\input texinfo @c -*-texinfo-*- @settitle Internals of GNU CC @setfilename internals @ifinfo This file documents the internals of the GNU compiler. Copyright (C) 1988 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. @ignore Permission is granted to process this file through Tex and print the results, provided the printed document carries copying permission notice identical to this one except for the removal of this paragraph (this paragraph not being relevant to the printed manual). @end ignore 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 CC General Public License'' is included exactly as in the original, 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 translations of this manual into another language, under the above conditions for modified versions, except that the section entitled ``GNU CC General Public License'' and this permission notice may be included in translations approved by the Free Software Foundation instead of in the original English. @end ifinfo @setchapternewpage odd @titlepage @center @titlefont{Internals of GNU CC} @sp 2 @center Richard M. Stallman @sp 3 @center last updated 24 April 1988 @sp 1 @center for version 1.21 @page @vskip 0pt plus 1filll Copyright @copyright{} 1988 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 CC General Public License'' is included exactly as in the original, 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 translations of this manual into another language, under the above conditions for modified versions, except that the section entitled ``GNU CC General Public License'' may be included in a translation approved by the author instead of in the original English. @end titlepage @page @ifinfo @node Top, Copying,, (DIR) @ichapter Introduction This manual documents how to run, install and port the GNU C compiler, as well as its new features and incompatibilities, and how to report bugs. @end ifinfo @menu * Copying:: GNU CC General Public License says how you can copy and share GNU CC. * Contributors:: People who have contributed to GNU CC. * Options:: Command options supported by @samp{gcc}. * Installation:: How to configure, compile and install GNU CC. * Trouble:: If you have trouble installing GNU CC. * Incompatibilities:: Incompatibilities of GNU CC. * Extensions:: GNU extensions to the C language. * Bugs:: How to report bugs (if you want to get them fixed). * Portability:: Goals of GNU CC's portability features. * Interface:: Function-call interface of GNU CC output. * Passes:: Order of passes, what they do, and what each file is for. * RTL:: The intermediate representation that most passes work on. * Machine Desc:: How to write machine description instruction patterns. * Machine Macros:: How to write the machine description C macros. @end menu @node Copying, Contributors, Top, Top @unnumbered GNU CC GENERAL PUBLIC LICENSE @center (Clarified 11 Feb 1988) The license agreements of most software companies keep you at the mercy of those companies. By contrast, our general public license is intended to give everyone the right to share GNU CC. To make sure that you get the rights we want you to have, we need to make restrictions that forbid anyone to deny you these rights or to ask you to surrender the rights. Hence this license agreement. Specifically, we want to make sure that you have the right to give away copies of GNU CC, that you receive source code or else can get it if you want it, that you can change GNU CC or use pieces of it in new free programs, and that you know you can do these things. To make sure that everyone has such rights, we have to forbid you to deprive anyone else of these rights. For example, if you distribute copies of GNU CC, you must give the recipients all the rights that you have. You must make sure that they, too, receive or can get the source code. And you must tell them their rights. Also, for our own protection, we must make certain that everyone finds out that there is no warranty for GNU CC. If GNU CC is modified by someone else and passed on, we want its recipients to know that what they have is not what we distributed, so that any problems introduced by others will not reflect on our reputation. Therefore we (Richard Stallman and the Free Software Foundation, Inc.) make the following terms which say what you must do to be allowed to distribute or change GNU CC. @unnumberedsec COPYING POLICIES @enumerate @item You may copy and distribute verbatim copies of GNU CC source code as you receive it, in any medium, provided that you conspicuously and appropriately publish on each copy a valid copyright notice ``Copyright @copyright{} 1988 Free Software Foundation, Inc.'' (or with whatever year is appropriate); keep intact the notices on all files that refer to this License Agreement and to the absence of any warranty; and give any other recipients of the GNU CC program a copy of this License Agreement along with the program. You may charge a distribution fee for the physical act of transferring a copy. @item You may modify your copy or copies of GNU CC or any portion of it, and copy and distribute such modifications under the terms of Paragraph 1 above, provided that you also do the following: @itemize @bullet @item cause the modified files to carry prominent notices stating that you changed the files and the date of any change; and @item cause the whole of any work that you distribute or publish, that in whole or in part contains or is a derivative of GNU CC or any part thereof, to be licensed at no charge to all third parties on terms identical to those contained in this License Agreement (except that you may choose to grant more extensive warranty protection to some or all third parties, at your option). @item You may charge a distribution fee for the physical act of transferring a copy, and you may at your option offer warranty protection in exchange for a fee. @end itemize Mere aggregation of another unrelated program with this program (or its derivative) on a volume of a storage or distribution medium does not bring the other program under the scope of these terms. @item You may copy and distribute GNU CC (or a portion or derivative of it, under Paragraph 2) in object code or executable form under the terms of Paragraphs 1 and 2 above provided that you also do one of the following: @itemize @bullet @item accompany it with the complete corresponding machine-readable source code, which must be distributed under the terms of Paragraphs 1 and 2 above; or, @item accompany it with a written offer, valid for at least three years, to give any third party free (except for a nominal shipping charge) a complete machine-readable copy of the corresponding source code, to be distributed under the terms of Paragraphs 1 and 2 above; or, @item accompany it with the information you received as to where the corresponding source code may be obtained. (This alternative is allowed only for noncommercial distribution and only if you received the program in object code or executable form alone.) @end itemize For an executable file, complete source code means all the source code for all modules it contains; but, as a special exception, it need not include source code for modules which are standard libraries that accompany the operating system on which the executable file runs. @item You may not copy, sublicense, distribute or transfer GNU CC except as expressly provided under this License Agreement. Any attempt otherwise to copy, sublicense, distribute or transfer GNU CC is void and your rights to use the program under this License agreement shall be automatically terminated. However, parties who have received computer software programs from you with this License Agreement will not have their licenses terminated so long as such parties remain in full compliance. @item If you wish to incorporate parts of GNU CC into other free programs whose distribution conditions are different, write to the Free Software Foundation at 675 Mass Ave, Cambridge, MA 02139. We have not yet worked out a simple rule that can be stated here, but we will often permit this. We will be guided by the two goals of preserving the free status of all derivatives of our free software and of promoting the sharing and reuse of software. @end enumerate Your comments and suggestions about our licensing policies and our software are welcome! Please contact the Free Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, or call (617) 876-3296. @unnumberedsec NO WARRANTY BECAUSE GNU CC IS LICENSED FREE OF CHARGE, WE PROVIDE ABSOLUTELY NO WARRANTY, TO THE EXTENT PERMITTED BY APPLICABLE STATE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING, FREE SOFTWARE FOUNDATION, INC, RICHARD M. STALLMAN AND/OR OTHER PARTIES PROVIDE GNU CC "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF GNU CC IS WITH YOU. SHOULD GNU CC PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR OR CORRECTION. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW WILL RICHARD M. STALLMAN, THE FREE SOFTWARE FOUNDATION, INC., AND/OR ANY OTHER PARTY WHO MAY MODIFY AND REDISTRIBUTE GNU CC AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY LOST PROFITS, LOST MONIES, OR OTHER SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS) GNU CC, EVEN IF YOU HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES, OR FOR ANY CLAIM BY ANY OTHER PARTY. @node Contributors, Options, Copying, Top @unnumbered Contributors to GNU CC In addition to Richard Stallman, several people have written parts of GNU CC. @itemize @bullet @item The idea of using RTL and some of the optimization ideas came from the U. of Arizona Portable Optimizer, written by Jack Davidson and Christopher Fraser. See ``Register Allocation and Exhaustive Peephole Optimization'', Software Practice and Experience 14 (9), Sept. 1984, 857-866. @item Paul Rubin wrote most of the preprocessor. @item Leonard Tower wrote parts of the parser, RTL generator, RTL definitions, and of the Vax machine description. @item Ted Lemon wrote parts of the RTL reader and printer. @item Nobuyuki Hikichi of Software Research Associates, Tokyo, contributed the support for the SONY NEWS machine. @item Charles LaBrec contributed the support for the Integrated Solutions 68020 system. @item Michael Tiemann of MCC wrote the description of the National Semiconductor 32000 series cpu, with some contributions from Jan Stein of the Chalmers Computer Club. Tiemann also wrote the code for inline function integration. @item Robert Brown implemented the support for Encore 32000 systems. @item Michael Kashtan of SRI adapted GNU CC to the Vomit-Making System. @item Alex Crain provided changes for the 3b1. @item Chris Hanson and another person who should remind me of his name assisted in making GNU CC work on HP-UX for the 9000 series 300. @end itemize @node Options, Installation, Contributors, Top @chapter GNU CC Command Options The GNU C compiler uses a command syntax much like the Unix C compiler. The @code{gcc} program accepts options and file names as operands. Multiple single-letter options may @emph{not} be grouped: @samp{-dr} is very different from @samp{-d -r}. When you invoke GNU CC, it normally does preprocessing, compilation, assembly and linking. File names which end in @samp{.c} are taken as C source to be preprocessed and compiled; compiler output files plus any input files with names ending in @samp{.s} are assembled; then the resulting object files, plus any other input files, are linked together to produce an executable. Command options allow you to stop this process at an intermediate stage. For example, the @samp{-c} option says not to run the linker. Then the output consists of object files output by the assembler. Other command options are passed on to one stage. Some options control the preprocessor and others the compiler itself. Yet other options control the assembler and linker; these are not documented here because the GNU assembler and linker are not yet released. Here are the options to control the overall compilation process, including those that say whether to link, whether to assemble, and so on. @table @samp @item -o @var{file} Place output in file @var{file}. This applies regardless to whatever sort of output is being produced, whether it be an executable file, an object file, an assembler file or preprocessed C code. If @samp{-o} is not specified, the default is to put an executable file in @file{a.out}, the object file @file{@var{source}.c} in @file{@var{source}.o}, an assembler file in @file{@var{source}.s}, and preprocessed C on standard output.@refill @item -c Compile or assemble the source files, but do not link. Produce object files with names made by replacing @samp{.c} or @samp{.s} with @samp{.o} at the end of the input file names. Do nothing at all for object files specified as input. @item -S Compile into assembler code but do not assemble. The assembler output file name is made by replacing @samp{.c} with @samp{.s} at the end of the input file name. Do nothing at all for assembler source files or object files specified as input. @item -E Run only the C preprocessor. Preprocess all the C source files specified and output the results to standard output. @item -v Compiler driver program prints the commands it executes as it runs the preprocessor, compiler proper, assembler and linker. Some of these are directed to print their own version numbers. @item -B@var{prefix} Compiler driver program tries @var{prefix} as a prefix for each program it tries to run. These programs are @file{cpp}, @file{cc1}, @file{as} and @file{ld}. For each subprogram to be run, the compiler driver first tries the @samp{-B} prefix, if any. If that name is not found, or if @samp{-B} was not specified, the driver tries two standard prefixes, which are @file{/usr/lib/gcc-} and @file{/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 @samp{PATH} environment variable. The run-time support file @file{gnulib} is also searched for using the @samp{-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. Most of the time, on most machines, you can do without it. @end table These options control the details of C compilation itself. @table @samp @item -ansi Support all ANSI standard C programs. This turns off certain features of GNU C that are incompatible with ANSI C, such as the @code{asm}, @code{inline} and @code{typeof} keywords, and predefined macros such as @code{unix} and @code{vax} that identify the type of system you are using. It also enables the undesirable and rarely used ANSI trigraph feature. The @samp{-ansi} option does not cause non-ANSI programs to be rejected gratuitously. For that, @samp{-pedantic} is required in addition to @samp{-ansi}. The macro @code{__STRICT_ANSI__} is predefined when the @samp{-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. @item -traditional Attempt to support some aspects of traditional C compilers. Specifically: @itemize @bullet @item All @code{extern} declarations take effect globally even if they are written inside of a function definition. This includes implicit declarations of functions. @item The keywords @code{typeof}, @code{inline}, @code{signed}, @code{const} and @code{volatile} are not recognized.@refill @item Comparisons between pointers and integers are always allowed. @item Integer types @code{unsigned short} and @code{unsigned char} promote to @code{unsigned int}. @item In the preprocessor, comments convert to nothing at all, rather than to a space. This allows traditional token concatenation. @item In the preprocessor, single and double quote characters are ignored when scanning macro definitions, so that macro arguments can be replaced even within a string or character constant. Quote characters are also ignored when skipping text inside a failing conditional directive. @end itemize @item -O Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large function. Without @samp{-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 @samp{-O}, only variables declared @code{register} are allocated in registers. The resulting compiled code is a little worse than produced by PCC without @samp{-O}. With @samp{-O}, the compiler tries to reduce code size and execution time. Some of the @samp{-f} options described below turn specific kinds of optimization on or off. @item -g Produce debugging information in DBX format. Unlike most other C compilers, GNU CC allows you to use @samp{-g} with @samp{-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. @item -gg Produce debugging information in GDB's own format. This requires the GNU assembler and linker in order to work. @item -w Inhibit all warning messages. @item -W Print extra warning messages for these events: @itemize @bullet @item 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. They occur only for variables that are candidates for register allocation. Therefore, they do not occur for a variable that is declared @code{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 the flow analysis pass 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: @example @{ int x; switch (y) @{ case 1: x = 1; break; case 2: x = 4; break; case 3: x = 5; @} foo (x); @} @end example @noindent If the value of @code{y} is always 1, 2 or 3, then @code{x} is always initialized, but GNU CC doesn't know this. Here is another common case: @example @{ int save_y; if (change_y) save_y = y, y = new_y; @dots{} if (change_y) y = save_y; @} @end example @noindent This has no bug because @code{x} is used only if it is set. @item A nonvolatile automatic variable might be changed by a call to @code{longjmp}. These warnings as well are possible only in optimizing compilation. The compiler sees only the calls to @code{setjmp}. It cannot know where @code{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 @code{longjmp} cannot in fact be called at the place which would cause a problem. @item 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 inspire such a warning: @example foo (a) @{ if (a > 0) return a; @} @end example Spurious warnings can occur because GNU CC does not realize that certain functions (including @code{abort} and @code{longjmp}) will never return. @end itemize In the future, other useful warnings may also be enabled by this option. @item -Wimplicit Warn whenever a function is implicitly declared. @item -Wreturn-type Warn whenever a function is defined with a return-type that defaults to @code{int}. Also warn about any @code{return} statement with no return-value in a function whose return-type is not @code{void}. @item -Wcomment Warn whenever a comment-start sequence @samp{/*} appears in a comment. @item -Wall All of the above @samp{-W} options combined. @item -p Generate extra code to write profile information suitable for the analysis program @code{prof}. @item -pg Generate extra code to write profile information suitable for the analysis program @code{gprof}. @item -l@var{library} Search a standard list of directories for a library named @var{library}, which is actually a file named @file{lib@var{library}.a}. The linker 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 @samp{-L}. Normally the files found this way are library files---archive files whose members are object files. The linker handles an archive file by 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 an @samp{-l} option and the full file name of the file that is found is syntactic and the fact that several directories are searched. @item -L@var{dir} Add directory @var{dir} to the list of directories to be searched for @samp{-l}. @item -nostdlib Don't use the standard system libraries and startup files when linking. Only the files you specify (plus @file{gnulib}) will be passed to the linker. @item -m@var{machinespec} Machine-dependent option specifying something about the type of target machine. These options are defined by the macro @code{TARGET_SWITCHES} in the machine description. The default for the options is also defined by that macro, which enables you to change the defaults.@refill These are the @samp{-m} options defined in the 68000 machine description: @table @samp @item -m68020 Generate output for a 68020 (rather than a 68000). This is the default if you use the unmodified sources. @item -m68000 Generate output for a 68000 (rather than a 68020). @item -m68881 Generate output containing 68881 instructions for floating point. This is the default if you use the unmodified sources. @item -msoft-float Generate output containing library calls for floating point. @item -mshort Consider type @code{int} to be 16 bits wide, like @code{short int}. @item -mnobitfield Do not use the bit-field instructions. @samp{-m68000} implies @samp{-mnobitfield}. @item -mbitfield Do use the bit-field instructions. @samp{-m68020} implies @samp{-mbitfield}. This is the default if you use the unmodified sources. @item -mrtd Use a different function-calling convention, in which functions that take a fixed number of arguments return with the @code{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 @code{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 @code{rtd} instruction is supported by the 68010 and 68020 processors, but not by the 68000. @end table These @samp{-m} options are defined in the Vax machine description: @table @samp @item -munix Do not output certain jump instructions (@code{aobleq} and so on) that the Unix assembler for the Vax cannot handle across long ranges. @item -mgnu Do output those jump instructions, on the assumption that you will assemble with the GNU assembler. @item -mg Output code for g-format floating point numbers instead of d-format. @end table @item -f@var{flag} Specify machine-independent flags. These are the flags: @table @samp @item -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 @code{double} is supposed to have. For most programs, the excess precision does only good, but a few programs rely on the precise definition of IEEE floating point. Use @samp{-ffloat-store} for such programs. @item -fno-asm Do not recognize @code{asm}, @code{inline} or @code{typeof} as a keyword. These words may then be used as identifiers. @item -fno-defer-pop Always pop the arguments to each function call as soon as that function returns. Normally the compiler (when optimizing) lets arguments accumulate on the stack for several function calls and pops them all at once. @item -fcombine-regs Allow the combine pass to combine an instruction that copies one register into another. This might or might not produce better code when used in addition to @samp{-O}. I am interested in hearing about the difference this makes. @item -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. @item -fforce-addr Force memory address constants to be copied into registers before doing arithmetic on them. This may produce better code just as @samp{-fforce-mem} may. I am interested in hearing about the difference this makes. @item -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. @strong{It also makes debugging impossible.} 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 @code{FRAME_POINTER_REQUIRED} controls whether a target machine supports this flag. @xref{Registers}.@refill @item -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 @code{static}, then the function is normally not output as assembler code in its own right. @item -fkeep-inline-functions Even if all calls to a given function are integrated, and the function is declared @code{static}, nevertheless output a separate run-time callable version of the function. @item -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. Writing into string constants is a very bad idea; ``constants'' should be constant. @item -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. @item -fvolatile Consider all memory references through pointers to be volatile. @item -funsigned-char Let the type @code{char} be the unsigned, like @code{unsigned char}. Each kind of machine has a default for what @code{char} should be. It is either like @code{unsigned char} by default or like @code{signed char} by default. (Actually, at present, the default is always signed.) The type @code{char} is always a distinct type from either @code{signed char} or @code{unsigned char}, even though its behavior is always just like one of those two. @item -fsigned-char Let the type @code{char} be signed, like @code{signed char}. @item -ffixed-@var{reg} Treat the register named @var{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). @var{reg} must be the name of a register. The register names accepted are machine-specific and are defined in the @code{REGISTER_NAMES} macro in the machine description macro file. @item -fcall-used-@var{reg} Treat the register named @var{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 @var{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. @item -fcall-saved-@var{reg} Treat the register named @var{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 @var{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 are may be returned. @end table @item -d@var{letters} Says to make debugging dumps at times specified by @var{letters}. Here are the possible letters: @table @samp @item r Dump after RTL generation. @item j Dump after first jump optimization. @item J Dump after last jump optimization. @item s Dump after CSE (including the jump optimization that sometimes follows CSE). @item L Dump after loop optimization. @item f Dump after flow analysis. @item c Dump after instruction combination. @item l Dump after local register allocation. @item g Dump after global register allocation. @item m Print statistics on memory usage, at the end of the run. @end table @item -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 @samp{-ansi}). However, without this option, certain GNU extensions and traditional C features are supported as well. With this option, they are rejected. There is no reason to @i{use} this option; it exists only to satisfy pedants. @end table These options control the C preprocessor, which is run on each C source file before actual compilation. If you use the @samp{-E} option, nothing is done except C preprocessing. Some of these options make sense only together with @samp{-E} because they request preprocessor output that is not suitable for actual compilation. @table @samp @item -C Tell the preprocessor not to discard comments. Used with the @samp{-E} option. @item -I@var{dir} Search directory @var{dir} for include files. @item -I- Any directories specified with @samp{-I} options before the @samp{-I-} option are searched only for the case of @samp{#include "@var{file}"}; they are not searched for @samp{#include <@var{file}>}. If additional directories are specified with @samp{-I} options after the @samp{-I-}, these directories are searched for all @samp{#include} directives. (Ordinarily @emph{all} @samp{-I} directories are used this way.) In addition, the @samp{-I-} option inhibits the use of the current directory as the first search directory for @samp{#include "@var{file}"}. Therefore, the current directory is searched only if it is requested explicitly with @samp{-I.