or be a part of the machine-dependent stack frame: OUTGOING_REG_PARM_STACK_SPACE says which. OUTGOING_REG_PARM_STACK_SPACE Define this if it is the responsibility of the caller to allocate the area reserved for arguments passed in registers. If ACCUMULATE_OUTGOING_ARGS is defined, this macro controls whether the space for these arguments counts in the value of current_function_outgoing_args_size. STACK_PARMS_IN_REG_PARM_AREA Define this macro if REG_PARM_STACK_SPACE is defined but stack parameters don't skip the area specified by REG_PARM_STACK_SPACE. Normally, when a parameter is not passed in registers, it is placed on the stack beyond the REG_PARM_STACK_SPACE area. Defining this macro suppresses this behavior and causes the parameter to be passed on the stack in its natural location. RETURN_POPS_ARGS (funtype, stack-size) A C expression that should indicate the number 298 Using GNU CC of bytes of its own arguments that a function pops on returning, or 0 if the function pops no arguments and the caller must therefore pop them all after the function returns. funtype is a C variable whose value is a tree node that describes the function in question. Normally it is a node of type FUNCTION_TYPE that describes the data type of the function. From this it is possible to obtain the data types of the value and arguments (if known). When a call to a library function is being considered, funtype will contain an identifier node for the library function. Thus, if you need to distinguish among various library functions, you can do so by their names. Note that ``library function'' in this context means a function used to perform arithmetic, whose name is known specially in the compiler and was not mentioned in the C code being compiled. stack-size is the number of bytes of arguments passed on the stack. If a variable number of bytes is passed, it is zero, and argument popping will always be the responsibility of the calling function. On the Vax, all functions always pop their arguments, so the definition of this macro is stack-size. On the 68000, using the standard calling convention, no functions pop their arguments, so the value of the macro is always 0 in this case. But an alternative calling convention is available in which functions that take a fixed number of arguments pop them but other functions (such as printf) pop nothing (the caller pops all). When this convention is in use, funtype is examined to determine whether a function takes a fixed number of arguments. 16.7.5. Passing Arguments in Registers This section describes the macros which let you control how various types of arguments are passed in registers or how they are arranged in the stack. FUNCTION_ARG (cum, mode, type, named) A C expression that controls whether a function argument is passed in a register, and which Using GNU CC 299 register. The arguments are cum, which summarizes all the previous arguments; mode, the machine mode of the argument; type, the data type of the argument as a tree node or 0 if that is not known (which happens for C support library functions); and named, which is 1 for an ordinary argument and 0 for nameless arguments that correspond to `...' in the called function's prototype. The value of the expression should either be a reg RTX for the hard register in which to pass the argument, or zero to pass the argument on the stack. For machines like the Vax and 68000, where normally all arguments are pushed, zero suffices as a definition. The usual way to make the ANSI library `stdarg.h' work on a machine where some arguments are usually passed in registers, is to cause nameless arguments to be passed on the stack instead. This is done by making FUNCTION_ARG return 0 whenever named is 0. You may use the macro MUST_PASS_IN_STACK (mode, type) in the definition of this macro to determine if this argument is of a type that must be passed in the stack. If REG_PARM_STACK_SPACE is not defined and FUNCTION_ARG returns non-zero for such an argument, the compiler will abort. If REG_PARM_STACK_SPACE is defined, the argument will be computed in the stack and then loaded into a register. FUNCTION_INCOMING_ARG (cum, mode, type, named) Define this macro if the target machine has ``register windows'', so that the register in which a function sees an arguments is not necessarily the same as the one in which the caller passed the argument. For such machines, FUNCTION_ARG computes the register in which the caller passes the value, and FUNCTION_INCOMING_ARG should be defined in a similar fashion to tell the function being called where the arguments will arrive. If FUNCTION_INCOMING_ARG is not defined, FUNCTION_ARG serves both purposes. 300 Using GNU CC FUNCTION_ARG_PARTIAL_NREGS (cum, mode, type, named) A C expression for the number of words, at the beginning of an argument, must be put in registers. The value must be zero for arguments that are passed entirely in registers or that are entirely pushed on the stack. On some machines, certain arguments must be passed partially in registers and partially in memory. On these machines, typically the first n words of arguments are passed in registers, and the rest on the stack. If a multi-word argument (a double or a structure) crosses that boundary, its first few words must be passed in registers and the rest must be pushed. This macro tells the compiler when this occurs, and how many of the words should go in registers. FUNCTION_ARG for these arguments should return the first register to be used by the caller for this argument; likewise FUNCTION_INCOMING_ARG, for the called function. FUNCTION_ARG_PASS_BY_REFERENCE (cum, mode, type, named) A C expression that indicates when an argument must be passed by reference. If nonzero for an argument, a copy of that argument is made in memory and a pointer to the argument is passed instead of the argument itself. The pointer is passed in whatever way is appropriate for passing a pointer to that type. On machines where REG_PARM_STACK_SPACE is not defined, a suitable definition of this macro might be #define FUNCTION_ARG_PASS_BY_REFERENCE(CUM, MODE, TYPE, NAMED) \ MUST_PASS_IN_STACK (MODE, TYPE) CUMULATIVE_ARGS A C type for declaring a variable that is used as the first argument of FUNCTION_ARG and other related values. For some target machines, the type int suffices and can hold the number of bytes of argument so far. There is no need to record in CUMULATIVE_ARGS anything about the arguments that have been passed on the stack. The compiler has other variables to keep track of that. For target Using GNU CC 301 machines on which all arguments are passed on the stack, there is no need to store anything in CUMULATIVE_ARGS; however, the data structure must exist and should not be empty, so use int. INIT_CUMULATIVE_ARGS (cum, fntype, libname) A C statement (sans semicolon) for initializing the variable cum for the state at the beginning of the argument list. The variable has type CUMULATIVE_ARGS. The value of fntype is the tree node for the data type of the function which will receive the args, or 0 if the args are to a compiler support library function. When processing a call to a compiler support library function, libname identifies which one. It is a symbol_ref rtx which contains the name of the function, as a string. libname is 0 when an ordinary C function call is being processed. Thus, each time this macro is called, either libname or fntype is nonzero, but never both of them at once. INIT_CUMULATIVE_INCOMING_ARGS (cum, fntype, libname) Like INIT_CUMULATIVE_ARGS but overrides it for the purposes of finding the arguments for the function being compiled. If this macro is undefined, INIT_CUMULATIVE_ARGS is used instead. The argument libname exists for symmetry with INIT_CUMULATIVE_ARGS. The value passed for libname is always 0, since library routines with special calling conventions are never compiled with GNU CC. FUNCTION_ARG_ADVANCE (cum, mode, type, named) A C statement (sans semicolon) to update the summarizer variable cum to advance past an argument in the argument list. The values mode, type and named describe that argument. Once this is done, the variable cum is suitable for analyzing the following argument with FUNCTION_ARG, etc. This macro need not do anything if the argument in question was passed on the stack. The compiler knows how to track the amount of stack space used for arguments without any special help. 302 Using GNU CC FUNCTION_ARG_PADDING (mode, type) If defined, a C expression which determines whether, and in which direction, to pad out an argument with extra space. The value should be of type enum direction: either upward to pad above the argument, downward to pad below, or none to inhibit padding. This macro does not control the amount of padding; that is always just enough to reach the next multiple of FUNCTION_ARG_BOUNDARY. This macro has a default definition which is right for most systems. For little-endian machines, the default is to pad upward. For big-endian machines, the default is to pad downward for an argument of constant size shorter than an int, and upward otherwise. FUNCTION_ARG_BOUNDARY (mode, type) If defined, a C expression that gives the alignment boundary, in bits, of an argument with the specified mode and type. If it is not defined, PARM_BOUNDARY is used for all arguments. FUNCTION_ARG_REGNO_P (regno) A C expression that is nonzero if regno is the number of a hard register in which function arguments are sometimes passed. This does not include implicit arguments such as the static chain and the structure-value address. On many machines, no registers can be used for this purpose since all function arguments are pushed on the stack. 16.7.6. How Scalar Function Values Are Returned This section discusses the macros that control return- ing scalars as values---values that can fit in registers. TRADITIONAL_RETURN_FLOAT Define this macro if `-traditional' should not cause functions declared to return float to convert the value to double. FUNCTION_VALUE (valtype, func) A C expression to create an RTX representing the place where a function returns a value of data type valtype. valtype is a tree node representing a data type. Write TYPE_MODE (valtype) to get the machine mode used to represent that type. On many Using GNU CC 303 machines, only the mode is relevant. (Actually, on most machines, scalar values are returned in the same place regardless of mode). If the precise function being called is known, func is a tree node (FUNCTION_DECL) for it; otherwise, func is a null pointer. This makes it possible to use a different value-returning convention for specific functions when all their calls are known. FUNCTION_VALUE is not used for return vales with aggregate data types, because these are returned in another way. See STRUCT_VALUE_REGNUM and related macros, below. FUNCTION_OUTGOING_VALUE (valtype, func) Define this macro if the target machine has ``register windows'' so that the register in which a function returns its value is not the same as the one in which the caller sees the value. For such machines, FUNCTION_VALUE computes the register in which the caller will see the value, and FUNCTION_OUTGOING_VALUE should be defined in a similar fashion to tell the function where to put the value. If FUNCTION_OUTGOING_VALUE is not defined, FUNCTION_VALUE serves both purposes. FUNCTION_OUTGOING_VALUE is not used for return vales with aggregate data types, because these are returned in another way. See STRUCT_VALUE_REGNUM and related macros, below. LIBCALL_VALUE (mode) A C expression to create an RTX representing the place where a library function returns a value of mode mode. If the precise function being called is known, func is a tree node (FUNCTION_DECL) for it; otherwise, func is a null pointer. This makes it possible to use a different value-returning convention for specific functions when all their calls are known. Note that ``library function'' in this context means a compiler support routine, used to perform arithmetic, whose name is known specially by the compiler and was not mentioned in the C code being compiled. 304 Using GNU CC The definition of LIBRARY_VALUE need not be concerned aggregate data types, because none of the library functions returns such types. FUNCTION_VALUE_REGNO_P (regno) A C expression that is nonzero if regno is the number of a hard register in which the values of called function may come back. A register whose use for returning values is limited to serving as the second of a pair (for a value of type double, say) need not be recognized by this macro. So for most machines, this definition suffices: #define FUNCTION_VALUE_REGNO_P(N) ((N) == 0) If the machine has register windows, so that the caller and the called function use different registers for the return value, this macro should recognize only the caller's register numbers. 16.7.7. How Large Values Are Returnd When a function value's mode is BLKmode (and in some other cases), the value is not returned according to FUNCTION_VALUE (see section Scalar Return). Instead, the caller passes the address of a block of memory in which the value should be stored. This address is called the struc- ture value address. This section describes how to control returning struc- ture values in memory. RETURN_IN_MEMORY (type) A C expression which can inhibit the returning of certain function values in registers, based on the type of value. A nonzero value says to return the function value in memory, just as large structures are always returned. Here type will be a C expression of type tree, representing the data type of the value. Note that values of mode BLKmode are returned in memory regardless of this macro. Also, the option `-fpcc-struct-return' takes effect regardless of this macro. On most systems, it is possible to leave the macro undefined; this causes a default Using GNU CC 305 definition to be used, whose value is the constant 0. STRUCT_VALUE_REGNUM If the structure value address is passed in a register, then STRUCT_VALUE_REGNUM should be the number of that register. STRUCT_VALUE If the structure value address is not passed in a register, define STRUCT_VALUE as an expression returning an RTX for the place where the address is passed. If it returns 0, the address is passed as an ``invisible'' first argument. STRUCT_VALUE_INCOMING_REGNUM On some architectures the place where the structure value address is found by the called function is not the same place that the caller put it. This can be due to register windows, or it could be because the function prologue moves it to a different place. If the incoming location of the structure value address is in a register, define this macro as the register number. STRUCT_VALUE_INCOMING If the incoming location is not a register, define STRUCT_VALUE_INCOMING as an expression for an RTX for where the called function should find the value. If it should find the value on the stack, define this to create a mem which refers to the frame pointer. A definition of 0 means that the address is passed as an ``invisible'' first argument. PCC_STATIC_STRUCT_RETURN Define this macro if the usual system convention on the target machine for returning structures and unions is for the called function to return the address of a static variable containing the value. GNU CC does not normally use this convention, even if it is the usual one, but does use it if `- fpcc-struct-value' is specified. Do not define this if the usual system convention is for the caller to pass an address to the subroutine. 