1 @c Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999,
2 @c 2000, 2001, 2002, 2003 Free Software Foundation, Inc.
3 @c This is part of the GCC manual.
4 @c For copying conditions, see the file gcc.texi.
8 Copyright @copyright{} 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997,
9 1998, 1999, 2000, 2001, 2002, 2003 Free Software Foundation, Inc.
11 Permission is granted to copy, distribute and/or modify this document
12 under the terms of the GNU Free Documentation License, Version 1.2 or
13 any later version published by the Free Software Foundation; with the
14 Invariant Sections being ``GNU General Public License'' and ``Funding
15 Free Software'', the Front-Cover texts being (a) (see below), and with
16 the Back-Cover Texts being (b) (see below). A copy of the license is
17 included in the gfdl(7) man page.
19 (a) The FSF's Front-Cover Text is:
23 (b) The FSF's Back-Cover Text is:
25 You have freedom to copy and modify this GNU Manual, like GNU
26 software. Copies published by the Free Software Foundation raise
27 funds for GNU development.
29 @c Set file name and title for the man page.
31 @settitle GNU project C and C++ compiler
33 gcc [@option{-c}|@option{-S}|@option{-E}] [@option{-std=}@var{standard}]
34 [@option{-g}] [@option{-pg}] [@option{-O}@var{level}]
35 [@option{-W}@var{warn}@dots{}] [@option{-pedantic}]
36 [@option{-I}@var{dir}@dots{}] [@option{-L}@var{dir}@dots{}]
37 [@option{-D}@var{macro}[=@var{defn}]@dots{}] [@option{-U}@var{macro}]
38 [@option{-f}@var{option}@dots{}] [@option{-m}@var{machine-option}@dots{}]
39 [@option{-o} @var{outfile}] @var{infile}@dots{}
41 Only the most useful options are listed here; see below for the
42 remainder. @samp{g++} accepts mostly the same options as @samp{gcc}.
45 gpl(7), gfdl(7), fsf-funding(7),
46 cpp(1), gcov(1), g77(1), as(1), ld(1), gdb(1), adb(1), dbx(1), sdb(1)
47 and the Info entries for @file{gcc}, @file{cpp}, @file{g77}, @file{as},
48 @file{ld}, @file{binutils} and @file{gdb}.
51 For instructions on reporting bugs, see
52 @w{@uref{http://gcc.gnu.org/bugs.html}}. Use of the @command{gccbug}
53 script to report bugs is recommended.
56 See the Info entry for @command{gcc}, or
57 @w{@uref{http://gcc.gnu.org/onlinedocs/gcc/Contributors.html}},
58 for contributors to GCC@.
63 @chapter GCC Command Options
64 @cindex GCC command options
65 @cindex command options
66 @cindex options, GCC command
68 @c man begin DESCRIPTION
69 When you invoke GCC, it normally does preprocessing, compilation,
70 assembly and linking. The ``overall options'' allow you to stop this
71 process at an intermediate stage. For example, the @option{-c} option
72 says not to run the linker. Then the output consists of object files
73 output by the assembler.
75 Other options are passed on to one stage of processing. Some options
76 control the preprocessor and others the compiler itself. Yet other
77 options control the assembler and linker; most of these are not
78 documented here, since you rarely need to use any of them.
80 @cindex C compilation options
81 Most of the command line options that you can use with GCC are useful
82 for C programs; when an option is only useful with another language
83 (usually C++), the explanation says so explicitly. If the description
84 for a particular option does not mention a source language, you can use
85 that option with all supported languages.
87 @cindex C++ compilation options
88 @xref{Invoking G++,,Compiling C++ Programs}, for a summary of special
89 options for compiling C++ programs.
91 @cindex grouping options
92 @cindex options, grouping
93 The @command{gcc} program accepts options and file names as operands. Many
94 options have multi-letter names; therefore multiple single-letter options
95 may @emph{not} be grouped: @option{-dr} is very different from @w{@samp{-d
98 @cindex order of options
99 @cindex options, order
100 You can mix options and other arguments. For the most part, the order
101 you use doesn't matter. Order does matter when you use several options
102 of the same kind; for example, if you specify @option{-L} more than once,
103 the directories are searched in the order specified.
105 Many options have long names starting with @samp{-f} or with
106 @samp{-W}---for example, @option{-fforce-mem},
107 @option{-fstrength-reduce}, @option{-Wformat} and so on. Most of
108 these have both positive and negative forms; the negative form of
109 @option{-ffoo} would be @option{-fno-foo}. This manual documents
110 only one of these two forms, whichever one is not the default.
114 @xref{Option Index}, for an index to GCC's options.
117 * Option Summary:: Brief list of all options, without explanations.
118 * Overall Options:: Controlling the kind of output:
119 an executable, object files, assembler files,
120 or preprocessed source.
121 * Invoking G++:: Compiling C++ programs.
122 * C Dialect Options:: Controlling the variant of C language compiled.
123 * C++ Dialect Options:: Variations on C++.
124 * Objective-C Dialect Options:: Variations on Objective-C.
125 * Language Independent Options:: Controlling how diagnostics should be
127 * Warning Options:: How picky should the compiler be?
128 * Debugging Options:: Symbol tables, measurements, and debugging dumps.
129 * Optimize Options:: How much optimization?
130 * Preprocessor Options:: Controlling header files and macro definitions.
131 Also, getting dependency information for Make.
132 * Assembler Options:: Passing options to the assembler.
133 * Link Options:: Specifying libraries and so on.
134 * Directory Options:: Where to find header files and libraries.
135 Where to find the compiler executable files.
136 * Spec Files:: How to pass switches to sub-processes.
137 * Target Options:: Running a cross-compiler, or an old version of GCC.
138 * Submodel Options:: Specifying minor hardware or convention variations,
139 such as 68010 vs 68020.
140 * Code Gen Options:: Specifying conventions for function calls, data layout
142 * Environment Variables:: Env vars that affect GCC.
143 * Precompiled Headers:: Compiling a header once, and using it many times.
144 * Running Protoize:: Automatically adding or removing function prototypes.
150 @section Option Summary
152 Here is a summary of all the options, grouped by type. Explanations are
153 in the following sections.
