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1. GNU CC Command Options When you invoke GNU CC, it normally does preprocessing, compilation, assembly and linking. The ``overall options'' allow you to stop this process at an intermediate stage. For example, the `-c' option says not to run the linker. Then the output consists of object files output by the assembler. Other options are passed on to one stage of processing. Some options control the preprocessor and others the com- piler itself. Yet other options control the assembler and linker; most of these are not documented here, since you rarely need to use any of them. The GNU C compiler uses a command syntax much like the Unix C compiler. The gcc program accepts options and file names as operands. Multiple single-letter options may not be grouped: `-dr' is very different from `-d -r' You can mix options and other arguments. For the most part, the order you use doesn't matter; gcc reorders the command-line options so that the choices specified by option flags are applied to all input files. Order does matter when you use several options of the same kind; for example, if you specify `-L' more than once, the directories are searched in the order specified. Many options have long names starting with `-f' or with `-W'---for example, `-fforce-mem', `-fstrength-reduce', `- Wformat' and so on. Most of these have both positive and negative forms; the negative form of `-ffoo' would be `- fno-foo'. This manual documents only one of these two forms, whichever one is not the default. Here is a summary of all the options, grouped by type. Explanations are in the following sections. Overall Options See section Overall Options,,Options Controlling the Kind of Output. -c -S -E -o file -pipe -v -x language Language Options See section Dialect Options,,Options Control- ling Dialect. -ansi -fbuiltin -fcond-mismatch -fno-asm -fsigned-bitfields -fsigned-char -funsigned-bitfields -funsigned-char -fwritable-strings -traditional -traditional-cpp -trigraphs Warning Options See section Warning Options,,Options to Re- quest or Suppress Warnings. -fsyntax-only -pedantic -pedantic-errors -w -W -Wall -Waggregate-return -Wcast-align -Wcast-qual -Wcomment -Wconversion -Werror -Wformat -Wid-clash-len -Wimplicit -Wmissing-prototypes -Wno-parentheses -Wpointer-arith -Wreturn-type -Wshadow -Wstrict-prototypes -Wswitch -Wtraditional -Wtrigraphs -Wuninitialized -Wunused -Wwrite-strings -Wchar-subscripts Debugging Options See section Debugging Options,,Options for De- bugging Your Program or GCC. -a -dletters -fpretend-float -g -ggdb -gdwarf -gstabs -gstabs+ -gcoff -p -pg -save-temps Optimization Options See section Optimize Options,,Options that Control Optimization. -fcaller-saves -fcse-follow-jumps -fdelayed-branch -fexpensive-optimizations -ffloat-store -fforce-addr -fforce-mem -finline -finline-functions -fkeep-inline-functions -fno-defer-pop -fno-function-cse -fomit-frame-pointer -frerun-cse-after-loop -fschedule-insns -fschedule-insns2 -fstrength-reduce -fthread-jumps -funroll-all-loops -funroll-loops -O -O2 Preprocessor Options See section Preprocessor Options,,Options Con- trolling the Preprocessor. -C -dD -dM -dN -Dmacro[=defn] -E -H -include file -imacros file -M -MD -MM -MMD -nostdinc -P -trigraphs -Umacro Linker Options See section Link Options,,Options for Linking. object-file-name -llibrary -nostdlib -static Directory Options See section Directory Options,,Options for Directory Search. -Bprefix -Idir -I- -Ldir Target Options See section Target Options,,Target Machine and Compiler Version. -b machine -V version Machine Dependent Options See section Submodel Options,,Hardware Models and Configurations. M680x0 Options -m68000 -m68020 -m68881 -mbitfield -mc68000 -mc68020 -mfpa -mnobitfield -mrtd -mshort -msoft-float VAX Options -mg -mgnu -munix SPARC Options -mfpu -mno-epilogue Convex Options -margcount -mc1 -mc2 -mnoargcount AMD29K Options -m29000 -m29050 -mbw -mdw -mkernel-registers -mlarge -mnbw -mnodw -msmall -mstack-check -muser-registers M88K Options -m88000 -m88100 -m88110 -mbig-pic -mcheck-zero-division -mhandle-large-shift -midentify-revision -mno-check-zero-division -mno-ocs-debug-info -mno-ocs-frame-position -mno-optimize-arg-area -mno-underscores -mocs-debug-info -mocs-frame-position -moptimize-arg-area -mshort-data-num -msvr3 -msvr4 -mtrap-large-shift -muse-div-instruction -mversion-03.00 -mwarn-passed-structs RS/6000 Options -mfp-in-toc -mno-fop-in-toc RT Options -mcall-lib-mul -mfp-arg-in-fpregs -mfp-arg-in-gregs -mfull-fp-blocks -mhc-struct-return -min-line-mul -mminimum-fp-blocks -mnohc-struct-return MIPS Options -mcpu=cpu type -mips2 -mips3 -mint64 -mlong64 -mlonglong128 -mmips-as -mgas -mrnames -mno-rnames -mgpopt -mno-gpopt -mstats -mno-stats -mmemcpy -mno-memcpy -mno-mips-tfile -mmips-tfile -msoft-float -mhard-float -mabicalls -mno-abicalls -mhalf-pic -mno-half-pic -G num Code Generation Options See section Code Gen Options,,Options for Code Generation Conventions. -fcall-saved-reg -fcall-used-reg -ffixed-reg -fno-common -fpcc-struct-return -fpic -fPIC -fshared-data -fshort-enums -fshort-double -fvolatile 1.1. Options Controlling the Kind of Output Compilation can involve up to four stages: preprocess- ing, compilation proper, assembly and linking, always in that order. The first three stages apply to an individual source file, and end by producing an object file; linking combines all the object files (those newly compiled, and those specified as input) into an executable file. For any given input file, the file name suffix deter- mines what kind of compilation is done: file.c C source code which must be preprocessed. file.i C source code which should not be preprocessed. file.m Objective-C source code file.h C header file (not to be compiled or linked). file.cc file.cxx file.C C++ source code which must be preprocessed. file.s Assembler code. file.S Assembler code which must be preprocessed. other An object file to be fed straight into linking. Any file name with no recognized suffix is treated this way. You can specify the input language explicitly with the `-x' option: -x language Specify explicitly the language for the following input files (rather than choosing a default based on the file name suffix). This option applies to all following input files until the next `-x' op- tion. Possible values of language are `c', `objective-c', `c-header', `c++', `cpp-output', `assembler', and `assembler-with-cpp'. -x none Turn off any specification of a language, so that subsequent files are handled according to their file name suffixes (as they are if `-x' has not been used at all). If you only want some of the stages of compilation, you can use `-x' (or filename suffixes) to tell gcc where to start, and one of the options `-c', `-S', or `-E' to say where gcc is to stop. Note that some combinations (for example, `-x cpp-output -E' instruct gcc to do nothing at all. -c Compile or assemble the source files, but do not link. The linking stage simply is not done. The ultimate output is in the form of an object file for each source file. By default, the object file name for a source file is made by replacing the suffix `.c', `.i', `.s', etc., with `.o'. Unrecognized input files, not requiring compila- tion or assembly, are ignored. -S Stop after the stage of compilation proper; do not assemble. The output is in the form of an assem- bler code file for each non-assembler input file specified. By default, the assembler file name for a source file is made by replacing the suffix `.c', `.i', etc., with `.s'. Input files that don't require compilation are ig- nored. -E Stop after the preprocessing stage; do not run the compiler proper. The output is in the form of preprocessed source code, which is sent to the standard output. Input files which don't require preprocessing are ignored. -o file Place output in file file. This applies regard- less to whatever sort of output is being produced, whether it be an executable file, an object file, an assembler file or preprocessed C code. Since only one output file can be specified, it does not make sense to use `-o' when compiling more than one input file, unless you are producing an executable file as output. If `-o' is not specified, the default is to put an executable file in `a.out', the object file for `source.suffix' in `source.o', its assembler file in `source.s', and all preprocessed C source on standard output. -v Print (on standard error output) the commands exe- cuted to run the stages of compilation. Also print the version number of the compiler driver program and of the preprocessor and the compiler proper. -pipe Use pipes rather than temporary files for communi- cation between the various stages of compilation. This fails to work on some systems where the as- sembler is unable to read from a pipe; but the GNU assembler has no trouble. 