306 Using GNU CC 16.7.8. Caller-Saves Register Allocation If you enable it, GNU CC can save registers around function calls. This makes it possible to use call- clobbered registers to hold variables that must live across calls. DEFAULT_CALLER_SAVES Define this macro if function calls on the target machine do not preserve any registers; in other words, if CALL_USED_REGISTERS has 1 for all registers. This macro enables `-fcaller-saves' by default. Eventually that option will be enabled by default on all machines and both the option and this macro will be eliminated. CALLER_SAVE_PROFITABLE (refs, calls) A C expression to determine whether it is worthwhile to consider placing a pseudo-register in a call-clobbered hard register and saving and restoring it around each function call. The expression should be 1 when this is worth doing, and 0 otherwise. If you don't define this macro, a default is used which is good on most machines: 4 * calls < refs. 16.7.9. Function Entry and Exit This section describes the macros that output function entry (prologue) and exit (epilogue) code. FUNCTION_PROLOGUE (file, size) A C compound statement that outputs the assembler code for entry to a function. The prologue is responsible for setting up the stack frame, initializing the frame pointer register, saving registers that must be saved, and allocating size additional bytes of storage for the local variables. size is an integer. file is a stdio stream to which the assembler code should be output. The label for the beginning of the function need not be output by this macro. That has already been done when the macro is run. To determine which registers to save, the macro can refer to the array regs_ever_live: element r is nonzero if hard register r is used anywhere within the function. This implies the function prologue should save register r, provided it is Using GNU CC 307 not one of the call-used registers. (FUNCTION_EPILOGUE must likewise use regs_ever_live.) On machines that have ``register windows'', the function entry code does not save on the stack the registers that are in the windows, even if they are supposed to be preserved by function calls; instead it takes appropriate steps to ``push'' the register stack, if any non-call-used registers are used in the function. On machines where functions may or may not have frame-pointers, the function entry code must vary accordingly; it must set up the frame pointer if one is wanted, and not otherwise. To determine whether a frame pointer is in wanted, the macro can refer to the variable frame_pointer_needed. The variable's value will be 1 at run time in a function that needs a frame pointer. See section Elimination. The function entry code is responsible for allocating any stack space required for the function. This stack space consists of the regions listed below. In most cases, these regions are allocated in the order listed, with the last listed region closest to the top of the stack (the lowest address if STACK_GROWS_DOWNWARD is defined, and the highest address if it is not defined). You can use a different order for a machine if doing so is more convenient or required for compatibility reasons. Except in cases where required by standard or by a debugger, there is no reason why the stack layout used by GCC need agree with that used by other compilers for a machine. o+ A region of current_function_pretend_args_size bytes of uninitialized space just underneath the first argument arriving on the stack. (This may not be at the very start of the allocated stack region if the calling sequence has pushed anything else since pushing the stack arguments. But usually, on such machines, nothing else has been pushed yet, because the function prologue itself does all the pushing.) This region is used on machines where an argument may be passed partly in registers and partly in memory, and, in some cases to support the features in `varargs.h' and `stdargs.h'. 308 Using GNU CC o+ An area of memory used to save certain registers used by the function. The size of this area, which may also include space for such things as the return address and pointers to previous stack frames, is machine-specific and usually depends on which registers have been used in the function. Machines with register windows often do not require a save area. o+ A region of at least size bytes, possibly rounded up to an allocation boundary, to contain the local variables of the function. On some machines, this region and the save area may occur in the opposite order, with the save area closer to the top of the stack. o+ Optionally, in the case that ACCUMULATE_OUTGOING_ARGS is defined, a region of current_function_outgoing_args_size bytes to be used for outgoing argument lists of the function. See section Stack Arguments. Normally, it is necessary for FUNCTION_PROLOGUE and FUNCTION_EPILOGUE to treat leaf functions specially. The C variable leaf_function is nonzero for such a function. EXIT_IGNORE_STACK Define this macro as a C expression that is nonzero if the return instruction or the function epilogue ignores the value of the stack pointer; in other words, if it is safe to delete an instruction to adjust the stack pointer before a return from the function. Note that this macro's value is relevant only for functions for which frame pointers are maintained. It is never safe to delete a final stack adjustment in a function that has no frame pointer, and the compiler knows this regardless of EXIT_IGNORE_STACK. FUNCTION_EPILOGUE (file, size) A C compound statement that outputs the assembler code for exit from a function. The epilogue is responsible for restoring the saved registers and stack pointer to their values when the function was called, and returning control to the caller. This macro takes the same arguments as the macro FUNCTION_PROLOGUE, and the registers to restore are determined from regs_ever_live and Using GNU CC 309 CALL_USED_REGISTERS in the same way. On some machines, there is a single instruction that does all the work of returning from the function. On these machines, give that instruction the name `return' and do not define the macro FUNCTION_EPILOGUE at all. Do not define a pattern named `return' if you want the FUNCTION_EPILOGUE to be used. If you want the target switches to control whether return instructions or epilogues are used, define a `return' pattern with a validity condition that tests the target switches appropriately. If the `return' pattern's validity condition is false, epilogues will be used. On machines where functions may or may not have frame-pointers, the function exit code must vary accordingly. Sometimes the code for these two cases is completely different. To determine whether a frame pointer is in wanted, the macro can refer to the variable frame_pointer_needed. The variable's value will be 1 at run time in a function that needs a frame pointer. Normally, it is necessary for FUNCTION_PROLOGUE and FUNCTION_EPILOGUE to treat leaf functions specially. The C variable leaf_function is nonzero for such a function. See section Leaf Functions. On some machines, some functions pop their arguments on exit while others leave that for the caller to do. For example, the 68020 when given `-mrtd' pops arguments in functions that take a fixed number of arguments. Your definition of the macro RETURN_POPS_ARGS decides which functions pop their own arguments. FUNCTION_EPILOGUE needs to know what was decided. The variable current_function_pops_args is the number of bytes of its arguments that a function should pop. See section Scalar Return. DELAY_SLOTS_FOR_EPILOGUE Define this macro if the function epilogue contains delay slots to which instructions from the rest of the function can be ``moved''. The definition should be a C expression whose value is an integer representing the number of delay slots there. 310 Using GNU CC ELIGIBLE_FOR_EPILOGUE_DELAY (insn, n) A C expression that returns 1 if insn can be placed in delay slot number n of the epilogue. The argument n is an integer which identifies the delay slot now being considered (since different slots may have different rules of eligibility). It is never negative and is always less than the number of epilogue delay slots (what DELAY_SLOTS_FOR_EPILOGUE returns). If you reject a particular insn for a given delay slot, in principle, it may be reconsidered for a subsequent delay slot. Also, other insns may (at least in principle) be considered for the so far unfilled delay slot. The insns accepted to fill the epilogue delay slots are put in an RTL list made with insn_list objects, stored in the variable current_function_epilogue_delay_list. The insn for the first delay slot comes first in the list. Your definition of the macro FUNCTION_EPILOGUE should fill the delay slots by outputting the insns in this list, usually by calling final_scan_insn. You need not define this macro if you did not define DELAY_SLOTS_FOR_EPILOGUE. 16.7.10. Generating Code for Profiling FUNCTION_PROFILER (file, labelno) A C statement or compound statement to output to file some assembler code to call the profiling subroutine mcount. Before calling, the assembler code must load the address of a counter variable into a register where mcount expects to find the address. The name of this variable is `LP' followed by the number labelno, so you would generate the name using `LP%d' in a fprintf. The details of how the address should be passed to mcount are determined by your operating system environment, not by GNU CC. To figure them out, compile a small program for profiling using the system's installed C compiler and look at the assembler code that results. PROFILE_BEFORE_PROLOGUE Define this macro if the code for function profiling should come before the function prologue. Normally, the profiling code comes Using GNU CC 311 after. FUNCTION_BLOCK_PROFILER (file, labelno) A C statement or compound statement to output to file some assembler code to initialize basic-block profiling for the current object module. This code should call the subroutine __bb_init_func once per object module, passing it as its sole argument the address of a block allocated in the object module. The name of the block is a local symbol made with this statement: ASM_GENERATE_INTERNAL_LABEL (buffer, "LPBX", 0); Of course, since you are writing the definition of ASM_GENERATE_INTERNAL_LABEL as well as that of this macro, you can take a short cut in the definition of this macro and use the name that you know will result. The first word of this block is a flag which will be nonzero if the object module has already been initialized. So test this word first, and do not call __bb_init_func if the flag is nonzero. BLOCK_PROFILER (file, blockno) A C statement or compound statement to increment the count associated with the basic block number blockno. Basic blocks are numbered separately from zero within each compilation. The count associated with block number blockno is at index blockno in a vector of words; the name of this array is a local symbol made with this statement: ASM_GENERATE_INTERNAL_LABEL (buffer, "LPBX", 2); Of course, since you are writing the definition of ASM_GENERATE_INTERNAL_LABEL as well as that of this macro, you can take a short cut in the definition of this macro and use the name that you know will result. 312 Using GNU CC 16.8. Implementing the Varargs Macros GNU CC comes with an implementation of `varargs.h' and `stdarg.h' that work without change on machines that pass arguments on the stack. Other machines require their own implementations of varargs, and the two machine independent header files must have conditionals to include it. ANSI `stdarg.h' differs from traditional `varargs.h' mainly in the calling convention for va_start. The tradi- tional implementation takes just one argument, which is the variable in which to store the argument pointer. The ANSI implementation takes an additional first argument, which is the last named argument of the function. However, it should not use this argument. The way to find the end of the named arguments is with the built-in functions described below. __builtin_saveregs () Use this built-in function to save the argument registers in memory so that the varargs mechanism can access them. Both ANSI and traditional versions of va_start must use __builtin_saveregs, unless you use SETUP_INCOMING_VARARGS (see below) instead. On some machines, __builtin_saveregs is open-coded under the control of the macro EXPAND_BUILTIN_SAVEREGS. On other machines, it calls a routine written in assembler language, found in `libgcc2.c'. Regardless of what code is generated for the call to __builtin_saveregs, it appears at the beginning of the function, not where the call to __builtin_saveregs is written. This is because the registers must be saved before the function starts to use them for its own purposes. __builtin_args_info (category) Use this built-in function to find the first anonymous arguments in registers. In general, a machine may have several categories of registers used for arguments, each for a particular category of data types. (For example, on some machines, floating-point registers are used for floating-point arguments while other arguments are passed in the general registers.) To make non-varargs functions use the proper calling convention, you have defined the CUMULATIVE_ARGS data type to record how many registers in each category have been used so far Using GNU CC 313 __builtin_args_info accesses the same data structure of type CUMULATIVE_ARGS after the ordinary argument layout is finished with it, with category specifying which word to access. Thus, the value indicates the first unused register in a given category. Normally, you would use __builtin_args_info in the implementation of va_start, accessing each category just once and storing the value in the va_list object. This is because va_list will have to update the values, and there is no way to alter the values accessed by __builtin_args_info. __builtin_next_arg () This is the equivalent of __builtin_args_info, for stack arguments. It returns the address of the first anonymous stack argument, as type void *. If ARGS_GROW_DOWNWARD, it returns the address of the location above the first anonymous stack argument. Use it in va_start to initialize the pointer for fetching arguments from the stack. __builtin_classify_type (object) Since each machine has its own conventions for which data types are passed in which kind of register, your implementation of va_arg has to embody these conventions. The easiest way to categorize the specified data type is to use __builtin_classify_type together with sizeof and __alignof__. __builtin_classify_type ignores the value of object, considering only its data type. It returns an integer describing what kind of type that is---integer, floating, pointer, structure, and so on. The file `typeclass.h' defines an enumeration that you can use to interpret the values of __builtin_classify_type. These machine description macros help implement varargs: EXPAND_BUILTIN_SAVEREGS (args) If defined, is a C expression that produces the machine-specific code for a call to __builtin_saveregs. This code will be moved to the very beginning of the function, before any parameter access are made. The return value of this function should be an RTX that contains the 314 Using GNU CC value to use as the return of __builtin_saveregs. The argument args is a tree_list containing the arguments that were passed to __builtin_saveregs. If this macro is not defined, the compiler will output an ordinary call to the library function `__builtin_saveregs'. SETUP_INCOMING_VARARGS (args_so_far, mode, type, pretend_args_size, second_time) This macro offers an alternative to using __builtin_saveregs and defining the macro EXPAND_BUILTIN_SAVEREGS. Use it to store the anonymous register arguments into the stack so that all the arguments appear to have been passed consecutively on the stack. Once this is done, you can use the standard implementation of varargs that works for machines that pass all their arguments on the stack. The argument args_so_far is the CUMULATIVE_ARGS data structure, containing the values that obtain after processing of the named arguments. The arguments mode and type describe the last named argument---its machine mode and its data type as a tree node. The macro implementation should do two things: first, push onto the stack all the argument registers not used for the named arguments, and second, store the size of the data thus pushed into the int-valued variable whose name is supplied as the argument pretend_args_size. The value that you store here will serve as additional offset for setting up the stack frame. Because you must generate code to push the anonymous arguments at compile time without knowing their data types, SETUP_INCOMING_VARARGS is only useful on machines that have just a single category of argument register and use it uniformly for all data types. If the argument second_time is nonzero, it means that the arguments of the function are being analyzed for the second time. This happens for an inline function, which is not actually compiled until the end of the source file. The macro SETUP_INCOMING_VARARGS should not generate any instructions in this case. Using GNU CC 315 16.9. Trampolines for Nested Functions A trampoline is a small piece of code that is created at run time when the address of a nested function is taken. It normally resides on the stack, in the stack frame of the containing function. These macros tell GNU CC how to gen- erate code to allocate and initialize a trampoline. The instructions in the trampoline must do two things: load a constant address into the static chain register, and jump to the real address of the nested function. On CISC machines such as the m68k, this requires two instructions, a move immediate and a jump. Then the two addresses exist in the trampoline as word-long immediate operands. On RISC machines, it is often necessary to load each address into a register in two parts. Then pieces of each address form separate immediate operands. The code generated to initialize the trampoline must store the variable parts---the static chain value and the function address---into the immediate operands of the instructions. On a CISC machine, this is simply a matter of copying each address to a memory reference at the proper offset from the start of the trampoline. On a RISC machine, it may be necessary to take out pieces of the address and store them separately. TRAMPOLINE_TEMPLATE (file) A C statement to output, on the stream file, assembler code for a block of data that contains the constant parts of a trampoline. This code should not include a label---the label is taken care of automatically. TRAMPOLINE_SIZE A C expression for the size in bytes of the trampoline, as an integer. TRAMPOLINE_ALIGNMENT Alignment required for trampolines, in bits. If you don't define this macro, the value of BIGGEST_ALIGNMENT is used for aligning trampolines. INITIALIZE_TRAMPOLINE (addr, fnaddr, static_chain) A C statement to initialize the variable parts of a trampoline. addr is an RTX for the address of the trampoline; fnaddr is an RTX for the address of the nested function; static_chain is an RTX for the static chain value that should be passed to the function when it is called. 