}. Specifying both @samp{-I-} and @samp{-I.} allows you to control precisely which directories are searched before the current one and which are searched after. @item -nostdinc Do not search the standard system directories for header files. Only the directories you have specified with @samp{-I} options (and the current directory, if appropriate) are searched. Between @samp{-nostdinc} and @samp{-I-}, you can eliminate all directories from the search path except those you specify. @item -M Tell the preprocessor to output a rule suitable for @code{make} describing the dependencies of each source file. For each source file, the preprocessor outputs one @code{make}-rule whose target is the object file name for that source file and whose dependencies are all the files @samp{#include}d in it. This rule may be a single line or may be continued with @samp{\}-newline if it is long. @samp{-M} implies @samp{-E}. @item -MM Like @samp{-M} but the output mentions only the user-header files included with @samp{#include "@var{file}"}. System header files included with @samp{#include <@var{file}>} are omitted. @samp{-MM} implies @samp{-E}. @item -D@var{macro} Define macro @var{macro} with the empty string as its definition. @item -D@var{macro}=@var{defn} Define macro @var{macro} as @var{defn}. @item -U@var{macro} Undefine macro @var{macro}. @item -T Support ANSI C trigraphs. You don't want to know about this brain-damage. The @samp{-ansi} option also has this effect. @end table @node Installation, Trouble, Options, Top @chapter Installing GNU CC Here is the procedure for installing GNU CC on a Unix system. @menu * VMS Install:: See below for installation on VMS. @end menu @iftex (See below for VMS.) @end iftex @enumerate @item Edit @file{Makefile}. If you are using HPUX, you must make a few changes described in comments at the beginning of the file. @item Choose configuration files. @itemize @bullet @item Make a symbolic link named @file{config.h} to the top-level config file for the machine you are using (@pxref{Config}). This file is responsible for defining information about the host machine. It includes @file{tm.h}. The file's name should be @file{config-@var{machine}.h}. On VMS, use @file{config-vms.h} rather than @file{config-vax.h}. On the HP 9000 series 300, use @file{config-hp9k3.h} rather than @file{config-m68k.h}.@refill If your system does not support symbolic links, you might want to set up @file{config.h} to contain a @samp{#include} command which refers to the appropriate file. @item Make a symbolic link named @file{tm.h} to the machine-description macro file for your machine (its name should be @file{tm-@var{machine}.h}). For the 68000/68020, do not use @file{tm-m68k.h} directly; instead use one of the files @file{tm-sun3.h}, @file{tm-sun2.h}, @file{tm-isi68.h}, @file{tm-news800.h} or @file{tm-3b1.h}. Each of those files includes @file{tm-m68k.h} but sets up a few things differently as appropriate to the specific model of machine.@refill There are two files you can use for a 680x0 running HPUX: @file{tm-hp9k320.h} and @file{tm-hp9k320g.h}. Use the former if you are installing GNU CC alone. The latter is for another option where GNU CC together with the GNU assembler, linker, debugger and other utilities are used to replace all of HPUX that deals with compilation. Not all of the pieces of GNU software needed for this mode of operation are as yet in distribution; full instructions will appear here in the future.@refill For the 32000, use @file{tm-sequent.h} if you are using a Sequent machine, or @file{tm-encore.h} for an Encore machine; otherwise, perhaps @file{tm-ns32k.h} will work for you. For the vax, use @file{tm-vax.h} on Unix, or @file{tm-vms.h} on VMS.@refill @item Make a symbolic link named @file{md} to the machine description pattern file (its name should be @file{@var{machine}.md}). @item Make a symbolic link named @file{aux-output.c} to the output subroutine file for your machine (its name should be @file{output-@var{machine}.c}). @end itemize @item Make sure the Bison parser generator is installed. (This is unnecessary if the Bison output file @file{parse.tab.c} is more recent than @file{parse.y} and you do not plan to change @file{parse.y}.) Note that if you have an old version of Bison you may get an error from the line with the @samp{%expect} directive. If so, simply remove that line from @file{parse.y} and proceed. @item If you are using a Sun, make sure the environment variable @code{FLOAT_OPTION} is not set. If this option were set to @code{f68881} when @file{gnulib} is compiled, the resulting code would demand to be linked with a special startup file and will not link properly without special pains. @item Build the compiler. Just type @samp{make} in the compiler directory. @item Move the first-stage object files and executables into a subdirectory with this command: @example make stage1 @end example The files are moved into a subdirectory named @file{stage1}. Once installation is complete, you may wish to delete these files with @code{rm -r stage1}. @item Recompile the compiler with itself, with this command: @example make CC=stage1/gcc CFLAGS="-g -O -Bstage1/" @end example On a 68000 or 68020 system lacking floating point hardware, unless you have selected a @file{tm.h} file that expects by default that there is no such hardware, do this instead: @example make CC=stage1/gcc CFLAGS="-g -O -Bstage1/ -msoft-float" @end example @item If you wish to test the compiler by compiling it with itself one more time, do this: @example make stage2 make CC=stage2/gcc CFLAGS="-g -O -Bstage2/" foreach file (*.o) cmp $file stage2/$file end @end example This will notify you if any of these stage 3 object files differs from those of stage 2. 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 (@pxref{Bugs}). @item Install the compiler driver, the compiler's passes and run-time support. You can use the following command: @example make install @end example @noindent This copies the files @file{cc1}, @file{cpp} and @file{gnulib} to files @file{gcc-cc1}, @file{gcc-cpp} and @file{gcc-gnulib} in directory @file{/usr/local/lib}, which is where the compiler driver program looks for them. It also copies the driver program @file{gcc} into the directory @file{/usr/local}, so that it appears in typical execution search paths.@refill @strong{Warning: the GNU CPP may not work for @file{ioctl.h}, @file{ttychars.h} and other system header files unless the @samp{-traditional} option is used.} The bug is in the header files: at least on some machines, they rely on behavior that is incompatible with ANSI C. This behavior consists of substituting for macro argument names when they appear inside of character constants. The @samp{-traditional} option tells GNU CC to behave the way these headers expect. Because of this problem, you might prefer to configure GNU CC to use the system's own C preprocessor. To do so, make the file @file{/usr/local/lib/gcc-cpp} a link to @file{/lib/cpp}. Alternatively, on Sun systems and 4.3BSD at least, you can correct the include files by running the shell script @file{fixincludes}. This installs modified, corrected copies of the files @file{ioctl.h} and @file{ttychars.h} in a special directory where only GNU CC will normally look for them. The file @file{/usr/include/vaxuba/qvioctl.h} used in the X window system needs a similar correction. @end enumerate If you cannot install the compiler's passes and run-time support in @file{/usr/local/lib}, you can alternatively use the @samp{-B} option to specify a prefix by which they may be found. The compiler concatenates the prefix with the names @file{cpp}, @file{cc1} and @file{gnulib}. Thus, you can put the files in a directory @file{/usr/foo/gcc} and specify @samp{-B/usr/foo/gcc/} when you run GNU CC. @node VMS Install,, Installation, Installation @section Installing GNU CC on VMS The VMS version of GNU CC is distributed in an unusual tape format which consists of several tape files. The first is a command file; the second is an executable program which reads Unix tar format; the third is another command file which uses this program to read the remainder of the tape. To load the tape, it suffices to mount it @samp{/foreign} and then do @samp{@@mta0:} to execute the command file at the beginning of the tape. The tape contains executables and object files as well as sources, so no compilation is necessary unless you change the sources. (This is a good thing, since you probably don't have any other C compiler.) If you must recompile, here is how: @enumerate @item Copy the file @file{tm-vms.h} to @file{tm.h}, @file{config-vms.h} to @file{config.h}, @file{vax.md} to @file{md.} and @file{output-vax.c} to @file{aux-output.c}.@refill @item Type @samp{@@make} to do recompile everything. @end enumerate To install the @samp{GCC} command so you can use the compiler easily, in the same manner as you use the VMS C compiler, you must install the VMS CLD file for GNU CC as follows: @enumerate @item Define the VMS logical names @samp{GNU_CC} and @samp{GNU_CC_INCLUDE} to point to the directories where the GNU CC executables (@samp{gcc-cpp}, @samp{gcc-cc1}, etc.) and the C include files are kept. This should be done with the commands:@refill @example $ assign /super /system disk:[gcc] gnu_cc $ assign /super /system disk:[gcc.include] gnu_cc_include @end example @noindent 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. @item Install the @samp{GCC} command with the command line: @example $ set command /table=sys$library:dcltables gnu_cc:gcc @end example @noindent Now you can invoke the compiler with a command like @samp{gcc /verbose file.c}, which is equivalent to the command @samp{gcc -v -c file.c} in Unix. @end enumerate @node Trouble, Incompatibilities, Installation, Top @chapter Trouble in Installation Here are some of the things that have caused trouble for people installing GNU CC. @itemize @item On certain systems, defining certain environment variables such as @samp{CC} can interfere with the functioning of @code{make}. @end itemize @node Incompatibilities, Extensions, Trouble, Top @chapter Incompatibilities of GNU CC There are several noteworthy incompatibilities between GNU C and most existing (non-ANSI) versions of C. Ultimately our intention is that the @samp{-traditional} option will eliminate most of these incompatibilities by telling GNU C to behave like the other C compilers. @itemize @bullet @item 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 @code{mktemp} with a string constant argument. The function @code{mktemp} always alters the string its argument points to. Another consequence is that @code{sscanf} does not work on some systems when passed a string constant as its format control string. This is because @code{sscanf} incorrectly tries to write into the string constant. The best solution to these problems is to change the program to use @code{char}-array variables with initialization strings for these purposes instead of string constants. But if this is not possible, you can use the @samp{-fwritable-strings} flag, which directs GNU CC to handle string constants the same way most C compilers do. @item GNU CC does not substitute macro arguments when they appear inside of string constants. For example, the following macro in GNU CC @example #define foo(a) "a" @end example @noindent will produce output @samp{"a"} regardless of what the argument @var{a} is. The @samp{-traditional} option directs GNU CC to handle such cases (among others) in the old-fashioned (non-ANSI) fashion. @item When you use @code{setjmp} and @code{longjmp}, the only automatic variables guaranteed to remain valid are those declared @code{volatile}. This is a consequence of automatic register allocation. Consider this function: @example jmp_buf j; foo () @{ int a, b; a = fun1 (); if (setjmp (j)) return a; a = fun2 (); /* @r{@code{longjmp (j)} may be occur in @code{fun3}.} */ return a + fun3 (); @} @end example Here @code{a} may or may not be restored to its first value when the @code{longjmp} occurs. If @code{a} is allocated in a register, then its first value is restored; otherwise, it keeps the last value stored in it. If you use the @samp{-W} option with the @samp{-O} option, you will get a warning when GNU CC thinks such a problem might be possible. @item 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 @code{extern} declaration affects all the rest of the file even if it happens within a block. The @samp{-traditional} option directs GNU C to treat all @code{extern} declarations as global, like traditional compilers. @item In traditional C, you can combine @code{long}, etc., with a typedef name, as shown here: @example typedef int foo; typedef long foo bar; @end example In ANSI C, this is not allowed: @code{long} and other type modifiers require an explicit @code{int}. Because this criterion is expressed by Bison grammar rules rather than C code, the @samp{-traditional} flag cannot alter it. @item When compiling functions that return structures or unions, GNU CC output code 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 GCC 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 in a special, fixed register. PCC usually handles all sizes of structures and unions by returning the address of a block of static storage containing the value. This method is not used in GCC because it is slower and nonreentrant. On systems where PCC works this way, you may be able to make GCC-compiled code call such functions that were compiled with PCC by declaring them to return a pointer to the structure or union instead of the structure or union itself. For example, instead of this: @example struct foo nextfoo (); @end example @noindent write this: @example struct foo *nextfoo (); #define nextfoo *nextfoo @end example @noindent (Note that this assumes you are using the GNU preprocessor, so that the ANSI antirecursion rules for macro expansions are effective.) @end itemize @node Extensions, Bugs, Incompatibilities, Top @chapter GNU Extensions to the C Language GNU C provides several language features not found in ANSI standard C. (The @samp{-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 conditional compilation, check for a predefined macro @code{__GNUC__}, which is always defined under GNU CC. @menu * Statement Exprs:: Putting statements and declarations inside expressions. * Naming Types:: Giving a name to the type of some expression. * Typeof:: @code{typeof}: referring to the type of an expression. * Lvalues:: Using @samp{?:}, @samp{,} and casts in lvalues. * Conditionals:: Omitting the middle operand of a @samp{?:} expression. * Zero-Length:: Zero-length arrays. * Variable-Length:: Arrays whose length is computed at run time. * Subscripting:: Any array can be subscripted, even if not an lvalue. * Pointer Arith:: Arithmetic on @code{void}-pointers and function pointers. * Constructors:: Constructor expressions give structures, unions or arrays as values. * Dollar Signs:: Dollar sign is allowed in identifiers. * Alignment:: Inquiring about the alignment of a type or variable. * Inline:: Defining inline functions (as fast as macros). * Extended Asm:: Assembler instructions with C expressions as operands. (With them you can define ``built-in'' functions.) * Asm Labels:: Specifying the assembler name to use for a C symbol. @end menu @node Statement Exprs, Naming Types, Extensions, Extensions @section Statements and Declarations inside of Expressions A compound statement in parentheses may appear inside an expression in GNU C. This allows you to declare variables within an expression. For example: @example (@{ int y = foo (); int z; if (y > 0) z = y; else z = - y; z; @}) @end example @noindent is a valid (though slightly more complex than necessary) expression for the absolute value of @code{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: @example #define max(a,b) ((a) > (b) ? (a) : (b)) @end example @noindent But this definition computes either @var{a} or @var{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 @code{int}), you can define the macro safely as follows: @example #define maxint(a,b) \ (@{int _a = (a), _b = (b); _a > _b ? _a : _b; @}) @end example Embedded statements are not allowed in constant expressions, such as the value of an enumeration constant, the width of a bit field, or the initial value of a static variable. If you don't know the type of the operand, you can still do this, but you must use @code{typeof} (@pxref{Typeof}) or type naming (@pxref{Naming Types}). @node Naming Types, Typeof, Statement Exprs, Extensions @section Naming an Expression's Type You can give a name to the type of an expression using a @code{typedef} declaration with an initializer. Here is how to define @var{name} as a type name for the type of @var{exp}: @example typedef @var{name} = @var{exp}; @end example 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: @example #define max(a,b) \ (@{typedef _ta = (a), _tb = (b); \ _ta _a = (a); _tb _b = (b); \ _a > _b ? _a : _b; @}) @end example 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 @code{a} and @code{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. @node Typeof, Lvalues, Naming Types, Extensions @section Referring to a Type with @code{typeof} Another way to refer to the type of an expression is with @code{typeof}. The syntax of using of this keyword looks like @code{sizeof}, but the construct acts semantically like a type name defined with @code{typedef}. There are two ways of writing the argument to @code{typeof}: with an expression or with a type. Here is an example with an expression: @example typeof (x[0](1)) @end example @noindent This assumes that @code{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: @example typeof (int *) @end example @noindent Here the type described is that of pointers to @code{int}. A @code{typeof}-construct can be used anywhere a typedef name could be used. For example, you can use it in a declaration, in a cast, or inside of @code{sizeof} or @code{typeof}. @itemize @bullet @item This declares @code{y} with the type of what @code{x} points to. @example typeof (*x) y; @end example @item This declares @code{y} as an array of such values. @example typeof (*x) y[4]; @end example @item This declares @code{y} as an array of pointers to characters: @example typeof (typeof (char *)[4]) y; @end example @noindent It is equivalent to the following traditional C declaration: @example char *y[4]; @end example To see the meaning of the declaration using @code{typeof}, and why it might be a useful way to write, let's rewrite it with these macros: @example #define pointer(T) typeof(T *) #define array(T, N) typeof(T [N]) @end example @noindent Now the declaration can be rewritten this way: @example array (pointer (char), 4) y; @end example @noindent Thus, @samp{array (pointer (char), 4)} is the type of arrays of 4 pointers to @code{char}. @end itemize @node Lvalues, Conditionals, Typeof, Extensions @section Generalized Lvalues Compound expressions, conditional 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: @example (a, b) += 5 a, (b += 5) @end example Similarly, the address of the compound expression can be taken. These two expressions are equivalent: @example &(a, b) a, &b @end example 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: @example (a ? b : c) = 5 (a ? b = 5 : (c = 5)) @end example A cast is a valid lvalue if its operand is valid. Taking the address of the cast is the same as taking the address without a cast, except for the type of the result. For example, these two expressions are equivalent (but the second may be valid when the type of @samp{a} does not permit a cast to @samp{int *}). @example &(int *)a (int **)&a @end example 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 converter back to the specified type to become the value of the assignment. Thus, if @samp{a} has type @samp{char *}, the following two expressions are equivalent: @example (int)a = 5 (int)(a = (char *)5) @end example An assignment-with-arithmetic operation such as @samp{+=} applied to a cast performs the arithmetic using the type resulting from the cast, and then continues as in the previous case. Therefore, these two expressions are equivalent: @example (int)a += 5 (int)(a = (char *) ((int)a + 5)) @end example @node Conditionals, Zero-Length, Lvalues, Extensions @section Conditional Expressions with Omitted Middle-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 @example x ? : y @end example @noindent has the value of @code{x} if that is nonzero; otherwise, the value of @code{y}. This example is perfectly equivalent to @example x ? x : y @end example @noindent 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. @node Zero-Length, Variable-Length, Conditionals, Extensions @section 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: @example struct line @{ int length; char contents[0]; @}; @{ struct line *thisline = (struct line *) malloc (sizeof (struct line) + this_length); thisline->length = thislength; @} @end example In standard C, you would have to give @code{contents} a length of 1, which means either you waste space or complicate the argument to @code{malloc}. @node Variable-Length, Subscripting, Zero-Length, Extensions @section 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 that time and deallocated when the brace-level is exited. For example: @example 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); @} @end example You can also define structure types containing variable-length arrays, and use them even for arguments or function values, as shown here: @example int foo; struct entry @{ char data[foo]; @}; struct entry tester (struct entry arg) @{ struct entry new; int i; for (i = 0; i < foo; i++) new.data[i] = arg.data[i] + 1; return new; @} @end example @noindent (Eventually there will be a way to say that the size of the array is another member of the same structure.) The length of an array is computed on entry to the brace-level where the array is declared and is remembered for the scope of the array in case you access it with @code{sizeof}. Jumping or breaking out of the scope of the array name will also deallocate the storage. Jumping into the scope is not allowed; you will get an error message for it. You can use the function @code{alloca} to get an effect much like variable-length arrays. The function @code{alloca} is available 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 @code{alloca} exists until the containing @emph{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 @code{alloca} in the same function, deallocation of a variable-length array will also deallocate anything more recently allocated with @code{alloca}.) @node Subscripting, Pointer Arith, Variable-Length, Extensions @section Non-Lvalue Arrays May Have Subscripts Subscripting is allowed on arrays that are not lvalues, even though the unary @samp{&} operator is not. For example, this is valid in GNU C though not valid in other C dialects: @example struct foo @{int a[4];@}; struct foo f(); bar (int index) @{ return f().a[index]; @} @end example @node Pointer Arith, Initializers, Subscripting, Extensions @section Arithmetic on @code{void}-Pointers and Function Pointers In GNU C, addition and subtraction operations are supported on pointers to @code{void} and on pointers to functions. This is done by treating the size of a @code{void} or of a function as 1. A consequence of this is that @code{sizeof} is also allowed on @code{void} and on function types, and returns 1. @node Initializers, Constructors, Pointer Arith, Extensions @section Non-Constant Initializers The elements of an aggregate initializer are not required to be constant expressions in GNU C. Here is an example of an initializer with run-time varying elements: @example foo (float f, float g) @{ float beat_freqs[2] = @{ f-g, f+g @}; @dots{} @} @end example @node Constructors, Dollar Signs, Initializers, Extensions @section 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. The type must be a structure, union or array type. Assume that @code{struct foo} and @code{structure} are declared as shown: @example struct foo @{int a; char b[2];@} structure; @end example @noindent Here is an example of constructing a @samp{struct foo} with a constructor: @example structure = ((struct foo) @{x + y, 'a', 0@}); @end example @noindent This is equivalent to writing the following: @example @{ struct foo temp = @{x + y, 'a', 0@}; structure = temp; @} @end example You can also construct an array. If all the elements of the constructor are (made up of) simple constant expressions, suitable for use in initializers, then the constructor is an lvalue and can be coerced to a pointer to its first element, as shown here: @example char **foo = (char *[]) @{ "x", "y", "z" @}; @end example Array constructors whose elements are not simple constants are not very useful, because the constructor is not an lvalue. There are only two valid ways to use it: to subscript it, or initialize an array variable with it. The former is probably slower than a @code{switch} statement, while the latter does the same thing an ordinary C initializer would do. @example output = ((int[]) @{ 2, x, 28 @}) [input]; @end example @node Dollar Signs, Alignment, Constructors, Extensions @section 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. @node Alignment, Inline, Dollar Signs, Extensions @section Inquiring about the Alignment of a Type or Variable The keyword @code{__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 @code{sizeof}. For example, if the target machine requires a @code{double} value to be aligned on an 8-byte boundary, then @code{__alignof (double)} is 8. This is true on many RISC machines. On more traditional machine designs, @code{__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, @code{__alignof} reports the @emph{recommended} alignment of a type. When the operand of @code{__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 structure and inherits alignment from that structure. For example, after this declaration: @example struct foo @{ int x; char y; @} foo1; @end example @noindent the value of @code{__alignof (foo1.y)} is probably 2 or 4, the same as @code{__alignof (int)}, even though the data type of @code{foo1.y} does not itself demand any alignment.