156 @item Overall Options
157 @xref{Overall Options,,Options Controlling the Kind of Output}.
158 @gccoptlist{-c -S -E -o @var{file} -pipe -pass-exit-codes @gol
159 -x @var{language} -v -### --help --target-help --version}
161 @item C Language Options
162 @xref{C Dialect Options,,Options Controlling C Dialect}.
163 @gccoptlist{-ansi -std=@var{standard} -aux-info @var{filename} @gol
164 -fno-asm -fno-builtin -fno-builtin-@var{function} @gol
165 -fhosted -ffreestanding -fms-extensions @gol
166 -trigraphs -no-integrated-cpp -traditional -traditional-cpp @gol
167 -fallow-single-precision -fcond-mismatch @gol
168 -fsigned-bitfields -fsigned-char @gol
169 -funsigned-bitfields -funsigned-char @gol
172 @item C++ Language Options
173 @xref{C++ Dialect Options,,Options Controlling C++ Dialect}.
174 @gccoptlist{-fabi-version=@var{n} -fno-access-control -fcheck-new @gol
175 -fconserve-space -fno-const-strings @gol
176 -fno-elide-constructors @gol
177 -fno-enforce-eh-specs -fexternal-templates @gol
178 -falt-external-templates @gol
179 -ffor-scope -fno-for-scope -fno-gnu-keywords @gol
180 -fno-implicit-templates @gol
181 -fno-implicit-inline-templates @gol
182 -fno-implement-inlines -fms-extensions @gol
183 -fno-nonansi-builtins -fno-operator-names @gol
184 -fno-optional-diags -fpermissive @gol
185 -frepo -fno-rtti -fstats -ftemplate-depth-@var{n} @gol
186 -fuse-cxa-atexit -fvtable-gc -fno-weak -nostdinc++ @gol
187 -fno-default-inline -Wabi -Wctor-dtor-privacy @gol
188 -Wnon-virtual-dtor -Wreorder @gol
189 -Weffc++ -Wno-deprecated @gol
190 -Wno-non-template-friend -Wold-style-cast @gol
191 -Woverloaded-virtual -Wno-pmf-conversions @gol
192 -Wsign-promo -Wsynth}
194 @item Objective-C Language Options
195 @xref{Objective-C Dialect Options,,Options Controlling Objective-C Dialect}.
196 @gccoptlist{-fconstant-string-class=@var{class-name} @gol
197 -fgnu-runtime -fnext-runtime -gen-decls @gol
198 -Wno-protocol -Wselector -Wundeclared-selector}
200 @item Language Independent Options
201 @xref{Language Independent Options,,Options to Control Diagnostic Messages Formatting}.
202 @gccoptlist{-fmessage-length=@var{n} @gol
203 -fdiagnostics-show-location=@r{[}once@r{|}every-line@r{]}}
205 @item Warning Options
206 @xref{Warning Options,,Options to Request or Suppress Warnings}.
207 @gccoptlist{-fsyntax-only -pedantic -pedantic-errors @gol
208 -w -Wextra -Wall -Waggregate-return @gol
209 -Wcast-align -Wcast-qual -Wchar-subscripts -Wcomment @gol
210 -Wconversion -Wno-deprecated-declarations @gol
211 -Wdisabled-optimization -Wno-div-by-zero -Werror @gol
212 -Wfloat-equal -Wformat -Wformat=2 @gol
213 -Wformat-nonliteral -Wformat-security @gol
214 -Wimplicit -Wimplicit-int @gol
215 -Wimplicit-function-declaration @gol
216 -Werror-implicit-function-declaration @gol
217 -Wimport -Winline -Winvalid-pch -Wno-endif-labels @gol
218 -Wno-invalid-offsetof @gol
219 -Wlarger-than-@var{len} -Wlong-long @gol
220 -Wmain -Wmissing-braces @gol
221 -Wmissing-format-attribute -Wmissing-noreturn @gol
222 -Wno-multichar -Wno-format-extra-args -Wno-format-y2k @gol
223 -Wno-import -Wnonnull -Wpacked -Wpadded @gol
224 -Wparentheses -Wpointer-arith -Wredundant-decls @gol
225 -Wreturn-type -Wsequence-point -Wshadow @gol
226 -Wsign-compare -Wstrict-aliasing @gol
227 -Wswitch -Wswitch-default -Wswitch-enum @gol
228 -Wsystem-headers -Wtrigraphs -Wundef -Wuninitialized @gol
229 -Wunknown-pragmas -Wunreachable-code @gol
230 -Wunused -Wunused-function -Wunused-label -Wunused-parameter @gol
231 -Wunused-value -Wunused-variable -Wwrite-strings}
233 @item C-only Warning Options
234 @gccoptlist{-Wbad-function-cast -Wmissing-declarations @gol
235 -Wmissing-prototypes -Wnested-externs @gol
236 -Wstrict-prototypes -Wtraditional}
238 @item Debugging Options
239 @xref{Debugging Options,,Options for Debugging Your Program or GCC}.
240 @gccoptlist{-d@var{letters} -dumpspecs -dumpmachine -dumpversion @gol
241 -fdump-unnumbered -fdump-translation-unit@r{[}-@var{n}@r{]} @gol
242 -fdump-class-hierarchy@r{[}-@var{n}@r{]} @gol
243 -fdump-tree-original@r{[}-@var{n}@r{]} @gol
244 -fdump-tree-optimized@r{[}-@var{n}@r{]} @gol
245 -fdump-tree-inlined@r{[}-@var{n}@r{]} @gol
246 -feliminate-dwarf2-dups -feliminate-unused-debug-types @gol
247 -fmem-report -fprofile-arcs @gol
248 -frandom-seed=@var{string} -fsched-verbose=@var{n} @gol
249 -ftest-coverage -ftime-report @gol
250 -g -g@var{level} -gcoff -gdwarf -gdwarf-1 -gdwarf-1+ -gdwarf-2 @gol
251 -ggdb -gstabs -gstabs+ -gvms -gxcoff -gxcoff+ @gol
252 -p -pg -print-file-name=@var{library} -print-libgcc-file-name @gol
253 -print-multi-directory -print-multi-lib @gol
254 -print-prog-name=@var{program} -print-search-dirs -Q @gol
257 @item Optimization Options
258 @xref{Optimize Options,,Options that Control Optimization}.