316 Using GNU CC ALLOCATE_TRAMPOLINE (fp) A C expression to allocate run-time space for a trampoline. The expression value should be an RTX representing a memory reference to the space for the trampoline. If this macro is not defined, by default the trampoline is allocated as a stack slot. This default is right for most machines. The exceptions are machines where it is impossible to execute instructions in the stack area. On such machines, you may have to implement a separate stack, using this macro in conjunction with FUNCTION_PROLOGUE and FUNCTION_EPILOGUE. fp points to a data structure, a struct function, which describes the compilation status of the immediate containing function of the function which the trampoline is for. Normally (when ALLOCATE_TRAMPOLINE is not defined), the stack slot for the trampoline is in the stack frame of this containing function. Other allocation strategies probably must do something analogous with this information. Implementing trampolines is difficult on many machines because they have separate instruction and data caches. Writing into a stack location fails to clear the memory in the instruction cache, so when the program jumps to that location, it executes the old contents. Here are two possible solutions. One is to clear the relevant parts of the instruction cache whenever a trampo- line is set up. The other is to make all trampolines ident- ical, by having them jump to a standard subroutine. The former technique makes trampoline execution faster; the latter makes initialization faster. To clear the instruction cache when a trampoline is initialized, define the following macros which describe the shape of the cache. INSN_CACHE_SIZE The total size in bytes of the cache. INSN_CACHE_LINE_WIDTH The length in bytes of each cache line. The cache is divided into cache lines which are disjoint slots, each holding a contiguous chunk of data fetched from memory. Each time data is brought into the cache, an entire line is read at once. The data loaded into a cache line is always Using GNU CC 317 aligned on a boundary equal to the line size. INSN_CACHE_DEPTH The number of alternative cache lines that can hold any particular memory location. To use a standard subroutine, define the following macro. In addition, you must make sure that the instruc- tions in a trampoline fill an entire cache line with identi- cal instructions, or else ensure that the beginning of the trampoline code is always aligned at the same point in its cache line. Look in `m68k.h' as a guide. TRANSFER_FROM_TRAMPOLINE Define this macro if trampolines need a special subroutine to do their work. The macro should expand to a series of asm statements which will be compiled with GNU CC. They go in a library function named __transfer_from_trampoline. If you need to avoid executing the ordinary prologue code of a compiled C function when you jump to the subroutine, you can do so by placing a special label of your own in the assembler code. Use one asm statement to generate an assembler label, and another to make the label global. Then trampolines can use that label to jump directly to your special assembler code. 16.10. Implicit Calls to Library Routines MULSI3_LIBCALL A C string constant giving the name of the function to call for multiplication of one signed full-word by another. If you do not define this macro, the default name is used, which is __mulsi3, a function defined in `libgcc.a'. DIVSI3_LIBCALL A C string constant giving the name of the function to call for division of one signed full- word by another. If you do not define this macro, the default name is used, which is __divsi3, a function defined in `libgcc.a'. UDIVSI3_LIBCALL A C string constant giving the name of the function to call for division of one unsigned full-word by another. If you do not define this macro, the default name is used, which is __udivsi3, a function defined in `libgcc.a'. 318 Using GNU CC MODSI3_LIBCALL A C string constant giving the name of the function to call for the remainder in division of one signed full-word by another. If you do not define this macro, the default name is used, which is __modsi3, a function defined in `libgcc.a'. UMODSI3_LIBCALL A C string constant giving the name of the function to call for the remainder in division of one unsigned full-word by another. If you do not define this macro, the default name is used, which is __umodsi3, a function defined in `libgcc.a'. MULDI3_LIBCALL A C string constant giving the name of the function to call for multiplication of one signed double-word by another. If you do not define this macro, the default name is used, which is __muldi3, a function defined in `libgcc.a'. DIVDI3_LIBCALL A C string constant giving the name of the function to call for division of one signed double-word by another. If you do not define this macro, the default name is used, which is __divdi3, a function defined in `libgcc.a'. UDIVDI3_LIBCALL A C string constant giving the name of the function to call for division of one unsigned full-word by another. If you do not define this macro, the default name is used, which is __udivdi3, a function defined in `libgcc.a'. MODDI3_LIBCALL A C string constant giving the name of the function to call for the remainder in division of one signed double-word by another. If you do not define this macro, the default name is used, which is __moddi3, a function defined in `libgcc.a'. UMODDI3_LIBCALL A C string constant giving the name of the function to call for the remainder in division of one unsigned full-word by another. If you do not define this macro, the default name is used, which is __umoddi3, a function defined in `libgcc.a'. TARGET_MEM_FUNCTIONS Define this macro if GNU CC should generate calls to the System V (and ANSI C) library functions memcpy and memset rather than the BSD functions Using GNU CC 319 bcopy and bzero. LIBGCC_NEEDS_DOUBLE Define this macro if only float arguments cannot be passed to library routines (so they must be converted to double). This macro affects both how library calls are generated and how the library routines in `libgcc1.c' accept their arguments. It is useful on machines where floating and fixed point arguments are passed differently, such as the i860. FLOAT_ARG_TYPE Define this macro to override the type used by the library routines to pick up arguments of type float. (By default, they use a union of float and int.) The obvious choice would be float---but that won't work with traditional C compilers that expect all arguments declared as float to arrive as double. To avoid this conversion, the library routines ask for the value as some other type and then treat it as a float. On some systems, no other type will work for this. For these systems, you must use LIBGCC_NEEDS_DOUBLE instead, to force conversion of the values double before they are passed. FLOATIFY (passed-value) Define this macro to override the way library routines redesignate a float argument as a float instead of the type it was passed as. The default is an expression which takes the float field of the union. FLOAT_VALUE_TYPE Define this macro to override the type used by the library routines to return values that ought to have type float. (By default, they use int.) The obvious choice would be float---but that won't work with traditional C compilers gratuitously convert values declared as float into double. INTIFY (float-value) Define this macro to override the way the value of a float-returning library routine should be packaged in order to return it. These functions are actually declared to return type FLOAT_VALUE_TYPE (normally int). 320 Using GNU CC These values can't be returned as type float because traditional C compilers would gratuitously convert the value to a double. A local variable named intify is always available when the macro INTIFY is used. It is a union of a float field named f and a field named i whose type is FLOAT_VALUE_TYPE or int. If you don't define this macro, the default definition works by copying the value through that union. SItype Define this macro as the name of the data type corresponding to SImode in the system's own C compiler. You need not define this macro if that type is int, as it usually is. perform_... Define these macros to supply explicit C statements to carry out various arithmetic operations on types float and double in the library routines in `libgcc1.c'. See that file for a full list of these macros and their arguments. On most machines, you don't need to define any of these macros, because the C compiler that comes with the system takes care of doing them. NEXT_OBJC_RUNTIME Define this macro to generate code for Objective C message sending using the calling convention of the NeXT system. This calling convention involves passing the object, the selector and the method arguments all at once to the method-lookup library function. The default calling convention passes just the object and the selector to the lookup function, which returns a pointer to the method. 16.11. Addressing Modes HAVE_POST_INCREMENT Define this macro if the machine supports post- increment addressing. Using GNU CC 321 HAVE_PRE_INCREMENT HAVE_POST_DECREMENT HAVE_PRE_DECREMENT Similar for other kinds of addressing. CONSTANT_ADDRESS_P (x) A C expression that is 1 if the RTX x is a constant which is a valid address. On most machines, this can be defined as CONSTANT_P (x), but a few machines are more restrictive in which constant addresses are supported. CONSTANT_P accepts integer-values expressions whose values are not explicitly known, such as symbol_ref, label_ref, and high expressions and const arithmetic expressions, in addition to const_int and const_double expressions. MAX_REGS_PER_ADDRESS A number, the maximum number of registers that can appear in a valid memory address. Note that it is up to you to specify a value equal to the maximum number that GO_IF_LEGITIMATE_ADDRESS would ever accept. GO_IF_LEGITIMATE_ADDRESS (mode, x, label) A C compound statement with a conditional goto label; executed if x (an RTX) is a legitimate memory address on the target machine for a memory operand of mode mode. It usually pays to define several simpler macros to serve as subroutines for this one. Otherwise it may be too complicated to understand. This macro must exist in two variants: a strict variant and a non-strict one. The strict variant is used in the reload pass. It must be defined so that any pseudo-register that has not been allocated a hard register is considered a memory reference. In contexts where some kind of register is required, a pseudo-register with no hard register must be rejected. The non-strict variant is used in other passes. It must be defined to accept all pseudo-registers in every context where some kind of register is required. Compiler source files that want to use the strict variant of this macro define the macro 322 Using GNU CC REG_OK_STRICT. You should use an #ifdef REG_OK_STRICT conditional to define the strict variant in that case and the non-strict variant otherwise. Typically among the subroutines used to define GO_IF_LEGITIMATE_ADDRESS are subroutines to check for acceptable registers for various purposes (one for base registers, one for index registers, and so on). Then only these subroutine macros need have two variants; the higher levels of macros may be the same whether strict or not. Normally, constant addresses which are the sum of a symbol_ref and an integer are stored inside a const RTX to mark them as constant. Therefore, there is no need to recognize such sums specifically as legitimate addresses. Normally you would simply recognize any const as legitimate. Usually PRINT_OPERAND_ADDRESS is not prepared to handle constant sums that are not marked with const. It assumes that a naked plus indicates indexing. If so, then you must reject such naked constant sums as illegitimate addresses, so that none of them will be given to PRINT_OPERAND_ADDRESS. On some machines, whether a symbolic address is legitimate depends on the section that the address refers to. On these machines, define the macro ENCODE_SECTION_INFO to store the information into the symbol_ref, and then check for it here. When you see a const, you will have to look inside it to find the symbol_ref in order to determine the section. See section Assembler Format. The best way to modify the name string is by adding text to the beginning, with suitable punctuation to prevent any ambiguity. Allocate the new name in saveable_obstack. You will have to modify ASM_OUTPUT_LABELREF to remove and decode the added text and output the name accordingly. You can check the information stored here into the symbol_ref in the definitions of GO_IF_LEGITIMATE_ADDRESS and PRINT_OPERAND_ADDRESS. REG_OK_FOR_BASE_P (x) A C expression that is nonzero if x (assumed to be a reg RTX) is valid for use as a base register. Using GNU CC 323 For hard registers, it should always accept those which the hardware permits and reject the others. Whether the macro accepts or rejects pseudo registers must be controlled by REG_OK_STRICT as described above. This usually requires two variant definitions, of which REG_OK_STRICT controls the one actually used. REG_OK_FOR_INDEX_P (x) A C expression that is nonzero if x (assumed to be a reg RTX) is valid for use as an index register. The difference between an index register and a base register is that the index register may be scaled. If an address involves the sum of two registers, neither one of them scaled, then either one may be labeled the ``base'' and the other the ``index''; but whichever labeling is used must fit the machine's constraints of which registers may serve in each capacity. The compiler will try both labelings, looking for one that is valid, and will reload one or both registers only if neither labeling works. LEGITIMIZE_ADDRESS (x, oldx, mode, win) A C compound statement that attempts to replace x with a valid memory address for an operand of mode mode. win will be a C statement label elsewhere in the code; the macro definition may use GO_IF_LEGITIMATE_ADDRESS (mode, x, win); to avoid further processing if the address has become legitimate. x will always be the result of a call to break_out_memory_refs, and oldx will be the operand that was given to that function to produce x. The code generated by this macro should not alter the substructure of x. If it transforms x into a more legitimate form, it should assign x (which will always be a C variable) a new value. It is not necessary for this macro to come up with a legitimate address. The compiler has standard ways of doing so in all cases. In fact, it is safe for this macro to do nothing. 