@refill @node Inline, Extended Asm, Alignment, Extensions @section An Inline Function is As Fast As a Macro By declaring a function @code{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 @code{inline} keyword in its declaration, like this: @example inline int inc (int *a) @{ (*a)++; @} @end example You can also make all ``simple enough'' functions inline with the option @samp{-finline-functions}. Note that certain usages in a function definition can make it unsuitable for inline substitution. When a function is both inline and @code{static}, if all calls to the function are integrated into the caller, 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 @samp{-fkeep-inline-functions}. Some calls cannot be integrated for various reasons (in particular, 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. When an inline function is not @code{static}, then the compiler 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-@code{static} inline function is always compiled on its own in the usual fashion. @node Extended Asm, Asm Labels, Inline, Extensions @section Assembler Instructions with C Expression Operands In an assembler instruction using @code{asm}, you can now specify the operands of the instruction using C expressions. This means no more guessing which registers or memory locations 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 @code{fsinx} instruction: @example asm ("fsinx %1,%0" : "=f" (result) : "f" (angle)); @end example @noindent Here @code{angle} is the C expression for the input operand while @code{result} is that of the output operand. Each has @samp{"f"} as its operand constraint, saying that a floating-point register is required. The constraints use the same language used in the machine description (@pxref{Constraints}). 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 number of operands is limited to the maximum number of operands in any instruction pattern in the machine description. Output operand expressions must be lvalues, and there must be at least one of them. The compiler can check this. The input operands need not be lvalues, and there need not be any. The compiler cannot check whether the operands have data types that are reasonable for the instruction being executed. The output operands must be write-only; GNU CC will assume that the values in these operands before the instruction are dead and need not be generated. For an operand that is read-write, you must logically split its function into two separate operands, one input operand and one write-only output 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) @samp{combine} instruction with @code{bar} as its read-only source operand and @code{foo} as its read-write destination: @example asm ("combine %2,%0" : "=r" (foo) : "0" (foo), "g" (bar)); @end example @noindent The constraint @samp{"0"} for operand 1 says that it must occupy the same location as operand 0. Therefore it is not necessary to substitute operand 1 into the assembler code output. Usually the most convenient way to use these @code{asm} instructions is to encapsulate them in macros that look like functions. For example, @example #define sin(x) \ (@{ double __value, __arg = (x); \ asm ("fsinx %1,%0": "=f" (__value): "f" (__arg)); \ __value; @}) @end example @noindent Here the variable @code{__arg} is used to make sure that the instruction operates on a proper @code{double} value, and to accept only those arguments @code{x} which can convert automatically to a @code{double}. Another way to make sure the instruction operates on the correct data type is to use a cast in the @code{asm}. This is different from using a variable @code{__arg} in that it converts more different types. For example, if the desired type were @code{int}, casting the argument to @code{int} would accept a pointer with no complaint, while assigning the argument to an @code{int} variable named @code{__arg} would warn about using a pointer unless the caller explicitly casts it. GNU CC assumes for optimization purposes that these instructions have 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 happens to be found in a register. You can prevent an @code{asm} instruction from being deleted, moved or combined by writing the keyword @code{volatile} after the @code{asm}. For example: @example #define set_priority(x) \ asm volatile ("set_priority %1": \ "=m" (*(char *)0): "g" (x)) @end example @noindent Note that we have supplied an output operand which is not actually used in the instruction. This is because @code{asm} requires at least one output operand. This requirement exists for internal implementation reasons and we might be able to relax it in the future. In this case output operand has the additional benefit effect of giving the appearance of writing in memory. As a result, GNU CC will assume that data previously fetched from memory must be fetched again if needed again later. This may be desirable if you have not employed the @code{volatile} keyword on all the variable declarations that ought to have it. @node Asm Labels,,Extended Asm, Extensions @section 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 @code{asm} keyword after the declarator as follows: @example int foo asm ("myfoo") = 2; @end example @noindent This specifies that the name to be used for the variable @code{foo} in the assembler code should be @samp{myfoo} rather than the usual @samp{_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 @code{asm} in this way in a function @emph{definition}; but you can get the same effect by writing a declaration for the function before its definition and putting @code{asm} there, like this: @example extern func () asm ("FUNC"); func (x, y) int x, y; @dots{} @end example 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. @node Bugs, Portability, Extensions, Top @chapter Reporting Bugs Your bug reports play an essential role in making GNU CC reliable. Reporting a bug may help you by bringing a solution to your problem, or it may not. But in any case the important 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. @menu * Criteria: Bug Criteria. Have you really found a bug? * Reporting: Bug Reporting. How to report a bug effectively. @end menu @node Bug Criteria, Bug Reporting, Bugs, Bugs @section Have You Found a Bug? If you are not sure whether you have found a bug, here are some guidelines: @itemize @bullet @item If the compiler gets a fatal signal, for any input whatever, that is a compiler bug. Reliable compilers never crash. @item If the compiler produces invalid assembly code, for any input whatever (except an @code{asm} statement), that is a compiler bug, unless the compiler reports errors (not just warnings) which would ordinarily prevent the assembler from being run. @item 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 (@pxref{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 @samp{x;} at the end of a function instead of @samp{return x;}, with the same results. But the value of the function is undefined if @samp{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 @samp{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. 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. @item 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: @example int foo (char); int foo (x) char x; @{ @dots{} @} @end example @noindent The prototype says to pass a @code{char}, while the definition says to pass an @code{int} and treat the value as a @code{char}. This is what the ANSI standard says, and it makes sense. @item 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''. @item If you are an experienced user of C compilers, your suggestions for improvement of GNU CC are welcome in any case. @end itemize @node Bug Reporting,, Bug Criteria, Bugs @section How to Report Bugs Send bug reports for GNU C to one of these addresses: @example bug-gcc@@prep.ai.mit.edu @{ucbvax|mit-eddie|uunet@}!prep.ai.mit.edu!bug-gcc @end example As a last resort, snail them to: @example GNU Compiler Bugs 545 Tech Sq Cambridge, MA 02139 @end example The fundamental principle of reporting bugs usefully is this: @strong{report all the facts}. If you are not sure whether to mention a fact or leave it out, mention 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, probably 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 an exact example. If you want to enable me to fix the bug, you should include all these things: @itemize @bullet @item The version of GNU CC. You can get this by running it with the @samp{-v} option. Without this, I won't know whether there is any point in looking for the bug in the current version of GNU CC. @item 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 (@file{cc1}), run your source file through the C preprocessor by doing @samp{gcc -E @var{sourcefile} > @var{outfile}}, then include the contents of @var{outfile} in the bug report. (Any @samp{-I}, @samp{-D} or @samp{-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. @item The command arguments you gave GNU CC to compile that example and observe the bug. For example, did you use @samp{-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. @item The names of the files that you used for @file{tm.h} and @file{md} when you installed the compiler. @item The type of machine you are using, and the operating system name and version number. @item 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. 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 @i{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. In cases where GNU CC generates incorrect code, if you send me a small complete sample program I will find the error myself by running the program under a debugger. If you send me a large example or a part of a larger program, I cannot do this; you must debug the compiled program and narrow the problem down to one source line. Tell me which source line it is, and what you believe is incorrect about the code generated for that line. @item If you send me examples of output from GNU CC, please use @samp{-g} when you make them. The debugging information includes source line numbers which are essential for correlating the output with the input. @end itemize Here are some things that are not necessary: @itemize @bullet @item 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. Of course, it can't hurt if you can find a simpler example that triggers the same bug. Errors in the output will be easier to spot, running under the debugger will take less time, etc. An easy 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. However, simplification is not necessary; if you don't want to do this, report the bug anyway. @item 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, because I might see problems with your patch and decide to fix the problem another way. Sometimes with a program as complicated as GNU CC it is very hard to construct an example that will make the program go through a certain point in the code. If you don't send me the example, I won't be able to verify that the bug is fixed. @item 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 using the debugger to find the facts. They also don't serve a useful purpose. @end itemize @node Portability, Interface, Bugs, Top @chapter GNU CC and Portability The main goal of GNU CC was to make a good, fast compiler 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 compiler 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 syntax tree, strategies that are usable for different combinations 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 happens because the compiler will call @code{abort}. Fortunately, the new strategies can be added in a machine-independent fashion, and will affect only the target machines that need them. @node Interface, Passes, Portability, Top @chapter 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 (@pxref{Machine Macros}). However, returning of structure and union values is done differently. As a result, functions compiled with PCC returning such types cannot be called from code compiled with GNU CC, and vice versa. This usually does not cause trouble because the Unix library routines don't return structures and unions. Structures and unions that are 1, 2, 4 or 8 bytes long are returned in the same registers used for @code{int} or @code{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 in a register. This method is faster than the one normally used by PCC and is also reentrant. The register used for passing the address is specified by the machine-description macro @code{STRUCT_VALUE_REGNUM}. GNU CC always passes arguments on the stack. At some point it will be extended to pass arguments in registers, for machines which use that as the standard calling convention. This will make it possible to use such a convention on other machines as well. However, that would render it completely incompatible with PCC. We will probably do this once we have a complete GNU system so we can compile the libraries with GNU CC. If you use @code{longjmp}, beware of automatic variables. ANSI C says that automatic variables that are not declared @code{volatile} have undefined values after a @code{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 @code{longjmp}, and you don't want to write @code{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: @example @{ int careful; &careful; @dots{} @} @end example Code compiled with GNU CC may call certain library routines. The routines needed on the Vax and 68000 are in the file @file{gnulib.c}. You must compile this file with the standard C compiler, not with GNU CC, and then link it with each program you compile with GNU CC. (In actuality, many programs will not need it.) The usual function call interface is used for calling the library routines. Some standard parts of the C library, such as @code{bcopy}, are also called automatically. @node Passes, RTL, Interface, Top @chapter Passes and Files of the Compiler The overall control structure of the compiler is in @file{toplev.c}. This file is responsible for initialization, decoding arguments, opening and closing files, and sequencing 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 converted to RTL; then the storage for the tree for the statement is reclaimed. Storage for types (and the expressions for their sizes), declarations, and a representation of the binding contours and how they nest, remains until the function 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 @code{rest_of_compilation} or @code{rest_of_decl_compilation} in @file{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 @code{rest_of_compilation}. When that function returns from compiling a function definition, the storage used for that function definition's compilation is entirely freed, unless it is an inline function (@pxref{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 @samp{-d} options. @itemize @bullet @item 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. C 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 associative-law simplifications are also done during this pass. The source files for parsing are @file{parse.y}, @file{decl.c}, @file{typecheck.c}, @file{stor-layout.c}, @file{fold-const.c}, and @file{tree.c}. The last three are intended to be language-independent. There are also header files @file{parse.h}, @file{c-tree.h}, @file{tree.h} and @file{tree.def}. The last two define the format of the tree representation.@refill @item 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 @code{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 @code{switch} statements. The source files for RTL generation are @file{stmt.c}, @file{expr.c}, @file{explow.c}, @file{expmed.c}, @file{optabs.c} and @file{emit-rtl.c}. Also, the file @file{insn-emit.c}, generated from the machine description by the program @code{genemit}, is used in this pass. The header files @file{expr.h} is used for communication within this pass.@refill The header files @file{insn-flags.h} and @file{insn-codes.h}, generated from the machine description by the programs @code{genflags} and @code{gencodes}, tell this pass which standard names are available for use and which patterns correspond to them.@refill 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 option @samp{-dr} causes a debugging dump of the RTL code after this pass. This dump file's name is made by appending @samp{.rtl} to the input file name. @item 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.) 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 no-op move instructions are done together with the optimizations described above. The source file of this pass is @file{jump.c}. The option @samp{-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 @samp{.jump} to the input file name. @item Register scan. This pass finds the first and last use of each register, as a guide for common subexpression elimination. Its source is in @file{regclass.c}. @item Common subexpression elimination. This pass also does constant propagation. Its source file is @file{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 @samp{-ds} causes a debugging dump of the RTL code after this pass. This dump file's name is made by appending @samp{.cse} to the input file name. @item Loop optimization. This pass moves constant expressions out of loops. Its source file is @file{loop.c}. The option @samp{-dL} causes a debugging dump of the RTL code after this pass. This dump file's name is made by appending @samp{.loop} to the input file name. @item 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 @file{stupid.c}. @item Data flow analysis (@file{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 @samp{-df} causes a debugging dump of the RTL code after this pass. This dump file's name is made by appending @samp{.flow} to the input file name. If stupid register allocation is in use, this dump file reflects the full results of such allocation. @item Instruction combination (@file{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 @samp{-dc} causes a debugging dump of the RTL code after this pass. This dump file's name is made by appending @samp{.combine} to the input file name. @item Register class preferencing. The RTL code is scanned to find out which register class is best for each pseudo register. The source file is @file{regclass.c}. @item Local register allocation (@file{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 @samp{-dl} causes a debugging dump of the RTL code after this pass. This dump file's name is made by appending @samp{.lreg} to the input file name. @item Global register allocation (@file{global-alloc.c}). This pass allocates hard registers for the remaining pseudo registers (those whose life spans are not contained in one basic block). @item 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. Source files are @file{reload.c} and @file{reload1.c}, plus the header @file{reload.h} used for communication between them. The option @samp{-dg} causes a debugging dump of the RTL code after this pass. This dump file's name is made by appending @samp{.greg} to the input file name. @item Jump optimization is repeated, this time including cross-jumping and deletion of no-op move instructions. Machine-specific peephole optimizations are performed at the same time. The option @samp{-dJ} causes a debugging dump of the RTL code after this pass. This dump file's name is made by appending @samp{.jump2} to the input file name. @item Final. This pass outputs the assembler code for the function. It is also responsible for identifying spurious test and compare instructions. The function entry and exit sequences are generated directly as assembler code in this pass; they never exist as RTL. The source files are @file{final.c} plus @file{insn-output.c}; the latter is generated automatically from the machine description by the tool @file{genoutput}. The header file @file{conditions.h} is used for communication between these files. @item 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 @file{dbxout.c} for DBX symbol table format and @file{symout.c} for GDB's own symbol table format. @end itemize Some additional files are used by all or many passes: @itemize @bullet @item Every pass uses @file{machmode.def}, which defines the machine modes. @item All the passes that work with RTL use the header files @file{rtl.h} and @file{rtl.def}, and subroutines in file @file{rtl.c}. The tools @code{gen*} also use these files to read and work with the machine description RTL. @item Several passes refer to the header file @file{insn-config.h} which contains a few parameters (C macro definitions) generated automatically from the machine description RTL by the tool @code{genconfig}. @item Several passes use the instruction recognizer, which consists of @file{recog.c} and @file{recog.h}, plus the files @file{insn-recog.c} and @file{insn-extract.c} that are generated automatically from the machine description by the tools @file{genrecog} and @file{genextract}.@refill @item Several passes use the header files @file{regs.h} which defines the information recorded about pseudo register usage, and @file{basic-block.h} which defines the information recorded about basic blocks. @item @file{hard-reg-set.h} defines the type @code{HARD_REG_SET}, a bit-vector with a bit for each hard register, and some macros to manipulate it. This type is just @code{int} if the machine has few enough hard registers; otherwise it is an array of @code{int} and some of the macros expand into loops. @end itemize @node RTL, Machine Desc, Passes, Top @chapter RTL Representation Most of the work of the compiler is done on an intermediate 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. @menu * RTL Objects:: Expressions vs vectors vs strings vs integers. * Accessors:: Macros to access expression operands or vector elts. * Flags:: Other flags in an RTL expression. * Machine Modes:: Describing the size and format of a datum. * Constants:: Expressions with constant values. * Regs and Memory:: Expressions representing register contents or memory. * Arithmetic:: Expressions representing arithmetic on other expressions. * Comparisons:: Expressions representing comparison of expressions. * Bit Fields:: Expressions representing bit-fields in memory or reg. * Conversions:: Extending, truncating, floating or fixing. * RTL Declarations:: Declaring volatility, constancy, etc. * Side Effects:: Expressions for storing in registers, etc. * Incdec:: Embedded side-effects for autoincrement addressing. * Assembler:: Representing @code{asm} with operands. * Insns:: Expression types for entire insns. * Calls:: RTL representation of function call insns. * Sharing:: Some expressions are unique; others *must* be copied. @end menu @node RTL Objects, Accessors, RTL, RTL @section 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 structure, but it is usually referred to with a pointer; a type that is given the typedef name @code{rtx}. An integer is simply an @code{int}, and a string is a @code{char *}. Within RTL code, strings appear only inside @samp{symbol_ref} expressions, but they appear in other contexts in the RTL expressions that make up machine descriptions. Their written form uses decimal digits. A string is a sequence of characters. In core it is represented as a @code{char *} in usual C fashion, and it is written in C syntax as well. However, strings in RTL may never be null. If you write an empty string in a machine description, 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. A vector contains an arbitrary, specified number of pointers to expressions. The number of elements in the vector is explicitly present in the vector. The written form of a vector consists of square brackets (@samp{[@dots{}]}) surrounding the elements, in sequence and with whitespace separating them. Vectors of length zero are not created; null pointers are used instead. Expressions are classified by @dfn{expression codes} (also called RTX codes). The expression code is a name defined in @file{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 @code{GET_CODE (@var{x})} and altered with @code{PUT_CODE (@var{x}, @var{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 expression. For example, in an expression of code @samp{subreg}, the first operand is to be regarded as an expression and the second operand as an integer. In an expression of code @samp{plus}, there are two operands, both of which are to be regarded as expressions. In a @samp{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 @samp{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: @samp{const_int}. In a few contexts a null pointer is valid where an expression is normally wanted. The written form of this is @samp{(nil)}. @node Accessors, Flags, RTL Objects, RTL @section Access to Operands For each expression type @file{rtl.def} specifies the number of contained objects and their kinds, with four possibilities: @samp{e} for expression (actually a pointer to an expression), @samp{i} for integer, @samp{s} for string, and @samp{E} for vector of expressions. The sequence of letters for an expression code is called its @dfn{format}. Thus, the format of @samp{subreg} is @samp{ei}.@refill Two other format characters are used occasionally: @samp{u} and @samp{0}. @samp{u} is equivalent to @samp{e} except that it is printed differently in debugging dumps, and @samp{0} means a slot whose contents do not fit any normal category. @samp{0} slots are not printed at all in dumps, and are often used in special ways by small parts of the compiler.@refill There are macros to get the number of operands and the format of an expression code: @table @code @item GET_RTX_LENGTH (@var{code}) Number of operands of an RTX of code @var{code}. @item GET_RTX_FORMAT (@var{code}) The format of an RTX of code @var{code}, as a C string. @end table Operands of expressions are accessed using the macros @code{XEXP}, @code{XINT} and @code{XSTR}. Each of these macros takes two arguments: an expression-pointer (RTX) and an operand number (counting from zero). Thus,@refill @example XEXP (@var{x}, 2) @end example @noindent accesses operand 2 of expression @var{x}, as an expression. @example XINT (@var{x}, 2) @end example @noindent accesses the same operand as an integer. @code{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 @var{x} is a @samp{subreg} expression, you know that it has two operands which can be correctly accessed as @code{XEXP (@var{x}, 0)} and @code{XINT (@var{x}, 1)}. If you did @code{XINT (@var{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 @code{(int) XEXP (@var{x}, 0)}. @code{XEXP (@var{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 @code{XEXP (@var{x}, 28)} either, but this will access memory past the end of the expression with unpredictable results.