259 @gccoptlist{-falign-functions=@var{n} -falign-jumps=@var{n} @gol
260 -falign-labels=@var{n} -falign-loops=@var{n} @gol
261 -fbranch-probabilities -fcaller-saves -fcprop-registers @gol
262 -fcse-follow-jumps -fcse-skip-blocks -fdata-sections @gol
263 -fdelayed-branch -fdelete-null-pointer-checks @gol
264 -fexpensive-optimizations -ffast-math -ffloat-store @gol
265 -fforce-addr -fforce-mem -ffunction-sections @gol
266 -fgcse -fgcse-lm -fgcse-sm -floop-optimize -fcrossjumping @gol
267 -fif-conversion -fif-conversion2 @gol
268 -finline-functions -finline-limit=@var{n} -fkeep-inline-functions @gol
269 -fkeep-static-consts -fmerge-constants -fmerge-all-constants @gol
270 -fmove-all-movables -fnew-ra -fno-branch-count-reg @gol
271 -fno-default-inline -fno-defer-pop @gol
272 -fno-function-cse -fno-guess-branch-probability @gol
273 -fno-inline -fno-math-errno -fno-peephole -fno-peephole2 @gol
274 -funsafe-math-optimizations -ffinite-math-only @gol
275 -fno-trapping-math -fno-zero-initialized-in-bss @gol
276 -fomit-frame-pointer -foptimize-register-move @gol
277 -foptimize-sibling-calls -fprefetch-loop-arrays @gol
278 -freduce-all-givs -fregmove -frename-registers @gol
279 -freorder-blocks -freorder-functions @gol
280 -frerun-cse-after-loop -frerun-loop-opt @gol
281 -fschedule-insns -fschedule-insns2 @gol
282 -fno-sched-interblock -fno-sched-spec -fsched-spec-load @gol
283 -fsched-spec-load-dangerous -fsched2-use-superblocks @gol
284 -fsched2-use-traces -fsignaling-nans @gol
285 -fsingle-precision-constant -fssa -fssa-ccp -fssa-dce @gol
286 -fstrength-reduce -fstrict-aliasing -ftracer -fthread-jumps @gol
287 -funroll-all-loops -funroll-loops -fpeel-loops @gol
288 -funswitch-loops -fold-unroll-loops -fold-unroll-all-loops @gol
289 --param @var{name}=@var{value}
290 -O -O0 -O1 -O2 -O3 -Os}
292 @item Preprocessor Options
293 @xref{Preprocessor Options,,Options Controlling the Preprocessor}.
294 @gccoptlist{-A@var{question}=@var{answer} @gol
295 -A-@var{question}@r{[}=@var{answer}@r{]} @gol
296 -C -dD -dI -dM -dN @gol
297 -D@var{macro}@r{[}=@var{defn}@r{]} -E -H @gol
298 -idirafter @var{dir} @gol
299 -include @var{file} -imacros @var{file} @gol
300 -iprefix @var{file} -iwithprefix @var{dir} @gol
301 -iwithprefixbefore @var{dir} -isystem @var{dir} @gol
302 -M -MM -MF -MG -MP -MQ -MT -nostdinc -P -remap @gol
303 -trigraphs -undef -U@var{macro} -Wp,@var{option} @gol
304 -Xpreprocessor @var{option}}
306 @item Assembler Option
307 @xref{Assembler Options,,Passing Options to the Assembler}.
308 @gccoptlist{-Wa,@var{option} -Xassembler @var{option}}
311 @xref{Link Options,,Options for Linking}.
312 @gccoptlist{@var{object-file-name} -l@var{library} @gol
313 -nostartfiles -nodefaultlibs -nostdlib -pie @gol
314 -s -static -static-libgcc -shared -shared-libgcc -symbolic @gol
315 -Wl,@var{option} -Xlinker @var{option} @gol
318 @item Directory Options
319 @xref{Directory Options,,Options for Directory Search}.
320 @gccoptlist{-B@var{prefix} -I@var{dir} -I- -L@var{dir} -specs=@var{file}}
323 @c I wrote this xref this way to avoid overfull hbox. -- rms
324 @xref{Target Options}.
325 @gccoptlist{-V @var{version} -b @var{machine}}
327 @item Machine Dependent Options
328 @xref{Submodel Options,,Hardware Models and Configurations}.