324 Using GNU CC But often a machine-dependent strategy can generate better code. GO_IF_MODE_DEPENDENT_ADDRESS (addr, label) A C statement or compound statement with a conditional goto label; executed if memory address x (an RTX) can have different meanings depending on the machine mode of the memory reference it is used for. Autoincrement and autodecrement addresses typically have mode-dependent effects because the amount of the increment or decrement is the size of the operand being addressed. Some machines have other mode-dependent addresses. Many RISC machines have no mode-dependent addresses. You may assume that addr is a valid address for the machine. LEGITIMATE_CONSTANT_P (x) A C expression that is nonzero if x is a legitimate constant for an immediate operand on the target machine. You can assume that x satisfies CONSTANT_P, so you need not check this. In fact, `1' is a suitable definition for this macro on machines where anything CONSTANT_P is valid. LEGITIMATE_PIC_OPERAND_P (x) A C expression that is nonzero if x is a legitimate immediate operand on the target machine when generating position independent code. You can assume that x satisfies CONSTANT_P, so you need not check this. You can also assume flag_pic is true, so you need not check it either. You need not define this macro if all constants (including SYMBOL_REF) can be immediate operands when generating position independent code. 16.12. Condition Code Status The file `conditions.h' defines a variable cc_status to describe how the condition code was computed (in case the interpretation of the condition code depends on the instruc- tion that it was set by). This variable contains the RTL expressions on which the condition code is currently based, and several standard flags. Using GNU CC 325 Sometimes additional machine-specific flags must be defined in the machine description header file. It can also add additional machine-specific information by defining CC_STATUS_MDEP. CC_STATUS_MDEP C code for a data type which is used for declaring the mdep component of cc_status. It defaults to int. This macro is not used on machines that do not use cc0. CC_STATUS_MDEP_INIT A C expression to initialize the mdep field to ``empty''. The default definition does nothing, since most machines don't use the field anyway. If you want to use the field, you should probably define this macro to initialize it. This macro is not used on machines that do not use cc0. NOTICE_UPDATE_CC (exp, insn) A C compound statement to set the components of cc_status appropriately for an insn insn whose body is exp. It is this macro's responsibility to recognize insns that set the condition code as a byproduct of other activity as well as those that explicitly set (cc0). This macro is not used on machines that do not use cc0. If there are insns that do not set the condition code but do alter other machine registers, this macro must check to see whether they invalidate the expressions that the condition code is recorded as reflecting. For example, on the 68000, insns that store in address registers do not set the condition code, which means that usually NOTICE_UPDATE_CC can leave cc_status unaltered for such insns. But suppose that the previous insn set the condition code based on location `a4@(102)' and the current insn stores a new value in `a4'. Although the condition code is not changed by this, it will no longer be true that it reflects the contents of `a4@(102)'. Therefore, NOTICE_UPDATE_CC must alter cc_status in this case to say that nothing is known about the condition code value. 326 Using GNU CC The definition of NOTICE_UPDATE_CC must be prepared to deal with the results of peephole optimization: insns whose patterns are parallel RTXs containing various reg, mem or constants which are just the operands. The RTL structure of these insns is not sufficient to indicate what the insns actually do. What NOTICE_UPDATE_CC should do when it sees one is just to run CC_STATUS_INIT. A possible definition of NOTICE_UPDATE_CC is to call a function that looks at an attribute (see section Insn Attributes) named, for example, `cc'. This avoids having detailed information about patterns in two places, the `md' file and in NOTICE_UPDATE_CC. EXTRA_CC_MODES A list of names to be used for additional modes for condition code values in registers (see section Jump Patterns). These names are added to enum machine_mode and all have class MODE_CC. By convention, they should start with `CC' and end with `mode'. You should only define this macro if your machine does not use cc0 and only if additional modes are required. EXTRA_CC_NAMES A list of C strings giving the names for the modes listed in EXTRA_CC_MODES. For example, the Sparc defines this macro and EXTRA_CC_MODES as #define EXTRA_CC_MODES CC_NOOVmode, CCFPmode #define EXTRA_CC_NAMES "CC_NOOV", "CCFP" This macro is not required if EXTRA_CC_MODES is not defined. SELECT_CC_MODE (op, x) Returns a mode from class MODE_CC to be used when comparison operation code op is applied to rtx x. For example, on the Sparc, SELECT_CC_MODE is defined as (see see section Jump Patterns for a description of the reason for this definition) #define SELECT_CC_MODE(OP,X) \ (GET_MODE_CLASS (GET_MODE (X)) == MODE_FLOAT ? CCFPmode \ Using GNU CC 327 : (GET_CODE (X) == PLUS || GET_CODE (X) == MINUS \ || GET_CODE (X) == NEG) \ ? CC_NOOVmode : CCmode) This macro is not required if EXTRA_CC_MODES is not defined. 16.13. Describing Relative Costs of Operations These macros let you describe the relative speed of various operations on the target machine. CONST_COSTS (x, code) A part of a C switch statement that describes the relative costs of constant RTL expressions. It must contain case labels for expression codes const_int, const, symbol_ref, label_ref and const_double. Each case must ultimately reach a return statement to return the relative cost of the use of that kind of constant value in an expression. The cost may depend on the precise value of the constant, which is available for examination in x. code is the expression code---redundant, since it can be obtained with GET_CODE (x). RTX_COSTS (x, code) Like CONST_COSTS but applies to nonconstant RTL expressions. This can be used, for example, to indicate how costly a multiply instruction is. In writing this macro, you can use the construct COSTS_N_INSNS (n) to specify a cost equal to n fast instructions. This macro is optional; do not define it if the default cost assumptions are adequate for the target machine. ADDRESS_COST (address) An expression giving the cost of an addressing mode that contains address. If not defined, the cost is computed from the address expression and the CONST_COSTS values. For most CISC machines, the default cost is a good approximation of the true cost of the addressing mode. However, on RISC machines, all instructions normally have the same length and execution time. Hence all addresses will have equal costs. 328 Using GNU CC In cases where more than one form of an address is known, the form with the lowest cost will be used. If multiple forms have the same, lowest, cost, the one that is the most complex will be used. For example, suppose an address that is equal to the sum of a register and a constant is used twice in the same basic block. When this macro is not defined, the address will be computed in a register and memory references will be indirect through that register. On machines where the cost of the addressing mode containing the sum is no higher than that of a simple indirect reference, this will produce an additional instruction and possibly require an additional register. Proper specification of this macro eliminates this overhead for such machines. Similar use of this macro is made in strength reduction of loops. address need not be valid as an address. In such a case, the cost is not relevant and can be any value; invalid addresses need not be assigned a different cost. On machines where an address involving more than one register is as cheap as an address computation involving only one register, defining ADDRESS_COST to reflect this can cause two registers to be live over a region of code where only one would have been if ADDRESS_COST were not defined in that manner. This effect should be considered in the definition of this macro. Equivalent costs should probably only be given to addresses with different numbers of registers on machines with lots of registers. This macro will normally either not be defined or be defined as a constant. REGISTER_MOVE_COST (from, to) A C expression for the cost of moving data from a register in class from to one in class to. The classes are expressed using the enumeration values such as GENERAL_REGS. A value of 2 is the default; other values are interpreted relative to that. It is not required that the cost always equal 2 when from is the same as to; on some machines it is expensive to move between registers if they are not general registers. Using GNU CC 329 If reload sees an insn consisting of a single set between two hard registers, and if REGISTER_MOVE_COST applied to their classes returns a value of 2, reload does not check to ensure that the constraints of the insn are met. Setting a cost of other than 2 will allow reload to verify that the constraints are met. You should do this if the `movm' pattern's constraints do not allow such copying. MEMORY_MOVE_COST (m) A C expression for the cost of moving data of mode m between a register and memory. A value of 2 is the default; this cost is relative to those in REGISTER_MOVE_COST. If moving between registers and memory is more expensive than between two registers, you should define this macro to express the relative cost. BRANCH_COST A C expression for the cost of a branch instruction. A value of 1 is the default; other values are interpreted relative to that. Here are additional macros which do not specify precise relative costs, but only that certain actions are more expensive than GNU CC would ordinarily expect. SLOW_BYTE_ACCESS Define this macro as a C expression which is nonzero if accessing less than a word of memory (i.e. a char or a short) is no faster than accessing a word of memory, i.e., if such access require more than one instruction or if there is no difference in cost between byte and (aligned) word loads. When this macro is not defined, the compiler will access a field by finding the smallest containing object; when it is defined, a fullword load will be used if alignment permits. Unless bytes accesses are faster than word accesses, using word accesses is preferable since it may eliminate subsequent memory access if subsequent accesses occur to other fields in the same word of the structure, but to different bytes. SLOW_ZERO_EXTEND Define this macro if zero-extension (of a char or short to an int) can be done faster if the destination is a register that is known to be 330 Using GNU CC zero. If you define this macro, you must have instruction patterns that recognize RTL structures like this: (set (strict_low_part (subreg:QI (reg:SI ...) 0)) ...) and likewise for HImode. SLOW_UNALIGNED_ACCESS Define this macro if unaligned accesses have a cost many times greater than aligned accesses, for example if they are emulated in a trap handler. When this macro is defined, the compiler will act as if STRICT_ALIGNMENT were defined when generating code for block moves. This can cause significantly more instructions to be produced. Therefore, do not define this macro if unaligned accesses only add a cycle or two to the time for a memory access. DONT_REDUCE_ADDR Define this macro to inhibit strength reduction of memory addresses. (On some machines, such strength reduction seems to do harm rather than good.) MOVE_RATIO The number of scalar move insns which should be generated instead of a string move insn or a library call. Increasing the value will always make code faster, but eventually incurs high cost in increased code size. If you don't define this, a reasonable default is used. NO_FUNCTION_CSE Define this macro if it is as good or better to call a constant function address than to call an address kept in a register. NO_RECURSIVE_FUNCTION_CSE Define this macro if it is as good or better for a function to call itself with an explicit address than to call an address kept in a register. Using GNU CC 331 16.14. Dividing the Output into Sections (Texts, Data, ...) An object file is divided into sections containing dif- ferent types of data. In the most common case, there are three sections: the text section, which holds instructions and read-only data; the data section, which holds initial- ized writable data; and the bss section, which holds unini- tialized data. Some systems have other kinds of sections. The compiler must tell the assembler when to switch sections. These macros control what commands to output to tell the assembler this. You can also define additional sections. TEXT_SECTION_ASM_OP A C string constant for the assembler operation that should precede instructions and read-only data. Normally ".text" is right. DATA_SECTION_ASM_OP A C string constant for the assembler operation to identify the following data as writable initialized data. Normally ".data" is right. SHARED_SECTION_ASM_OP If defined, a C string constant for the assembler operation to identify the following data as shared data. If not defined, DATA_SECTION_ASM_OP will be used. INIT_SECTION_ASM_OP If defined, a C string constant for the assembler operation to identify the following data as initialization code. If not defined, GNU CC will assume such a section does not exist. EXTRA_SECTIONS A list of names for sections other than the standard two, which are in_text and in_data. You need not define this macro on a system with no other sections (that GCC needs to use). EXTRA_SECTION_FUNCTIONS One or more functions to be defined in `varasm.c'. These functions should do jobs analogous to those of text_section and data_section, for your additional sections. Do not define this macro if you do not define EXTRA_SECTIONS. READONLY_DATA_SECTION On most machines, read-only variables, constants, and jump tables are placed in the text section. 332 Using GNU CC If this is not the case on your machine, this macro should be defined to be the name of a function (either data_section or a function defined in EXTRA_SECTIONS) that switches to the section to be used for read-only items. If these items should be placed in the text section, this macro should not be defined. SELECT_SECTION (exp, reloc) A C statement or statements to switch to the appropriate section for output of exp. You can assume that exp is either a VAR_DECL node or a constant of some sort. reloc indicates whether the initial value of exp requires link-time relocations. Select the section by calling text_section or one of the alternatives for other sections. Do not define this macro if you put all read-only variables and constants in the read-only data section (usually the text section). SELECT_RTX_SECTION (mode, rtx) A C statement or statements to switch to the appropriate section for output of rtx in mode mode. You can assume that rtx is some kind of constant in RTL. The argument mode is redundant except in the case of a const_int rtx. Select the section by calling text_section or one of the alternatives for other sections. Do not define this macro if you put all constants in the read-only data section. JUMP_TABLES_IN_TEXT_SECTION Define this macro if jump tables (for tablejump insns) should be output in the text section, along with the assembler instructions. Otherwise, the readonly data section is used. This macro is irrelevant if there is no separate readonly data section. ENCODE_SECTION_INFO (decl) Define this macro if references to a symbol must be treated differently depending on something about the variable or function named by the symbol (such as what section it is in). The macro definition, if any, is executed immediately after the rtl for decl has been created and stored in DECL_RTL (decl). The value Using GNU CC 333 of the rtl will be a mem whose address is a symbol_ref. The usual thing for this macro to do is to record a flag in the symbol_ref (such as SYMBOL_REF_FLAG) or to store a modified name string in the symbol_ref (if one bit is not enough information). 