@refill Access to operands which are vectors is more complicated. You can use the macro @code{XVEC} to get the vector-pointer itself, or the macros @code{XVECEXP} and @code{XVECLEN} to access the elements and length of a vector. @table @code @item XVEC (@var{exp}, @var{idx}) Access the vector-pointer which is operand number @var{idx} in @var{exp}. @item XVECLEN (@var{exp}, @var{idx}) Access the length (number of elements) in the vector which is in operand number @var{idx} in @var{exp}. This value is an @code{int}. @item XVECEXP (@var{exp}, @var{idx}, @var{eltnum}) Access element number @var{eltnum} in the vector which is in operand number @var{idx} in @var{exp}. This value is an RTX. It is up to you to make sure that @var{eltnum} is not negative and is less than @code{XVECLEN (@var{exp}, @var{idx})}. @end table 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. @node Flags, Machine Modes, Accessors, RTL @section Flags in an RTL Expression RTL expressions contain several flags (one-bit bit-fields) that are used in certain types of expression. @table @code @item used 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 (@pxref{Sharing}). @item volatil This flag is used in @samp{mem} and @samp{reg} expressions and in insns. In RTL dump files, it is printed as @samp{/v}. In a @samp{mem} expression, it is 1 if the memory reference is volatile. Volatile memory references may not be deleted, reordered or combined. In a @samp{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. @item in_struct This flag is used in @samp{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 RTL dump, this flag is represented as @samp{/s}. @item unchanging This flag is used in @samp{reg} and @samp{mem} expressions. 1 means that the value of the expression never changes (at least within the current function). In an RTL dump, this flag is represented as @samp{/u}. @end table @node Machine Modes, Constants, Flags, RTL @section 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, @code{enum machine_mode}, defined in @file{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 @samp{mode} which appear at the end of each machine mode name are omitted. For example, @code{(reg:SI 38)} is a @samp{reg} expression with machine mode @code{SImode}. If the mode is @code{VOIDmode}, it is not written at all. Here is a table of machine modes. @table @code @item QImode ``Quarter-Integer'' mode represents a single byte treated as an integer. @item HImode ``Half-Integer'' mode represents a two-byte integer. @item SImode ``Single Integer'' mode represents a four-byte integer. @item DImode ``Double Integer'' mode represents an eight-byte integer. @item TImode ``Tetra Integer'' (?) mode represents a sixteen-byte integer. @item SFmode ``Single Floating'' mode represents a single-precision (four byte) floating point number. @item DFmode ``Double Floating'' mode represents a double-precision (eight byte) floating point number. @item TFmode ``Tetra Floating'' mode represents a quadruple-precision (sixteen byte) floating point number. @item 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, @code{BLKmode} will not appear in RTL. @item VOIDmode Void mode means the absence of a mode or an unspecified mode. For example, RTL expressions of code @samp{const_int} have mode @code{VOIDmode} because they can be taken to have whatever mode the context requires. In debugging dumps of RTL, @code{VOIDmode} is expressed by the absence of any mode. @item EPmode ``Entry Pointer'' mode is intended to be used for function variables in Pascal and other block structured languages. Such values contain both a function address and a static chain pointer for access to automatic variables of outer levels. This mode is only partially implemented since C does not use it. @item CSImode@r{, @dots{}} ``Complex Single Integer'' mode stands for a complex number represented as a pair of @code{SImode} integers. Any of the integer and floating modes may have @samp{C} prefixed to its name to obtain a complex number mode. For example, there are @code{CQImode}, @code{CSFmode}, and @code{CDFmode}. Since C does not support complex numbers, these machine modes are only partially implemented. @item BImode This is the machine mode of a bit-field in a structure. It is used only in the syntax tree, never in RTL, and in the syntax tree it appears only in declaration nodes. In C, it appears only in @code{FIELD_DECL} nodes for structure fields defined with a bit size. @end table The machine description defines @code{Pmode} as a C macro which expands into the machine mode used for addresses. Normally this is @code{SImode}. The only modes which a machine description @i{must} support are @code{QImode}, @code{SImode}, @code{SFmode} and @code{DFmode}. The compiler will attempt to use @code{DImode} for two-word structures and unions, but it would not be hard to program it to avoid this. Likewise, you can arrange for the C type @code{short int} to avoid using @code{HImode}. In the long term it would be desirable to make the set of available machine modes machine-dependent and eliminate all assumptions about specific machine modes or their uses from the machine-independent code of the compiler. Here are some C macros that relate to machine modes: @table @code @item GET_MODE (@var{x}) Returns the machine mode of the RTX @var{x}. @item PUT_MODE (@var{x}, @var{newmode}) Alters the machine mode of the RTX @var{x} to be @var{newmode}. @item GET_MODE_SIZE (@var{m}) Returns the size in bytes of a datum of mode @var{m}. @item GET_MODE_BITSIZE (@var{m}) Returns the size in bits of a datum of mode @var{m}. @item GET_MODE_UNIT_SIZE (@var{m}) Returns the size in bits of the subunits of a datum of mode @var{m}. This is the same as @code{GET_MODE_SIZE} except in the case of complex modes and @code{EPmode}. For them, the unit size is the size of the real or imaginary part, or the size of the function pointer or the context pointer. @end table @node Constants, Regs and Memory, Machine Modes, RTL @section Constant Expression Types The simplest RTL expressions are those that represent constant values. @table @code @item (const_int @var{i}) This type of expression represents the integer value @var{i}. @var{i} is customarily accessed with the macro @code{INTVAL} as in @code{INTVAL (@var{exp})}, which is equivalent to @code{XINT (@var{exp}, 0)}. There is only one expression object for the integer value zero; it is the value of the variable @code{const0_rtx}. Likewise, the only expression for integer value one is found in @code{const1_rtx}. Any attempt to create an expression of code @samp{const_int} and value zero or one will return @code{const0_rtx} or @code{const1_rtx} as appropriate. @item (const_double:@var{m} @var{i0} @var{i1}) Represents a floating point constant value of mode @var{m}. The two integers @var{i0} and @var{i1} together contain the bits of a @code{double} value. To convert them to a @code{double}, do @example union @{ double d; int i[2];@} u; u.i[0] = XINT (x, 0); u.i[1] = XINT (x, 1); @end example @noindent and then refer to @code{u.d}. The value of the constant is represented as a double in this fashion even if the value represented is single-precision. The global variables @code{dconst0_rtx} and @code{fconst0_rtx} hold @samp{const_double} expressions with value 0, in modes @code{DFmode} and @code{SFmode}, respectively. @item (symbol_ref @var{symbol}) Represents the value of an assembler label for data. @var{symbol} is a string that describes the name of the assembler label. If it starts with a @samp{*}, the label is the rest of @var{symbol} not including the @samp{*}. Otherwise, the label is @var{symbol}, prefixed with @samp{_}. @item (label_ref @var{label}) Represents the value of an assembler label for code. It contains one operand, an expression, which must be a @samp{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. @item (const @var{exp}) Represents a constant that is the result of an assembly-time arithmetic computation. The operand, @var{exp}, is an expression that contains only constants (@samp{const_int}, @samp{symbol_ref} and @samp{label_ref} expressions) combined with @samp{plus} and @samp{minus}. However, not all combinations are valid, since the assembler cannot do arbitrary arithmetic on relocatable symbols. @end table @node Regs and Memory, Arithmetic, Constants, RTL @section Registers and Memory Here are the RTL expression types for describing access to machine registers and to main memory. @table @code @item (reg:@var{m} @var{n}) For small values of the integer @var{n} (less than @code{FIRST_PSEUDO_REGISTER}), this stands for a reference to machine register number @var{n}: a @dfn{hard register}. For larger values of @var{n}, it stands for a temporary value or @dfn{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. The symbol @code{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. Each pseudo register number used in a function's RTL code is represented by a unique @samp{reg} expression. @var{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. 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 @samp{subreg} expression is used. A @samp{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. Such multi-word hardware register @samp{reg} expressions may not be live across the boundary of a basic block. The lifetime analysis pass does not know how to record properly that several consecutive registers are actually live there, and therefore register allocation would be confused. The CSE pass must go out of its way to make sure the situation does not arise. @item (subreg:@var{m} @var{reg} @var{wordnum}) @samp{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 @samp{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 @samp{subreg}. In such a case, @var{wordnum} is zero. The other use of @samp{subreg} is to extract the individual registers of a multi-register value. Machine modes such as @code{DImode} and @code{EPmode} indicate values longer than a word, values which usually require two consecutive registers. To access one of the registers, use a @samp{subreg} with mode @code{SImode} and a @var{wordnum} that says which register. The compilation parameter @code{WORDS_BIG_ENDIAN}, if defined, says that word number zero is the most significant part; otherwise, it is the least significant part. Note that it is not valid to access a @code{DFmode} value in @code{SFmode} using a @samp{subreg}. On some machines the most significant part of a @code{DFmode} value does not have the same format as a single-precision floating value. @item (cc0) This refers to the machine's condition code register. It has no operands and may not have a machine mode. It 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 (@samp{const_int} with value zero; that is to say, @code{const0_rtx}). There is only one expression object of code @samp{cc0}; it is the value of the variable @code{cc0_rtx}. Any attempt to create an expression of code @samp{cc0} will return @code{cc0_rtx}. One special thing about the condition code register is that instructions can set it 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 @code{NOTICE_UPDATE_CC}). Only instructions whose sole purpose is to set the condition code, and instructions that use the condition code, need mention @code{(cc0)}. @item (pc) This represents the machine's program counter. It has no operands and may not have a machine mode. @code{(pc)} may be validly used only in certain specific contexts in jump instructions. There is only one expression object of code @samp{pc}; it is the value of the variable @code{pc_rtx}. Any attempt to create an expression of code @samp{pc} will return @code{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. @item (mem:@var{m} @var{addr}) This RTX represents a reference to main memory at an address represented by the expression @var{addr}. @var{m} specifies how large a unit of memory is accessed. @end table @node Arithmetic, Comparisons, Regs and Memory, RTL @section RTL Expressions for Arithmetic @table @code @item (plus:@var{m} @var{x} @var{y}) Represents the sum of the values represented by @var{x} and @var{y} carried out in machine mode @var{m}. This is valid only if @var{x} and @var{y} both are valid for mode @var{m}. @item (minus:@var{m} @var{x} @var{y}) Like @samp{plus} but represents subtraction. @item (minus @var{x} @var{y}) Represents the result of subtracting @var{y} from @var{x} for purposes of comparison. The absence of a machine mode in the @samp{minus} expression indicates that 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 @code{(cc0)}. And that is the only way this kind of expression may validly be used: as a value to be stored in the condition codes. @item (neg:@var{m} @var{x}) Represents the negation (subtraction from zero) of the value represented by @var{x}, carried out in mode @var{m}. @var{x} must be valid for mode @var{m}. @item (mult:@var{m} @var{x} @var{y}) Represents the signed product of the values represented by @var{x} and @var{y} carried out in machine mode @var{m}. If @var{x} and @var{y} are both valid for mode @var{m}, this is ordinary size-preserving multiplication. Alternatively, both @var{x} and @var{y} may be valid for a different, narrower mode. This represents the kind of multiplication that generates a product wider than the operands. Widening multiplication and same-size multiplication are completely distinct and supported by different machine instructions; machines may support one but not the other.@refill @samp{mult} may be used for floating point division as well. Then @var{m} is a floating point machine mode. @item (umult:@var{m} @var{x} @var{y}) Like @samp{mult} but represents unsigned multiplication. It may be used in both same-size and widening forms, like @samp{mult}. @samp{umult} is used only for fixed-point multiplication. @item (div:@var{m} @var{x} @var{y}) Represents the quotient in signed division of @var{x} by @var{y}, carried out in machine mode @var{m}. If @var{m} is a floating-point mode, it represents the exact quotient; otherwise, the integerized quotient. If @var{x} and @var{y} are both valid for mode @var{m}, this is ordinary size-preserving division. Some machines have division instructions in which the operands and quotient widths are not all the same; such instructions are represented by @samp{div} expressions in which the machine modes are not all the same. @item (udiv:@var{m} @var{x} @var{y}) Like @samp{div} but represents unsigned division. @item (mod:@var{m} @var{x} @var{y}) @itemx (umod:@var{m} @var{x} @var{y}) Like @samp{div} and @samp{udiv} but represent the remainder instead of the quotient. @item (not:@var{m} @var{x}) Represents the bitwise complement of the value represented by @var{x}, carried out in mode @var{m}, which must be a fixed-point machine mode. @var{x} must be valid for mode @var{m}, which must be a fixed-point mode. @item (and:@var{m} @var{x} @var{y}) Represents the bitwise logical-and of the values represented by @var{x} and @var{y}, carried out in machine mode @var{m}. This is valid only if @var{x} and @var{y} both are valid for mode @var{m}, which must be a fixed-point mode. @item (ior:@var{m} @var{x} @var{y}) Represents the bitwise inclusive-or of the values represented by @var{x} and @var{y}, carried out in machine mode @var{m}. This is valid only if @var{x} and @var{y} both are valid for mode @var{m}, which must be a fixed-point mode. @item (xor:@var{m} @var{x} @var{y}) Represents the bitwise exclusive-or of the values represented by @var{x} and @var{y}, carried out in machine mode @var{m}. This is valid only if @var{x} and @var{y} both are valid for mode @var{m}, which must be a fixed-point mode. @item (lshift:@var{m} @var{x} @var{c}) Represents the result of logically shifting @var{x} left by @var{c} places. @var{x} must be valid for the mode @var{m}, a fixed-point machine mode. @var{c} must be valid for a fixed-point mode; 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 @var{c} is @code{QImode} regardless of @var{m}. On some machines, negative values of @var{c} may be meaningful; this is why logical left shift and arithmetic left shift are distinguished. For example, Vaxes have no right-shift instructions, and right shifts are represented as left-shift instructions whose counts happen to be negative constants or else computed (in a previous instruction) by negation. @item (ashift:@var{m} @var{x} @var{c}) Like @samp{lshift} but for arithmetic left shift. @item (lshiftrt:@var{m} @var{x} @var{c}) @itemx (ashiftrt:@var{m} @var{x} @var{c}) Like @samp{lshift} and @samp{ashift} but for right shift. @item (rotate:@var{m} @var{x} @var{c}) @itemx (rotatert:@var{m} @var{x} @var{c}) Similar but represent left and right rotate. @item (abs:@var{m} @var{x}) Represents the absolute value of @var{x}, computed in mode @var{m}. @var{x} must be valid for @var{m}. @item (sqrt:@var{m} @var{x}) Represents the square root of @var{x}, computed in mode @var{m}. @var{x} must be valid for @var{m}. Most often @var{m} will be a floating point mode. @item (ffs:@var{m} @var{x}) Represents the one plus the index of the least significant 1-bit in @var{x}, represented as an integer of mode @var{m}. (The value is zero if @var{x} is zero.) The mode of @var{x} need not be @var{m}; depending on the target machine, various mode combinations may be valid. @end table @node Comparisons, Bit Fields, Arithmetic, RTL @section Comparison Operations Comparison operators test a relation on two operands and are considered to represent the value 1 if the relation holds, or zero if it does not. The mode of the comparison is determined 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. Inequality comparisons come in two flavors, signed and unsigned. Thus, there are distinct expression codes @samp{gt} and @samp{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 actually @code{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. The comparison operators may be used to compare the condition codes @code{(cc0)} against zero, as in @code{(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 above example stands for 1 if the condition codes were set to say ``zero'' or ``equal'', 0 otherwise. Although the same comparison operators are used for this as may be used in other contexts on actual data, no confusion can result since the machine description would never allow both kinds of uses in the same context. @table @code @item (eq @var{x} @var{y}) 1 if the values represented by @var{x} and @var{y} are equal, otherwise 0. @item (ne @var{x} @var{y}) 1 if the values represented by @var{x} and @var{y} are not equal, otherwise 0. @item (gt @var{x} @var{y}) 1 if the @var{x} is greater than @var{y}. If they are fixed-point, the comparison is done in a signed sense. @item (gtu @var{x} @var{y}) Like @samp{gt} but does unsigned comparison, on fixed-point numbers only. @item (lt @var{x} @var{y}) @item (ltu @var{x} @var{y}) Like @samp{gt} and @samp{gtu} but test for ``less than''. @item (ge @var{x} @var{y}) @item (geu @var{x} @var{y}) Like @samp{gt} and @samp{gtu} but test for ``greater than or equal''. @item (le @var{x} @var{y}) @item (leu @var{x} @var{y}) Like @samp{gt} and @samp{gtu} but test for ``less than or equal''. @item (if_then_else @var{cond} @var{then} @var{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, @var{cond} is a comparison expression. This expression represents a choice, according to @var{cond}, between the value represented by @var{then} and the one represented by @var{else}. On most machines, @samp{if_then_else} expressions are valid only to express conditional jumps. @end table @node Bit Fields, Conversions, Comparisons, RTL @section 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 a assignment, indicating insertion of a value into the specified bit field. @table @code @item (sign_extract:SI @var{loc} @var{size} @var{pos}) This represents a reference to a sign-extended bit-field contained or starting in @var{loc} (a memory or register reference). The bit field is @var{size} bits wide and starts at bit @var{pos}. The compilation option @code{BITS_BIG_ENDIAN} says which end of the memory unit @var{pos} counts from. Which machine modes are valid for @var{loc} depends on the machine, but typically @var{loc} should be a single byte when in memory or a full word in a register. @item (zero_extract:SI @var{loc} @var{size} @var{pos}) Like @samp{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. @end table @node Conversions, RTL Declarations, Bit Fields, RTL @section 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 @code{(plus:SI (reg:QI 34) (reg:SI 80))} because the @samp{plus} operation requires two operands of the same machine mode. Therefore, the byte-sized operand is enclosed in a conversion operation, as in @example (plus:SI (sign_extend:SI (reg:QI 34)) (reg:SI 80)) @end example 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 conversion operation code says how to do it. @table @code @item (sign_extend:@var{m} @var{x}) Represents the result of sign-extending the value @var{x} to machine mode @var{m}. @var{m} must be a fixed-point mode and @var{x} a fixed-point value of a mode narrower than @var{m}. @item (zero_extend:@var{m} @var{x}) Represents the result of zero-extending the value @var{x} to machine mode @var{m}. @var{m} must be a fixed-point mode and @var{x} a fixed-point value of a mode narrower than @var{m}. @item (float_extend:@var{m} @var{x}) Represents the result of extending the value @var{x} to machine mode @var{m}. @var{m} must be a floating point mode and @var{x} a floating point value of a mode narrower than @var{m}. @item (truncate:@var{m} @var{x}) Represents the result of truncating the value @var{x} to machine mode @var{m}. @var{m} must be a fixed-point mode and @var{x} a fixed-point value of a mode wider than @var{m}. @item (float_truncate:@var{m} @var{x}) Represents the result of truncating the value @var{x} to machine mode @var{m}. @var{m} must be a floating point mode and @var{x} a floating point value of a mode wider than @var{m}. @item (float:@var{m} @var{x}) Represents the result of converting fixed point value @var{x}, regarded as signed, to floating point mode @var{m}. @item (unsigned_float:@var{m} @var{x}) Represents the result of converting fixed point value @var{x}, regarded as unsigned, to floating point mode @var{m}. @item (fix:@var{m} @var{x}) When @var{m} is a fixed point mode, represents the result of converting floating point value @var{x} to mode @var{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. @item (unsigned_fix:@var{m} @var{x}) Represents the result of converting floating point value @var{x} to fixed point mode @var{m}, regarded as unsigned. How rounding is done is not specified. @item (fix:@var{m} @var{x}) When @var{m} is a floating point mode, represents the result of converting floating point value @var{x} (valid for mode @var{m}) to an integer, still represented in floating point mode @var{m}, by rounding towards zero. @end table @node RTL Declarations, Side Effects, Conversions, RTL @section Declarations Declaration expression codes do not represent arithmetic operations but rather state assertions about their operands. @table @code @item (strict_low_part (subreg:@var{m} (reg:@var{n} @var{r}) 0)) This expression code is used in only one context: operand 0 of a @samp{set} expression. In addition, the operand of this expression must be a @samp{subreg} expression. The presence of @samp{strict_low_part} says that the part of the register which is meaningful in mode @var{n}, but is not part of mode @var{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 @var{m} is less than a word. @end table @node Side Effects, Incdec, RTL Declarations, RTL @section 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. @table @code @item (set @var{lval} @var{x}) Represents the action of storing the value of @var{x} into the place represented by @var{lval}. @var{lval} must be an expression representing a place that can be stored in: @samp{reg} (or @samp{subreg} or @samp{strict_low_part}), @samp{mem}, @samp{pc} or @samp{cc0}.@refill If @var{lval} is a @samp{reg}, @samp{subreg} or @samp{mem}, it has a machine mode; then @var{x} must be valid for that mode.@refill If @var{lval} is a @samp{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 @var{lval} is a @samp{subreg} whose machine mode is narrower than @code{SImode}, the rest of the register can be changed in an undefined way. If @var{lval} is a @samp{strict_low_part} of a @samp{subreg}, then the part of the register specified by the machine mode of the @samp{subreg} is given the value @var{x} and the rest of the register is not changed.@refill If @var{lval} is @code{(cc0)}, it has no machine mode, and @var{x} may have any mode. This represents a ``test'' or ``compare'' instruction.@refill If @var{lval} is @code{(pc)}, we have a jump instruction, and the possibilities for @var{x} are very limited. It may be a @samp{label_ref} expression (unconditional jump). It may be an @samp{if_then_else} (conditional jump), in which case either the second or the third operand must be @code{(pc)} (for the case which does not jump) and the other of the two must be a @samp{label_ref} (for the case which does jump). @var{x} may also be a @samp{mem} or @code{(plus:SI (pc) @var{y})}, where @var{y} may be a @samp{reg} or a @samp{mem}; these unusual patterns are used to represent jumps through branch tables.@refill @item (return) 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 @samp{return} expression code is never used. @item (call @var{function} @var{nargs}) Represents a function call. @var{function} is a @samp{mem} expression whose address is the address of the function to be called. @var{nargs} is an expression representing the number of words of argument. Each machine has a standard machine mode which @var{function} must have. The machine description defines macro @code{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. @item (clobber @var{x}) Represents the storing or possible storing of an unpredictable, undescribed value into @var{x}, which must be a @samp{reg} or @samp{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. @var{x} may also be null---a null C pointer, no expression at all. Such a @code{(clobber (null))} expression 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 @samp{clobber} expressions to indicate this fact. Also, each function call is assumed to have the potential to alter any memory location. @item (use @var{x}) Represents the use of the value of @var{x}. It indicates that the value in @var{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 instructions whose only effect is to store a value in @var{x}. @var{x} must be a @samp{reg} expression. @item (parallel [@var{x0} @var{x1} @dots{}]) Represents several side effects performed in parallel. The square brackets stand for a vector; the operand of @samp{parallel} is a vector of expressions. @var{x0}, @var{x1} and so on are individual side effects---expressions of code @samp{set}, @samp{call}, @samp{return}, @samp{clobber} or @samp{use}.