1.2. Options Controlling Dialect The following options control the dialect of C that the compiler accepts: -ansi Support all ANSI standard C programs. This turns off certain features of GNU C that are incompatible with ANSI C, such as the asm, inline and typeof keywords, and predefined macros such as unix and vax that identify the type of system you are using. It also enables the undesirable and rarely used ANSI trigraph feature, and disallows `$' as part of identifiers. The alternate keywords __asm__, __extension__, __inline__ and __typeof__ continue to work despite `-ansi'. You would not want to use them in an ANSI C program, of course, but it useful to put them in header files that might be included in compilations done with `-ansi'. Alternate prede- fined macros such as __unix__ and __vax__ are also available, with or without `-ansi'. The `-ansi' option does not cause non-ANSI pro- grams to be rejected gratuitously. For that, `- pedantic' is required in addition to `-ansi'. See section Warning Options. The macro __STRICT_ANSI__ is predefined when the `-ansi' option is used. Some header files may no- tice this macro and refrain from declaring certain functions or defining certain macros that the ANSI standard doesn't call for; this is to avoid in- terfering with any programs that might use these names for other things. -fno-asm Do not recognize asm, inline or typeof as a key- word. These words may then be used as identif- iers. You can use __asm__, __inline__ and __typeof__ instead. `-ansi' implies `-fno-asm'. -fno-builtin Don't recognize non-ANSI built-in functions. `- ansi' also has this effect. Currently, the only function affected is alloca. -trigraphs Support ANSI C trigraphs. You don't want to know about this brain-damage. The `-ansi' option im- plies `-trigraphs'. -traditional Attempt to support some aspects of traditional C compilers. Specifically: o+ All extern declarations take effect globally even if they are written inside of a function definition. This includes implicit declara- tions of functions. o+ The keywords typeof, inline, signed, const and volatile are not recognized. (You can still use the alternative keywords such as __typeof__, __inline__, and so on.) o+ Comparisons between pointers and integers are always allowed. o+ Integer types unsigned short and unsigned char promote to unsigned int. o+ Out-of-range floating point literals are not an error. o+ String ``constants'' are not necessarily con- stant; they are stored in writable space, and identical looking constants are allocated separately. (This is the same as the effect of `-fwritable-strings'.) o+ All automatic variables not declared register are preserved by longjmp. Ordinarily, GNU C follows ANSI C: automatic variables not de- clared volatile may be clobbered. o+ In the preprocessor, comments convert to nothing at all, rather than to a space. This allows traditional token concatenation. o+ In the preprocessor, macro arguments are recognized within string constants in a macro definition (and their values are stringified, though without additional quote marks, when they appear in such a context). The prepro- cessor always considers a string constant to end at a newline. o+ The predefined macro __STDC__ is not defined when you use `-traditional', but __GNUC__ is (since the GNU extensions which __GNUC__ in- dicates are not affected by `-traditional'). If you need to write header files that work differently depending on whether `- traditional' is in use, by testing both of these predefined macros you can distinguish four situations: GNU C, traditional GNU C, other ANSI C compilers, and other old C com- pilers. -traditional-cpp Attempt to support some aspects of traditional C preprocessors. This includes the last three items in the table immediately above, but none of the other effects of `-traditional'. -fcond-mismatch Allow conditional expressions with mismatched types in the second and third arguments. The value of such an expression is void. -funsigned-char Let the type char be unsigned, like unsigned char. Each kind of machine has a default for what char should be. It is either like unsigned char by de- fault or like signed char by default. Ideally, a portable program should always use signed char or unsigned char when it depends on the signedness of an object. But many programs have been written to use plain char and expect it to be signed, or expect it to be unsigned, depend- ing on the machines they were written for. This option, and its inverse, let you make such a pro- gram work with the opposite default. The type char is always a distinct type from each of signed char or unsigned char, even though its behavior is always just like one of those two. -fsigned-char Let the type char be signed, like signed char. Note that this is equivalent to `-fno-unsigned- char', which is the negative form of `-funsigned- char'. Likewise, `-fno-signed-char' is equivalent to `-funsigned-char'. -fsigned-bitfields -funsigned-bitfields -fno-signed-bitfields -fno-unsigned-bitfields These options control whether a bitfield is signed or unsigned, when the declaration does not use ei- ther signed or unsigned. By default, such a bit- field is signed, because this is consistent: the basic integer types such as int are signed types. However, when `-traditional' is used, bitfields are all unsigned no matter what. -fwritable-strings Store string constants in the writable data seg- ment and don't uniquize them. This is for compa- tibility with old programs which assume they can write into string constants. `-traditional' also has this effect. Writing into string constants is a very bad idea; ``constants'' should be constant. 1.3. Options to Request or Suppress Warnings Warnings are diagnostic messages that report construc- tions which are not inherently erroneous but which are risky or suggest there may have been an error. You can request many specific warnings with options beginning `-W', for example `-Wimplicit' to request warnings on implicit declarations. Each of these specific warning options also has a negative form beginning `-Wno-' to turn off warnings; for example, `-Wno-implicit'. This manual lists only one of the two forms, whichever is not the default. These options control the amount and kinds of warnings produced by GNU CC: -fsyntax-only Check the code for syntax errors, but don't emit any output. -w Inhibit all warning messages. -pedantic Issue all the warnings demanded by strict ANSI standard C; reject all programs that use forbidden extensions. Valid ANSI standard C programs should compile properly with or without this option (though a rare few will require `-ansi'). However, without this option, certain GNU extensions and tradition- al C features are supported as well. With this option, they are rejected. `-pedantic' does not cause warning messages for use of the alternate keywords whose names begin and end with `__'. Pedantic warnings are also disabled in the expression that follows __exten- sion__. However, only system header files should use these escape routes; application programs should avoid them. See section Alternate Key- words. This option is not intended to be useful; it ex- ists only to satisfy pedants who would otherwise claim that GNU CC fails to support the ANSI stan- dard. Some users try to use `-pedantic' to check pro- grams for strict ANSI C conformance. They soon find that it does not do quite what they want: it finds some non-ANSI practices, but not all---only those for which ANSI C requires a diagnostic. A feature to report any failure to conform to ANSI C might be useful in some instances, but would re- quire considerable additional work and would be quite different from `-pedantic'. We recommend, rather, that users take advantage of the exten- sions of GNU C and disregard the limitations of other compilers. Aside from certain supercomput- ers and obsolete small machines, there is less and less reason ever to use any other C compiler other than for bootstrapping GNU CC. -pedantic-errors Like `-pedantic', except that errors are produced rather than warnings. -W Print extra warning messages for these events: o+ A nonvolatile automatic variable might be changed by a call to longjmp. These warnings as well are possible only in optimizing com- pilation. The compiler sees only the calls to setjmp. It cannot know where longjmp will be called; in fact, a signal handler could call it at any point in the code. As a result, you may get a warning even when there is in fact no problem because longjmp cannot in fact be called at the place which would cause a prob- lem. o+ A function can return either with or without a value. (Falling off the end of the func- tion body is considered returning without a value.) For example, this function would evoke such a warning: foo (a) { if (a > 0) return a; } o+ An expression-statement contains no side effects. o+ An unsigned value is compared against zero with `>' or `<='. -Wimplicit Warn whenever a function or parameter is im- plicitly declared. -Wreturn-type Warn whenever a function is defined with a return-type that defaults to int. Also warn about any return statement with no return- value in a function whose return-type is not void. -Wunused Warn whenever a local variable is unused aside from its declaration, whenever a function is declared static but never defined, and whenev- er a statement computes a result that is ex- plicitly not used. -Wswitch Warn whenever a switch statement has an index of enumeral type and lacks a case for one or more of the named codes of that enumeration. (The presence of a default label prevents this warning.) case labels outside the enumeration range also provoke warnings when this option is used. -Wcomment Warn whenever a comment-start sequence `/*' appears in a comment. -Wtrigraphs Warn if any trigraphs are encountered (assum- ing they are enabled). -Wformat Check calls to printf and scanf, etc., to make sure that the arguments supplied have types appropriate to the format string specified. -Wchar-subscripts Warn if an array subscript has type char. This is a common cause of error, as program- mers often forget that this type is signed on some