330 @emph{M680x0 Options}
331 @gccoptlist{-m68000 -m68020 -m68020-40 -m68020-60 -m68030 -m68040 @gol
332 -m68060 -mcpu32 -m5200 -m68881 -mbitfield -mc68000 -mc68020 @gol
333 -mnobitfield -mrtd -mshort -msoft-float -mpcrel @gol
334 -malign-int -mstrict-align}
336 @emph{M68hc1x Options}
337 @gccoptlist{-m6811 -m6812 -m68hc11 -m68hc12 -m68hcs12 @gol
338 -mauto-incdec -minmax -mlong-calls -mshort @gol
339 -msoft-reg-count=@var{count}}
342 @gccoptlist{-mg -mgnu -munix}
345 @gccoptlist{-mcpu=@var{cpu-type} @gol
346 -mtune=@var{cpu-type} @gol
347 -mcmodel=@var{code-model} @gol
349 -mapp-regs -mbroken-saverestore -mcypress @gol
350 -mfaster-structs -mflat @gol
351 -mfpu -mhard-float -mhard-quad-float @gol
352 -mimpure-text -mlive-g0 -mno-app-regs @gol
353 -mno-faster-structs -mno-flat -mno-fpu @gol
354 -mno-impure-text -mno-stack-bias -mno-unaligned-doubles @gol
355 -msoft-float -msoft-quad-float -msparclite -mstack-bias @gol
356 -msupersparc -munaligned-doubles -mv8}
359 @gccoptlist{-mapcs-frame -mno-apcs-frame @gol
360 -mapcs-26 -mapcs-32 @gol
361 -mapcs-stack-check -mno-apcs-stack-check @gol
362 -mapcs-float -mno-apcs-float @gol
363 -mapcs-reentrant -mno-apcs-reentrant @gol
364 -msched-prolog -mno-sched-prolog @gol
365 -mlittle-endian -mbig-endian -mwords-little-endian @gol
366 -malignment-traps -mno-alignment-traps @gol
367 -msoft-float -mhard-float -mfpe @gol
368 -mthumb-interwork -mno-thumb-interwork @gol
369 -mcpu=@var{name} -march=@var{name} -mfpe=@var{name} @gol
370 -mstructure-size-boundary=@var{n} @gol
371 -mabort-on-noreturn @gol
372 -mlong-calls -mno-long-calls @gol
373 -msingle-pic-base -mno-single-pic-base @gol
374 -mpic-register=@var{reg} @gol
375 -mnop-fun-dllimport @gol
376 -mcirrus-fix-invalid-insns -mno-cirrus-fix-invalid-insns @gol
377 -mpoke-function-name @gol
379 -mtpcs-frame -mtpcs-leaf-frame @gol
380 -mcaller-super-interworking -mcallee-super-interworking}
382 @emph{MN10200 Options}
385 @emph{MN10300 Options}
386 @gccoptlist{-mmult-bug -mno-mult-bug @gol
387 -mam33 -mno-am33 @gol
390 @emph{M32R/D Options}
391 @gccoptlist{-m32rx -m32r -mcode-model=@var{model-type} @gol
392 -msdata=@var{sdata-type} -G @var{num}}
395 @gccoptlist{-m88000 -m88100 -m88110 -mbig-pic @gol
396 -mcheck-zero-division -mhandle-large-shift @gol
397 -midentify-revision -mno-check-zero-division @gol
398 -mno-ocs-debug-info -mno-ocs-frame-position @gol
399 -mno-optimize-arg-area -mno-serialize-volatile @gol
400 -mno-underscores -mocs-debug-info @gol
401 -mocs-frame-position -moptimize-arg-area @gol
402 -mserialize-volatile -mshort-data-@var{num} -msvr3 @gol
403 -msvr4 -mtrap-large-shift -muse-div-instruction @gol
404 -mversion-03.00 -mwarn-passed-structs}
406 @emph{RS/6000 and PowerPC Options}
407 @gccoptlist{-mcpu=@var{cpu-type} @gol
408 -mtune=@var{cpu-type} @gol
409 -mpower -mno-power -mpower2 -mno-power2 @gol
410 -mpowerpc -mpowerpc64 -mno-powerpc @gol
411 -maltivec -mno-altivec @gol
412 -mpowerpc-gpopt -mno-powerpc-gpopt @gol
413 -mpowerpc-gfxopt -mno-powerpc-gfxopt @gol
414 -mnew-mnemonics -mold-mnemonics @gol
415 -mfull-toc -mminimal-toc -mno-fp-in-toc -mno-sum-in-toc @gol
416 -m64 -m32 -mxl-call -mno-xl-call -mpe @gol
417 -malign-power -malign-natural @gol
418 -msoft-float -mhard-float -mmultiple -mno-multiple @gol
419 -mstring -mno-string -mupdate -mno-update @gol
420 -mfused-madd -mno-fused-madd -mbit-align -mno-bit-align @gol
421 -mstrict-align -mno-strict-align -mrelocatable @gol
422 -mno-relocatable -mrelocatable-lib -mno-relocatable-lib @gol
423 -mtoc -mno-toc -mlittle -mlittle-endian -mbig -mbig-endian @gol
424 -mdynamic-no-pic @gol
425 -mcall-sysv -mcall-netbsd @gol
426 -maix-struct-return -msvr4-struct-return @gol
427 -mabi=altivec -mabi=no-altivec @gol
428 -mabi=spe -mabi=no-spe @gol
429 -misel=yes -misel=no @gol
430 -mspe=yes -mspe=no @gol
431 -mfloat-gprs=yes -mfloat-gprs=no @gol
432 -mprototype -mno-prototype @gol
433 -msim -mmvme -mads -myellowknife -memb -msdata @gol
434 -msdata=@var{opt} -mvxworks -mwindiss -G @var{num} -pthread}
436 @emph{Darwin Options}
437 @gccoptlist{-all_load -allowable_client -arch -arch_errors_fatal @gol
438 -arch_only -bind_at_load -bundle -bundle_loader @gol
439 -client_name -compatibility_version -current_version @gol
440 -dependency-file -dylib_file -dylinker_install_name @gol
441 -dynamic -dynamiclib -exported_symbols_list @gol
442 -filelist -flat_namespace -force_cpusubtype_ALL @gol
443 -force_flat_namespace -headerpad_max_install_names @gol
444 -image_base -init -install_name -keep_private_externs @gol
445 -multi_module -multiply_defined -multiply_defined_unused @gol
446 -noall_load -nomultidefs -noprebind -noseglinkedit @gol
447 -pagezero_size -prebind -prebind_all_twolevel_modules @gol
448 -private_bundle -read_only_relocs -sectalign @gol