@refill ``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, @example (parallel [(set (reg:SI 1) (mem:SI (reg:SI 1))) (set (mem:SI (reg:SI 1)) (reg:SI 1))]) @end example @noindent says unambiguously that the values of hard register 1 and the memory location addressed by it are interchanged. In both places where @code{(reg:SI 1)} appears as a memory address it refers to the value in register 1 @emph{before} the execution of the instruction. @item (sequence [@var{insns} @dots{}]) Represents a sequence of insns. Each of the @var{insns} that appears in the vector is suitable for appearing in the chain of insns, so it must be an @samp{insn}, @samp{jump_insn}, @samp{call_insn}, @samp{code_label}, @samp{barrier} or @samp{note}. A @samp{sequence} RTX never appears in an actual insn. It represents the sequence of insns that result from a @samp{define_expand} @emph{before} those insns are passed to @code{emit_insn} to insert them in the chain of insns. When actually inserted, the individual sub-insns are separated out and the @samp{sequence} is forgotten. @end table Three expression codes appear in place of a side effect, as the body of an insn, though strictly speaking they do not describe side effects as such: @table @code @item (asm_input @var{s}) Represents literal assembler code as described by the string @var{s}. @item (addr_vec:@var{m} [@var{lr0} @var{lr1} @dots{}]) Represents a table of jump addresses. The vector elements @var{lr0}, etc., are @samp{label_ref} expressions. The mode @var{m} specifies how much space is given to each address; normally @var{m} would be @code{Pmode}. @item (addr_diff_vec:@var{m} @var{base} [@var{lr0} @var{lr1} @dots{}]) Represents a table of jump addresses expressed as offsets from @var{base}. The vector elements @var{lr0}, etc., are @samp{label_ref} expressions and so is @var{base}. The mode @var{m} specifies how much space is given to each address-difference.@refill @end table @node Incdec, Assembler, Side Effects, RTL @section Embedded Side-Effects on Addresses Four special side-effect expression codes appear as memory addresses. @table @code @item (pre_dec:@var{m} @var{x}) Represents the side effect of decrementing @var{x} by a standard amount and represents also the value that @var{x} has after being decremented. @var{x} must be a @samp{reg} or @samp{mem}, but most machines allow only a @samp{reg}. @var{m} must be the machine mode for pointers on the machine in use. The amount @var{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:@refill @example (mem:DF (pre_dec:SI (reg:SI 39))) @end example @noindent This says to decrement pseudo register 39 by the length of a @code{DFmode} value and use the result to address a @code{DFmode} value. @item (pre_inc:@var{m} @var{x}) Similar, but specifies incrementing @var{x} instead of decrementing it. @item (post_dec:@var{m} @var{x}) Represents the same side effect as @samp{pre_decrement} but a different value. The value represented here is the value @var{x} has @i{before} being decremented. @item (post_inc:@var{m} @var{x}) Similar, but specifies incrementing @var{x} instead of decrementing it. @end table These embedded side effect expressions must be used with care. Instruction patterns may not use them. Until the @samp{flow} pass of the compiler, they may occur only to represent pushes onto the stack. The @samp{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. Explicit popping of the stack could be represented with these embedded side effect operators, but that would not be safe; the instruction combination pass could move the popping past pushes, thus changing the meaning of the code. An instruction that can be represented with an embedded side effect could also be represented using @samp{parallel} containing an additional @samp{set} to describe how the address register 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 additional parallel stores would require doubling the number of entries in the machine description. @node Assembler, Insns, IncDec, RTL @section Assembler Instructions as Expressions The RTX code @samp{asm_operands} represents a value produced by a user-specified assembler instruction. It is used to represent an @code{asm} statement with arguments. An @code{asm} statement with a single output operand, like this: @example asm ("foo %1,%2,%0" : "a" (outputvar) : "g" (x + y), "di" (*z)); @end example @noindent is represented using a single @samp{asm_operands} RTX which represents the value that is stored in @code{outputvar}: @example (set @var{rtx-for-outputvar} (asm_operands "foo %1,%2,%0" "a" 0 [@var{rtx-for-addition-result} @var{rtx-for-*z}] [(asm_input:@var{m1} "g") (asm_input:@var{m2} "di")])) @end example @noindent Here the operands of the @samp{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 @var{m1} is the mode of the sum @code{x+y}; @var{m2} is that of @code{*z}. When an @code{asm} statement has multiple output values, its insn has several such @samp{set} RTX's inside of a @samp{parallel}. Each @samp{set} contains a @samp{asm_operands}; all of these share the same assembler template and vectors, but each contains the constraint for the respective output operand. They are also distinguished by the output-operand index number, which is 0, 1, @dots{} for successive output operands. @node Insns, Calls, Assembler, RTL @section Insns The RTL representation of the code for a function is a doubly-linked chain of objects called @dfn{insns}. Insns are expressions with special codes that are used for no other purpose. Some insns are actual instructions; others represent dispatch tables for @code{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, 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 @code{XEXP} and @code{XINT}, but instead three special macros are always used: @table @code @item INSN_UID (@var{i}) Accesses the unique id of insn @var{i}. @item PREV_INSN (@var{i}) Accesses the chain pointer to the insn preceding @var{i}. If @var{i} is the first insn, this is a null pointer. @item NEXT_INSN (@var{i}) Accesses the chain pointer to the insn following @var{i}. If @var{i} is the last insn, this is a null pointer. @end table The @code{NEXT_INSN} and @code{PREV_INSN} pointers must always correspond: if @var{i} is not the first insn, @example NEXT_INSN (PREV_INSN (@var{insn})) == @var{insn} @end example @noindent is always true. Every insn has one of the following six expression codes: @table @samp @item insn The expression code @samp{insn} is used for instructions that do not jump and do not do function calls. Insns with code @samp{insn} have four additional fields beyond the three mandatory ones listed above. These four are described in a table below. @item jump_insn The expression code @samp{jump_insn} is used for instructions that may jump (or, more generally, may contain @samp{label_ref} expressions). @samp{jump_insn} insns have the same extra fields as @samp{insn} insns, accessed in the same way. @item call_insn The expression code @samp{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. @samp{call_insn} insns have the same extra fields as @samp{insn} insns, accessed in the same way. @item code_label A @samp{code_label} insn represents a label that a jump insn can jump to. It contains one special field of data in addition to the three standard ones. It is used to hold the @dfn{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 @samp{L@var{n}} where @var{n} is the label number. @item barrier Barriers are placed in the instruction stream after unconditional jump instructions to indicate that the jumps are unconditional. They contain no information beyond the three standard fields. @item note @samp{note} insns are used to represent additional debugging and declarative information. They contain two nonstandard fields, an integer which is accessed with the macro @code{NOTE_LINE_NUMBER} and a string accessed with @code{NOTE_SOURCE_FILE}. If @code{NOTE_LINE_NUMBER} is positive, the note represents the position of a source line and @code{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, @code{NOTE_LINE_NUMBER} is not really a line number but a code with one of the following values (and @code{NOTE_SOURCE_FILE} must contain a null pointer): @table @code @item NOTE_INSN_DELETED Such a note is completely ignorable. Some passes of the compiler delete insns by altering them into notes of this kind. @item NOTE_INSN_BLOCK_BEG @itemx 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. @item NOTE_INSN_LOOP_BEG @itemx NOTE_INSN_LOOP_END These types of notes indicate the position of the beginning and end of a @code{while} or @code{for} loop. They enable the loop optimizer to find loops quickly. @end table @end table Here is a table of the extra fields of @samp{insn}, @samp{jump_insn} and @samp{call_insn} insns: @table @code @item PATTERN (@var{i}) An expression for the side effect performed by this insn. @item REG_NOTES (@var{i}) A list (chain of @samp{expr_list} expressions) giving information about the usage of registers in this insn. This list is set up by the flow analysis pass; it is a null pointer until then. @item LOG_LINKS (@var{i}) A list (chain of @samp{insn_list} expressions) of previous ``related'' insns: insns which store into registers values that are used for the first time in this insn. (An additional constraint is that neither a jump nor a label may come between the related insns). This list is set up by the flow analysis pass; it is a null pointer until then. @item INSN_CODE (@var{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 is not used on an insn whose pattern consists of a single @samp{use}, @samp{clobber}, @samp{asm}, @samp{addr_vec} or @samp{addr_diff_vec} expression. @end table The @code{LOG_LINKS} field of an insn is a chain of @samp{insn_list} expressions. Each of these has two operands: the first is an insn, and the second is another @samp{insn_list} expression (the next one in the chain). The last @samp{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 @samp{insn_list} expressions). Their order is not significant. The @code{REG_NOTES} field of an insn is a similar chain but of @samp{expr_list} expressions instead of @samp{insn_list}. There are four kinds of register notes, which are distinguished by the machine mode of the @samp{expr_list}, which a register note is really understood as being an @code{enum reg_note}. The first operand @var{op} of the @samp{expr_list} is data whose meaning depends on the kind of note. Here are the four kinds: @table @code @item REG_DEAD The register @var{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. @item REG_INC The register @var{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 @code{POST_INC}, @code{PRE_INC}, @code{POST_DEC} or @code{PRE_DEC} RTX. @item REG_EQUIV The register that is set by this insn will be equal to @var{op} at run time, and could validly be replaced in all its occurrences by @var{op}. (``Validly'' here refers to the data flow of the program; simple replacement may make some insns invalid.) The value which the insn explicitly copies into the register may look different from @var{op}, but they will be equal at run time. 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. @item REG_EQUAL The register that is set by this insn will be equal to @var{op} at run time at the end of this insn (but not necessarily elsewhere in the function). The RTX @var{op} is typically an arithmetic expression. For example, when a sequence of insns such as a library call is used to perform an arithmetic operation, this kind of note is attached to the insn that produces or copies the final value. It tells the CSE pass how to think of that value. @item REG_RETVAL This insn copies the value of a library call, and @var{op} is the first insn that was generated to set up the arguments for the library call. Flow analysis uses this note to delete all of a library call whose result is dead. @item REG_WAS_0 The register @var{op} contained zero before this insn. You can rely on this note if it is present; its absence implies nothing. @end table (The only difference between the expression codes @samp{insn_list} and @samp{expr_list} is that the first operand of an @samp{insn_list} is assumed to be an insn and is printed in debugging dumps as the insn's unique id; the first operand of an @samp{expr_list} is printed in the ordinary way as an expression.) @node Calls, Sharing, Insns, RTL @section RTL Representation of Function-Call Insns Insns that call subroutines have the RTL expression code @samp{call_insn}. These insns must satisfy special rules, and their bodies must use a special RTL expression code, @samp{call}. A @samp{call} expression has two operands, as follows: @example (call @var{nbytes} (mem:@var{fm} @var{addr})) @end example @noindent Here @var{nbytes} is an operand that represents the number of bytes of argument data being passed to the subroutine, @var{fm} is a machine mode (which must equal as the definition of the @code{FUNCTION_MODE} macro in the machine description) and @var{addr} represents the address of the subroutine. For a subroutine that returns no value, the @samp{call} RTX as shown above is the entire body of the insn. For a subroutine that returns a value whose mode is not @code{BLKmode}, the value is returned in a hard register. If this register's number is @var{r}, then the body of the call insn looks like this: @example (set (reg:@var{m} @var{r}) (call @var{nbytes} (mem:@var{fm} @var{addr}))) @end example @noindent This RTL expression makes it clear (to the optimizer passes) that the appropriate register receives a useful value in this insn. Immediately after RTL generation, if the value of the subroutine is actually used, this call insn is always followed closely by an insn which refers to the register @var{r}. This remains true through all the optimizer passes until cross jumping occurs. The following insn has one of two forms. Either it copies the value into a pseudo-register, like this: @example (set (reg:@var{m} @var{p}) (reg:@var{m} @var{r})) @end example @noindent or (in the case where the calling function will simply return whatever value the call produced, and no operation is needed to do this): @example (use (reg:@var{m} @var{r})) @end example @noindent Between the call insn and this following insn there may intervene only a stack-adjustment insn (and perhaps some @samp{note} insns). When a subroutine returns a @code{BLKmode} value, it is handled 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. @node Sharing,, Calls, RTL @section Structure Sharing Assumptions The compiler assumes that certain kinds of RTL expressions 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 certain 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, no RTL objects are common to two functions. @itemize @bullet @item Each pseudo-register has only a single @samp{reg} object to represent it, and therefore only a single machine mode. @item For any symbolic label, there is only one @samp{symbol_ref} object referring to it. @item There is only one @samp{const_int} expression with value zero, and only one with value one. @item There is only one @samp{pc} expression. @item There is only one @samp{cc0} expression. @item There is only one @samp{const_double} expression with mode @code{SFmode} and value zero, and only one with mode @code{DFmode} and value zero. @item No @samp{label_ref} 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 @samp{label_ref} is seen it is distinct from all others that are seen. @item Only one @samp{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. @item 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. @item 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 @code{unshare_all_rtl} in @file{emit-rtl.c}, after which the above rules are guaranteed to be followed. @item During the combiner pass, shared structure with an insn can exist temporarily. However, the shared structure is copied before the combiner is finished with the insn. This is done by @code{copy_substitutions} in @samp{combine.c}. @end itemize @node Machine Desc, Machine Macros, RTL, Top @chapter Machine Descriptions A machine description has two parts: a file of instruction patterns (@file{.md} file) and a C header file of macro definitions. The @file{.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. @menu * Patterns:: How to write instruction patterns. * Example:: An explained example of a @samp{define_insn} pattern. * RTL Template:: The RTL template defines what insns match a pattern. * Output Template:: The output template says how to make assembler code from such an insn. * Output Statement:: For more generality, write C code to output the assembler code. * Constraints:: When not all operands are general operands. * Standard Names:: Names mark patterns to use for code generation. * Pattern Ordering:: When the order of patterns makes a difference. * Dependent Patterns:: Having one pattern may make you need another. * Jump Patterns:: Special considerations for patterns for jump insns. * Peephole Definitions::Defining machine-specific peephole optimizations. * Expander Definitions::Generating a sequence of several RTL insns for a standard operation. @end menu @node Patterns, Example, Machine Desc, Machine Desc @section Everything about Instruction Patterns Each instruction pattern contains an incomplete RTL expression, with pieces to be filled in later, operand constraints that restrict how the pieces can be filled in, and an output pattern or C code to generate the assembler output, all wrapped up in a @samp{define_insn} expression. A @samp{define_insn} is an RTL expression containing four operands: @enumerate @item 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. @item The @dfn{RTL template} (@pxref{RTL Template}) is a vector of incomplete RTL expressions which show what the instruction should look like. It is incomplete because it may contain @samp{match_operand} and @samp{match_dup} expressions that stand for operands of the instruction. If the vector has only one element, that element is what the instruction should look like. If the vector has multiple elements, then the instruction looks like a @samp{parallel} expression containing that many elements as described. @item 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 @code{operands}. @item The @dfn{output template}: a string that says how to output matching insns as assembler code. @samp{%} in this string specifies where to substitute the value of an operand. @xref{Output Template}. When simple substitution isn't general enough, you can specify a piece of C code to compute the output. @xref{Output Statement}. @end enumerate @node Example, RTL Template, Patterns, Machine Desc @section Example of @samp{define_insn} Here is an actual example of an instruction pattern, for the 68000/68020. @example (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\"; @}") @end example This is an instruction that sets the condition codes based on the value of a general operand. It has no condition, so any insn whose RTL description has the form shown may be handled according to this pattern. The name @samp{tstsi} means ``test a @code{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. @samp{"rm"} is an operand constraint. Its meaning is explained below. @node RTL Template, Output Template, Example, Machine Desc @section 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. @table @code @item (match_operand:@var{m} @var{n} @var{testfn} @var{constraint}) This expression is a placeholder for operand number @var{n} of the insn. When constructing an insn, operand number @var{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 @var{n}; but it must satisfy @var{testfn} 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 @samp{match_operand} expression in the pattern for each expression number, and they must appear in order of increasing expression number. @var{testfn} is a string that is the name of a C function that accepts two arguments, a machine mode and an expression. During matching, the function will be called with @var{m} as the mode argument and the putative operand as the other argument. If it returns zero, this instruction pattern fails to match. @var{testfn} may be an empty string; then it means no test is to be done on the operand. Most often, @var{testfn} is @code{"general_operand"}. It checks that the putative operand is either a constant, a register or a memory reference, and that it is valid for mode @var{m}. For an operand that must be a register, @var{testfn} should be @code{"register_operand"}. This prevents GNU CC from creating insns that have memory references in these operands, insns which would only have to be taken apart in the reload pass. For an operand that must be a constant, either @var{testfn} should be @code{"immediate_operand"}, or the instruction pattern's extra condition should check for constants, or both. @var{constraint} is explained later (@pxref{Constraints}). @item (match_dup @var{n}) This expression is also a placeholder for operand number @var{n}. It is used when the operand needs to appear more than once in the insn. In construction, @samp{match_dup} behaves exactly like @samp{match_operand}: the operand is substituted into the insn being constructed. But in matching, @samp{match_dup} behaves differently. It assumes that operand number @var{n} has already been determined by a @samp{match_operand} appearing earlier in the recognition template, and it matches only an identical-looking expression. @item (address (match_operand:@var{m} @var{n} "address_operand" "")) This complex of expressions is a placeholder for an operand number @var{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. @samp{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 @samp{p} in the constraint serves this purpose. @var{m} is the machine mode of the @emph{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 @code{Pmode}, which normally is @code{SImode}), so there is no point in mentioning it; thus, no machine mode is written in the @samp{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 @samp{address} expression. @end table @node Output Template, Output Statement, RTL Template, Machine Desc @section Output Templates and Operand Substitution The @dfn{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 @samp{%} is used to specify where to substitute an operand; it can also be used to identify places different variants of the assembler require different syntax. In the simplest case, a @samp{%} followed by a digit @var{n} says to output operand @var{n} at that point in the string. @samp{%} followed by a letter and a digit says to output an operand in an alternate fashion. Four letters have standard, built-in meanings described below. The machine description macro @code{PRINT_OPERAND} can define additional letters with nonstandard meanings. @samp{%c@var{digit}} can be used to substitute an operand that is a constant value without the syntax that normally indicates an immediate operand. @samp{%n@var{digit}} is like @samp{%c@var{digit}} except that the value of the constant is negated before printing. @samp{%a@var{digit}} 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. @samp{%l@var{digit}} is used to substitute a @code{label_ref} into a jump instruction. @samp{%} followed by a punctuation character specifies a substitution that does not use an operand. Only one case is standard: @samp{%%} outputs a @samp{%} into the assembler code. Other nonstandard cases can be defined in the @code{PRINT_OPERAND} macro. The template may generate multiple assembler instructions. Write the text for the instructions, with @samp{\;} between them. When the RTL contains two operand 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. One use of nonstandard letters or punctuation following @samp{%} 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 example, the opcode @samp{movel} in MIT syntax is @samp{move.l} in Motorola syntax. The same file of patterns is used for both kinds of output syntax, but the character sequence @samp{%.} is used in each place where Motorola syntax wants a period. The @code{PRINT_OPERAND} macro for Motorola syntax defines the sequence to output a period; the macro for MIT syntax defines it to do nothing. @node Output Statement, Constraints, Output Template, Machine Desc @section 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 example, 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 @samp{*}, then it is not an output template but rather a piece of C program that should compute a template. It should execute a @code{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 @samp{\}. The operands may be found in the array @code{operands}, whose C data type is @code{rtx []}. It is possible to output an assembler instruction and then go on to output or compute more of them, using the subroutine @code{output_asm_insn}. This receives two arguments: a template-string and a vector of operands. The vector may be @code{operands}, or it may be another array of @code{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 determined mostly by which alternative was matched. When this is so, the C code can test the variable @code{which_alternative}, which is the ordinal number of the alternative that was actually satisfied (0 for the first, 1 for the second alternative, etc.). For example, suppose there are two opcodes for storing zero, @samp{clrreg} for registers and @samp{clrmem} for memory locations. Here is how a pattern could use @code{which_alternative} to choose between them: @example (define_insn "" [(set (match_operand:SI 0 "general_operand" "r,m") (const_int 0))] "" "* return (which_alternative == 0 ? \"clrreg %0\" : \"clrmem %0\"); ") @end example @node Constraints, Standard Names, Output Statement, Machine Desc @section Operand Constraints Each @samp{match_operand} in an instruction pattern can specify a constraint for the type of operands allowed. Constraints 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. @menu * Simple Constraints:: Basic use of constraints. * Multi-Alternative:: When an insn has two alternative constraint-patterns. * Class Preferences:: Constraints guide which hard register to put things in. * Modifiers:: More precise control over effects of constraints. * No Constraints:: Describing a clean machine without constraints. @end menu @node Simple Constraints, Multi-Alternative, Constraints, Constraints @subsection 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: @table @asis @item @samp{m} A memory operand is allowed, with any kind of address that the machine supports in general. @item @samp{o} A memory operand is allowed, but only if the address is @dfn{offsetable}. 