machines. -Wuninitialized An automatic variable is used without first being initialized. These warnings are possible only in optimizing compilation, because they require data flow information that is computed only when optim- izing. If you don't specify `-O', you simply won't get these warnings. These warnings occur only for variables that are candidates for register allocation. Therefore, they do not occur for a variable that is declared volatile, or whose address is taken, or whose size is other than 1, 2, 4 or 8 bytes. Also, they do not occur for struc- tures, 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 compu- tations may be deleted by data flow analysis before the warnings are printed. These warnings are made optional because GNU CC is not smart enough to see all the reasons why the code might be correct despite appear- ing to have an error. Here is one example of how this can happen: { int x; switch (y) { case 1: x = 1; break; case 2: x = 4; break; case 3: x = 5; } foo (x); } If the value of y is always 1, 2 or 3, then x is always initialized, but GNU CC doesn't know this. Here is another common case: { int save_y; if (change_y) save_y = y, y = new_y; ... if (change_y) y = save_y; } This has no bug because save_y is used only if it is set. Some spurious warnings can be avoided if you declare as volatile all the functions you use that never return. See section Function At- tributes. -Wall All of the above `-W' options combined. These are all the options which pertain to usage that we recommend avoiding and that we believe is easy to avoid, even in conjunction with macros. The remaining `-W...' options are not implied by `- Wall' because they warn about constructions that we consider reasonable to use, on occasion, in clean programs. -Wtraditional Warn about certain constructs that behave dif- ferently in traditional and ANSI C. o+ Macro arguments occurring within string con- stants in the macro body. These would sub- stitute the argument in traditional C, but are part of the constant in ANSI C. o+ A function declared external in one block and then used after the end of the block. o+ A switch statement has an operand of type long. -Wshadow Warn whenever a local variable shadows another lo- cal variable. -Wid-clash-len Warn whenever two distinct identifiers match in the first len characters. This may help you prepare a program that will compile with certain obsolete, brain-damaged compilers. -Wpointer-arith Warn about anything that depends on the ``size of'' a function type or of void. GNU C assigns these types a size of 1, for convenience in calcu- lations with void * pointers and pointers to func- tions. -Wcast-qual Warn whenever a pointer is cast so as to remove a type qualifier from the target type. For example, warn if a const char * is cast to an ordinary char *. -Wcast-align Warn whenever a pointer is cast such that the re- quired alignment of the target is increased. For example, warn if a char * is cast to an int * on machines where integers can only be accessed at two- or four-byte boundaries. -Wwrite-strings Give string constants the type const char[length] so that copying the address of one into a non- const char * pointer will get a warning. These warnings will help you find at compile time code that can try to write into a string constant, but only if you have been very careful about using const in declarations and prototypes. Otherwise, it will just be a nuisance; this is why we did not make `-Wall' request these warnings. -Wconversion Warn if a prototype causes a type conversion that is different from what would happen to the same argument in the absence of a prototype. This in- cludes conversions of fixed point to floating and vice versa, and conversions changing the width or signedness of a fixed point argument except when the same as the default promotion. -Waggregate-return Warn if any functions that return structures or unions are defined or called. (In languages where you can return an array, this also elicits a warn- ing.) -Wstrict-prototypes Warn if a function is declared or defined without specifying the argument types. (An old-style function definition is permitted without a warning if preceded by a declaration which specifies the argument types.) -Wmissing-prototypes Warn if a global function is defined without a previous prototype declaration. This warning is issued even if the definition itself provides a prototype. The aim is to detect global functions that fail to be declared in header files. -Wredundant-decls Warn if anything is declared more than once in the same scope, even in cases where multiple declara- tion is valid and changes nothing. -Wnested-externs Warn if an extern declaration is encountered within an function. -Wno-parentheses Disable warnings that parentheses are suggested around an expression. -Werror Make all warnings into errors. 1.4. Options for Debugging Your Program or GNU CC GNU CC has various special options that are used for debugging either your program or GCC: -g Produce debugging information in the operating system's native format (stabs or COFF or DWARF). GDB can work with this debugging information. On most systems that use stabs format, `-g' en- ables use of extra debugging information that only GDB can use; this extra information makes debug- ging work better in GDB but will probably make DBX crash or refuse to read the program. If you want to control for certain whether to generate the ex- tra information, use `-gstabs+' or `-gstabs' (see below). Unlike most other C compilers, GNU CC allows you to use `-g' with `-O'. The shortcuts taken by op- timized 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 exe- cuted 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 op- timizer for programs that might have bugs. The following options are useful when GNU CC is generated with the capability for more than one debugging format. -ggdb Produce debugging information in the native format (if that is supported), including GDB extensions if at all possible. -gstabs Produce debugging information in stabs format (if that is supported), without GDB extensions. This is the format used by DBX on most BSD systems. -gstabs+ Produce debugging information in stabs format (if that is supported), using GDB extensions. The use of these extensions is likely to make DBX crash or refuse to read the program. -gcoff Produce debugging information in COFF format (if that is supported). This is the format used by SDB on COFF systems. -gdwarf Produce debugging information in DWARF format (if that is supported). This is the format used by SDB on systems that use DWARF. -glevel -ggdblevel -gstabslevel -gcofflevel -gdwarflevel Request debugging information and also use level to specify how much information. The default lev- el is 2. Level 1 produces minimal information, enough for making backtraces in parts of the program that you don't plan to debug. This includes descriptions of functions and external variables, but no infor- mation about local variables and no line numbers. Level 3 includes extra information, such as all the macro definitions present in the program. Some debuggers support macro expansion when you use `-g3'. -p Generate extra code to write profile information suitable for the analysis program prof. -pg Generate extra code to write profile information suitable for the analysis program gprof. -a Generate extra code to write profile information for basic blocks, which will record the number of times each basic block is executed. This data could be analyzed by a program like tcov. Note, however, that the format of the data is not what tcov expects. Eventually GNU gprof should be ex- tended to process this data. -dletters Says to make debugging dumps during compilation at times specified by letters. This is used for de- bugging the compiler. The file names for most of the dumps are made by appending a word to the source file name (e.g. `foo.c.rtl' or `foo.c.jump'). Here are the possible letters for use in letters, and their meanings: `M' Dump all macro definitions, at the end of preprocessing, and write no output. `N' Dump all macro names, at the end of prepro- cessing. `D' Dump all macro definitions, at the end of preprocessing, in addition to normal output. `y' Dump debugging information during parsing, to standard error. `r' Dump after RTL generation, to `file.rtl'. `x' Just generate RTL for a function instead of compiling it. Usually used with `r'. `j' Dump after first jump optimization, to `file.jump'. `s' Dump after CSE (including the jump optimiza- tion that sometimes follows CSE), to `file.cse'. `L' Dump after loop optimization, to `file.loop'. `t' Dump after the second CSE pass (including the jump optimization that sometimes follows CSE), to `file.cse2'. `f' Dump after flow analysis, to `file.flow'. `c' Dump after instruction combination, to `file.combine'. `S' Dump after the first instruction scheduling pass, to `file.sched'. `l' Dump after local register allocation, to `file.lreg'. `g' Dump after global register allocation, to `file.greg'. `R' Dump after the second instruction scheduling pass, to `file.sched2'. `J' Dump after last jump optimization, to `file.jump2'. `d' Dump after delayed branch scheduling, to `file.dbr'. `k' Dump after conversion from registers to stack, to `file.stack'. `a' Produce all the dumps listed above. `m' Print statistics on memory usage, at the end of the run, to standard error. `p' Annotate the assembler output with a comment indicating which pattern and alternative was used. -fpretend-float When running a cross-compiler, pretend that the target machine uses the same floating point format as the host machine. This causes incorrect output of the actual floating constants, but the actual instruction sequence will probably be the same as GNU CC would make when running on the target machine. -save-temps Store the usual ``temporary'' intermediate files permanently; place them in the current directory and name them based on the source file. Thus, compiling `foo.c' with `-c -save-temps' would pro- duce files `foo.cpp' and `foo.s', as well as `foo.o'. 