449 -sectobjectsymbols -whyload -seg1addr @gol
450 -sectcreate -sectobjectsymbols -sectorder @gol
451 -seg_addr_table -seg_addr_table_filename -seglinkedit @gol
452 -segprot -segs_read_only_addr -segs_read_write_addr @gol
453 -single_module -static -sub_library -sub_umbrella @gol
454 -twolevel_namespace -umbrella -undefined @gol
455 -unexported_symbols_list -weak_reference_mismatches @gol
459 @gccoptlist{-mcall-lib-mul -mfp-arg-in-fpregs -mfp-arg-in-gregs @gol
460 -mfull-fp-blocks -mhc-struct-return -min-line-mul @gol
461 -mminimum-fp-blocks -mnohc-struct-return}
464 @gccoptlist{-mabicalls -march=@var{cpu-type} -mtune=@var{cpu=type} @gol
465 -mcpu=@var{cpu-type} -membedded-data -muninit-const-in-rodata @gol
466 -membedded-pic -mfp32 -mfp64 -mfused-madd -mno-fused-madd @gol
467 -mgas -mgp32 -mgp64 @gol
468 -mgpopt -mhalf-pic -mhard-float -mint64 -mips1 @gol
469 -mips2 -mips3 -mips4 -mips32 -mips32r2 -mips64 @gol
470 -mlong64 -mlong32 -mlong-calls -mmemcpy @gol
471 -mmips-as -mmips-tfile -mno-abicalls @gol
472 -mno-embedded-data -mno-uninit-const-in-rodata @gol
473 -mno-embedded-pic -mno-gpopt -mno-long-calls @gol
474 -mno-memcpy -mno-mips-tfile -mno-rnames -mno-stats @gol
475 -mrnames -msoft-float @gol
476 -m4650 -msingle-float -mmad @gol
477 -EL -EB -G @var{num} -nocpp @gol
478 -mabi=32 -mabi=n32 -mabi=64 -mabi=eabi -mabi-fake-default @gol
479 -mfix7000 -mno-crt0 -mflush-func=@var{func} -mno-flush-func @gol
480 -mbranch-likely -mno-branch-likely}
482 @emph{i386 and x86-64 Options}
483 @gccoptlist{-mtune=@var{cpu-type} -march=@var{cpu-type} @gol
484 -mfpmath=@var{unit} @gol
485 -masm=@var{dialect} -mno-fancy-math-387 @gol
486 -mno-fp-ret-in-387 -msoft-float -msvr3-shlib @gol
487 -mno-wide-multiply -mrtd -malign-double @gol
488 -mpreferred-stack-boundary=@var{num} @gol
489 -mmmx -msse -msse2 -m3dnow @gol
490 -mthreads -mno-align-stringops -minline-all-stringops @gol
491 -mpush-args -maccumulate-outgoing-args -m128bit-long-double @gol
492 -m96bit-long-double -mregparm=@var{num} -momit-leaf-frame-pointer @gol
494 -mcmodel=@var{code-model} @gol
498 @gccoptlist{-march=@var{architecture-type} @gol
499 -mbig-switch -mdisable-fpregs -mdisable-indexing @gol
500 -mfast-indirect-calls -mgas -mgnu-ld -mhp-ld @gol
501 -mjump-in-delay -mlinker-opt -mlong-calls @gol
502 -mlong-load-store -mno-big-switch -mno-disable-fpregs @gol
503 -mno-disable-indexing -mno-fast-indirect-calls -mno-gas @gol
504 -mno-jump-in-delay -mno-long-load-store @gol
505 -mno-portable-runtime -mno-soft-float @gol
506 -mno-space-regs -msoft-float -mpa-risc-1-0 @gol
507 -mpa-risc-1-1 -mpa-risc-2-0 -mportable-runtime @gol
508 -mschedule=@var{cpu-type} -mspace-regs -msio -mwsio @gol
509 -nolibdld -static -threads}
511 @emph{Intel 960 Options}
512 @gccoptlist{-m@var{cpu-type} -masm-compat -mclean-linkage @gol
513 -mcode-align -mcomplex-addr -mleaf-procedures @gol
514 -mic-compat -mic2.0-compat -mic3.0-compat @gol
515 -mintel-asm -mno-clean-linkage -mno-code-align @gol
516 -mno-complex-addr -mno-leaf-procedures @gol
517 -mno-old-align -mno-strict-align -mno-tail-call @gol
518 -mnumerics -mold-align -msoft-float -mstrict-align @gol
521 @emph{DEC Alpha Options}
522 @gccoptlist{-mno-fp-regs -msoft-float -malpha-as -mgas @gol
523 -mieee -mieee-with-inexact -mieee-conformant @gol
524 -mfp-trap-mode=@var{mode} -mfp-rounding-mode=@var{mode} @gol
525 -mtrap-precision=@var{mode} -mbuild-constants @gol
526 -mcpu=@var{cpu-type} -mtune=@var{cpu-type} @gol
527 -mbwx -mmax -mfix -mcix @gol
528 -mfloat-vax -mfloat-ieee @gol
529 -mexplicit-relocs -msmall-data -mlarge-data @gol
530 -msmall-text -mlarge-text @gol
531 -mmemory-latency=@var{time}}
533 @emph{DEC Alpha/VMS Options}
534 @gccoptlist{-mvms-return-codes}
536 @emph{H8/300 Options}
537 @gccoptlist{-mrelax -mh -ms -mn -mint32 -malign-300}
540 @gccoptlist{-m1 -m2 -m2e -m3 -m3e @gol
541 -m4-nofpu -m4-single-only -m4-single -m4 @gol
542 -m5-64media -m5-64media-nofpu @gol
543 -m5-32media -m5-32media-nofpu @gol
544 -m5-compact -m5-compact-nofpu @gol
545 -mb -ml -mdalign -mrelax @gol
546 -mbigtable -mfmovd -mhitachi -mnomacsave @gol
547 -mieee -misize -mpadstruct -mspace @gol
548 -mprefergot -musermode}
550 @emph{System V Options}
551 @gccoptlist{-Qy -Qn -YP,@var{paths} -Ym,@var{dir}}
554 @gccoptlist{-EB -EL @gol
555 -mmangle-cpu -mcpu=@var{cpu} -mtext=@var{text-section} @gol
556 -mdata=@var{data-section} -mrodata=@var{readonly-data-section}}
558 @emph{TMS320C3x/C4x Options}
559 @gccoptlist{-mcpu=@var{cpu} -mbig -msmall -mregparm -mmemparm @gol
560 -mfast-fix -mmpyi -mbk -mti -mdp-isr-reload @gol
561 -mrpts=@var{count} -mrptb -mdb -mloop-unsigned @gol
562 -mparallel-insns -mparallel-mpy -mpreserve-float}
565 @gccoptlist{-mlong-calls -mno-long-calls -mep -mno-ep @gol
566 -mprolog-function -mno-prolog-function -mspace @gol