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 offsetable; 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 offsetable. More complicated indirect/indexed addresses may or may not be offsetable 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 @samp{o} is valid only when accompanied by both @samp{<} (if the target machine has predecrement addressing) and @samp{>} (if the target machine has preincrement addressing). @item @samp{<} A memory operand with autodecrement addressing (either predecrement or postdecrement) is allowed. @item @samp{>} A memory operand with autoincrement addressing (either preincrement or postincrement) is allowed. @item @samp{r} A register operand is allowed provided that it is in a general register. @item @samp{d}, @samp{a}, @samp{f}, @dots{} Other letters can be defined in machine-dependent fashion to stand for particular classes of registers. @samp{d}, @samp{a} and @samp{f} are defined on the 68000/68020 to stand for data, address and floating point registers. @item @samp{i} An immediate integer operand (one with constant value) is allowed. This includes symbolic constants whose values will be known only at assembly time. @item @samp{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 @samp{n} rather than @samp{i}. @item @samp{I}, @samp{J}, @samp{K}, @dots{} Other letters in the range @samp{I} through @samp{M} 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, @samp{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. @item @samp{F} An immediate floating operand (expression code @samp{const_double}) is allowed. @item @samp{G}, @samp{H} @samp{G} and @samp{H} may be defined in a machine-dependent fashion to permit immediate floating operands in particular ranges of values. @item @samp{s} An immediate integer operand whose value is not an explicit integer is allowed. 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 @samp{s} instead of @samp{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 -32 and 31, 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 @samp{moveq} instruction. We arrange for this to happen by defining the letter @samp{K} to mean ``any integer outside the range -32 to 31'', and then specifying @samp{Ks} in the operand constraints. @item @samp{g} Any register, memory or immediate integer operand is allowed, except for registers that are not general registers. @item @samp{@var{n}} (a digit) An operand that matches operand number @var{n} is allowed. If a digit is used together with letters, the digit should come last. This is called a @dfn{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 matching constraint must appear in an input-only operand and the operand that it matches must be an output-only operand with a lower number. 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, @code{*x} as an input operand will match @code{*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. @item @samp{p} An operand that is a valid memory address is allowed. This is for ``load address'' and ``push address'' instructions. If @samp{p} is used in the constraint, the test-function in the @samp{match_operand} must be @code{address_operand}. @end table 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 constraint will be satisfied. Usually this is done by copying an operand into a register. Contrast, therefore, the two instruction patterns that follow: @example (define_insn "" [(set (match_operand:SI 0 "general_operand" "r") (plus:SI (match_dup 0) (match_operand:SI 1 "general_operand" "r")))] "" "@dots{}") @end example @noindent which has two operands, one of which must appear in two places, and @example (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")))] "" "@dots{}") @end example @noindent which has three operands, two of which are required by a constraint to be identical. If we are considering an insn of the form @example (insn @var{n} @var{prev} @var{next} (set (reg:SI 3) (plus:SI (reg:SI 6) (reg:SI 109))) @dots{}) @end example @noindent 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: @example (insn @var{n2} @var{prev} @var{n} (set (reg:SI 3) (reg:SI 6)) @dots{}) (insn @var{n} @var{n2} @var{next} (set (reg:SI 3) (plus:SI (reg:SI 3) (reg:SI 109))) @dots{}) @end example Because insns that don't fit the constraints are fixed up by loading operands into registers, every instruction pattern's constraints must permit the case where all the operands are in registers. It need not permit all classes of registers; the compiler knows how to copy registers into other registers of the proper class in order to make an instruction valid. But if no registers are permitted, the compiler will be stymied: it does not know how to save a register in memory in order to make an instruction valid. Instruction patterns that reject registers can be made valid by attaching a condition-expression that refuses to match an insn at all if the crucial operand is a register. @node Multi-Alternative, Class Preferences, Simple Constraints, Constraints @subsection 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 alternatives. 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: @example (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")))] @dots{}) @end example The first alternative has @samp{m} (memory) for operand 0, @samp{0} for operand 1 (meaning it must match operand 0), and @samp{dKs} for operand 2. The second alternative has @samp{d} (data register) for operand 0, @samp{0} for operand 1, and @samp{dmKs} for operand 2. The @samp{=} and @samp{%} in the constraint for operand 0 are not part of any alternative; their meaning is explained in the next section. 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 alternative requiring the least copying is chosen. If two alternatives need the same amount of copying, the one that comes first is chosen. These choices can be altered with the @samp{?} and @samp{!} characters: @table @samp @item ? Disparage slightly the alternative that the @samp{?} appears in, as a choice when no alternative applies exactly. The compiler regards this alternative as one unit more costly for each @samp{?} that appears in it. @item ! Disparage severely the alternative that the @samp{!} appears in. When operands must be copied into registers, the compiler will never choose this alternative as the one to strive for. @end table When an insn pattern has multiple alternatives in its constraints, often the appearance of the assembler code determined mostly by which alternative was matched. When this is so, the C code for writing the assembler code can use the variable @code{which_alternative}, which is the ordinal number of the alternative that was actually satisfied (0 for the first, 1 for the second alternative, etc.). For example: @example (define_insn "" [(set (match_operand:SI 0 "general_operand" "r,m") (const_int 0))] "" "* return (which_alternative == 0 ? \"clrreg %0\" : \"clrmem %0\"); ") @end example @node Class Preferences, Modifiers, Multi-Alternative, Constraints @subsection Register Class Preferences The operand constraints have another function: they enable the compiler to decide which kind of hardware register 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 @samp{d} and @samp{a} that specify classes of registers. The pseudo register is put in whichever class gets the most ``votes''. The constraint letters @samp{g} and @samp{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. @node Modifiers, No Constraints, Class Preferences, Constraints @subsection Constraint Modifier Characters @table @samp @item = Means that this operand is write-only for this instruction: the previous value is discarded and replaced by output data. @item + 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. @samp{=} identifies an output; @samp{+} identifies an operand that is both input and output; all other operands are assumed to be input only. @item & 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 used as an input operand or as part of any memory address. @samp{&} applies only to the alternative in which it is written. In constraints with multiple alternatives, sometimes one alternative requires @samp{&} while others do not. See, for example, the @samp{movdf} insn of the 68000. @samp{&} does not obviate the need to write @samp{=}. @item % 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: @example (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")))] @dots{}) @end example Note that in previous versions of GNU CC the @samp{%} constraint modifier always applied to operands 1 and 2 regardless of which operand it was written in. The usual custom was to write it in operand 0. Now it must be in operand 1 if the operands to be exchanged are 1 and 2. @item # 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. @item * Says that the following character should be ignored when choosing register preferences. @samp{*} has no effect on the meaning of the constraint as a constraint. 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, @samp{*} is used so that the @samp{d} constraint letter (for data register) is ignored when computing register preferences. @example (define_insn "extendhisi2" [(set (match_operand:SI 0 "general_operand" "=*d,a") (sign_extend:SI (match_operand:HI 1 "general_operand" "0,g")))] @dots{}) @end example @end table @node No Constraints,, Modifiers, Constraints @subsection 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 @samp{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 @samp{p}. For such machines, instead of writing @samp{g} and @samp{p} for all the constraints, you can choose to write a description with empty constraints. Then you write @samp{""} for the constraint in every @samp{match_operand}. Address operands are identified by writing an @samp{address} expression around the @samp{match_operand}, not by their constraints. When the machine description has just empty constraints, certain parts of compilation are skipped, making the compiler faster. @node Standard Names, Pattern Ordering, Constraints, Machine Desc @section Standard Names for Patterns Used in Generation Here is a table of the instruction names that are meaningful 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. @table @asis @item @samp{mov@var{m}} Here @var{m} is 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, @samp{movsi} moves full-word data. If operand 0 is a @samp{subreg} with mode @var{m} of a register whose natural mode is wider than @var{m}, the effect of this instruction is to store the specified value in the part of the register that corresponds to mode @var{m}. 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 @emph{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 have a reload. Therefore, when given such a pair of operands, the pattern must generate RTL which needs no temporary registers---no registers other than the operands. For example, if you support the pattern with a @code{define_expand}, then in such a case you mustn't call @code{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 @file{spur.md} to see how the requirement is satisfied. 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, while on other machines explicit memory references will get optional reloads. @item @samp{movstrict@var{m}} Like @samp{mov@var{m}} except that if operand 0 is a @samp{subreg} with mode @var{m} of a register whose natural mode is wider, the @samp{movstrict@var{m}} instruction is guaranteed not to alter any of the register except the part which belongs to mode @var{m}. @item @samp{add@var{m}3} Add operand 2 and operand 1, storing the result in operand 0. All operands must have mode @var{m}. This can be used even on two-address machines, by means of constraints requiring operands 1 and 0 to be the same location. @item @samp{sub@var{m}3}, @samp{mul@var{m}3}, @samp{umul@var{m}3}, @samp{div@var{m}3}, @samp{udiv@var{m}3}, @samp{mod@var{m}3}, @samp{umod@var{m}3}, @samp{and@var{m}3}, @samp{ior@var{m}3}, @samp{xor@var{m}3} Similar, for other arithmetic operations. @item @samp{andcb@var{m}3} Bitwise logical-and operand 1 with the complement of operand 2 and store the result in operand 0. @item @samp{mulhisi3} Multiply operands 1 and 2, which have mode @code{HImode}, and store a @code{SImode} product in operand 0. @item @samp{mulqihi3}, @samp{mulsidi3} Similar widening-multiplication instructions of other widths. @item @samp{umulqihi3}, @samp{umulhisi3}, @samp{umulsidi3} Similar widening-multiplication instructions that do unsigned multiplication. @item @samp{divmod@var{m}4} 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. @item @samp{udivmod@var{m}4} Similar, but does unsigned division. @item @samp{divmod@var{m}@var{n}4} Like @samp{divmod@var{m}4} except that only the dividend has mode @var{m}; the divisor, quotient and remainder have mode @var{n}. For example, the Vax has a @samp{divmoddisi4} instruction (but it is omitted from the machine description, because it is so slow that it is faster to compute remainders by the circumlocution that the compiler will use if this instruction is not available). @item @samp{ashl@var{m}3} 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 @code{SImode}, not mode @var{m}. @item @samp{ashr@var{m}3}, @samp{lshl@var{m}3}, @samp{lshr@var{m}3}, @samp{rotl@var{m}3}, @samp{rotr@var{m}3} 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 @samp{ashl@var{m}3} and leave @samp{lshl@var{m}3} undefined. @item @samp{neg@var{m}2} Negate operand 1 and store the result in operand 0. @item @samp{abs@var{m}2} Store the absolute value of operand 1 into operand 0. @item @samp{sqrt@var{m}2} Store the square root of operand 1 into operand 0. @item @samp{ffs@var{m}2} Store into operand 0 one plus the index of the least significant 1-bit of operand 1. If operand 1 is zero, store zero. @var{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. @item @samp{one_cmpl@var{m}2} Store the bitwise-complement of operand 1 into operand 0. @item @samp{cmp@var{m}} Compare operand 0 and operand 1, and set the condition codes. The RTL pattern should look like this: @example (set (cc0) (minus (match_operand:@var{m} 0 @dots{}) (match_operand:@var{m} 1 @dots{}))) @end example Each such definition in the machine description, for integer mode @var{m}, must have a corresponding @samp{tst@var{m}} pattern, because optimization can simplify the compare into a test when operand 1 is zero. @item @samp{tst@var{m}} Compare operand 0 against zero, and set the condition codes. The RTL pattern should look like this: @example (set (cc0) (match_operand:@var{m} 0 @dots{})) @end example @item @samp{movstr@var{m}} Block move instruction. The addresses of the destination and source strings are the first two operands, and both are in mode @code{Pmode}. The number of bytes to move is the third operand, in mode @var{m}. @item @samp{cmpstr@var{m}} Block compare instruction, with operands like @samp{movstr@var{m}} except that the two memory blocks are compared byte by byte in lexicographic order. The effect of the instruction is to set the condition codes. @item @samp{float@var{m}@var{n}2} Convert operand 1 (valid for fixed point mode @var{m}) to floating point mode @var{n} and store in operand 0 (which has mode @var{n}). @item @samp{fix@var{m}@var{n}2} Convert operand 1 (valid for floating point mode @var{m}) to fixed point mode @var{n} as a signed number and store in operand 0 (which has mode @var{n}). This instruction's result is defined only when the value of operand 1 is an integer. @item @samp{fixuns@var{m}@var{n}2} Convert operand 1 (valid for floating point mode @var{m}) to fixed point mode @var{n} as an unsigned number and store in operand 0 (which has mode @var{n}). This instruction's result is defined only when the value of operand 1 is an integer. @item @samp{ftrunc@var{m}2} Convert operand 1 (valid for floating point mode @var{m}) to an integer value, still represented in floating point mode @var{m}, and store it in operand 0 (valid for floating point mode @var{m}). @item @samp{fix_trunc@var{m}@var{n}2} Like @samp{fix@var{m}@var{n}2} but works for any floating point value of mode @var{m} by converting the value to an integer. @item @samp{fixuns_trunc@var{m}@var{n}2} Like @samp{fixuns@var{m}@var{n}2} but works for any floating point value of mode @var{m} by converting the value to an integer. @item @samp{trunc@var{m}@var{n}} Truncate operand 1 (valid for mode @var{m}) to mode @var{n} and store in operand 0 (which has mode @var{n}). Both modes must be fixed point or both floating point. @item @samp{extend@var{m}@var{n}} Sign-extend operand 1 (valid for mode @var{m}) to mode @var{n} and store in operand 0 (which has mode @var{n}). Both modes must be fixed point or both floating point. @item @samp{zero_extend@var{m}@var{n}} Zero-extend operand 1 (valid for mode @var{m}) to mode @var{n} and store in operand 0 (which has mode @var{n}). Both modes must be fixed point. @item @samp{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 @code{Simode}. Operand 1 may have mode @code{QImode} or @code{SImode}; often @code{SImode} is allowed only for registers. Operands 2 and 3 must be valid for @code{SImode}. 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. @item @samp{extzv} Like @samp{extv} except that the bit-field value is zero-extended. @item @samp{insv} Store operand 3 (which must be valid for @code{SImode}) 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 @code{QImode} or @code{SImode}; often @code{SImode} is allowed only for registers. Operands 1 and 2 must be valid for @code{SImode}. The RTL generation pass generates this instruction only with constants for operands 1 and 2. @item @samp{s@var{cond}} Store zero or nonzero in the operand according to the condition codes. Value stored is nonzero iff the condition @var{cond} is true. @var{cond} is the name of a comparison operation expression code, such as @samp{eq}, @samp{lt} or @samp{leu}. You specify the mode that the operand must have when you write the @code{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. Otherwise the instruction is not suitable and must be omitted from the machine description. You must tell the compiler exactly which value is stored by defining the macro @code{STORE_FLAG_VALUE}. @item @samp{b@var{cond}} Conditional branch instruction. Operand 0 is a @samp{label_ref} that refers to the label to jump to. Jump if the condition codes meet condition @var{cond}. @item @samp{call} Subroutine call instruction. Operand 1 is the number of bytes of arguments pushed (in mode @code{SImode}), and operand 0 is the function to call. Operand 0 should be a @samp{mem} RTX whose address is the address of the function. @item @samp{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. @item @samp{casesi} Instruction to jump through a dispatch table, including bounds checking. This instruction takes five operands: @enumerate @item The index to dispatch on, which has mode @code{SImode}. @item The lower bound for indices in the table, an integer constant. @item The upper bound for indices in the table, an integer constant. @item A label to jump to if the index has a value outside the bounds. (If the machine-description macro @code{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 @samp{casesi} instruction, but it is always provided as an operand.) @item A label that precedes the table itself. @end enumerate The table is a @samp{addr_vec} or @samp{addr_diff_vec} inside of a @samp{jump_insn}. The number of elements in the table is one plus the difference between the upper bound and the lower bound. @item @samp{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 @samp{casesi} pattern. This pattern requires two operands: the address or offset, and a label which should immediately precede the jump table. If the macro @code{CASE_VECTOR_PC_RELATIVE} is defined then the first operand is an absolute address to jump to; otherwise, it is an offset which counts from the address of the table. The @samp{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. @end table @node Pattern Ordering, Dependent Patterns, Standard Names, Machine Desc @section 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 patterns (patterns that will match fewer things) and faster 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 example, 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 ordering it would be possible to make the pattern for convert-a-byte smart enough to deal properly with any constant value. @node Dependent Patterns, Jump Patterns, Pattern Ordering, Machine Desc @section Interdependence of Patterns Every machine description must have a named pattern for each of the conditional branch names @samp{b@var{cond}}. The recognition template must always have the form @example (set (pc) (if_then_else (@var{cond} (cc0) (const_int 0)) (label_ref (match_operand 0 "" "")) (pc))) @end example @noindent In addition, every machine description must have an anonymous pattern for each of the possible reverse-conditional branches. These patterns look like @example (set (pc) (if_then_else (@var{cond} (cc0) (const_int 0)) (pc) (label_ref (match_operand 0 "" "")))) @end example @noindent They are necessary because jump optimization can turn direct-conditional branches into reverse-conditional branches. The compiler does more with RTL than just create it from patterns and recognize the patterns: it can perform arithmetic expression codes when constant values for their operands can be determined. As a result, sometimes having one pattern can require other patterns. For example, the Vax has no `and' instruction, but it has `and not' instructions. Here is the definition of one of them: @example (define_insn "andcbsi2" [(set (match_operand:SI 0 "general_operand" "") (and:SI (match_dup 0) (not:SI (match_operand:SI 1 "general_operand" ""))))] "" "bicl2 %1,%0") @end example @noindent If operand 1 is an explicit integer constant, an instruction constructed using that pattern can be simplified into an `and' like this: @example (set (reg:SI 41) (and:SI (reg:SI 41) (const_int 0xffff7fff))) @end example @noindent (where the integer constant is the one's complement of what appeared in the original instruction). To avoid a fatal error, the compiler must have a pattern that recognizes such an instruction. Here is what is used: @example (define_insn "" [(set (match_operand:SI 0 "general_operand" "") (and:SI (match_dup 0) (match_operand:SI 1 "general_operand" "")))] "GET_CODE (operands[1]) == CONST_INT" "* @{ operands[1] = gen_rtx (CONST_INT, VOIDmode, ~INTVAL (operands[1])); return \"bicl2 %1,%0\"; @}") @end example @noindent Whereas a pattern to match a general `and' instruction is impossible to support on the Vax, this pattern is possible because it matches only a constant second argument: a special case that can be output as an `and not' instruction. A ``compare'' instruction whose RTL looks like this: @example (set (cc0) (minus @var{operand} (const_int 0))) @end example @noindent may be simplified by optimization into a ``test'' like this: @example (set (cc0) @var{operand}) @end example @noindent So in the machine description, each ``compare'' pattern for an integer mode must have a corresponding ``test'' pattern that will match the result of such simplification. 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 @example (set (match_operand:SI 0 @dots{}) (extend:SI (match_operand:HI 1 @dots{}))) (set (match_operand:SI 0 @dots{}) (extend:SI (match_operand:QI 1 @dots{}))) @end example @noindent 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 (@code{HImode}, here). If the pattern matches the @code{QImode} instruction, the results will be incorrect if the constant value does not actually fit that mode. Such instructions to extend constants are rarely generated because they are optimized away, but they do occasionally happen in nonoptimized compilations. @node Jump Patterns, Peephole Definitions, Dependent Patterns, Machine Desc @section Defining Jump Instruction Patterns GNU CC assumes that the machine has a condition code. A comparison insn sets the condition code, recording the results of both signed and unsigned comparison 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 flavors. 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 outputting code for the compare instruction, peek ahead at the following branch using @code{NEXT_INSN (insn)}. (The variable @code{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 output 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 generates a pair of consecutive RTL 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 @code{NOTICE_UPDATE_CC} to do @code{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 remembered operands. It also works to define patterns for compare-and-branch instructions. In optimizing compilation, the pair of compare and branch instructions will be combined accoprding 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. @node Peephole Definitions, Expander Definitions, Jump Patterns, Machine Desc @section Defining Machine-Specific Peephole Optimizers In addition to instruction patterns the @file{md} file may contain definitions of machine-specific peephole optimizations. The combiner does not notice certain peephole optimizations when the data flow in the program does not suggest that it should try them. For example, sometimes two consecutive insns related in purpose can be combined even though the second one does not appear to use a register computed in the first one. A machine-specific peephole optimizer can detect such opportunities. A definition looks like this: @example (define_peephole [@var{insn-pattern-1} @var{insn-pattern-2} @dots{}] "@var{condition}" "@var{template}") @end example In this skeleton, @var{insn-pattern-1} and so on are patterns to match consecutive instructions. The optimization applies to a sequence of instructions when @var{insn-pattern-1} matches the first one, @var{insn-pattern-2} matches the next, and so on.@refill @var{insn-pattern-1} and so on look @emph{almost} like the second operand of @code{define_insn}. There is one important difference: this pattern is an RTX, not a vector. If the @code{define_insn} pattern would be a vector of one element, the @var{insn-pattern} should be just that element, no vector. If the @code{define_insn} pattern would have multiple elements then the @var{insn-pattern} must place the vector inside an explicit @code{parallel} RTX.