1.5. Options That Control Optimization These options control various sorts of optimizations: -O Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large func- tion. Without `-O', the compiler's goal is to reduce the cost of compilation and to make debugging produce the expected results. Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the program counter to any oth- er statement in the function and get exactly the results you would expect from the source code. Without `-O', only variables declared register are allocated in registers. The resulting compiled code is a little worse than produced by PCC without `-O'. With `-O', the compiler tries to reduce code size and execution time. When `-O' is specified, `-fthread-jumps' and `- fdelayed-branch' are turned on. On some machines other flags may also be turned on. -O2 Highly optimize. All supported optimizations that do not involve a space-speed tradeoff are per- formed. As compared to `-O', this option will in- crease both compilation time and the performance of the generated code. All `-fflag' options that control optimization are turned on when `-O2' is specified, except for `- funroll-loops' and `-funroll-all-loops'. Options of the form `-fflag' specify machine- independent flags. Most flags have both positive and nega- tive forms; the negative form of `-ffoo' would be `-fno- foo'. In the table below, only one of the forms is listed- --the one which is not the default. You can figure out the other form by either removing `no-' or adding it. -ffloat-store Do not store floating point variables in regis- ters. This prevents undesirable excess precision on machines such as the 68000 where the floating registers (of the 68881) keep more precision than a double is supposed to have. For most programs, the excess precision does only good, but a few programs rely on the precise de- finition of IEEE floating point. Use `-ffloat- store' for such programs. -fno-defer-pop Always pop the arguments to each function call as soon as that function returns. For machines which must pop arguments after a function call, the com- piler normally lets arguments accumulate on the stack for several function calls and pops them all at once. -fforce-mem Force memory operands to be copied into registers before doing arithmetic on them. This may produce better code by making all memory references poten- tial common subexpressions. When they are not common subexpressions, instruction combination should eliminate the separate register-load. I am interested in hearing about the difference this makes. -fforce-addr Force memory address constants to be copied into registers before doing arithmetic on them. This may produce better code just as `-fforce-mem' may. I am interested in hearing about the difference this makes. -fomit-frame-pointer Don't keep the frame pointer in a register for functions that don't need one. This avoids the instructions to save, set up and restore frame pointers; it also makes an extra register avail- able in many functions. It also makes debugging impossible on some machines. INTERNALS On some machines, such as the Vax, this flag has no effect, because the standard calling sequence automatically handles the frame pointer and nothing is saved by pretending it doesn't ex- ist. The machine-description macro FRAME_POINTER_REQUIRED controls whether a target machine supports this flag. See section Regis- ters. INTERNALS On some machines, such as the Vax, this flag has no effect, because the standard calling sequence automatically handles the frame pointer and nothing is saved by pretending it doesn't ex- ist. The machine-description macro FRAME_POINTER_REQUIRED controls whether a target machine supports this flag. See section Registers,,Register Usage, gcc.info, Using and Porting GCC. -finline Pay attention to the inline keyword. Normally the negation of this option `-fno-inline' is used to keep the compiler from expanding any functions in- line. However, the opposite effect may be desir- able when compiling without optimization, since inline expansion is turned off in that case. -finline-functions Integrate all simple functions into their callers. The compiler heuristically decides which functions are simple enough to be worth integrating in this way. If all calls to a given function are integrated, and the function is declared static, then the function is normally not output as assembler code in its own right. -fcaller-saves Enable values to be allocated in registers that will be clobbered by function calls, by emitting extra instructions to save and restore the regis- ters around such calls. Such allocation is done only when it seems to result in better code than would otherwise be produced. This option is enabled by default on certain machines, usually those which have no call- preserved registers to use instead. -fkeep-inline-functions Even if all calls to a given function are in- tegrated, and the function is declared static, nevertheless output a separate run-time callable version of the function. -fno-function-cse Do not put function addresses in registers; make each instruction that calls a constant function contain the function's address explicitly. This option results in less efficient code, but some strange hacks that alter the assembler output may be confused by the optimizations performed when this option is not used. The following options control specific optimizations. The `-O2' option turns on all of these optimizations except `-funroll-loops' and `-funroll-all-loops'. The `-O' option usually turns on the `-fthread-jumps' and `-fdelayed-branch' options, but specific machines may change the default optim- izations. You can use the following flags in the rare cases when ``fine-tuning'' of optimizations to be performed is desired. -fstrength-reduce Perform the optimizations of loop strength reduc- tion and elimination of iteration variables. -fthread-jumps Perform optimizations where we check to see if a jump branches to a location where another com- parison subsumed by the first is found. If so, the first branch is redirected to either the des- tination of the second branch or a point immedi- ately following it, depending on whether the con- dition is known to be true or false. -fcse-follow-jumps In common subexpression elimination, scan through jump instructions in certain cases. This is not as powerful as completely global CSE, but not as slow either. -frerun-cse-after-loop Re-run common subexpression elimination after loop optimizations has been performed. -fexpensive-optimizations Perform a number of minor optimizations that are relatively expensive. -fdelayed-branch If supported for the target machine, attempt to reorder instructions to exploit instruction slots available after delayed branch instructions. -fschedule-insns If supported for the target machine, attempt to reorder instructions to eliminate execution stalls due to required data being unavailable. This helps machines that have slow floating point or memory load instructions by allowing other in- structions to be issued until the result of the load or floating point instruction is required. -fschedule-insns2 Similar to `-fschedule-insns', but requests an ad- ditional pass of instruction scheduling after re- gister allocation has been done. This is espe- cially useful on machines with a relatively small number of registers and where memory load instruc- tions take more than one cycle. -funroll-loops Perform the optimization of loop unrolling. This is only done for loops whose number of iterations can be determined at compile time or run time. `-funroll-loop' implies `-fstrength-reduce' and `-frerun-cse-after-loop'. -funroll-all-loops Perform the optimization of loop unrolling. This is done for all loops and usually makes programs run more slowly. `-funroll-all-loops' implies `- fstrength-reduce' and `-frerun-cse-after-loop'. -fno-peephole Disable any machine-specific peephole optimiza- tions. 