567 -mtda=@var{n} -msda=@var{n} -mzda=@var{n} @gol
568 -mapp-regs -mno-app-regs @gol
569 -mdisable-callt -mno-disable-callt @gol
574 @gccoptlist{-m32032 -m32332 -m32532 -m32081 -m32381 @gol
575 -mmult-add -mnomult-add -msoft-float -mrtd -mnortd @gol
576 -mregparam -mnoregparam -msb -mnosb @gol
577 -mbitfield -mnobitfield -mhimem -mnohimem}
580 @gccoptlist{-mmcu=@var{mcu} -msize -minit-stack=@var{n} -mno-interrupts @gol
581 -mcall-prologues -mno-tablejump -mtiny-stack}
584 @gccoptlist{-mhardlit -mno-hardlit -mdiv -mno-div -mrelax-immediates @gol
585 -mno-relax-immediates -mwide-bitfields -mno-wide-bitfields @gol
586 -m4byte-functions -mno-4byte-functions -mcallgraph-data @gol
587 -mno-callgraph-data -mslow-bytes -mno-slow-bytes -mno-lsim @gol
588 -mlittle-endian -mbig-endian -m210 -m340 -mstack-increment}
591 @gccoptlist{-mlibfuncs -mno-libfuncs -mepsilon -mno-epsilon -mabi=gnu @gol
592 -mabi=mmixware -mzero-extend -mknuthdiv -mtoplevel-symbols @gol
593 -melf -mbranch-predict -mno-branch-predict -mbase-addresses @gol
594 -mno-base-addresses -msingle-exit -mno-single-exit}
597 @gccoptlist{-mbig-endian -mlittle-endian -mgnu-as -mgnu-ld -mno-pic @gol
598 -mvolatile-asm-stop -mb-step -mregister-names -mno-sdata @gol
599 -mconstant-gp -mauto-pic -minline-float-divide-min-latency @gol
600 -minline-float-divide-max-throughput @gol
601 -minline-int-divide-min-latency @gol
602 -minline-int-divide-max-throughput -mno-dwarf2-asm @gol
603 -mfixed-range=@var{register-range}}
606 @gccoptlist{-mextmem -mextmemory -monchip -mno-asm-optimize @gol
607 -masm-optimize -mbranch-cost=@var{n} -mcond-exec=@var{n}}
609 @emph{S/390 and zSeries Options}
610 @gccoptlist{-mtune=@var{cpu-type} -march=@var{cpu-type} @gol
611 -mhard-float -msoft-float -mbackchain -mno-backchain @gol
612 -msmall-exec -mno-small-exec -mmvcle -mno-mvcle @gol
613 -m64 -m31 -mdebug -mno-debug -mesa -mzarch}
616 @gccoptlist{-mcpu=@var{cpu} -march=@var{cpu} -mtune=@var{cpu} @gol
617 -mmax-stack-frame=@var{n} -melinux-stacksize=@var{n} @gol
618 -metrax4 -metrax100 -mpdebug -mcc-init -mno-side-effects @gol
619 -mstack-align -mdata-align -mconst-align @gol
620 -m32-bit -m16-bit -m8-bit -mno-prologue-epilogue -mno-gotplt @gol
621 -melf -maout -melinux -mlinux -sim -sim2}
623 @emph{PDP-11 Options}
624 @gccoptlist{-mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45 -m10 @gol
625 -mbcopy -mbcopy-builtin -mint32 -mno-int16 @gol
626 -mint16 -mno-int32 -mfloat32 -mno-float64 @gol
627 -mfloat64 -mno-float32 -mabshi -mno-abshi @gol
628 -mbranch-expensive -mbranch-cheap @gol
629 -msplit -mno-split -munix-asm -mdec-asm}
631 @emph{Xstormy16 Options}
634 @emph{Xtensa Options}
635 @gccoptlist{-mbig-endian -mlittle-endian @gol
636 -mdensity -mno-density @gol
637 -mconst16 -mno-const16 @gol
639 -maddx -mno-addx @gol
640 -mmac16 -mno-mac16 @gol
641 -mmul16 -mno-mul16 @gol
642 -mmul32 -mno-mul32 @gol
644 -mminmax -mno-minmax @gol
645 -msext -mno-sext @gol
646 -mbooleans -mno-booleans @gol
647 -mhard-float -msoft-float @gol
648 -mfused-madd -mno-fused-madd @gol
649 -mtext-section-literals -mno-text-section-literals @gol
650 -mtarget-align -mno-target-align @gol
651 -mlongcalls -mno-longcalls}
654 @gccoptlist{-mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64 @gol
655 -mhard-float -msoft-float @gol
656 -malloc-cc -mfixed-cc -mdword -mno-dword @gol
657 -mdouble -mno-double @gol
658 -mmedia -mno-media -mmuladd -mno-muladd @gol
659 -mlibrary-pic -macc-4 -macc-8 @gol
660 -mpack -mno-pack -mno-eflags -mcond-move -mno-cond-move @gol
661 -mscc -mno-scc -mcond-exec -mno-cond-exec @gol
662 -mvliw-branch -mno-vliw-branch @gol
663 -mmulti-cond-exec -mno-multi-cond-exec -mnested-cond-exec @gol
664 -mno-nested-cond-exec -mtomcat-stats @gol
667 @item Code Generation Options
668 @xref{Code Gen Options,,Options for Code Generation Conventions}.
669 @gccoptlist{-fcall-saved-@var{reg} -fcall-used-@var{reg} @gol
670 -ffixed-@var{reg} -fexceptions @gol
671 -fnon-call-exceptions -funwind-tables @gol
672 -fasynchronous-unwind-tables @gol
673 -finhibit-size-directive -finstrument-functions @gol
674 -fno-common -fno-ident -fno-gnu-linker @gol
675 -fpcc-struct-return -fpic -fPIC -fpie -fPIE @gol
676 -freg-struct-return -fshared-data -fshort-enums @gol
677 -fshort-double -fshort-wchar @gol
678 -fverbose-asm -fpack-struct -fstack-check @gol
679 -fstack-limit-register=@var{reg} -fstack-limit-symbol=@var{sym} @gol
680 -fargument-alias -fargument-noalias @gol
681 -fargument-noalias-global -fleading-underscore @gol
682 -ftls-model=@var{model} @gol
683 -ftrapv -fwrapv -fbounds-check}
687 * Overall Options:: Controlling the kind of output:
688 an executable, object files, assembler files,
689 or preprocessed source.
690 * C Dialect Options:: Controlling the variant of C language compiled.