@refill The operands of the instructions are matched with @code{match_operands} and @code{match_dup}, as usual). What is not usual is that the operand numbers apply to all the instruction patterns in the definition. So, you can check for identical operands in two instructions by using @code{match_operand} in one instruction and @code{match_dup} in the other. The operand constraints used in @code{match_operand} patterns do not have any direct effect on the applicability of the optimization, but they will be validated afterward, so write constraints that are sure to fit whenever the optimization is applied. It is safe to use @code{"g"} for each operand. Once a sequence of instructions matches the patterns, the @var{condition} is checked. This is a C expression which makes the final decision whether to perform the optimization (do so if the expression is nonzero). If @var{condition} is omitted (in other words, the string is empty) then the optimization is applied to every sequence of instructions that matches the patterns. The defined peephole optimizations are applied after register allocation is complete. Therefore, the optimizer 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 @var{condition} is to write @code{operands[@var{i}]} for operand number @var{i} (as matched by @code{(match_operand @var{i} @dots{})}). Use the variable @code{insn} to refer to the last of the insns being matched; use @code{PREV_INSN} to find the preceding insns (but be careful to skip over any @samp{note} insns that intervene).@refill When optimizing computations with intermediate results, you can use @var{condition} to match only when the intermediate results are not used elsewhere. Use the C expression @code{dead_or_set_p (@var{insn}, @var{op})}, where @var{insn} is the insn in which you expect the value to be used for the last time (from the value of @code{insn}, together with use of @code{PREV_INSN}), and @var{op} is the intermediate value (from @code{operands[@var{i}]}).@refill Applying the optimization means replacing the sequence of instructions with one new instruction. The @var{template} controls ultimate output of assembler code for this combined instruction. It works exactly like the template of a @code{define_insn}. Operand numbers in this template are the same ones used in matching the original sequence of instructions. The result of a defined peephole optimizer does not need to match any of the instruction patterns, and it does not have an opportunity to match them. The peephole optimizer definition itself serves as the instruction pattern to control how the instruction is output. Defined peephole optimizers are run in the last jump optimization pass, so the instructions they produce are never combined or rearranged automatically in any way. Here is an example, taken from the 68000 machine description: @example (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 @} ") @end example The effect of this optimization is to change @example jbsr _foobar addql #4,sp movel d1,sp@@- movel d0,sp@@- fmoved sp@@+,fp0 @end example @noindent into @example jbsr _foobar movel d1,sp@@ movel d0,sp@@- fmoved sp@@+,fp0 @end example @node Expander Definitions,, Peephole Definitions, Machine Desc @section 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 @samp{define_expand} to specify how to generate the sequence of RTL. A @samp{define_expand} is an RTL expression that looks almost like a @samp{define_insn}; but, unlike the latter, a @samp{define_expand} is used only for RTL generation and it can produce more than one RTL insn. A @samp{define_expand} RTX has four operands: @itemize @bullet @item The name. Each @samp{define_expand} must have a name, since the only use for it is to refer to it by name. @item The RTL template. This is just like the RTL template for a @samp{define_peephole} in that it is a vector of RTL expressions each being one insn. @item 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 @samp{define_insn} that has a standard name. @item 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 @samp{emit_insn}, etc. Any such insns precede the ones that come from the RTL template. @end itemize 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 need to be specified in order to generate RTL from the pattern, should be described with a @samp{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 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 @samp{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 @samp{define_expand}. Internal operands are substituted into the RTL template with @samp{match_dup}, never with @samp{match_operand}. The values of the internal operands are not passed in as arguments by the compiler when it requests use of this pattern. Instead, they are computed within the pattern, in the preparation statements. These statements compute the values and store them into the appropriate elements of @code{operands} so that @samp{match_dup} can find them. There are two special macros defined for use in the preparation statements: @code{DONE} and @code{FAIL}. Use them with a following semicolon, as a statement. @table @code @item DONE Use the @code{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 @code{emit_insn} within the preparation statements; the RTL template will not be generated. @item 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 operations (addition, multiplication, shifting, etc.). Do not emit any insns explicitly with @code{emit_insn} before failing. @end table Here is an example, the definition of left-shift for the SPUR chip: @example (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 || (unsigned) INTVAL (operands[2]) > 3) FAIL; @}") @end example @noindent This example uses @samp{define_expand} so that it can generate an RTL insn for shifting when the shift-count is in the supported 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 there would be possible to use a @samp{define_insn} in that case. Here is another case (zero-extension on the 68000) which makes more use of the power of @samp{define_expand}: @example (define_expand "zero_extendhisi2" [(set (match_operand:SI 0 "general_operand" "") (const_int 0)) (set (strict_low_part (subreg:HI (match_operand:SI 0 "general_operand" "") 0)) (match_operand:HI 1 "general_operand" ""))] "" "operands[1] = make_safe_from (operands[1], operands[0]);") @end example @noindent 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 @code{make_safe_from} copies the @code{operands[1]} into a temporary register if it refers to @code{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 @samp{and}-ing the result against a halfword mask. But this mask cannot be represented by a @samp{const_int} because the constant value is too large to be legitimate on this machine. So it must be copied into a register with @code{force_reg} and then the register used in the @samp{and}. @example (define_expand "zero_extendhisi2" [(set (match_operand:SI 0 "register_operand" "") (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)); ") @end example @node Machine Macros, Config, Machine Desc, Top @chapter Machine Description Macros The other half of the machine description is a C header file conventionally given the name @file{tm-@var{machine}.h}. The file @file{tm.h} should be a link to it. The header file @file{config.h} includes @file{tm.h} and most compiler source files include @file{config.h}. @menu * Run-time Target:: Defining -m options like -m68000 and -m68020. * Storage Layout:: Defining sizes and alignments of data types. * Registers:: Naming and describing the hardware registers. * Register Classes:: Defining the classes of hardware registers. * Stack Layout:: Defining which way the stack grows and by how much. * Library Names:: Specifying names of subroutines to call automatically. * Addressing Modes:: Defining addressing modes valid for memory operands. * Condition Code:: Defining how insns update the condition code. * Assembler Format:: Defining how to write insns and pseudo-ops to output. * Misc:: Everything else. @end menu @node Run-time Target, Storage Layout, Machine Macros, Machine Macros @section Run-time Target Specification @table @code @item CPP_PREDEFINES Define this to be a string constant containing @samp{-D} options to define the predefined macros that identify this machine and system. For example, on the Sun, one can use the value @example "-Dmc68000 -Dsun -Dunix" @end example @item extern int target_flags; This declaration should be present. @item TARGET_@dots{} 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 @code{TARGET_68020} that tests a bit in @code{target_flags}. Define a macro @code{TARGET_@var{featurename}} for each such option. Its definition should test a bit in @code{target_flags}; for example: @example #define TARGET_68020 (target_flags & 1) @end example One place where these macros are used is in the condition-expressions of instruction patterns. Note how @code{TARGET_68020} appears frequently in the 68000 machine description file, @file{m68k.md}. Another place they are used is in the definitions of the other macros in the @file{tm-@var{machine}.h} file. @item TARGET_SWITCHES This macro defines names of command options to set and clear bits in @code{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 @code{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 @samp{-m} to the specified name. One of the subgroupings should have a null string. The number in this grouping is the default value for @code{target_flags}. Any target options act starting with that value. Here is an example which defines @samp{-m68000} and @samp{-m68020} with opposite meanings, and picks the latter as the default: @example #define TARGET_SWITCHES \ @{ @{ "68020", 1@}, \ @{ "68000", -1@}, \ @{ "", 1@}@} @end example @end table Sometimes certain combinations of command options do not make sense on a particular target machine. You can define a macro @code{OVERRIDE_OPTIONS} to take account of this. This macro, if defined, is executed once just after all the command options have been parsed. @node Storage Layout, Registers, Run-time Target, Machine Macros @section 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 @code{target_flags}. @xref{Run-time Target}. @table @code @item BITS_BIG_ENDIAN Define this macro if the most significant bit in a byte has the lowest number. This means that bit-field instructions count from the most significant bit. If the machine has no bit-field instructions, this macro is irrelevant. @item BYTES_BIG_ENDIAN Define this macro if the most significant byte in a word has the lowest number. @item WORDS_BIG_ENDIAN Define this macro if, in a multiword object, the most significant word has the lowest number. @item BITS_PER_UNIT Number of bits in an addressable storage unit (byte); normally 8. @item BITS_PER_WORD Number of bits in a word; normally 32. @item UNITS_PER_WORD Number of storage units in a word; normally 4. @item POINTER_SIZE Width of a pointer, in bits. @item PARM_BOUNDARY Alignment required for function parameters on the stack, in bits. @item STACK_BOUNDARY Define this macro if you wish to preserve a certain alignment for the stack pointer at all times. The definition is a C expression for the desired alignment (measured in bits). @item FUNCTION_BOUNDARY Alignment required for a function entry point, in bits. @item BIGGEST_ALIGNMENT Biggest alignment that any data type can require on this machine, in bits. @item EMPTY_FIELD_ALIGNMENT Alignment in bits to be given to a structure bit field that follows an empty field such as @code{int : 0;}. @item 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. If you do not define this macro, the default is the same as @code{BITS_PER_UNIT}. @item 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. @end table @node Registers, Register Classes, Storage Layout, Machine Macros @section Register Usage @table @code @item FIRST_PSEUDO_REGISTER Number of hardware registers known to the compiler. They receive numbers 0 through @code{FIRST_PSEUDO_REGISTER-1}; thus, the first pseudo register's number really is assigned the number @code{FIRST_PSEUDO_REGISTER}. @item 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, the program counter on machines where that is considered one of the addressable registers, and any other numbered register with a standard use. This information is expressed as a sequence of numbers, separated by commas and surrounded by braces. The @var{n}th number is 1 if register @var{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 @code{CONDITIONAL_REGISTER_USAGE}, or by the user with the command options @samp{-ffixed-@var{reg}}, @samp{-fcall-used-@var{reg}} and @samp{-fcall-saved-@var{reg}}. @item CALL_USED_REGISTERS Like @code{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 @code{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. @item CONDITIONAL_REGISTER_USAGE Zero or more C statements that may conditionally modify two variables @code{fixed_regs} and @code{call_used_regs} (both of type @code{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. @item HARD_REGNO_REGS (@var{regno}, @var{mode}) A C expression for the number of consecutive hard registers, starting at register number @var{regno}, required to hold a value of mode @var{mode}. On a machine where all registers are exactly one word, a suitable definition of this macro is @example #define HARD_REGNO_NREGS(REGNO, MODE) \ ((GET_MODE_SIZE (MODE) + UNITS_PER_WORD - 1) \ / UNITS_PER_WORD)) @end example @item HARD_REGNO_MODE_OK (@var{regno}, @var{mode}) A C expression that is nonzero if it is permissible to store a value of mode @var{mode} in hard register number @var{regno} (or in several registers starting with that one). For a machine where all registers are equivalent, a suitable definition is @example #define HARD_REGNO_MODE_OK(REGNO, MODE) 1 @end example It is not necessary for this macro to check for fixed register numbers because the allocation mechanism considers them to be always occupied. 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 @emph{hold} a floating point machine mode, whether or not floating arithmetic can be done on it in those registers. The true significance of special floating registers is rather than non-floating-point machine modes @emph{may not} go in those registers. This is true if the floating registers normalize any value stored in them, because storing a non-floating value there would garble it. 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. Sometimes there are floating registers that 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 @code{GENERAL_REGS}, they will not be used unless some insn's constraint asks for one. It is obligatory to support floating point `move' instructions into and out of general registers, because unions and structures (which have modes @samp{SImode} or @samp{DImode}) can be in those registers and they may have floating point members. @item MODES_TIEABLE_P (@var{mode1}, @var{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 @var{mode1} and a value of mode @var{mode2}. If @code{HARD_REGNO_MODE_OK (@var{r}, @var{mode1})} and @code{HARD_REGNO_MODE_OK (@var{r}, @var{mode2})} are ever different for any @var{r}, then @code{MODES_TIEABLE_P (@var{mode1}, @var{mode2})} must be zero. @item PC_REGNUM If the program counter has a register number, define this as that register number. Otherwise, do not define it. @item STACK_POINTER_REGNUM The register number of the stack pointer register, which must also be a fixed register according to @code{FIXED_REGISTERS}. On many machines, the hardware determines which register this is. @item 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. @item 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, in the function @code{reload}, and it 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 do without a frame pointer. The compiler recognizes those cases and automatically gives the function a frame pointer regardless of what @code{FRAME_POINTER_REQUIRED} says. You don't need to worry about them.@refill In a function that does not require a frame pointer, the frame pointer register can be allocated for ordinary usage, provided it is not marked as a fixed register. See @code{FIXED_REGISTERS} for more information. @item 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. It must in any case be a fixed register according to @code{FIXED_REGISTERS}. @item STATIC_CHAIN_REGNUM The register number used for passing a function's static chain pointer. This is needed for languages such as Pascal and Algol where functions defined within other functions can access the local variables of the outer functions; it is not currently used because C does not provide this feature. The static chain register need not be a fixed register. @item STRUCT_VALUE_REGNUM When a function's value's mode is @code{BLKmode}, the value is not returned according to @code{FUNCTION_VALUE}. Instead, the caller passes the address of a block of memory in which the value should be stored. @code{STRUCT_VALUE_REGNUM} is the register in which this address is passed. @end table @node Register Classes, Stack Layout, Registers, Machine Macros @section 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 are described to the compiler using @dfn{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 @code{ALL_REGS} and contain all the registers. Another class must be named @code{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 @code{GENERAL_REGS}. There is nothing terribly special about the name, but the operand constraint letters @samp{r} and @samp{g} specify this class. If @code{GENERAL_REGS} is the same as @code{ALL_REGS}, just define it as a macro which expands to @code{ALL_REGS}. The way classes other than @code{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 constraints. 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 (coprocessor) register or a general register for a certain operand, you should define a class @code{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. @table @code @item enum reg_class An enumeral type that must be defined with all the register class names as enumeral values. @code{NO_REGS} must be first. @code{ALL_REGS} must be the last register class, followed by one more enumeral value, @code{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 @code{int}. The number serves as an index in many of the tables described below. @item 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. @item REG_CLASS_CONTENTS An initializer containing the contents of the register classes, as integers which are bit masks. The @var{n}th integer specifies the contents of class @var{n}. The way the integer @var{mask} is interpreted is that register @var{r} is in the class if @code{@var{mask} & (1 << @var{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 @code{HARD_REG_SET} which is defined in @file{hard-reg-set.h}. @item REGNO_REG_CLASS (@var{regno}) A C expression whose value is a register class containing hard register @var{regno}. In general there is more that one such class; choose a class which is @dfn{minimal}, meaning that no smaller class also contains the register. @item INDEX_REG_CLASS A macro whose definition is the name of the class to which a valid index register must belong. @item REG_CLASS_FROM_LETTER (@var{char}) A C expression which defines the machine-dependent operand constraint letters for register classes. If @var{char} is such a letter, the value should be the register class corresponding to it. Otherwise, the value should be @code{NO_REGS}. @item REGNO_OK_FOR_BASE_P (@var{num}) A C expression which is nonzero if register number @var{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. @item REGNO_OK_FOR_INDEX_P (@var{num}) A C expression which is nonzero if register number @var{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 labelings, looking for one that is valid, and reload one or both registers only if neither labeling works. @item PREFERRED_RELOAD_CLASS (@var{x}, @var{class}) A C expression that places additional restrictions on the register class to use when it is necessary to copy value @var{x} into a register in class @var{class}. The value is a register class; perhaps @var{class}, or perhaps another, smaller class. @var{class} is always safe as a value. In fact, the definition @example #define PREFERRED_RELOAD_CLASS(X,CLASS) CLASS @end example @noindent is always safe. However, sometimes returning a more restrictive class makes better code. For example, on the 68000, when @var{x} is an integer constant that is in range for a @samp{moveq} instruction, the value of this macro is always @code{DATA_REGS} as long as @var{class} includes the data registers. Requiring a data register guarantees that a @samp{moveq} will be used. @item CLASS_MAX_NREGS (@var{class}, @var{mode}) A C expression for the maximum number of consecutive registers of class @var{class} needed to hold a value of mode @var{mode}. This is closely related to the macro @code{HARD_REGNO_NREGS}. In fact, the value of the macro @code{CLASS_MAX_NREGS (@var{class}, @var{mode})} should be the maximum value of @code{HARD_REGNO_NREGS (@var{regno}, @var{mode})} for all @var{regno} values in the class @var{class}. This macro helps control the handling of multiple-word values in the reload pass. @end table Two other special macros describe which constants fit which constraint letters. @table @code @item CONST_OK_FOR_LETTER_P (@var{value}, @var{c}) A C expression that defines the machine-dependent operand constraint letters that specify particular ranges of integer values. If @var{c} is one of those letters, the expression should check that @var{value}, an integer, is in the appropriate range and return 1 if so, 0 otherwise. If @var{c} is not one of those letters, the value should be 0 regardless of @var{value}. @item CONST_DOUBLE_OK_FOR_LETTER_P (@var{value}, @var{c}) A C expression that defines the machine-dependent operand constraint letters that specify particular ranges of floating values. If @var{c} is one of those letters, the expression should check that @var{value}, an RTX of code @samp{const_double}, is in the appropriate range and return 1 if so, 0 otherwise. If @var{c} is not one of those letters, the value should be 0 regardless of @var{value}. @end table @node Stack Layout, Library Names, Register Classes, Machine Macros @section Describing Stack Layout @table @code @item 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 @dots{},'' it means that the compiler checks this macro only with @code{#ifdef} so the precise definition used does not matter. @item FRAME_GROWS_DOWNWARD Define this macro if the addresses of local variable slots are at negative offsets from the frame pointer. @item STARTING_FRAME_OFFSET Offset from the frame pointer to the first local variable slot to be allocated. If @code{FRAME_GROWS_DOWNWARD}, the next slot's offset is found by subtracting the length of the first slot from @code{STARTING_FRAME_OFFSET}. Otherwise, it is found by adding the length of the first slot to the value @code{STARTING_FRAME_OFFSET}. @item PUSH_ROUNDING (@var{npushed}) A C expression that is the number of bytes actually pushed onto the stack when an instruction attempts to push @var{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 @example #define PUSH_ROUNDING(BYTES) (BYTES) @end example @noindent 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 @example #define PUSH_ROUNDING(BYTES) (((BYTES) + 1) & ~1) @end example @item FIRST_PARM_OFFSET Offset from the argument pointer register to the first argument's address. @item RETURN_POPS_ARGS (@var{funtype}) A C expression that should be 1 if a function pops its own arguments on returning, or 0 if the function pops no arguments and the caller must therefore pop them all after the function returns. @var{funtype} is a C variable whose value is a tree node that describes the function in question. Normally it is a node of type @code{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, @var{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. On the Vax, all functions always pop their arguments, so the definition of this macro is 1. 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 @code{printf}) pop nothing (the caller pops all). When this convention is in use, @var{funtype} is examined to determine whether a function takes a fixed number of arguments. @item FUNCTION_VALUE (@var{valtype}, @var{func}) A C expression to create an RTX representing the place where a function returns a value of data type @var{valtype}. @var{valtype} is a tree node representing a data type. Write @code{TYPE_MODE (@var{valtype})} to get the machine mode used to represent that type. On many machines, only the mode is relevant. (Actually, on most machines, scalar values are returned in the same place regardless of mode).@refill If the precise function being called is known, @var{func} is a tree node (@code{FUNCTION_DECL}) for it; otherwise, @var{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.@refill @item FUNCTION_OUTGOING_VALUE (@var{valtype}, @var{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, @code{FUNCTION_VALUE} computes the register in which the caller will see the value, and @code{FUNCTION_OUTGOING_VALUE} should be defined in a similar fashion to tell the function where to put the value.@refill If @code{FUNCTION_OUTGOING_VALUE} is not defined, @code{FUNCTION_VALUE} serves both purposes.@refill @item LIBCALL_VALUE (@var{mode}) A C expression to create an RTX representing the place where a library function returns a value of mode @var{mode}. If the precise function being called is known, @var{func} is a tree node (@code{FUNCTION_DECL}) for it; otherwise, @var{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.@refill 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. @item FUNCTION_VALUE_REGNO_P (@var{regno}) A C expression that is nonzero if @var{regno} is the number of a hard register in which function values are sometimes returned. A register whose use for returning values is limited to serving as the second of a pair (for a value of type @code{double}, say) need not be recognized by this macro. So for most machines, this definition suffices: @example #define FUNCTION_VALUE_REGNO_P(N) ((N) == 0) @end example @item FUNCTION_ARG (@var{cum}, @var{mode}, @var{type}, @var{named}) A C expression that controls whether a function argument is passed in a register, and which register. The arguments are @var{cum}, which summarizes all the previous arguments; @var{mode}, the machine mode of the argument; @var{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 @var{named}, which is 1 for an ordinary argument and 0 for nameless arguments that correspond to @samp{...} in the called function's prototype. The value of the expression should either be a @samp{reg} RTX for the hard register in which to pass the argument, or zero to pass the argument on the stack. For the Vax and 68000, where normally all arguments are pushed, zero suffices as a definition. @item FUNCTION_INCOMING_ARG (@var{cum}, @var{mode}, @var{type}, @var{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, @code{FUNCTION_ARG} computes the register in which the caller passes the value, and @code{FUNCTION_INCOMING_ARG} should be defined in a similar fashion to tell the function being called where the arguments will arrive. If @code{FUNCTION_INCOMING_ARG} is not defined, @code{FUNCTION_ARG} serves both purposes.