1.6. Options Controlling the Preprocessor These options control the C preprocessor, which is run on each C source file before actual compilation. If you use the `-E' option, nothing is done except preprocessing. Some of these options make sense only together with `-E' because they cause the preprocessor out- put to be unsuitable for actual compilation. -include file Process file as input before processing the regu- lar input file. In effect, the contents of file are compiled first. Any `-D' and `-U' options on the command line are always processed before `- include file', regardless of the order in which they are written. All the `-include' and `- imacros' options are processed in the order in which they are written. -imacros file Process file as input, discarding the resulting output, before processing the regular input file. Because the output generated from file is discard- ed, the only effect of `-imacros file' is to make the macros defined in file available for use in the main input. Any `-D' and `-U' options on the command line are always processed before `-imacros file', regard- less of the order in which they are written. All the `-include' and `-imacros' options are pro- cessed in the order in which they are written. -nostdinc Do not search the standard system directories for header files. Only the directories you have specified with `-I' options (and the current directory, if appropriate) are searched. See sec- tion Directory Options, for information on `-I'. By using both `-nostdinc' and `-I-', you can limit the include-file search path to only those direc- tories you specify explicitly. -undef Do not predefine any nonstandard macros. (Includ- ing architecture flags). -E Run only the C preprocessor. Preprocess all the C source files specified and output the results to standard output or to the specified output file. -C Tell the preprocessor not to discard comments. Used with the `-E' option. -P Tell the preprocessor not to generate `#line' com- mands. Used with the `-E' option. -M Tell the preprocessor to output a rule suitable for make describing the dependencies of each ob- ject file. For each source file, the preprocessor outputs one make-rule whose target is the object file name for that source file and whose dependen- cies are all the files `#include'd in it. This rule may be a single line or may be continued with `\'-newline if it is long. The list of rules is printed on standard output instead of the prepro- cessed C program. `-M' implies `-E'. Another way to specify output of a make rule is by setting the environment variable DEPENDENCIES_OUTPUT (see section Environment Variables). -MM Like `-M' but the output mentions only the user header files included with `#include "file"'. System header files included with `#include <file>' are omitted. -MD Like `-M' but the dependency information is writ- ten to files with names made by replacing `.c' with `.d' at the end of the input file names. This is in addition to compiling the file as specified---`-MD' does not inhibit ordinary compi- lation the way `-M' does. The Mach utility `md' can be used to merge the `.d' files into a single dependency file suitable for using with the `make' command. -MMD Like `-MD' except mention only user header files, not system header files. -H Print the name of each header file used, in addi- tion to other normal activities. -Dmacro Define macro macro with the string `1' as its de- finition. -Dmacro=defn Define macro macro as defn. All instances of `-D' on the command line are processed before any `-U' options. -Umacro Undefine macro macro. `-U' options are evaluated after all `-D' options, but before any `-include' and `-imacros' options. -dM Tell the preprocessor to output only a list of the macro definitions that are in effect at the end of preprocessing. Used with the `-E' option. -dD Tell the preprocessing to pass all macro defini- tions into the output, in their proper sequence in the rest of the output. -dN Like `-dD' except that the macro arguments and contents are omitted. Only `#define name' is in- cluded in the output. -trigraphs Support ANSI C trigraphs. You don't want to know about this brain-damage. The `-ansi' option also has this effect. 1.7. Options for Linking These options come into play when the compiler links object files into an executable output file. They are mean- ingless if the compiler is not doing a link step. object-file-name A file name that does not end in a special recog- nized suffix is considered to name an object file or library. (Object files are distinguished from libraries by the linker according to the file con- tents.) If linking is done, these object files are used as input to the linker. -c -S -E If any of these options is used, then the linker is not run, and object file names should not be used as arguments. See section Overall Options. -llibrary Search the library named library when linking. It makes a difference where in the command you write this option; the linker searches processes libraries and object files in the order they are specified. Thus, `foo.o -lz bar.o' seaches li- brary `z' after file `foo.o' but before `bar.o'. If `bar.o' refers to functions in `z', those func- tions may not be loaded. The linker searches a standard list of directories for the library, which is actually a file named `liblibrary.a'. The linker then uses this file as if it had been specified precisely by name. The directories searched include several standard system directories plus any that you specify with `-L'. Normally the files found this way are library files---archive files whose members are object files. The linker handles an archive file by scanning through it for members which define sym- bols that have so far been referenced but not de- fined. But if the file that is found is an ordi- nary object file, it is linked in the usual fashion. The only difference between using an `- l' option and specifying a file name is that `-l' surrounds library with `lib' and `.a' and searches several directories. -nostdlib Don't use the standard system libraries and start- up files when linking. Only the files you specify will be passed to the linker. -static On systems that support dynamic linking, this prevents linking with the shared libraries. On other systems, this option has no effect. -dynamic On systems that support dynamic linking, you can use this option to request it explicitly. -shared Produce a shared object which can then be linked with other objects to form an executable. Only a few systems support this option. -symbolic Bind references to global symbols when building a shared object. Warn about any unresolved refer- ences (unless overridden by the link editor option `-Xlinker -z -Xlinker defs'). Only a few systems support this option. -Xlinker option Pass option as an option to the linker. You can use this to supply system-specific linker options which GNU CC does not know how to recognize. If you want to pass an option that takes an argu- ment, you must use `-Xlinker' twice, once for the option and once for the argument. For example, to pass `-assert definitions', you must write `- Xlinker -assert -Xlinker definitions'. It does not work to write `-Xlinker "-assert defini- tions"', because this passes the entire string as a single argument, which is not what the linker expects. 1.8. Options for Directory Search These options specify directories to search for header files, for libraries and for parts of the compiler: -Idir Append directory dir to the list of directories searched for include files. -I- Any directories you specify with `-I' options be- fore the `-I-' option are searched only for the case of `#include "file"'; they are not searched for `#include <file>'. If additional directories are specified with `-I' options after the `-I-', these directories are searched for all `#include' directives. (Ordi- narily all `-I' directories are used this way.) In addition, the `-I-' option inhibits the use of the current directory (where the current input file came from) as the first search directory for `#include "file"'. There is no way to override this effect of `-I-'. With `-I.' you can specify searching the directory which was current when the compiler was invoked. That is not exactly the same as what the preprocessor does by default, but it is often satisfactory. `-I-' does not inhibit the use of the standard system directories for header files. Thus, `-I-' and `-nostdinc' are independent. -Ldir Add directory dir to the list of directories to be searched for `-l'. -Bprefix This option specifies where to find the execut- ables, libraries and data files of the compiler itself. The compiler driver program runs one or more of the subprograms `cpp', `cc1', `as' and `ld'. It tries prefix as a prefix for each program it tries to run, both with and without `machine/version/' (see section Target Options). For each subprogram to be run, the compiler driver first tries the `-B' prefix, if any. If that name is not found, or if `-B' was not specified, the driver tries two standard prefixes, which are `/usr/lib/gcc/' and `/usr/local/lib/gcc/'. If neither of those results in a file name that is found, the unmodified program name is searched for using the directories specified in your `PATH' en- vironment variable. `-B' prefixes that effectively specify directory names also apply to libraries in the linker, be- cause the compiler translates these options into `-L' options for the linker. The run-time support file `libgcc.a' can also be searched for using the `-B' prefix, if needed. If it is not found there, the two standard prefixes above are tried, and that is all. The file is left out of the link if it is not found by those means. Another way to specify a prefix much like the `-B' prefix is to use the environment variable GCC_EXEC_PREFIX. See section Environment Vari- ables. 