16.15. Position Independent Code This section describes macros that help implement gen- eration of position independent code. Simply defining these macros is not enough to generate valid PIC; you must also add support to the macros GO_IF_LEGITIMATE_ADDRESS and LEGITIMIZE_ADDRESS, and PRINT_OPERAND_ADDRESS as well. You must modify the definition of `movsi' to do something appropriate when the source operand contains a symbolic address. You may also need to alter the handling of switch statements so that they use relative addresses. PIC_OFFSET_TABLE_REGNUM The register number of the register used to address a table of static data addresses in memory. In some cases this register is defined by a processor's ``application binary interface'' (ABI). When this macro is defined, RTL is generated for this register once, as with the stack pointer and frame pointer registers. If this macro is not defined, it is up to the machine-dependent files to allocate such a register (if necessary). FINALIZE_PIC By generating position-independent code, when two different programs (A and B) share a common library (libC.a), the text of the library can be shared whether or not the library is linked at the same address for both programs. In some of these environments, position-independent code requires not only the use of different addressing modes, but also special code to enable the use of these addressing modes. The FINALIZE_PIC macro serves as a hook to emit these special codes once the function is being compiled into assembly code, but not before. (It is not done before, because in the case of compiling an inline function, it would lead to multiple PIC prologues being included in functions which used inline functions and were compiled to assembly language.) 334 Using GNU CC 16.16. Defining the Output Assembler Language This section describes macros whose principal purpose is to describe how to write instructions in assembler language--rather than what the instructions do. 16.16.1. The Overall Framework of an Assembler File ASM_FILE_START (stream) A C expression which outputs to the stdio stream stream some appropriate text to go at the start of an assembler file. Normally this macro is defined to output a line containing `#NO_APP', which is a comment that has no effect on most assemblers but tells the GNU assembler that it can save time by not checking for certain assembler constructs. On systems that use SDB, it is necessary to output certain commands; see `attasm.h'. ASM_FILE_END (stream) A C expression which outputs to the stdio stream stream some appropriate text to go at the end of an assembler file. If this macro is not defined, the default is to output nothing special at the end of the file. Most systems don't require any definition. On systems that use SDB, it is necessary to output certain commands; see `attasm.h'. ASM_IDENTIFY_GCC (file) A C statement to output assembler commands which will identify the object file as having been compiled with GNU CC (or another GNU compiler). If you don't define this macro, the string `gcc_compiled.:' is output. This string is calculated to define a symbol which, on BSD systems, will never be defined for any other reason. GDB checks for the presence of this symbol when reading the symbol table of an executable. On non-BSD systems, you must arrange communication with GDB in some other fashion. If GDB is not used on your system, you can define this macro with an empty body. Using GNU CC 335 ASM_COMMENT_START A C string constant describing how to begin a comment in the target assembler language. The compiler assumes that the comment will end at the end of the line. ASM_APP_ON A C string constant for text to be output before each asm statement or group of consecutive ones. Normally this is "#APP", which is a comment that has no effect on most assemblers but tells the GNU assembler that it must check the lines that follow for all valid assembler constructs. ASM_APP_OFF A C string constant for text to be output after each asm statement or group of consecutive ones. Normally this is "#NO_APP", which tells the GNU assembler to resume making the time-saving assumptions that are valid for ordinary compiler output. ASM_OUTPUT_SOURCE_FILENAME (stream, name) A C statement to output COFF information or DWARF debugging information which indicates that filename name is the current source file to the stdio stream stream. This macro need not be defined if the standard form of output for the file format in use is appropriate. ASM_OUTPUT_SOURCE_LINE (stream, line) A C statement to output DBX or SDB debugging information before code for line number line of the current source file to the stdio stream stream. This macro need not be defined if the standard form of debugging information for the debugger in use is appropriate. ASM_OUTPUT_IDENT (stream, string) A C statement to output something to the assembler file to handle a `#ident' directive containing the text string. If this macro is not defined, nothing is output for a `#ident' directive. OBJC_PROLOGUE A C statement to output any assembler statements which are required to precede any Objective C object definitions or message sending. The statement is executed only when compiling an 336 Using GNU CC Objective C program. 16.16.2. Output of Data ASM_OUTPUT_LONG_DOUBLE (stream, value) ASM_OUTPUT_DOUBLE (stream, value) ASM_OUTPUT_FLOAT (stream, value) A C statement to output to the stdio stream stream an assembler instruction to assemble a floating- point constant of TFmode, DFmode or SFmode, respectively, whose value is value. value will be a C expression of type REAL_VALUE__TYPE, usually double. ASM_OUTPUT_QUADRUPLE_INT (stream, exp) ASM_OUTPUT_DOUBLE_INT (stream, exp) ASM_OUTPUT_INT (stream, exp) ASM_OUTPUT_SHORT (stream, exp) ASM_OUTPUT_CHAR (stream, exp) A C statement to output to the stdio stream stream an assembler instruction to assemble an integer of 16, 8, 4, 2 or 1 bytes, respectively, whose value is value. The argument exp will be an RTL expression which represents a constant value. Use `output_addr_const (stream, exp)' to output this value as an assembler expression. For sizes larger than UNITS_PER_WORD, if the action of a macro would be identical to repeatedly calling the macro corresponding to a size of UNITS_PER_WORD, once for each word, you need not define the macro. ASM_OUTPUT_BYTE (stream, value) A C statement to output to the stdio stream stream an assembler instruction to assemble a single byte containing the number value. ASM_BYTE_OP A C string constant giving the pseudo-op to use for a sequence of single-byte constants. If this macro is not defined, the default is "byte". ASM_OUTPUT_ASCII (stream, ptr, len) A C statement to output to the stdio stream stream an assembler instruction to assemble a string Using GNU CC 337 constant containing the len bytes at ptr. ptr will be a C expression of type char * and len a C expression of type int. If the assembler has a .ascii pseudo-op as found in the Berkeley Unix assembler, do not define the macro ASM_OUTPUT_ASCII. ASM_OUTPUT_POOL_PROLOGUE (file funname fundecl size) A C statement to output assembler commands to define the start of the constant pool for a function. funname is a string giving the name of the function. Should the return type of the function be required, it can be obtained via fundecl. size is the size, in bytes, of the constant pool that will be written immediately after this call. If no constant-pool prefix is required, the usual case, this macro need not be defined. ASM_OUTPUT_SPECIAL_POOL_ENTRY (file, x, mode, align, labelno, jumpto) A C statement (with or without semicolon) to output a constant in the constant pool, if it needs special treatment. (This macro need not do anything for RTL expressions that can be output normally.) The argument file is the standard I/O stream to output the assembler code on. x is the RTL expression for the constant to output, and mode is the machine mode (in case x is a `const_int'). align is the required alignment for the value x; you should output an assembler directive to force this much alignment. The argument labelno is a number to use in an internal label for the address of this pool entry. The definition of this macro is responsible for outputting the label definition at the proper place. Here is how to do this: ASM_OUTPUT_INTERNAL_LABEL (file, "LC", labelno); When you output a pool entry specially, you should end with a goto to the label jumpto. This will prevent the same pool entry from being output a second time in the usual manner. 338 Using GNU CC You need not define this macro if it would do nothing. ASM_OPEN_PAREN ASM_CLOSE_PAREN These macros are defined as C string constant, describing the syntax in the assembler for grouping arithmetic expressions. The following definitions are correct for most assemblers: #define ASM_OPEN_PAREN "(" #define ASM_CLOSE_PAREN ")" 16.16.3. Output of Uninitialized Variables Each of the macros in this section is used to do the whole job of outputting a single uninitialized variable. ASM_OUTPUT_COMMON (stream, name, size, rounded) A C statement (sans semicolon) to output to the stdio stream stream the assembler definition of a common-label named name whose size is size bytes. The variable rounded is the size rounded up to whatever alignment the caller wants. Use the expression assemble_name (stream, name) to output the name itself; before and after that, output the additional assembler syntax for defining the name, and a newline. This macro controls how the assembler definitions of uninitialized global variables are output. ASM_OUTPUT_ALIGNED_COMMON (stream, name, size, alignment) Like ASM_OUTPUT_COMMON except takes the required alignment as a separate, explicit argument. If you define this macro, it is used in place of ASM_OUTPUT_COMMON, and gives you more flexibility in handling the required alignment of the variable. ASM_OUTPUT_SHARED_COMMON (stream, name, size, rounded) If defined, it is similar to ASM_OUTPUT_COMMON, except that it is used when name is shared. If not defined, ASM_OUTPUT_COMMON will be used. Using GNU CC 339 ASM_OUTPUT_LOCAL (stream, name, size, rounded) A C statement (sans semicolon) to output to the stdio stream stream the assembler definition of a local-common-label named name whose size is size bytes. The variable rounded is the size rounded up to whatever alignment the caller wants. Use the expression assemble_name (stream, name) to output the name itself; before and after that, output the additional assembler syntax for defining the name, and a newline. This macro controls how the assembler definitions of uninitialized static variables are output. ASM_OUTPUT_ALIGNED_LOCAL (stream, name, size, alignment) Like ASM_OUTPUT_LOCAL except takes the required alignment as a separate, explicit argument. If you define this macro, it is used in place of ASM_OUTPUT_LOCAL, and gives you more flexibility in handling the required alignment of the variable. ASM_OUTPUT_SHARED_LOCAL (stream, name, size, rounded) If defined, it is similar to ASM_OUTPUT_LOCAL, except that it is used when name is shared. If not defined, ASM_OUTPUT_LOCAL will be used. 16.16.4. Output and Generation of Labels ASM_OUTPUT_LABEL (stream, name) A C statement (sans semicolon) to output to the stdio stream stream the assembler definition of a label named name. Use the expression assemble_name (stream, name) to output the name itself; before and after that, output the additional assembler syntax for defining the name, and a newline. ASM_DECLARE_FUNCTION_NAME (stream, name, decl) A C statement (sans semicolon) to output to the stdio stream stream any text necessary for declaring the name name of a function which is being defined. This macro is responsible for outputting the label definition (perhaps using ASM_OUTPUT_LABEL). The argument decl is the FUNCTION_DECL tree node representing the function. If this macro is not defined, then the function name is defined in the usual manner as a label (by means of ASM_OUTPUT_LABEL). 340 Using GNU CC ASM_DECLARE_FUNCTION_SIZE (stream, name, decl) A C statement (sans semicolon) to output to the stdio stream stream any text necessary for declaring the size of a function which is being defined. The argument name is the name of the function. The argument decl is the FUNCTION_DECL tree node representing the function. If this macro is not defined, then the function size is not defined. ASM_DECLARE_OBJECT_NAME (stream, name, decl) A C statement (sans semicolon) to output to the stdio stream stream any text necessary for declaring the name name of an initialized variable which is being defined. This macro must output the label definition (perhaps using ASM_OUTPUT_LABEL). The argument decl is the VAR_DECL tree node representing the variable. If this macro is not defined, then the variable name is defined in the usual manner as a label (by means of ASM_OUTPUT_LABEL). ASM_GLOBALIZE_LABEL (stream, name) A C statement (sans semicolon) to output to the stdio stream stream some commands that will make the label name global; that is, available for reference from other files. Use the expression assemble_name (stream, name) to output the name itself; before and after that, output the additional assembler syntax for making that name global, and a newline. ASM_OUTPUT_EXTERNAL (stream, decl, name) A C statement (sans semicolon) to output to the stdio stream stream any text necessary for declaring the name of an external symbol named name which is referenced in this compilation but not defined. The value of decl is the tree node for the declaration. This macro need not be defined if it does not need to output anything. The GNU assembler and most Unix assemblers don't require anything. ASM_OUTPUT_EXTERNAL_LIBCALL (stream, symref) A C statement (sans semicolon) to output on stream an assembler pseudo-op to declare a library function name external. The name of the library function is given by symref, which has type rtx and is a symbol_ref. Using GNU CC 341 This macro need not be defined if it does not need to output anything. The GNU assembler and most Unix assemblers don't require anything. ASM_OUTPUT_LABELREF (stream, name) A C statement (sans semicolon) to output to the stdio stream stream a reference in assembler syntax to a label named name. This should add `_' to the front of the name, if that is customary on your operating system, as it is in most Berkeley Unix systems. This macro is used in assemble_name. ASM_OUTPUT_LABELREF_AS_INT (file, label) Define this macro for systems that use the program collect2. The definition should be a C statement to output a word containing a reference to the label label. ASM_GENERATE_INTERNAL_LABEL (string, prefix, num) A C statement to store into the string string a label whose name is made from the string prefix and the number num. This string, when output subsequently by ASM_OUTPUT_LABELREF, should produce the same output that ASM_OUTPUT_INTERNAL_LABEL would produce with the same prefix and num. ASM_OUTPUT_INTERNAL_LABEL (stream, prefix, num) A C statement to output to the stdio stream stream a label whose name is made from the string prefix and the number num. These labels are used for internal purposes, and there is no reason for them to appear in the symbol table of the object file. On many systems, the letter `L' at the beginning of a label has this effect. The usual definition of this macro is as follows: fprintf (stream, "L%s%d:\n", prefix, num) ASM_FORMAT_PRIVATE_NAME (outvar, name, number) A C expression to assign to outvar (which is a variable of type char *) a newly allocated string made from the string name and the number number, with some suitable punctuation added. Use alloca to get space for the string. 342 Using GNU CC This string will be used as the argument to ASM_OUTPUT_LABELREF to produce an assembler label for an internal static variable whose name is name. Therefore, the string must be such as to result in valid assembler code. The argument number is different each time this macro is executed; it prevents conflicts between similarly-named internal static variables in different scopes. Ideally this string should not be a valid C identifier, to prevent any conflict with the user's own symbols. Most assemblers allow periods or percent signs in assembler symbols; putting at least one of these between the name and the number will suffice. OBJC_GEN_METHOD_LABEL (buf, is_inst, class_name, cat_name, sel_name) Define this macro to override the default assembler names used for Objective C methods. The default name is a unique method number followed by the name of the class (e.g. `_1_Foo'). For methods in categories, the name of the category is also included in the assembler name (e.g. `_1_Foo_Bar'). These names are safe on most systems, but make debugging difficult since the method's selector is not present in the name. Therefore, particular systems define other ways of computing names. buf is a buffer in which to store the name (256 chars max); is_inst specifies whether the method is an instance method or a class method; class_name is the name of the class; cat_name is the name of the category (or NULL if the method is not in a category); and sel_name is the name of the selector. On systems where the assembler can handle quoted names, you can use this macro to provide more human-readable names. 