691 * C++ Dialect Options:: Variations on C++.
692 * Objective-C Dialect Options:: Variations on Objective-C.
693 * Language Independent Options:: Controlling how diagnostics should be
695 * Warning Options:: How picky should the compiler be?
696 * Debugging Options:: Symbol tables, measurements, and debugging dumps.
697 * Optimize Options:: How much optimization?
698 * Preprocessor Options:: Controlling header files and macro definitions.
699 Also, getting dependency information for Make.
700 * Assembler Options:: Passing options to the assembler.
701 * Link Options:: Specifying libraries and so on.
702 * Directory Options:: Where to find header files and libraries.
703 Where to find the compiler executable files.
704 * Spec Files:: How to pass switches to sub-processes.
705 * Target Options:: Running a cross-compiler, or an old version of GCC.
708 @node Overall Options
709 @section Options Controlling the Kind of Output
711 Compilation can involve up to four stages: preprocessing, compilation
712 proper, assembly and linking, always in that order. The first three
713 stages apply to an individual source file, and end by producing an
714 object file; linking combines all the object files (those newly
715 compiled, and those specified as input) into an executable file.
717 @cindex file name suffix
718 For any given input file, the file name suffix determines what kind of
723 C source code which must be preprocessed.
726 C source code which should not be preprocessed.
729 C++ source code which should not be preprocessed.
732 Objective-C source code. Note that you must link with the library
733 @file{libobjc.a} to make an Objective-C program work.
736 Objective-C source code which should not be preprocessed.
739 C or C++ header file to be turned into a precompiled header.
743 @itemx @var{file}.cxx
744 @itemx @var{file}.cpp
745 @itemx @var{file}.CPP
746 @itemx @var{file}.c++
748 C++ source code which must be preprocessed. Note that in @samp{.cxx},
749 the last two letters must both be literally @samp{x}. Likewise,
750 @samp{.C} refers to a literal capital C@.
754 C++ header file to be turned into a precompiled header.
757 @itemx @var{file}.for
758 @itemx @var{file}.FOR
759 Fortran source code which should not be preprocessed.
762 @itemx @var{file}.fpp
763 @itemx @var{file}.FPP
764 Fortran source code which must be preprocessed (with the traditional
768 Fortran source code which must be preprocessed with a RATFOR
769 preprocessor (not included with GCC)@.
771 @xref{Overall Options,,Options Controlling the Kind of Output, g77,
772 Using and Porting GNU Fortran}, for more details of the handling of
775 @c FIXME: Descriptions of Java file types.
782 Ada source code file which contains a library unit declaration (a
783 declaration of a package, subprogram, or generic, or a generic
784 instantiation), or a library unit renaming declaration (a package,
785 generic, or subprogram renaming declaration). Such files are also
788 @itemx @var{file}.adb
789 Ada source code file containing a library unit body (a subprogram or
790 package body). Such files are also called @dfn{bodies}.
792 @c GCC also knows about some suffixes for languages not yet included:
801 Assembler code which must be preprocessed.
804 An object file to be fed straight into linking.
805 Any file name with no recognized suffix is treated this way.
809 You can specify the input language explicitly with the @option{-x} option:
812 @item -x @var{language}
813 Specify explicitly the @var{language} for the following input files
814 (rather than letting the compiler choose a default based on the file
815 name suffix). This option applies to all following input files until
816 the next @option{-x} option. Possible values for @var{language} are:
818 c c-header cpp-output
819 c++ c++-header c++-cpp-output
820 objective-c objective-c-header objc-cpp-output
821 assembler assembler-with-cpp
823 f77 f77-cpp-input ratfor
829 Turn off any specification of a language, so that subsequent files are
830 handled according to their file name suffixes (as they are if @option{-x}
831 has not been used at all).
833 @item -pass-exit-codes
834 @opindex pass-exit-codes
835 Normally the @command{gcc} program will exit with the code of 1 if any
836 phase of the compiler returns a non-success return code. If you specify
837 @option{-pass-exit-codes}, the @command{gcc} program will instead return with
838 numerically highest error produced by any phase that returned an error
842 If you only want some of the stages of compilation, you can use
843 @option{-x} (or filename suffixes) to tell @command{gcc} where to start, and
844 one of the options @option{-c}, @option{-S}, or @option{-E} to say where
845 @command{gcc} is to stop. Note that some combinations (for example,
846 @samp{-x cpp-output -E}) instruct @command{gcc} to do nothing at all.
851 Compile or assemble the source files, but do not link. The linking
852 stage simply is not done. The ultimate output is in the form of an
853 object file for each source file.
855 By default, the object file name for a source file is made by replacing
856 the suffix @samp{.c}, @samp{.i}, @samp{.s}, etc., with @samp{.o}.
858 Unrecognized input files, not requiring compilation or assembly, are
863 Stop after the stage of compilation proper; do not assemble. The output
864 is in the form of an assembler code file for each non-assembler input
867 By default, the assembler file name for a source file is made by
868 replacing the suffix @samp{.c}, @samp{.i}, etc., with @samp{.s}.
870 Input files that don't require compilation are ignored.
874 Stop after the preprocessing stage; do not run the compiler proper. The
875 output is in the form of preprocessed source code, which is sent to the
878 Input files which don't require preprocessing are ignored.
880 @cindex output file option
883 Place output in file @var{file}. This applies regardless to whatever
884 sort of output is being produced, whether it be an executable file,
885 an object file, an assembler file or preprocessed C code.
887 Since only one output file can be specified, it does not make sense to
888 use @option{-o} when compiling more than one input file, unless you are
889 producing an executable file as output.
891 If @option{-o} is not specified, the default is to put an executable file
892 in @file{a.out}, the object file for @file{@var{source}.@var{suffix}} in
893 @file{@var{source}.o}, its assembler file in @file{@var{source}.s}, and
894 all preprocessed C source on standard output.
898 Print (on standard error output) the commands executed to run the stages
899 of compilation. Also print the version number of the compiler driver
900 program and of the preprocessor and the compiler proper.
904 Like @option{-v} except the commands are not executed and all command
905 arguments are quoted. This is useful for shell scripts to capture the
906 driver-generated command lines.