1.9. Specifying Target Machine and Compiler Version By default, GNU CC compiles code for the same type of machine that you are using. However, it can also be installed as a cross-compiler, to compile for some other type of machine. In fact, several different configurations of GNU CC, for different target machines, can be installed side by side. Then you specify which one to use with the `-b' option. In addition, older and newer versions of GNU CC can be installed side by side. One of them (probably the newest) will be the default, but you may sometimes wish to use another. -b machine The argument machine specifies the target machine for compilation. This is useful when you have in- stalled GNU CC as a cross-compiler. The value to use for machine is the same as was specified as the machine type when configuring GNU CC as a cross-compiler. For example, if a cross- compiler was configured with `configure i386v', meaning to compile for an 80386 running System V, then you would specify `-b i386v' to run that cross compiler. When you do not specify `-b', it normally means to compile for the same type of machine that you are using. -V version The argument version specifies which version of GNU CC to run. This is useful when multiple ver- sions are installed. For example, version might be `2.0', meaning to run GNU CC version 2.0. The default version, when you do not specify `-V', is controlled by the way GNU CC is installed. Normally, it will be a version that is recommended for general use. The `-b' and `-V' options actually work by controlling part of the file name used for the executable files and libraries used for compilation. A given version of GNU CC, for a given target machine, is normally kept in the direc- tory `/usr/local/lib/gcc/machine/version'. It follows that sites can customize the effect of `-b' or `-V' either by changing the names of these directories or adding alternate names (or symbolic links). Thus, if `/usr/local/lib/gcc/80386' is a link to `/usr/local/lib/gcc/i386v', then `-b 80386' will be an alias for `-b i386v'. In one respect, the `-b' or `-V' do not completely change to a different compiler: the top-level driver program gcc that you originally invoked continues to run and invoke the other executables (preprocessor, compiler per se, assem- bler and linker) that do the real work. However, since no real work is done in the driver program, it usually does not matter that the driver program in use is not the one for the specified target and version. The only way that the driver program depends on the target machine is in the parsing and handling of special machine-specific options. However, this is controlled by a file which is found, along with the other executables, in the directory for the specified version and target machine. As a result, a single installed driver program adapts to any specified target machine and compiler version. The driver program executable does control one signifi- cant thing, however: the default version and target machine. Therefore, you can install different instances of the driver program, compiled for different targets or versions, under different names. For example, if the driver for version 2.0 is installed as ogcc and that for version 2.1 is installed as gcc, then the command gcc will use version 2.1 by default, while ogcc will use 2.0 by default. However, you can choose either version with either command with the `-V' option. 1.10. Specifying Hardware Models and Configurations Earlier we discussed the standard option `-b' which chooses among different installed compilers for completely different target machines, such as Vax vs. 68000 vs. 80386. In addition, each of these target machine types can have its own special options, starting with `-m', to choose among various hardware models or configurations---for exam- ple, 68010 vs 68020, floating coprocessor or none. A single installed version of the compiler can compile for any model or configuration, according to the options specified. INTERNALS These options are defined by the macro TARGET_SWITCHES in the machine description. The default for the options is also defined by that macro, which enables you to change the defaults. 1.10.1. M680x0 Options These are the `-m' options defined for the 68000 series. The default values for these options depends on which style of 68000 was selected when the compiler was con- figured; the defaults for the most common choices are given below. -m68020 -mc68020 Generate output for a 68020 (rather than a 68000). This is the default when the compiler is config- ured for 68020-based systems. -m68000 -mc68000 Generate output for a 68000 (rather than a 68020). This is the default when the compiler is config- ured for a 68000-based systems. -m68881 Generate output containing 68881 instructions for floating point. This is the default for most 68020 systems unless `-nfp' was specified when the compiler was configured. -mfpa Generate output containing Sun FPA instructions for floating point. -msoft-float Generate output containing library calls for floating point. Warning: the requisite libraries are not part of GNU CC. Normally the facilities of the machine's usual C compiler are used, but this can't be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. -mshort Consider type int to be 16 bits wide, like short int. -mnobitfield Do not use the bit-field instructions. `-m68000' implies `-mnobitfield'. -mbitfield Do use the bit-field instructions. `-m68020' im- plies `-mbitfield'. This is the default if you use the unmodified sources configured for a 68020. -mrtd Use a different function-calling convention, in which functions that take a fixed number of argu- ments return with the rtd instruction, which pops their arguments while returning. This saves one instruction in the caller since there is no need to pop the arguments there. This calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler. Also, you must provide function prototypes for all functions that take variable numbers of arguments (including printf); otherwise incorrect code will be generated for calls to those functions. In addition, seriously incorrect code will result if you call a function with too many arguments. (Normally, extra arguments are harmlessly ig- nored.) The rtd instruction is supported by the 68010 and 68020 processors, but not by the 68000. 1.10.2. VAX Options These `-m' options are defined for the Vax: -munix Do not output certain jump instructions (aobleq and so on) that the Unix assembler for the Vax cannot handle across long ranges. -mgnu Do output those jump instructions, on the assump- tion that you will assemble with the GNU assem- bler. -mg Output code for g-format floating point numbers instead of d-format. 1.10.3. SPARC Options These `-m' switches are supported on the Sparc: -mno-epilogue Generate separate return instructions for return statements. This has both advantages and disad- vantages; I don't recall what they are. 1.10.4. Convex Options These `-m' options are defined for the Convex: -mc1 Generate output for a C1. This is the default when the compiler is configured for a C1. -mc2 Generate output for a C2. This is the default when the compiler is configured for a C2. -margcount Generate code which puts an argument count in the word preceding each argument list. Some nonport- able Convex and Vax programs need this word. (De- buggers don't, except for functions with variable-length argument lists; this info is in the symbol table.) -mnoargcount Omit the argument count word. This is the default if you use the unmodified sources. 1.10.5. AMD29K Options These `-m' options are defined for the AMD Am29000: -mdw Generate code that assumes the DW bit is set, i.e., that byte and halfword operations are directly supported by the hardware. This is the default. -mnodw Generate code that assumes the DW bit is not set. -mbw Generate code that assumes the system supports byte and halfword write operations. This is the default. -mnbw Generate code that assumes the systems does not support byte and halfword write operations. `- mnbw' implies `-mnodw'. -msmall Use a small memory model that assumes that all function addresses are either within a single 256 KB segment or at an absolute address of less than 256K. This allows the call instruction to be used instead of a const, consth, calli sequence. -mlarge Do not assume that the call instruction can be used; this is the default. -m29050 Generate code for the Am29050. -m29000 Generate code for the Am29000. This is the de- fault. -mkernel-registers Generate references to registers gr64-gr95 instead of gr96-gr127. This option can be used when com- piling kernel code that wants a set of global re- gisters disjoint from that used by user-mode code. Note that when this option is used, register names in `-f' flags must use the normal, user-mode, names. -muser-registers Use the normal set of global registers, gr96- gr127. This is the default. -mstack-check Insert a call to __msp_check after each stack ad- justment. This is often used for kernel code. 1.10.6. M88K Options These `-m' options are defined for Motorola 88K archi- tectures: -m88000 Generate code that works well on both the m88100 and the m88110. -m88100 Generate code tha Generate code that works best for the m88100, but that also runs on the m88110. -m88110 Generate code that works best for the m88110, and may not run on the m88100. -midentify-revision Include an ident directive in the assembler output recording the source file name, compiler name and version, timestamp, and compilation flags used. -mno-underscores In assembler output, emit symbol names without ad- ding an underscore character at the beginning of each name. The default is to use an underscore as prefix on each name. -mocs-debug-info -mno-ocs-debug-info Include (or omit) additional debugging information (about registers used in each stack frame) as specified in the 88open Object Compatibility Stan- dard, ``OCS''. This extra information allows de- bugging of code that has had the frame pointer el- iminated. The default for DG/UX, SVr4, and Delta 88 SVr3.2 is to include this information; other 88k configurations omit this information by de- fault. -mocs-frame-position