16.16.5. Output of Initialization Routines The compiled code for certain languages includes con- structors (also called initialization routines)---functions to initialize data in the program when the program is started. These functions need to be called before the pro- gram is ``started''---that is to say, before main is called. Using GNU CC 343 Compiling some languages generates destructors (also called termination routines) that should be called when the program terminates. To make the initialization and termination functions work, the compiler must output something in the assembler code to cause those functions to be called at the appropri- ate time. When you port the compiler to a new system, you need to specify what assembler code is needed to do this. Here are the two macros you should define if necessary: ASM_OUTPUT_CONSTRUCTOR (stream, name) Define this macro as a C statement to output on the stream stream the assembler code to arrange to call the function named name at initialization time. Assume that name is the name of a C function generated automatically by the compiler. This function takes no arguments. Use the function assemble_name to output the name name; this performs any system-specific syntactic transformations such as adding an underscore. If you don't define this macro, nothing special is output to arrange to call the function. This is correct when the function will be called in some other manner---for example, by means of the collect program, which looks through the symbol table to find these functions by their names. If you want to use collect, then you need to arrange for it to be built and installed and used on your system. ASM_OUTPUT_DESTRUCTOR (stream, name) This is like ASM_OUTPUT_CONSTRUCTOR but used for termination functions rather than initialization functions. 16.16.6. Output of Assembler Instructions REGISTER_NAMES A C initializer containing the assembler's names for the machine registers, each one as a C string constant. This is what translates register numbers in the compiler into assembler language. ADDITIONAL_REGISTER_NAMES If defined, a C initializer for an array of structures containing a name and a register number. This macro defines additional names for 344 Using GNU CC hard registers, thus allowing the asm option in declarations to refer to registers using alternate names. ASM_OUTPUT_OPCODE (stream, ptr) Define this macro if you are using an unusual assembler that requires different names for the machine instructions. The definition is a C statement or statements which output an assembler instruction opcode to the stdio stream stream. The macro-operand ptr is a variable of type char * which points to the opcode name in its ``internal'' form---the form that is written in the machine description. The definition should output the opcode name to stream, performing any translation you desire, and increment the variable ptr to point at the end of the opcode so that it will not be output twice. In fact, your macro definition may process less than the entire opcode name, or more than the opcode name; but if you want to process text that includes `%'-sequences to substitute operands, you must take care of the substitution yourself. Just be sure to increment ptr over whatever text should not be output normally. If you need to look at the operand values, they can be found as the elements of recog_operand. If the macro definition does nothing, the instruction is output in the usual way. FINAL_PRESCAN_INSN (insn, opvec, noperands) If defined, a C statement to be executed just prior to the output of assembler code for insn, to modify the extracted operands so they will be output differently. Here the argument opvec is the vector containing the operands extracted from insn, and noperands is the number of elements of the vector which contain meaningful data for this insn. The contents of this vector are what will be used to convert the insn template into assembler code, so you can change the assembler output by changing the contents of the vector. This macro is useful when various assembler syntaxes share a single file of instruction patterns; by defining this macro differently, you can cause a large class of instructions to be Using GNU CC 345 output differently (such as with rearranged operands). Naturally, variations in assembler syntax affecting individual insn patterns ought to be handled by writing conditional output routines in those patterns. If this macro is not defined, it is equivalent to a null statement. PRINT_OPERAND (stream, x, code) A C compound statement to output to stdio stream stream the assembler syntax for an instruction operand x. x is an RTL expression. code is a value that can be used to specify one of several ways of printing the operand. It is used when identical operands must be printed differently depending on the context. code comes from the `%' specification that was used to request printing of the operand. If the specification was just `%digit' then code is 0; if the specification was `%ltr digit' then code is the ASCII code for ltr. If x is a register, this macro should print the register's name. The names can be found in an array reg_names whose type is char *[]. reg_names is initialized from REGISTER_NAMES. When the machine description has a specification `%punct' (a `%' followed by a punctuation character), this macro is called with a null pointer for x and the punctuation character for code. PRINT_OPERAND_PUNCT_VALID_P (code) A C expression which evaluates to true if code is a valid punctuation character for use in the PRINT_OPERAND macro. If PRINT_OPERAND_PUNCT_VALID_P is not defined, it means that no punctuation characters (except for the standard one, `%') are used in this way. PRINT_OPERAND_ADDRESS (stream, x) A C compound statement to output to stdio stream stream the assembler syntax for an instruction operand that is a memory reference whose address is x. x is an RTL expression. On some machines, the syntax for a symbolic address depends on the section that the address refers to. On these machines, define the macro ENCODE_SECTION_INFO to store the information into 346 Using GNU CC the symbol_ref, and then check for it here. See section Assembler Format. DBR_OUTPUT_SEQEND(file) A C statement, to be executed after all slot- filler instructions have been output. If necessary, call dbr_sequence_length to determine the number of slots filled in a sequence (zero if not currently outputting a sequence), to decide how many no-ops to output, or whatever. Don't define this macro if it has nothing to do, but it is helpful in reading assembly output if the extent of the delay sequence is made explicit (e.g. with white space). Note that output routines for instructions with delay slots must be prepared to deal with not being output as part of a sequence (i.e. when the scheduling pass is not run, or when no slot fillers could be found.) The variable final_sequence is null when not processing a sequence, otherwise it contains the sequence rtx being output. REGISTER_PREFIX LOCAL_LABEL_PREFIX USER_LABEL_PREFIX IMMEDIATE_PREFIX If defined, C string expressions to be used for the `%R', `%L', `%U', and `%I' options of asm_fprintf (see `final.c'). These are useful when a single `md' file must support multiple assembler formats. In that case, the various `tm.h' files can define these macros differently. ASM_OUTPUT_REG_PUSH (stream, regno) A C expression to output to stream some assembler code which will push hard register number regno onto the stack. The code need not be optimal, since this macro is used only when profiling. ASM_OUTPUT_REG_POP (stream, regno) A C expression to output to stream some assembler code which will pop hard register number regno off of the stack. The code need not be optimal, since this macro is used only when profiling. Using GNU CC 347 16.16.7. Output of Dispatch Tables ASM_OUTPUT_ADDR_DIFF_ELT (stream, value, rel) This macro should be provided on machines where the addresses in a dispatch table are relative to the table's own address. The definition should be a C statement to output to the stdio stream stream an assembler pseudo- instruction to generate a difference between two labels. value and rel are the numbers of two internal labels. The definitions of these labels are output using ASM_OUTPUT_INTERNAL_LABEL, and they must be printed in the same way here. For example, fprintf (stream, "\t.word L%d-L%d\n", value, rel) ASM_OUTPUT_ADDR_VEC_ELT (stream, value) This macro should be provided on machines where the addresses in a dispatch table are absolute. The definition should be a C statement to output to the stdio stream stream an assembler pseudo-instruction to generate a reference to a label. value is the number of an internal label whose definition is output using ASM_OUTPUT_INTERNAL_LABEL. For example, fprintf (stream, "\t.word L%d\n", value) ASM_OUTPUT_CASE_LABEL (stream, prefix, num, table) Define this if the label before a jump-table needs to be output specially. The first three arguments are the same as for ASM_OUTPUT_INTERNAL_LABEL; the fourth argument is the jump-table which follows (a jump_insn containing an addr_vec or addr_diff_vec). This feature is used on system V to output a swbeg statement for the table. If this macro is not defined, these labels are output with ASM_OUTPUT_INTERNAL_LABEL. 348 Using GNU CC ASM_OUTPUT_CASE_END (stream, num, table) Define this if something special must be output at the end of a jump-table. The definition should be a C statement to be executed after the assembler code for the table is written. It should write the appropriate code to stdio stream stream. The argument table is the jump-table insn, and num is the label-number of the preceding label. If this macro is not defined, nothing special is output at the end of the jump-table. 16.16.8. Assembler Commands for Alignment ASM_OUTPUT_ALIGN_CODE (file) A C expression to output text to align the location counter in the way that is desirable at a point in the code that is reached only by jumping. This macro need not be defined if you don't want any special alignment to be done at such a time. Most machine descriptions do not currently define the macro. ASM_OUTPUT_LOOP_ALIGN (file) A C expression to output text to align the location counter in the way that is desirable at the beginning of a loop. This macro need not be defined if you don't want any special alignment to be done at such a time. Most machine descriptions do not currently define the macro. ASM_OUTPUT_SKIP (stream, nbytes) A C statement to output to the stdio stream stream an assembler instruction to advance the location counter by nbytes bytes. Those bytes should be zero when loaded. nbytes will be a C expression of type int. ASM_NO_SKIP_IN_TEXT Define this macro if ASM_OUTPUT_SKIP should not be used in the text section because it fails put zeros in the bytes that are skipped. This is true on many Unix systems, where the pseudo--op to skip bytes produces no-op instructions rather than zeros when used in the text section. ASM_OUTPUT_ALIGN (stream, power) A C statement to output to the stdio stream stream Using GNU CC 349 an assembler command to advance the location counter to a multiple of 2 to the power bytes. power will be a C expression of type int. 16.17. Controlling Debugging Information Format DBX_REGISTER_NUMBER (regno) A C expression that returns the DBX register number for the compiler register number regno. In simple cases, the value of this expression may be regno itself. But sometimes there are some registers that the compiler knows about and DBX does not, or vice versa. In such cases, some register may need to have one number in the compiler and another for DBX. If two registers have consecutive numbers inside GNU CC, and they can be used as a pair to hold a multiword value, then they must have consecutive numbers after renumbering with DBX_REGISTER_NUMBER. Otherwise, debuggers will be unable to access such a pair, because they expect register pairs to be consecutive in their own numbering scheme. If you find yourself defining DBX_REGISTER_NUMBER in way that does not preserve register pairs, then what you must do instead is redefine the actual register numbering scheme. DBX_DEBUGGING_INFO Define this macro if GNU CC should produce debugging output for DBX in response to the `-g' option. SDB_DEBUGGING_INFO Define this macro if GNU CC should produce COFF- style debugging output for SDB in response to the `-g' option. DWARF_DEBUGGING_INFO Define this macro if GNU CC should produce dwarf format debugging output in response to the `-g' option. DEFAULT_GDB_EXTENSIONS Define this macro to control whether GNU CC should by default generate GDB's extended version of DBX debugging information (assuming DBX-format debugging information is enabled at all). If you don't define the macro, the default is 1: always generate the extended information. 350 Using GNU CC DEBUG_SYMS_TEXT Define this macro if all .stabs commands should be output while in the text section. DEBUGGER_AUTO_OFFSET (x) A C expression that returns the integer offset value for an automatic variable having address x (an RTL expression). The default computation assumes that x is based on the frame-pointer and gives the offset from the frame-pointer. This is required for targets that produce debugging output for DBX or COFF-style debugging output for SDB and allow the frame-pointer to be eliminated when the `-g' options is used. DEBUGGER_ARG_OFFSET (offset, x) A C expression that returns the integer offset value for an argument having address x (an RTL expression). The nominal offset is offset. ASM_STABS_OP A C string constant naming the assembler pseudo op to use instead of .stabs to define an ordinary debugging symbol. If you don't define this macro, .stabs is used. This macro applies only to DBX debugging information format. ASM_STABD_OP A C string constant naming the assembler pseudo op to use instead of .stabd to define a debugging symbol whose value is the current location. If you don't define this macro, .stabd is used. This macro applies only to DBX debugging information format. ASM_STABN_OP A C string constant naming the assembler pseudo op to use instead of .stabn to define a debugging symbol with no name. If you don't define this macro, .stabn is used. This macro applies only to DBX debugging information format. PUT_SDB_... Define these macros to override the assembler syntax for the special SDB assembler directives. See `sdbout.c' for a list of these macros and their arguments. If the standard syntax is used, you need not define them yourself. SDB_DELIM Some assemblers do not support a semicolon as a delimiter, even between SDB assembler directives. In that case, define this macro to be the Using GNU CC 351 delimiter to use (usually `\n'). It is not necessary to define a new set of PUT_SDB_op macros if this is the only change required. SDB_GENERATE_FAKE Define this macro to override the usual method of constructing a dummy name for anonymous structure and union types. See `sdbout.c' for more information. SDB_ALLOW_UNKNOWN_REFERENCES Define this macro to allow references to unknown structure, union, or enumeration tags to be emitted. Standard COFF does not allow handling of unknown references, MIPS ECOFF has support for it. SDB_ALLOW_FORWARD_REFERENCES Define this macro to allow references to structure, union, or enumeration tags that have not yet been seen to be handled. Some assemblers choke if forward tags are used, while some require it. DBX_NO_XREFS Define this macro if DBX on your system does not support the construct `xstagname'. On some systems, this construct is used to describe a forward reference to a structure named tagname. On other systems, this construct is not supported at all. DBX_CONTIN_LENGTH A symbol name in DBX-format debugging information is normally continued (split into two separate .stabs directives) when it exceeds a certain length (by default, 80 characters). On some operating systems, DBX requires this splitting; on others, splitting must not be done. You can inhibit splitting by defining this macro with the value zero. You can override the default splitting-length by defining this macro as an expression for the length you desire. DBX_CONTIN_CHAR Normally continuation is indicated by adding a `\' character to the end of a .stabs string when a continuation follows. To use a different character instead, define this macro as a character constant for the character you want to use. Do not define this macro if backslash is correct for your system. 