@refill @item FUNCTION_ARG_PARTIAL_NREGS (@var{cum}, @var{mode}, @var{type}, @var{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 @var{n} words of arguments are passed in registers, and the rest on the stack. If a multi-word argument (a @code{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. @code{FUNCTION_ARG} for these arguments should return the first register to be used by the caller for this argument; likewise @code{FUNCTION_INCOMING_ARG}, for the called function. @item CUMULATIVE_ARGS A C type for declaring a variable that is used as the first argument of @code{FUNCTION_ARG} and other related values. For some target machines, the type @code{int} suffices and can hold the number of bytes of argument so far. @item INIT_CUMULATIVE_ARGS (@var{cum}) A C statement (sans semicolon) for initializing the variable @var{cum} for the state at the beginning of the argument list. The variable has type @code{CUMULATIVE_ARGS}. @item FUNCTION_ARG_ADVANCE (@var{cum}, @var{mode}, @var{type}, @var{named}) Update the summarizer variable @var{cum} to advance past an argument in the argument list. The values @var{mode}, @var{type} and @var{named} describe that argument. Once this is done, the variable @var{cum} is suitable for analyzing the @emph{following} argument with @code{FUNCTION_ARG}, etc.@refill @item FUNCTION_ARG_REGNO_P (@var{regno}) A C expression that is nonzero if @var{regno} is the number of a hard register in which function arguments are sometimes passed. This does @emph{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. @item FUNCTION_PROLOGUE (@var{file}, @var{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 @var{size} additional bytes of storage for the local variables. @var{size} is an integer. @var{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 @code{regs_ever_live}: element @var{r} is nonzero if hard register @var{r} is used anywhere within the function. This implies the function prologue should save register @var{r}, but not if it is one of the call-used registers. 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 @code{frame_pointer_needed}. The variable's value will be 1 at run time in a function that needs a frame pointer. @item FUNCTION_PROFILER (@var{file}, @var{labelno}) A C statement or compound statement to output to @var{file} some assembler code to call the profiling subroutine @code{mcount}. Before calling, the assembler code must load the address of a counter variable into a register where @code{mcount} expects to find the address. The name of this variable is @samp{LP} followed by the number @var{labelno}, so you would generate the name using @samp{LP%d} in a @code{fprintf}. The details of how the address should be passed to @code{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. @item EXIT_IGNORES_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 for which frame pointers are maintained. It is never possible to delete a final stack adjustment in a function that has no frame pointer, and the compiler knows this regardless of @code{EXIT_IGNORES_STACK}. @item FUNCTION_EPILOGUE (@var{file}, @var{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 @code{FUNCTION_PROLOGUE}, and the registers to restore are determined from @code{regs_ever_live} and @code{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 @samp{return} and do not define the macro @code{FUNCTION_EPILOGUE} at all. 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 @code{frame_pointer_needed}. The variable's value will be 1 at run time in a function that needs a frame pointer. 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 @samp{-mrtd} pops arguments in functions that take a fixed number of arguments. Your definition of the macro @code{RETURN_POPS_ARGS} decides which functions pop their own arguments. @code{FUNCTION_EPILOGUE} needs to know what was decided. The variable @code{current_function_pops_args} is nonzero if the function should pop its own arguments. If so, use the variable @code{current_function_args_size} as the number of bytes to pop. @item FIX_FRAME_POINTER_ADDRESS (@var{addr}, @var{depth}) A C compound statement to alter a memory address that uses the frame pointer register so that it uses the stack pointer register instead. This must be done in the instructions that load parameter values into registers, when the reload pass determines that a frame pointer is not necessary for the function. @var{addr} will be a C variable name, and the updated address should be stored in that variable. @var{depth} will be the current depth of stack temporaries (number of bytes of arguments currently pushed). The change in offset between a frame-pointer-relative address and a stack-pointer-relative address must include @var{depth}. Even if your machine description specifies there will always be a frame pointer in the frame pointer register, you must still define @code{FIX_FRAME_POINTER_ADDRESS}, but the definition will never be executed at run time, so it may be empty. @end table @node Library Names, Addressing Modes, Stack Layout, Machine Macros @section Library Subroutine Names @table @code @item UDIVSI3_LIBCALL A C string constant giving the name of the function to call for division of a full-word by a full-word. If you do not define this macro, the default name is used, which is @code{_udivsi3}, a function defined in @file{gnulib}. @item UMODSI3_LIBCALL A C string constant giving the name of the function to call for the remainder in division of a full-word by a full-word. If you do not define this macro, the default name is used, which is @code{_umodsi3}, a function defined in @file{gnulib}. @item TARGET_MEM_FUNCTIONS Define this macro if GNU CC should generate calls to the System V (and ANSI C) library functions @code{memcpy} and @code{memset} rather than the BSD functions @code{bcopy} and @code{bzero}. @end table @node Addressing Modes, Misc, Library Names, Machine Macros @section Addressing Modes @table @code @item HAVE_POST_INCREMENT Define this macro if the machine supports post-increment addressing. @item HAVE_PRE_INCREMENT @itemx HAVE_POST_DECREMENT @itemx HAVE_PRE_DECREMENT Similar for other kinds of addressing. @item CONSTANT_ADDRESS_P (@var{x}) A C expression that is 1 if the RTX @var{x} is a constant whose value is an integer. This includes integers whose values are not explicitly known, such as @samp{symbol_ref} and @samp{label_ref} expressions and @samp{const} arithmetic expressions. On most machines, this can be defined as @code{CONSTANT_P (@var{x})}, but a few machines are more restrictive in which constant addresses are supported. @item MAX_REGS_PER_ADDRESS A number, the maximum number of registers that can appear in a valid memory address. @item GO_IF_LEGITIMATE_ADDRESS (@var{mode}, @var{x}, @var{label}) A C compound statement with a conditional @code{goto @var{label};} executed if @var{x} (an RTX) is a legitimate memory address on the target machine for a memory operand of mode @var{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 @code{REG_OK_STRICT}. You should use an @code{#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 @code{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.@refill @item LEGITIMIZE_ADDRESS (@var{x}, @var{oldx}, @var{mode}, @var{win}) A C compound statement that attempts to replace @var{x} with a valid memory address for an operand of mode @var{mode}. @var{win} will be a C statement label elsewhere in the code; the macro definition may use @example GO_IF_LEGITIMATE_ADDRESS (@var{mode}, @var{x}, @var{win}); @end example @noindent to avoid further processing if the address has become legitimate. @var{x} will always be the result of a call to @code{break_out_memory_refs}, and @var{oldx} will be the operand that was given to that function to produce @var{x}. The code generated by this macro should not alter the substructure of @var{x}. If it transforms @var{x} into a more legitimate form, it should assign @var{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. But often a machine-dependent strategy can generate better code. @item GO_IF_MODE_DEPENDENT_ADDRESS (@var{addr}, @var{label}) A C statement or compound statement with a conditional @code{goto @var{label};} executed if memory address @var{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 @var{addr} is a valid address for the machine. @item LEGITIMATE_CONSTANT_P (@var{x}) A C expression that is nonzero if @var{x} is a legitimate constant for an immediate operand on the target machine. You can assume that either @var{x} is a @samp{const_double} or it satisfies @code{CONSTANT_P}, so you need not check these things. In fact, @samp{1} is a suitable definition for this macro on machines where any @samp{const_double} is valid and anything @code{CONSTANT_P} is valid.@refill @end table @node Misc, Condition Code, Addressing Modes, Machine Macros @section Miscellaneous Parameters @table @code @item CASE_VECTOR_MODE An alias for a machine mode name. This is the machine mode that elements of a jump-table should have. @item CASE_VECTOR_PC_RELATIVE Define this macro if jump-tables should contain relative addresses. @item CASE_DROPS_THROUGH Define this if control falls through a @code{case} insn when the index value is out of range. This means the specified default-label is actually ignored by the @code{case} insn proper. @item 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, @code{FIX_ROUND_EXPR} is used.@refill @item 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. @item 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 @code{TRUNC_DIV_EXPR}, @code{FLOOR_DIV_EXPR}, @code{CEIL_DIV_EXPR} or @code{ROUND_DIV_EXPR}. These four division operators differ in how they round the result to an integer. @code{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.@refill @item DEFAULT_SIGNED_CHAR An expression whose value is 1 or 0, according to whether the type @code{char} should be signed or unsigned by default. The user can always override this default with the options @samp{-fsigned-char} and @samp{-funsigned-char}. @item SCCS_DIRECTIVE Define this if the preprocessor should ignore @code{#sccs} directives with no error message. @item MOVE_MAX The maximum number of bytes that a single instruction can move quickly from memory to memory. @item INT_TYPE_SIZE A C expression for the size in bits of the type @code{int} on the target machine. @item SLOW_BYTE_ACCESS Define this macro as a C expression which is nonzero if accessing less than a word of memory (i.e. a @code{char} or a @code{short}) is slow (requires more than one instruction). @item SLOW_ZERO_EXTEND Define this macro if zero-extension (of a @code{char} or @code{short} to an @code{int}) can be done faster if the destination is a register that is known to be zero. If you define this macro, you must have instruction patterns that recognize RTL structures like this: @example (set (strict-low-part (subreg:QI (reg:SI @dots{}) 0)) @dots{}) @end example @noindent and likewise for @code{HImode}. @item SHIFT_COUNT_TRUNCATED Define this macro if shift instructions ignore all but the lowest few bits of the shift count. It implies that a sign-extend or zero-extend instruction for the shift count can be omitted. @item TRULY_NOOP_TRUNCATION (@var{outprec}, @var{inprec}) A C expression which is nonzero if on this machine it is safe to ``convert'' an integer of @var{inprec} bits to one of @var{outprec} bits (where @var{outprec} is smaller than @var{inprec}) by merely operating on it as if it had only @var{outprec} bits. On many machines, this expression can be 1. @item 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. @item STORE_FLAG_VALUE A C expression for the value stored by a store-flag instruction (@code{s@var{cond}}) when the condition is true. This is usually 1 or -1; it is required to be an odd number. Do not define @code{STORE_FLAG_VALUE} if the machine has no store-flag instructions. @item Pmode An alias for the machine mode for pointers. Normally the definition can be @example #define Pmode SImode @end example @item FUNCTION_MODE An alias for the machine mode used for memory references to functions being called, in @samp{call} RTL expressions. On most machines this should be @code{QImode}. @item CONST_COST (@var{x}, @var{code}) A part of a C @code{switch} statement that describes the relative costs of constant RTL expressions. It must contain @code{case} labels for expression codes @samp{const_int}, @samp{const}, @samp{symbol_ref}, @samp{label_ref} and @samp{const_double}. Each case must ultimately reach a @code{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 @var{x}. @var{code} is the expression code---redundant, since it can be obtained with @code{GET_CODE (@var{x})}. @item DOLLARS_IN_IDENTIFIERS Define this if the character @samp{$} should be allowed in identifier names. @end table @node Condition Code, Assembler Format, Misc, Machine Macros @section Condition Code Information The file @file{conditions.h} defines a variable @code{cc_status} to describe how the condition code was computed (in case the interpretation of the condition code depends on the instruction that it was set by). This variable contains the RTL expressions on which the condition code is currently based, and several standard flags. Sometimes additional machine-specific flags must be defined in the machine description header file. It can also add additional machine-specific information by defining @code{CC_STATUS_MDEP}. @table @code @item CC_STATUS_MDEP C code for a data type which is used for declaring the @code{mdep} component of @code{cc_status}. It defaults to @code{int}. @item CC_STATUS_MDEP_INIT A C expression for the initial value of the @code{mdep} field. It defaults to 0. @item NOTICE_UPDATE_CC (@var{exp}) A C compound statement to set the components of @code{cc_status} appropriately for an insn whose body is @var{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 @code{(cc0)}. If there are insn 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 @code{NOTICE_UPDATE_CC} can leave @code{cc_status} unaltered for such insns. But suppose that the previous insn set the condition code based on location @samp{a4@@(102)} and the current insn stores a new value in @samp{a4}. Although the condition code is not changed by this, it will no longer be true that it reflects the contents of @samp{a4@@(102)}. Therefore, @code{NOTICE_UPDATE_CC} must alter @code{cc_status} in this case to say that nothing is known about the condition code value. @end table @node Assembler Format,, Condition Code, Machine Macros @section Output of Assembler Code @table @code @item 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 @file{tm-sun3.h} for an example of this. Do not define this macro if it does not need to do anything. @item 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. @item ASM_FILE_START A C string constant for text to be output at the start of each assembler output file. Normally this is @code{"#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. @item ASM_APP_ON A C string constant for text to be output before each @code{asm} statement or group of consecutive ones. Normally this is @code{"#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. @item ASM_APP_OFF A C string constant for text to be output after each @code{asm} statement or group of consecutive ones. Normally this is @code{"#NO_APP"}, which tells the GNU assembler to resume making the time-saving assumptions that are valid for ordinary compiler output. @item TEXT_SECTION_ASM_OP A C string constant for the assembler operation that should precede instructions and read-only data. Normally @code{".text"} is right. @item DATA_SECTION_ASM_OP A C string constant for the assembler operation to identify the following data as writable initialized data. Normally @code{".data"} is right. @item 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. @item DBX_REGISTER_NUMBER (@var{regno}) A C expression that returns the DBX register number for the compiler register number @var{regno}. In simple cases, the value of this expression may be @var{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. @item DBX_NO_XREFS Define this macro if DBX on your system does not support the construct @samp{xs@var{tagname}}. On some systems, this construct is used to describe a forward reference to a structure named @var{tagname}. On other systems, this construct is not supported at all. @item DBX_CONTIN_LENGTH A symbol name in DBX-format debugging information is normally continued (split into two separate @code{.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. @item DBX_CONTIN_CHAR Normally continuation is indicated by adding a @samp{\} character to the end of a @code{.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. @item ASM_OUTPUT_LABEL (@var{file}, @var{name}) A C statement (sans semicolon) to output to the stdio stream @var{file} the assembler definition of a label named @var{name}. Use the expression @code{assemble_name (@var{file}, @var{name})} to output the name itself; before and after that, output the additional assembler syntax for defining the name, and a newline. @item ASM_DECLARE_FUNCTION_NAME (@var{file}, @var{name}) A C statement (sans semicolon) to output to the stdio stream @var{file} any text necessary for declaring the name of a function which is being defined. This macro is responsible for outputting the label definition (perhaps using @code{ASM_OUTPUT_LABEL}). If this macro is not defined, then the function name is defined in the usual manner as a label (by means of @code{ASM_OUTPUT_LABEL}). @item ASM_GLOBALIZE_LABEL (@var{file}, @var{name}) A C statement (sans semicolon) to output to the stdio stream @var{file} some commands that will make the label @var{name} global; that is, available for reference from other files. Use the expression @code{assemble_name (@var{file}, @var{name})} to output the name itself; before and after that, output the additional assembler syntax for making that name global, and a newline. @item ASM_OUTPUT_EXTERNAL (@var{file}, @var{name}) A C statement (sans semicolon) to output to the stdio stream @var{file} any text necessary for declaring the name of an external symbol which is referenced in this compilation but not defined. 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. @item ASM_OUTPUT_LABELREF (@var{file}, @var{name}) A C statement to output to the stdio stream @var{file} a reference in assembler syntax to a label named @var{name}. The character @samp{_} should be added 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 @code{assemble_name}. @item ASM_OUTPUT_INTERNAL_LABEL (@var{file}, @var{prefix}, @var{num}) A C statement to output to the stdio stream @var{file} a label whose name is made from the string @var{prefix} and the number @var{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 @samp{L} at the beginning of a label has this effect. The usual definition of this macro is as follows: @example fprintf (@var{file}, "L%s%d:\n", @var{prefix}, @var{num}) @end example @item ASM_OUTPUT_CASE_LABEL (@var{file}, @var{prefix}, @var{num}, @var{table}) Define this if the label before a jump-table needs to be output specially. The first three arguments are the same as for @code{ASM_OUTPUT_INTERNAL_LABEL}; the fourth argument is the jump-table which follows (a @samp{jump_insn} containing an @samp{addr_vec} or @samp{addr_diff_vec}). This feature is used on system V to output a @code{swbeg} statement for the table. If this macro is not defined, these labels are output with @code{ASM_OUTPUT_INTERNAL_LABEL}. @item ASM_FORMAT_PRIVATE_NAME (@var{outvar}, @var{name}, @var{number}) A C expression to assign to @var{outvar} (which is a variable of type @code{char *}) a newly allocated string made from the string @var{name} and the number @var{number}, with some suitable punctuation added. Use @code{alloca} to get space for the string. This string will be used as the argument to @code{ASM_OUTPUT_LABELREF} to produce an assembler label for an internal static variable whose name is @var{name}. Therefore, the string must be such as to result in valid assembler code. The argument @var{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. @item ASM_OUTPUT_ADDR_DIFF_ELT (@var{file}, @var{value}, @var{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 @var{file} an assembler pseudo-instruction to generate a difference between two labels. @var{value} and @var{rel} are the numbers of two internal labels. The definitions of these labels are output using @code{ASM_OUTPUT_INTERNAL_LABEL}, and they must be printed in the same way here. For example, @example fprintf (@var{file}, "\t.word L%d-L%d\n", @var{value}, @var{rel}) @end example @item ASM_OUTPUT_ADDR_VEC_ELT (@var{file}, @var{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 @var{file} an assembler pseudo-instruction to generate a reference to a label. @var{value} is the number of an internal label whose definition is output using @code{ASM_OUTPUT_INTERNAL_LABEL}. For example, @example fprintf (@var{file}, "\t.word L%d\n", @var{value}) @end example @item ASM_OUTPUT_DOUBLE (@var{file}, @var{value}) A C statement to output to the stdio stream @var{file} an assembler instruction to assemble a @code{double} constant whose value is @var{value}. @var{value} will be a C expression of type @code{double}. @item ASM_OUTPUT_FLOAT (@var{file}, @var{value}) A C statement to output to the stdio stream @var{file} an assembler instruction to assemble a @code{float} constant whose value is @var{value}. @var{value} will be a C expression of type @code{float}. @item ASM_OUTPUT_INT (@var{file}, @var{exp}) @itemx ASM_OUTPUT_SHORT (@var{file}, @var{exp}) @itemx ASM_OUTPUT_CHAR (@var{file}, @var{exp}) A C statement to output to the stdio stream @var{file} an assembler instruction to assemble a @code{int}, @code{short} or @code{char} constant whose value is @var{value}. The argument @var{exp} will be an RTL expression which represents a constant value. Use @samp{output_addr_const (@var{exp})} to output this value as an assembler expression.@refill @item ASM_OUTPUT_BYTE (@var{file}, @var{value}) A C statement to output to the stdio stream @var{file} an assembler instruction to assemble a single byte containing the number @var{value}. @item ASM_OUTPUT_ASCII (@var{file}, @var{ptr}, @var{len}) A C statement to output to the stdio stream @var{file} an assembler instruction to assemble a string constant containing the @var{len} bytes at @var{ptr}. @var{ptr} will be a C expression of type @code{char *} and @var{len} a C expression of type @code{int}. If the assembler has a @code{.ascii} pseudo-op as found in the Berkeley Unix assembler, do not define the macro @code{ASM_OUTPUT_ASCII}. @item ASM_OUTPUT_SKIP (@var{file}, @var{nbytes}) A C statement to output to the stdio stream @var{file} an assembler instruction to advance the location counter by @var{nbytes} bytes. @var{nbytes} will be a C expression of type @code{int}. @item ASM_OUTPUT_ALIGN (@var{file}, @var{power}) A C statement to output to the stdio stream @var{file} an assembler instruction to advance the location counter to a multiple of 2 to the @var{power} bytes. @var{power} will be a C expression of type @code{int}. @item ASM_OUTPUT_COMMON (@var{file}, @var{name}, @var{size}) A C statement (sans semicolon) to output to the stdio stream @var{file} the assembler definition of a common-label named @var{name} whose size is @var{size} bytes. Use the expression @code{assemble_name (@var{file}, @var{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. @item ASM_OUTPUT_LOCAL (@var{file}, @var{name}, @var{size}) A C statement (sans semicolon) to output to the stdio stream @var{file} the assembler definition of a local-common-label named @var{name} whose size is @var{size} bytes. Use the expression @code{assemble_name (@var{file}, @var{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. @item TARGET_BELL A C constant expression for the integer value for escape sequence @samp{\a}. @item TARGET_BS @itemx TARGET_TAB @itemx TARGET_NEWLINE C constant expressions for the integer values for escape sequences @samp{\b}, @samp{\t} and @samp{\n}. @item TARGET_VT @itemx TARGET_FF @itemx TARGET_CR C constant expressions for the integer values for escape sequences @samp{\v}, @samp{\f} and @samp{\r}. @item ASM_OUTPUT_OPCODE (@var{file}, @var{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 @var{file}. The macro-operand @var{ptr} is a variable of type @code{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 @var{file}, performing any translation you desire, and increment the variable @var{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 @samp{%}-sequences to substitute operands, you must take care of the substitution yourself. Just be sure to increment @var{ptr} over whatever text should not be output normally. If the macro definition does nothing, the instruction is output in the usual way. @item PRINT_OPERAND (@var{file}, @var{x}, @var{code}) A C compound statement to output to stdio stream @var{file} the assembler syntax for an instruction operand @var{x}. @var{x} is an RTL expression. @var{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. @var{code} comes from the @samp{%} specification that was used to request printing of the operand. If the specification was just @samp{%@var{digit}} then @var{code} is 0; if the specification was @samp{%@var{ltr} @var{digit}} then @var{code} is the ASCII code for @var{ltr}. If @var{x} is a register, this macro should print the register's name. The names can be found in an array @code{reg_names} whose type is @code{char *[]}. @code{reg_names} is initialized from @code{REGISTER_NAMES}. When the machine description has a specification @samp{%@var{punct}} (a @samp{%} followed by a punctuation character), this macro is called with a null pointer for @var{x} and the punctuation character for @var{code}. @item PRINT_OPERAND_ADDRESS (@var{file}, @var{x}) A C compound statement to output to stdio stream @var{file} the assembler syntax for an instruction operand that is a memory reference whose address is @var{x}. @var{x} is an RTL expression. @item ASM_OPEN_PAREN @itemx 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: @example #define ASM_OPEN_PAREN "(" #define ASM_CLOSE_PAREN ")" @end example @end table @node Config,, Machine Macros, Top @chapter The Configuration File The configuration file @file{config-@var{machine}.h} contains macro definitions that describe the machine and system on which the compiler is running. Most of the values in it are actually the same on all machines that GNU CC runs on, so most all configuration files are identical. But there are some macros that vary: @table @code @item FAILURE_EXIT_CODE A C expression for the status code to be returned when the compiler exits after serious errors. @item SUCCESS_EXIT_CODE A C expression for the status code to be returned when the compiler exits without serious errors. @end table @contents @bye