When emitting COFF debugging information for au- tomatic variables and parameters stored on the stack, use the offset from the canonical frame ad- dress, which is the stack pointer (register 31) on entry to the function. The DG/UX, SVr4, Delta88 SVr3.2, and BCS configurations use `-mocs-frame- position'; other 88k configurations have the de- fault `-mno-ocs-frame-position'. -mno-ocs-frame-position When emitting COFF debugging information for au- tomatic variables and parameters stored on the stack, use the offset from the frame pointer re- gister (register 30). When this option is in ef- fect, the frame pointer is not eliminated when de- bugging information is selected by the -g switch. -moptimize-arg-area -mno-optimize-arg-area Control how to store function arguments in stack frames. `-moptimize-arg-area' saves space, but was ruled illegal by 88open. `-mno-optimize-arg- area' conforms to the 88open standards. By de- fault GNU CC does not optimize the argument area. -mshort-data-num Generate smaller data references by making them relative to r0, which allows loading a value using a single instruction (rather than the usual two). You control which data references are affected by specifying num with this option. For example, if you specify `-mshort-data-512', then the data references affected are those involving displace- ments of less than 512 bytes. `-mshort-data-num' is not effective for num greater than 64K. -msvr4 -msvr3 Turn on (`-msvr4') or off (`-msvr3') compiler ex- tensions related to System V release 4 (SVr4). This controls the following: 1. Which variant of the assembler syntax to emit (which you can select independently using `- mversion-03.00'). 2. `-msvr4' makes the C preprocessor recognize `#pragma weak' that is used on System V release 4. 3. `-msvr4' makes GNU CC issue additional de- claration directives used in SVr4. `-msvr3' is the default for all m88K configura- tions except the SVr4 configuration. -mversion-03.00 In the DG/UX configuration, there are two flavors of SVr4. This option modifies `-msvr4' to select whether the hybrid-COFF or real-ELF flavor is used. All other configurations ignore this op- tion. -mno-check-zero-division -mcheck-zero-division Early models of the 88K architecture had problems with division by zero; in particular, many of them didn't trap. Use these options to avoid including (or to include explicitly) additional code to detect division by zero and signal an exception. All GNU CC configurations for the 88K use `- mcheck-zero-division' by default. -muse-div-instruction Do not emit code to check both the divisor and dividend when doing signed integer division to see if either is negative, and adjust the signs so the divide is done using non-negative numbers. In- stead, rely on the operating system to calculate the correct value when the div instruction traps. This results in different behavior when the most negative number is divided by -1, but is useful when most or all signed integer divisions are done with positive numbers. -mtrap-large-shift -mhandle-large-shift Include code to detect bit-shifts of more than 31 bits; respectively, trap such shifts or emit code to handle them properly. By default GNU CC makes no special provision for large bit shifts. -mwarn-passed-structs Warn when a function passes a struct as an argu- ment or result. Structure-passing conventions have changed during the evolution of the C language, and are often the source of portability problems. By default, GNU CC issues no such warn- ing. 1.10.7. IBM RS/6000 Options Only one pair of `-m' options is defined for the IBM RS/6000: -mfp-in-toc -mno-fp-in-toc Control whether or not floating-point constants go in the Table of Contents (TOC), a table of all global variable and function addresses. By de- fault GNU CC puts floating-point constants there; if the TOC overflows, `-mno-fp-in-toc' will reduce the size of the TOC, which may avoid the overflow. 1.10.8. IBM RT Options These `-m' options are defined for the IBM RT PC: -min-line-mul Use an in-line code sequence for integer multi- plies. This is the default. -mcall-lib-mul Call lmul$$ for integer multiples. -mfull-fp-blocks Generate full-size floating point data blocks, in- cluding the minimum amount of scratch space recom- mended by IBM. This is the default. -mminimum-fp-blocks Do not include extra scratch space in floating point data blocks. This results in smaller code, but slower execution, since scratch space must be allocated dynamically. -mfp-arg-in-fpregs Use a calling sequence incompatible with the IBM calling convention in which floating point argu- ments are passed in floating point registers. Note that varargs.h and stdargs.h will not work with floating point operands if this option is specified. -mfp-arg-in-gregs Use the normal calling convention for floating point arguments. This is the default. -mhc-struct-return Return structures of more than one word in memory, rather than in a register. This provides compati- bility with the MetaWare HighC (hc) compiler. Use `-fpcc-struct-return' for compatibility with the Portable C Compiler (pcc). -mnohc-struct-return Return some structures of more than one word in registers, when convenient. This is the default. For compatibility with the IBM-supplied compilers, use either `-fpcc-struct-return' or `-mhc-struct- return'. 1.10.9. MIPS Options These `-m' options are defined for the MIPS family of computers: -mcpu=cpu type Assume the defaults for the machine type cpu type when scheduling insturctions. The default cpu type is `default', which picks the longest cycles times for any of the machines, in order that the code run at reasonable rates on all MIPS cpu's. Other choices for cpu type are `r2000', `r3000', `r4000', and `r6000'. While picking a specific cpu type will schedule things appropriately for that particular chip, the compiler will not gen- erate any code that does not meet level 1 of the MIPS ISA (instruction set architecture) without the `-mips2' or `-mips3' switches being used. -mips2 Issue instructions from level 2 of the MIPS ISA (branch likely, square root instructions). The `-mcpu=r4000' or `-mcpu=r6000' switch must be used in conjuction with `-mips2'. -mips3 Issue instructions from level 3 of the MIPS ISA (64 bit instructions). You must use the `- mcpu=r4000' switch along with `-mips3'. -mint64 -mlong64 -mlonglong128 These options don't work at present. -mmips-as Generate code for the MIPS assembler, and invoke `mips-tfile' to add normal debug information. This is the default for all platforms except for the OSF/1 reference platform, using the OSF/rose object format. If the either of the `-gstabs' or `-gstabs+' switches are used, the `mips-tfile' program will encapsulate the stabs within MIPS EC- OFF. -mgas Generate code for the GNU assembler. This is the default on the OSF/1 reference platform, using the OSF/rose object format. -mrnames -mno-rnames The `-mrnames' switch says to output code using the MIPS software names for the registers, instead of the hardware names (ie, a0 instead of $4). The GNU assembler does not support the `-mrnames' switch, and the MIPS assembler will be instructed to run the MIPS C preprocessor over the source file. The `-mno-rnames' switch is default. -mgpopt -mno-gpopt The `-mgpopt' switch says to write all of the data declarations before the instructions in the text section, to all the MIPS assembler to generate one word memory references instead of using two words for short global or static data items. This is on by default if optimization is selected. -mstats -mno-stats For each non-inline function processed, the `- mstats' switch causes the compiler to emit one line to the standard error file to print statis- tics about the program (number of registers saved, stack size, etc.). -mmemcpy -mno-memcpy The `-mmemcpy' switch makes all block moves call the appropriate string function (`memcpy' or `bcopy') instead of possibly generating inline code. -mmips-tfile -mno-mips-tfile The `-mno-mips-tfile' switch causes the compiler not postprocess the object file with the `mips- tfile' program, after the MIPS assembler has gen- erated it to add debug support. If `mips-tfile' is not run, then no local variables will be avail- able to the debugger. In addition, `stage2' and `stage3' objects will have the temporary file names passed to the assembler embedded in the ob- ject file, which means the objects will not com- pare the same. -msoft-float Generate output containing library calls for floating point. Warning: the requisite libraries are not part of GNU CC. Normally the facilities of the machine's usual C compiler are used, but this can't be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. -mhard-float Generate output containing floating point instruc- tions. This is the default if you use the unmodi- fied sources. -mfp64 Assume that the FR bit in the status word is on, and that there are 32 64-bit floating point regis- ters, instead of 32 32-bit floating point regis- ters. You must also specify the `-mcpu=r4000' and `-mips3' switches. -mfp32 Assume that there are 32 32-bit floating point re- gisters. This is the default. -mabicalls -mno-abicalls Emit the `.abicalls', `.cpload', and `.cprestore' pseudo operations that some System V.4 ports use for position independent code. -mhalf-pic -mno-half-pic Put pointers to extern references into the data section and load them up, rather than put the references in the text section. These options do not work at present. -G num Put global and static items less than or equal to num bytes into the small data or bss sections in- stead of the normal data or bss section. This al- lows the assembler to emit one word memory refer- ence instructions based on the global pointer (gp or $28), instead of the normal two words used. By default, num is 8 when the MIPS assembler is used, and 0 when the GNU assembler is used. The `-G num' switch is also passed to the assembler and linker. All modules should be compiled with the same `-G num' value. INTERNALS These options are defined by the macro TARGET_SWITCHES in the machine description. The default for the options is also defined by that macro, which enables you to change the defaults. 