352 Using GNU CC DBX_STATIC_STAB_DATA_SECTION Define this macro if it is necessary to go to the data section before outputting the `.stabs' pseudo-op for a non-global static variable. DBX_LBRAC_FIRST Define this macro if the N_LBRAC symbol for a block should precede the debugging information for variables and functions defined in that block. Normally, in DBX format, the N_LBRAC symbol comes first. DBX_FUNCTION_FIRST Define this macro if the DBX information for a function and its arguments should precede the assembler code for the function. Normally, in DBX format, the debugging information entirely follows the assembler code. DBX_OUTPUT_FUNCTION_END (stream, function) Define this macro if the target machine requires special output at the end of the debugging information for a function. The definition should be a C statement (sans semicolon) to output the appropriate information to stream. function is the FUNCTION_DECL node for the function. DBX_OUTPUT_STANDARD_TYPES (syms) Define this macro if you need to control the order of output of the standard data types at the beginning of compilation. The argument syms is a tree which is a chain of all the predefined global symbols, including names of data types. Normally, DBX output starts with definitions of the types for integers and characters, followed by all the other predefined types of the particular language in no particular order. On some machines, it is necessary to output different particular types first. To do this, define DBX_OUTPUT_STANDARD_TYPES to output those symbols in the necessary order. Any predefined types that you don't explicitly output will be output afterward in no particular order. Be careful not to define this macro so that it works only for C. There are no global variables to access most of the built-in types, because another language may have another set of types. The way to output a particular type is to look through syms to see if you can find it. Here is an example: Using GNU CC 353 { tree decl; for (decl = syms; decl; decl = TREE_CHAIN (decl)) if (!strcmp (IDENTIFIER_POINTER (DECL_NAME (decl)), "long int")) dbxout_symbol (decl); ... } This does nothing if the expected type does not exist. See the function init_decl_processing in source file `c-decl.c' to find the names to use for all the built-in C types. DBX_OUTPUT_MAIN_SOURCE_FILENAME (stream, name) A C statement to output DBX debugging information to the stdio stream stream which indicates that file name is the main source file---the file specified as the input file for compilation. This macro is called only once, at the beginning of compilation. This macro need not be defined if the standard form of output for DBX debugging information is appropriate. DBX_OUTPUT_MAIN_SOURCE_DIRECTORY (stream, name) A C statement to output DBX debugging information to the stdio stream stream which indicates that the current directory during compilation is named name. This macro need not be defined if the standard form of output for DBX debugging information is appropriate. DBX_OUTPUT_MAIN_SOURCE_FILE_END (stream, name) A C statement to output DBX debugging information at the end of compilation of the main source file name. If you don't define this macro, nothing special is output at the end of compilation, which is correct for most machines. DBX_OUTPUT_SOURCE_FILENAME (stream, name) A C statement to output DBX debugging information to the stdio stream stream which indicates that file name is the current source 354 Using GNU CC file. This output is generated each time input shifts to a different source file as a result of `#include', the end of an included file, or a `#line' command. This macro need not be defined if the standard form of output for DBX debugging information is appropriate. 16.18. Cross Compilation and Floating Point Format While all modern machines use 2's complement represen- tation for integers, there are a variety of representations for floating point numbers. This means that in a cross- compiler the representation of floating point numbers in the compiled program may be different from that used in the machine doing the compilation. Because different representation systems may offer dif- ferent amounts of range and precision, the cross compiler cannot safely use the host machine's floating point arith- metic. Therefore, floating point constants must be represented in the target machine's format. This means that the cross compiler cannot use atof to parse a floating point constant; it must have its own special routine to use instead. Also, constant folding must emulate the target machine's arithmetic (or must not be done at all). The macros in the following table should be defined only if you are cross compiling between different floating point formats. Otherwise, don't define them. Then default definitions will be set up which use double as the data type, == to test for equality, etc. You don't need to worry about how many times you use an operand of any of these macros. The compiler never uses operands which have side effects. REAL_VALUE_TYPE A macro for the C data type to be used to hold a floating point value in the target machine's format. Typically this would be a struct containing an array of int. REAL_VALUES_EQUAL (x, y) A macro for a C expression which compares for equality the two values, x and y, both of type REAL_VALUE_TYPE. Using GNU CC 355 REAL_VALUES_LESS (x, y) A macro for a C expression which tests whether x is less than y, both values being of type REAL_VALUE_TYPE and interpreted as floating point numbers in the target machine's representation. REAL_VALUE_LDEXP (x, scale) A macro for a C expression which performs the standard library function ldexp, but using the target machine's floating point representation. Both x and the value of the expression have type REAL_VALUE_TYPE. The second argument, scale, is an integer. REAL_VALUE_FIX (x) A macro whose definition is a C expression to convert the target-machine floating point value x to a signed integer. x has type REAL_VALUE_TYPE. REAL_VALUE_UNSIGNED_FIX (x) A macro whose definition is a C expression to convert the target-machine floating point value x to an unsigned integer. x has type REAL_VALUE_TYPE. REAL_VALUE_FIX_TRUNCATE (x) A macro whose definition is a C expression to convert the target-machine floating point value x to a signed integer, rounding toward 0. x has type REAL_VALUE_TYPE. REAL_VALUE_UNSIGNED_FIX_TRUNCATE (x) A macro whose definition is a C expression to convert the target-machine floating point value x to an unsigned integer, rounding toward 0. x has type REAL_VALUE_TYPE. REAL_VALUE_ATOF (string) A macro for a C expression which converts string, an expression of type char *, into a floating point number in the target machine's representation. The value has type REAL_VALUE_TYPE. REAL_INFINITY Define this macro if infinity is a possible floating point value, and therefore division by 0 is legitimate. REAL_VALUE_ISINF (x) A macro for a C expression which determines whether x, a floating point value, is infinity. The value has type int. By default, this is 356 Using GNU CC defined to call isinf. REAL_VALUE_ISNAN (x) A macro for a C expression which determines whether x, a floating point value, is a ``nan'' (not-a-number). The value has type int. By default, this is defined to call isnan. Define the following additional macros if you want to make floating point constant folding work while cross com- piling. If you don't define them, cross compilation is still possible, but constant folding will not happen for floating point values. REAL_ARITHMETIC (output, code, x, y) A macro for a C statement which calculates an arithmetic operation of the two floating point values x and y, both of type REAL_VALUE_TYPE in the target machine's representation, to produce a result of the same type and representation which is stored in output (which will be a variable). The operation to be performed is specified by code, a tree code which will always be one of the following: PLUS_EXPR, MINUS_EXPR, MULT_EXPR, RDIV_EXPR, MAX_EXPR, MIN_EXPR. The expansion of this macro is responsible for checking for overflow. If overflow happens, the macro expansion should execute the statement return 0;, which indicates the inability to perform the arithmetic operation requested. REAL_VALUE_NEGATE (x) A macro for a C expression which returns the negative of the floating point value x. Both x and the value of the expression have type REAL_VALUE_TYPE and are in the target machine's floating point representation. There is no way for this macro to report overflow, since overflow can't happen in the negation operation. REAL_VALUE_TRUNCATE (x) A macro for a C expression which converts the double-precision floating point value x to single-precision. Both x and the value of the expression have type REAL_VALUE_TYPE and are in the target machine's floating point representation. However, the value Using GNU CC 357 should have an appropriate bit pattern to be output properly as a single-precision floating constant. There is no way for this macro to report overflow. REAL_VALUE_TO_INT (low, high, x) A macro for a C expression which converts a floating point value x into a double-precision integer which is then stored into low and high, two variables of type int. REAL_VALUE_FROM_INT (x, low, high) A macro for a C expression which converts a double-precision integer found in low and high, two variables of type int, into a floating point value which is then stored into x. 16.19. Miscellaneous Parameters PREDICATE_CODES Optionally define this if you have added predicates to `machine.c'. This macro is called within an initializer of an array of structures. The first field in the structure is the name of a predicate and the second field is an arrary of rtl codes. For each predicate, list all rtl codes that can be in expressions matched by the predicate. The list should have a trailing comma. Here is an example of two entries in the list for a typical RISC machine: #define PREDICATE_CODES \ {"gen_reg_rtx_operand", {SUBREG, REG}}, \ {"reg_or_short_cint_operand", {SUBREG, REG, CONST_INT}}, Defining this macro does not affect the generated code (however, incorrect definitions that omit an rtl code that may be matched by the predicate can cause the compiler to malfunction). Instead, it allows the table built by `genrecog' to be more compact and efficient, thus speeding up the compiler. The most important predicates to include in the list specified by this macro are thoses used in the most insn patterns. CASE_VECTOR_MODE An alias for a machine mode name. This is the 358 Using GNU CC machine mode that elements of a jump-table should have. CASE_VECTOR_PC_RELATIVE Define this macro if jump-tables should contain relative addresses. CASE_DROPS_THROUGH Define this if control falls through a case insn when the index value is out of range. This means the specified default-label is actually ignored by the case insn proper. BYTE_LOADS_ZERO_EXTEND Define this macro if an instruction to load a value narrower than a word from memory into a register also zero-extends the value to the whole register. IMPLICIT_FIX_EXPR An alias for a tree code that should be used by default for conversion of floating point values to fixed point. Normally, FIX_ROUND_EXPR is used. FIXUNS_TRUNC_LIKE_FIX_TRUNC Define this macro if the same instructions that convert a floating point number to a signed fixed point number also convert validly to an unsigned one. EASY_DIV_EXPR An alias for a tree code that is the easiest kind of division to compile code for in the general case. It may be TRUNC_DIV_EXPR, FLOOR_DIV_EXPR, CEIL_DIV_EXPR or ROUND_DIV_EXPR. These four division operators differ in how they round the result to an integer. EASY_DIV_EXPR is used when it is permissible to use any of those kinds of division and the choice should be made on the basis of efficiency. MOVE_MAX The maximum number of bytes that a single instruction can move quickly from memory to memory. SHIFT_COUNT_TRUNCATED Defining this macro causes the compiler to omit a sign-extend, zero-extend, or bitwise `and' instruction that truncates the count of a shift operation to a width equal to the Using GNU CC 359 number of bits needed to represent the size of the object being shifted. On machines that have instructions that act on bitfields at variable positions, including `bit test' instructions, defining SHIFT_COUNT_TRUNCATED also causes truncation not to be applied to these instructions. If both types of instructions truncate the count (for shifts) and position (for bitfield operations), or if no variable-position bitfield instructions exist, you should define this macro. However, on some machines, such as the 80386, truncation only applies to shift operations and not bitfield operations. Do not define SHIFT_COUNT_TRUNCATED on such machines. Instead, add patterns to the `md' file that include the implied truncation of the shift instructions. TRULY_NOOP_TRUNCATION (outprec, inprec) A C expression which is nonzero if on this machine it is safe to ``convert'' an integer of inprec bits to one of outprec bits (where outprec is smaller than inprec) by merely operating on it as if it had only outprec bits. On many machines, this expression can be 1. It is reported that suboptimal code can result when TRULY_NOOP_TRUNCATION returns 1 for a pair of sizes for modes for which MODES_TIEABLE_P is 0. If this is the case, making TRULY_NOOP_TRUNCATION return 0 in such cases may improve things. STORE_FLAG_VALUE A C expression describing the value returned by a comparison operator and stored by a store-flag instruction (`scond') when the condition is true. This description must apply to all the `scond' patterns and all the comparison operators. A value of 1 or -1 means that the instruction implementing the comparison operator returns exactly 1 or -1 when the comparison is true and 0 when the comparison is false. Otherwise, the value indicates which bits of the result are guaranteed to be 1 when the 360 Using GNU CC comparison is true. This value is interpreted in the mode of the comparison operation, which is given by the mode of the first operand in the `scond' pattern. Either the low bit or the sign bit of STORE_FLAG_VALUE be on. Presently, only those bits are used by the compiler. If STORE_FLAG_VALUE is neither 1 or -1, the compiler will generate code that depends only on the specified bits. It can also replace comparison operators with equivalent operations if they cause the required bits to be set, even if the remaining bits are undefined. For example, on a machine whose comparison operators return an SImode value and where STORE_FLAG_VALUE is defined as `0x80000000', saying that just the sign bit is relevant, the expression (ne:SI (and:SI x (const_int power-of-2)) (const_int 0)) can be converted to (ashift:SI x (const_int n)) where n is the appropriate shift count to move the bit being tested into the sign bit. There is no way to describe a machine that always sets the low-order bit for a true value, but does not guarantee the value of any other bits, but we do not know of any machine that has such an instruction. If you are trying to port GNU CC to such a machine, include an instruction to perform a logical- and of the result with 1 in the pattern for the comparison operators and let us know (see section Bug Reporting). Often, a machine will have multiple instructions that obtain a value from a comparison (or the condition codes). Here are rules to guide the choice of value for STORE_FLAG_VALUE, and hence the instructions to be used: Using GNU CC 361 o+ Use the shortest sequence that yields a valid definition for STORE_FLAG_VALUE. It is more efficent for the compiler to ``normalize'' the value (convert it to, e.g., 1 or 0) than for the comparison operators to do so because there may be opportunities to combine the normalization with other operations. o+ For equal-length sequences, use a value of 1 or -1, with -1 being slightly preferred on machines with expensive jumps and 1 preferred on other machines. o+ As a second choice, choose a value of `0x80000001' if instructions exist that set both the sign and low-order bits but do not define the others. o+ Otherwise, use a value of `0x80000000'. You need not define STORE_FLAG_VALUE if the machine has no store-flag instructions. Pmode An alias for the machine mode for pointers. Normally the definition can be #define Pmode SImode FUNCTION_MODE An alias for the machine mode used for memory references to functions being called, in call RTL expressions. On most machines this should be QImode. INTEGRATE_THRESHOLD (decl) A C expression for the maximum number of instructions above which the function decl should not be inlined. decl is a FUNCTION_DECL node. The default definition of this macro is 64 plus 8 times the number of arguments that the function accepts. Some people think a larger threshold should be used on RISC machines. SCCS_DIRECTIVE Define this if the preprocessor should ignore #sccs directives and print no error message. 