1.11. Options for Code Generation Conventions These machine-independent options control the interface conventions used in code generation. Most of them have both positive and negative forms; the negative form of `-ffoo' would be `-fno-foo'. In the table below, only one of the forms is listed---the one which is not the default. You can figure out the other form by either removing `no-' or adding it. -fpcc-struct-return Use the same convention for returning struct and union values that is used by the usual C compiler on your system. This convention is less efficient for small structures, and on many machines it fails to be reentrant; but it has the advantage of allowing intercallability between GNU CC-compiled code and PCC-compiled code. -fshort-enums Allocate to an enum type only as many bytes as it needs for the declared range of possible values. Specifically, the enum type will be equivalent to the smallest integer type which has enough room. -fshort-double Use the same size for double as for float. -fshared-data Requests that the data and non-const variables of this compilation be shared data rather than private data. The distinction makes sense only on certain operating systems, where shared data is shared between processes running the same program, while private data exists in one copy per process. -fno-common Allocate even uninitialized global variables in the bss section of the object file, rather than generating them as common blocks. This has the effect that if the same variable is declared (without extern) in two different compilations, you will get an error when you link them. The only reason this might be useful is if you wish to verify that the program will work on other systems which always work this way. -fno-ident Ignore the `#ident' directive. -fno-gnu-linker Don't output global initializations such as C++ constructors and destructors in the form used by the GNU linker (on systems where the GNU linker is the standard method of handling them). Use this option when you want to use a ``collect'' program and a non-GNU linker. -finhibit-size-directive Don't output a .size assembler directive, or any- thing else that would cause trouble if the func- tion is split in the middle, and the two halves are placed at locations far apart in memory. This option is used when compiling `crtstuff.c'; you should not need to use it for anything else. -fvolatile Consider all memory references through pointers to be volatile. -fpic If supported for the target machines, generate position-independent code, suitable for use in a shared library. All addresses will be accessed through a global offset table (GOT). If the GOT size for the linked executable exceeds a machine- specific maximum size, you will get an error mes- sage from the linker indicating that `-fpic' does not work; recompile with `-fPIC' instead. (These maximums are 16k on the m88k, 8k on the Sparc, and 32k on the m68k and RS/6000. The 386 has no such limit.) Position-independent code requires special sup- port, and therefore works only on certain machines. Code generated for the IBM RS/6000 is always position-independent. -fPIC If supported for the target machine, emit position-independent code, suitable for dynamic linking and avoiding any limit on the size of the global offset table. This option makes a differ- ence on the m68k, m88k and the Sparc. Position-independent code requires special sup- port, and therefore works only on certain machines. -ffixed-reg Treat the register named reg as a fixed register; generated code should never refer to it (except perhaps as a stack pointer, frame pointer or in some other fixed role). reg must be the name of a register. The register names accepted are machine-specific and are de- fined in the REGISTER_NAMES macro in the machine description macro file. This flag does not have a negative form, because it specifies a three-way choice. -fcall-used-reg Treat the register named reg as an allocatable re- gister that is clobbered by function calls. It may be allocated for temporaries or variables that do not live across a call. Functions compiled this way will not save and restore the register reg. Use of this flag for a register that has a fixed pervasive role in the machine's execution model, such as the stack pointer or frame pointer, will produce disastrous results. This flag does not have a negative form, because it specifies a three-way choice. -fcall-saved-reg Treat the register named reg as an allocatable re- gister saved by functions. It may be allocated even for temporaries or variables that live across a call. Functions compiled this way will save and restore the register reg if they use it. Use of this flag for a register that has a fixed pervasive role in the machine's execution model, such as the stack pointer or frame pointer, will produce disastrous results. A different sort of disaster will result from the use of this flag for a register in which function values may be returned. This flag does not have a negative form, because it specifies a three-way choice. 1.12. Environment Variables Affecting GNU CC This section describes several environment variables that affect how GNU CC operates. They work by specifying directories or prefixes to use when searching for various kinds of files. INTERNALS Note that you can also specify places to search using options such as `-B', `-I' and `-L' (see sec- tion Directory Options). These take precedence over places specified using environment variables, which in turn take precedence over those specified by the configuration of GNU CC. INTERNALS Note that you can also specify places to search using options such as `-B', `-I' and `-L' (see section Directory Options). These take precedence over places specified using environment variables, which in turn take precedence over those specified by the configuration of GNU CC. See section Driver. TMPDIR If TMPDIR is set, it specifies the directory to use for temporary files. GNU CC uses temporary files to hold the output of one stage of compila- tion which is to be used as input to the next stage: for example, the output of the preproces- sor, which is the input to the compiler proper. GCC_EXEC_PREFIX If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the names of the subprograms executed by the compiler. No slash is added when this prefix is combined with the name of a subprogram, but you can specify a prefix that ends with a slash if you wish. If GNU CC cannot find the subprogram using the specified prefix, it tries looking in the usual places for the subprogram. Other prefixes specified with `-B' take precedence over this prefix. This prefix is also used for finding files such as `crt0.o' that are used for linking. In addition, the prefix is used in an unusual way in finding the directories to search for header files. For each of the standard directories whose name normally begins with `/usr/local/lib/gcc' (more precisely, with the value of GCC_INCLUDE_DIR), GNU CC tries replacing that be- ginning with the specified prefix to produce an alternate directory name. Thus, with `-Bfoo/', GNU CC will search `foo/bar' where it would nor- mally search `/usr/local/lib/bar'. These alter- nate directories are searched first; the standard directories come next. COMPILER_PATH The value of COMPILER_PATH is a colon-separated list of directories, much like PATH. GNU CC tries the directories thus specified when searching for subprograms, if it can't find the subprograms us- ing GCC_EXEC_PREFIX. LIBRARY_PATH The value of LIBRARY_PATH is a colon-separated list of directories, much like PATH. GNU CC tries the directories thus specified when searching for special linker files, if it can't find them using GCC_EXEC_PREFIX. Linking using GNU CC also uses these directories when searching for ordinary li- braries for the `-l' option (but directories specified with `-L' come first). C_INCLUDE_PATH C++_INCLUDE_PATH OBJC_INCLUDE_PATH variable's value is a colon-separated list of directories, much like PATH. When GNU CC searches for header files, it tries the directories listed in the variable for the language you are using, after the directories specified with `-I' but be- fore the standard header file directories. DEPENDENCIES_OUTPUT If this variable is set, its value specifies how to output dependencies for Make based on the header files processed by the compiler. This out- put looks much like the output from the `-M' op- tion (see section Preprocessor Options), but it goes to a separate file, and is in addition to the usual results of compilation. The value of DEPENDENCIES_OUTPUT can be just a file name, in which case the Make rules are writ- ten to that file, guessing the target name from the source file name. Or the value can have the form `file target', in which case the rules are written to file file using target as the target name.