362 Using GNU CC HANDLE_PRAGMA (stream) Define this macro if you want to implement any pragmas. If defined, it should be a C statement to be executed when #pragma is seen. The argument stream is the stdio input stream from which the source text can be read. It is generally a bad idea to implement new uses of #pragma. The only reason to define this macro is for compatibility with other compilers that do support #pragma for the sake of any user programs which already use it. HAVE_VPRINTF Define this if the library function vprintf is available on your system. DOLLARS_IN_IDENTIFIERS Define this macro to control use of the character `$' in identifier names. The value should be 0, 1, or 2. 0 means `$' is not allowed by default; 1 means it is allowed by default if `-traditional' is used; 2 means it is allowed by default provided `-ansi' is not used. 1 is the default; there is no need to define this macro in that case. DEFAULT_MAIN_RETURN Define this macro if the target system expects every program's main function to return a standard ``success'' value by default (if no other value is explicitly returned). The definition should be a C statement (sans semicolon) to generate the appropriate rtl instructions. It is used only when compiling the end of main. HAVE_ATEXIT Define this if the target system supports the function atexit from the ANSI C standard. If this is not defined, and INIT_SECTION_ASM_OP is not defined, a default exit function will be provided to support C++. EXIT_BODY Define this if your exit function needs to do something besides calling an external function _cleanup before terminating with _exit. The EXIT_BODY macro is only needed if netiher HAVE_ATEXIT nor INIT_SECTION_ASM_OP are defined. Using GNU CC 363 INTERNALS 364 Using GNU CC 17. The Configuration File The configuration file `xm-machine.h' contains macro definitions that describe the machine and system on which the compiler is running, unlike the definitions in `machine.h', which describe the machine for which the com- piler is producing output. Most of the values in `xm- machine.h' are actually the same on all machines that GNU CC runs on, so large parts of all configuration files are identical. But there are some macros that vary: USG Define this macro if the host system is System V. VMS Define this macro if the host system is VMS. FAILURE_EXIT_CODE A C expression for the status code to be returned when the compiler exits after serious errors. SUCCESS_EXIT_CODE A C expression for the status code to be returned when the compiler exits without serious errors. HOST_WORDS_BIG_ENDIAN Defined if the host machine stores words of multi-word values in big-endian order. (GNU CC does not depend on the host byte ordering within a word.) HOST_FLOAT_FORMAT A numeric code distinguishing the floating point format for the host machine. See TARGET_FLOAT_FORMAT in `Storage Layout' for the alternatives and default. HOST_BITS_PER_CHAR A C expression for the number of bits in char on the host machine. HOST_BITS_PER_SHORT A C expression for the number of bits in short on the host machine. HOST_BITS_PER_INT A C expression for the number of bits in int on the host machine. HOST_BITS_PER_LONG A C expression for the number of bits in long on the host machine. ONLY_INT_FIELDS Define this macro to indicate that the host Using GNU CC 365 compiler only supports int bit fields, rather than other integral types, including enum, as do most C compilers. EXECUTABLE_SUFFIX Define this macro if the host system uses a naming convention for executable files that involves a common suffix (such as, in some systems, `.exe') that must be mentioned explicitly when you run the program. OBSTACK_CHUNK_SIZE A C expression for the size of ordinary obstack chunks. If you don't define this, a usually- reasonable default is used. OBSTACK_CHUNK_ALLOC The function used to allocate obstack chunks. If you don't define this, xmalloc is used. OBSTACK_CHUNK_FREE The function used to free obstack chunks. If you don't define this, free is used. USE_C_ALLOCA Define this macro to indicate that the compiler is running with the alloca implemented in C. This version of alloca can be found in the file `alloca.c'; to use it, you must also alter the `Makefile' variable ALLOCA. (This is done automatically for the systems on which we know it is needed.) If you do define this macro, you should probably do it as follows: #ifndef __GNUC__ #define USE_C_ALLOCA #else #define alloca __builtin_alloca #endif so that when the compiler is compiled with GNU CC it uses the more efficient built-in alloca function. FUNCTION_CONVERSION_BUG Define this macro to indicate that the host compiler does not properly handle converting a function value to a pointer-to-function when 366 Using GNU CC it is used in an expression. In addition, configuration files for system V define bcopy, bzero and bcmp as aliases. Some files define alloca as a macro when compiled with GNU CC, in order to take advantage of the benefit of GNU CC's built-in alloca. Index INTERNALS Using GNU CC 367 Index Using GNU CC i Table of Contents GNU GENERAL PUBLIC LICENSE ............................ 2 Preamble .............................................. 2 TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION .................................................. 3 NO WARRANTY ........................................... 8 END OF TERMS AND CONDITIONS ........................... 9 Appendix: How to Apply These Terms to Your New Programs .................................................. 9 Contributors to GNU CC ................................ 10 1 Protect Your Freedom--- Fight ``Look And Feel'' .......................... 12 2 GNU CC Command Options ........................ 15 2.1 Options Controlling the Kind of Output ....... 19 2.2 Options Controlling Dialect .................. 21 2.3 Options to Request or Suppress Warnings ...... 24 2.4 Options for Debugging Your Program or GNU CC ..... 31 2.5 Options That Control Optimization ............ 35 2.6 Options Controlling the Preprocessor ......... 39 2.7 Options for Linking .......................... 42 2.8 Options for Directory Search ................. 43 2.9 Specifying Target Machine and Compiler Version .................................................. 45 2.10 Specifying Hardware Models and Configurations .................................................. 46 2.10.1 M680x0 Options .............................. 47 2.10.2 VAX Options ................................. 48 2.10.3 SPARC Options ............................... 49 2.10.4 Convex Options .............................. 49 2.10.5 AMD29K Options .............................. 49 2.10.6 M88K Options ................................ 50 2.10.7 IBM RS/6000 Options ......................... 53 2.10.8 IBM RT Options .............................. 54 2.10.9 MIPS Options ................................ 55 2.11 Options for Code Generation Conventions ..... 58 2.12 Environment Variables Affecting GNU CC ...... 61 3 Installing GNU CC ............................. 63 3.1 Compilation in a Separate Directory .......... 74 3.2 Installing GNU CC on the Sun ................. 75 3.3 Installing GNU CC on the 3b1 ................. 75 3.4 Installing GNU CC on SCO System V 3.2 ........ 76 3.5 Installing GNU CC on Unos .................... 76 3.6 Installing GNU CC on VMS ..................... 77 4 Known Causes of Trouble with GNU CC ........... 81 5 How To Get Help with GNU CC ................... 84 6 Incompatibilities of GNU CC ................... 84 7 GNU Extensions to the C Language .............. 89 7.1 ii Using GNU CC Statements and Declarations within Expressions .................................................. 89 7.2 Locally Declared Labels ...................... 90 7.3 Labels as Values ............................. 91 7.4 Nested Functions ............................. 92 7.5 Naming an Expression's Type .................. 94 7.6 Referring to a Type with typeof .............. 95 7.7 Generalized Lvalues .......................... 96 7.8 Conditional Expressions with Omitted Operands .................................................. 98 7.9 Double-Word Integers ......................... 98 7.10 Arrays of Length Zero ....................... 99 7.11 Arrays of Variable Length ................... 99 7.12 Non-Lvalue Arrays May Have Subscripts ....... 101 7.13 Arithmetic on void- and Function-Pointers .................................................. 101 7.14 Non-Constant Initializers ................... 101 7.15 Constructor Expressions ..................... 102 7.16 Labeled Elements in Initializers ............ 103 7.17 Case Ranges ................................. 105 7.18 Cast to a Union Type ........................ 105 7.19 Declaring Attributes of Functions ........... 106 7.20 Dollar Signs in Identifier Names ............ 108 7.21 The Character ESC in Constants .............. 108 7.22 Inquiring on Alignment of Types or Variables .................................................. 108 7.23 Specifying Attributes of Variables .......... 109 7.24 An Inline Function is As Fast As a Macro .................................................. 110 7.25 Assembler Instructions with C Expression Operands .................................................. 112 7.26 Controlling Names Used in Assembler Code .................................................. 116 7.27 Variables in Specified Registers ............ 117 7.27.1 Defining Global Register Variables .......... 118 7.27.2 Specifying Registers for Local Variables .................................................. 120 7.28 Alternate Keywords .......................... 120 7.29 Incomplete enum Types ....................... 121 8 Reporting Bugs ................................ 121 8.1 Have You Found a Bug? ........................ 122 8.2 How to Report Bugs ........................... 123 8.3 Certain Changes We Don't Want to Make ........ 128 9 Using GNU CC on VMS ........................... 132 10 Using GNU CC on VMS ........................... 133 10.1 Include Files and VMS ....................... 133 10.2 Global Declarations and VMS ................. 135 10.3 Other VMS Issues ............................ 137 11 GNU CC and Portability ........................ 139 12 Interfacing to GNU CC Output .................. 140 Using GNU CC iii 13 Passes and Files of the Compiler .............. 142 14 RTL Representation ............................ 151 14.1 RTL Object Types ............................ 151 14.2 Access to Operands .......................... 152 14.3 Flags in an RTL Expression .................. 155 14.4 Machine Modes ............................... 159 14.5 Constant Expression Types ................... 163 14.6 Registers and Memory ........................ 166 14.7 RTL Expressions for Arithmetic .............. 171 14.8 Comparison Operations ....................... 175 14.9 Bit Fields .................................. 177 14.10 Conversions ................................ 178 14.11 Declarations ............................... 179 14.12 Side Effect Expressions .................... 180 14.13 Embedded Side-Effects on Addresses ......... 186 14.14 Assembler Instructions as Expressions ...... 187 14.15 Insns ...................................... 188 14.16 RTL Representation of Function-Call Insns .................................................. 198 14.17 Structure Sharing Assumptions .............. 199 15 Machine Descriptions .......................... 201 15.1 Everything about Instruction Patterns ....... 201 15.2 Example of define_insn ...................... 202 15.3 RTL Template for Generating and Recognizing Insns .................................................. 203 15.4 Output Templates and Operand Substitution .................................................. 208 15.5 C Statements for Generating Assembler Output .................................................. 209 15.6 Operand Constraints ......................... 211 15.6.1 Simple Constraints .......................... 211 15.6.2 Multiple Alternative Constraints ............ 217 15.6.3 Register Class Preferences .................. 218 15.6.4 Constraint Modifier Characters .............. 218 15.6.5 Not Using Constraints ....................... 220 15.7 Standard Names for Patterns Used in Generation .................................................. 220 15.8 When the Order of Patterns Matters .......... 231 15.9 Interdependence of Patterns ................. 232 15.10 Defining Jump Instruction Patterns ......... 233 15.11 Canonicalization of Instructions ........... 236 15.12 Defining Machine- Specific Peephole Optimizers ..................... 238 15.13 Defining RTL Sequences for Code Generation ....... 241 15.14 Splitting Instructions into Multiple Instructions .................................................. 245 15.15 Instruction Attributes ..................... 246 15.15.1 Defining Attributes and their Values ....... 246 iv Using GNU CC 15.15.2 Attribute Expressions ...................... 248 15.15.3 Assigning Attribute Values to Insns ........ 251 15.15.4 Example of Attribute Specifications ........ 253 15.15.5 Computing the Length of an Insn ............ 254 15.15.6 Delay Slot Scheduling ...................... 255 15.15.7 Specifying Function Units .................. 257 16 Machine Description Macros .................... 260 16.1 Controlling the Compilation Driver, `gcc' .................................................. 260 16.2 Run-time Target Specification ............... 265 16.3 Storage Layout .............................. 268 16.4 Layout of Source Language Data Types ........ 273 16.5 Register Usage .............................. 276 16.5.1 Basic Characteristics of Registers .......... 276 16.5.2 Order of Allocation of Registers ............ 278 16.5.3 How Values Fit in Registers ................. 278 16.5.4 Handling Leaf Functions ..................... 281 16.5.5 Registers That Form a Stack ................. 282 16.5.6 Obsolete Macros for Controlling Register Usage .................................................. 282 16.6 Register Classes ............................ 283 16.7 Describing Stack Layout and Calling Conventions .................................................. 291 16.7.1 Basic Stack Layout .......................... 291 16.7.2 Registers That Address the Stack Frame .................................................. 292 16.7.3 Eliminating Frame Pointer and Arg Pointer .................................................. 294 16.7.4 Passing Function Arguments on the Stack .................................................. 296 16.7.5 Passing Arguments in Registers .............. 298 16.7.6 How Scalar Function Values Are Returned .................................................. 302 16.7.7 How Large Values Are Returnd ................ 304 16.7.8 Caller-Saves Register Allocation ............ 306 16.7.9 Function Entry and Exit ..................... 306 16.7.10 Generating Code for Profiling .............. 310 16.8 Implementing the Varargs Macros ............. 312 16.9 Trampolines for Nested Functions ............ 315 16.10 Implicit Calls to Library Routines ......... 317 16.11 Addressing Modes ........................... 320 16.12 Condition Code Status ...................... 324 16.13 Describing Relative Costs of Operations .................................................. 327 16.14 Dividing the Output into Sections (Texts, Data, ...) .................................................. 331 16.15 Position Independent Code .................. 333 16.16 Defining the Output Assembler Language .................................................. 334 16.16.1 Using GNU CC v The Overall Framework of an Assembler File ...... 334 16.16.2 Output of Data ............................. 336 16.16.3 Output of Uninitialized Variables .......... 338 16.16.4 Output and Generation of Labels ............ 339 16.16.5 Output of Initialization Routines .......... 342 16.16.6 Output of Assembler Instructions ........... 343 16.16.7 Output of Dispatch Tables .................. 347 16.16.8 Assembler Commands for Alignment ........... 348 16.17 Controlling Debugging Information Format .................................................. 349 16.18 Cross Compilation and Floating Point Format ...... 354 16.19 Miscellaneous Parameters ................... 357 17 The Configuration File ........................ 364 Index ................................................. 366 Index ................................................. 367