910 Use pipes rather than temporary files for communication between the
911 various stages of compilation. This fails to work on some systems where
912 the assembler is unable to read from a pipe; but the GNU assembler has
917 Print (on the standard output) a description of the command line options
918 understood by @command{gcc}. If the @option{-v} option is also specified
919 then @option{--help} will also be passed on to the various processes
920 invoked by @command{gcc}, so that they can display the command line options
921 they accept. If the @option{-Wextra} option is also specified then command
922 line options which have no documentation associated with them will also
927 Print (on the standard output) a description of target specific command
928 line options for each tool.
932 Display the version number and copyrights of the invoked GCC.
936 @section Compiling C++ Programs
938 @cindex suffixes for C++ source
939 @cindex C++ source file suffixes
940 C++ source files conventionally use one of the suffixes @samp{.C},
941 @samp{.cc}, @samp{.cpp}, @samp{.CPP}, @samp{.c++}, @samp{.cp}, or
942 @samp{.cxx}; C++ header files often use @samp{.hh} or @samp{.H}; and
943 preprocessed C++ files use the suffix @samp{.ii}. GCC recognizes
944 files with these names and compiles them as C++ programs even if you
945 call the compiler the same way as for compiling C programs (usually
946 with the name @command{gcc}).
950 However, C++ programs often require class libraries as well as a
951 compiler that understands the C++ language---and under some
952 circumstances, you might want to compile programs or header files from
953 standard input, or otherwise without a suffix that flags them as C++
954 programs. You might also like to precompile a C header file with a
955 @samp{.h} extension to be used in C++ compilations. @command{g++} is a
956 program that calls GCC with the default language set to C++, and
957 automatically specifies linking against the C++ library. On many
958 systems, @command{g++} is also installed with the name @command{c++}.
960 @cindex invoking @command{g++}
961 When you compile C++ programs, you may specify many of the same
962 command-line options that you use for compiling programs in any
963 language; or command-line options meaningful for C and related
964 languages; or options that are meaningful only for C++ programs.
965 @xref{C Dialect Options,,Options Controlling C Dialect}, for
966 explanations of options for languages related to C@.
967 @xref{C++ Dialect Options,,Options Controlling C++ Dialect}, for
968 explanations of options that are meaningful only for C++ programs.
970 @node C Dialect Options
971 @section Options Controlling C Dialect
972 @cindex dialect options
973 @cindex language dialect options
974 @cindex options, dialect
976 The following options control the dialect of C (or languages derived
977 from C, such as C++ and Objective-C) that the compiler accepts:
984 In C mode, support all ISO C90 programs. In C++ mode,
985 remove GNU extensions that conflict with ISO C++.
987 This turns off certain features of GCC that are incompatible with ISO
988 C90 (when compiling C code), or of standard C++ (when compiling C++ code),
989 such as the @code{asm} and @code{typeof} keywords, and
990 predefined macros such as @code{unix} and @code{vax} that identify the
991 type of system you are using. It also enables the undesirable and
992 rarely used ISO trigraph feature. For the C compiler,
993 it disables recognition of C++ style @samp{//} comments as well as
994 the @code{inline} keyword.
996 The alternate keywords @code{__asm__}, @code{__extension__},
997 @code{__inline__} and @code{__typeof__} continue to work despite
998 @option{-ansi}. You would not want to use them in an ISO C program, of
999 course, but it is useful to put them in header files that might be included
1000 in compilations done with @option{-ansi}. Alternate predefined macros
1001 such as @code{__unix__} and @code{__vax__} are also available, with or
1002 without @option{-ansi}.
1004 The @option{-ansi} option does not cause non-ISO programs to be
1005 rejected gratuitously. For that, @option{-pedantic} is required in
1006 addition to @option{-ansi}. @xref{Warning Options}.
1008 The macro @code{__STRICT_ANSI__} is predefined when the @option{-ansi}
1009 option is used. Some header files may notice this macro and refrain
1010 from declaring certain functions or defining certain macros that the
1011 ISO standard doesn't call for; this is to avoid interfering with any
1012 programs that might use these names for other things.
1014 Functions which would normally be built in but do not have semantics
1015 defined by ISO C (such as @code{alloca} and @code{ffs}) are not built-in
1016 functions with @option{-ansi} is used. @xref{Other Builtins,,Other
1017 built-in functions provided by GCC}, for details of the functions
1022 Determine the language standard. This option is currently only
1023 supported when compiling C or C++. A value for this option must be
1024 provided; possible values are
1029 ISO C90 (same as @option{-ansi}).
1031 @item iso9899:199409
1032 ISO C90 as modified in amendment 1.
1038 ISO C99. Note that this standard is not yet fully supported; see
1039 @w{@uref{http://gcc.gnu.org/c99status.html}} for more information. The
1040 names @samp{c9x} and @samp{iso9899:199x} are deprecated.
1043 Default, ISO C90 plus GNU extensions (including some C99 features).
1047 ISO C99 plus GNU extensions. When ISO C99 is fully implemented in GCC,
1048 this will become the default. The name @samp{gnu9x} is deprecated.
1051 The 1998 ISO C++ standard plus amendments.
1054 The same as @option{-std=c++98} plus GNU extensions. This is the
1055 default for C++ code.
1058 Even when this option is not specified, you can still use some of the
1059 features of newer standards in so far as they do not conflict with
1060 previous C standards. For example, you may use @code{__restrict__} even
1061 when @option{-std=c99} is not specified.
1063 The @option{-std} options specifying some version of ISO C have the same
1064 effects as @option{-ansi}, except that features that were not in ISO C90
1065 but are in the specified version (for example, @samp{//} comments and
1066 the @code{inline} keyword in ISO C99) are not disabled.
1068 @xref{Standards,,Language Standards Supported by GCC}, for details of
1069 these standard versions.
1071 @item -aux-info @var{filename}
1073 Output to the given filename prototyped declarations for all functions
1074 declared and/or defined in a translation unit, including those in header
1075 files. This option is silently ignored in any language other than C@.
1077 Besides declarations, the file indicates, in comments, the origin of
1078 each declaration (source file and line), whether the declaration was
1079 implicit, prototyped or unprototyped (@samp{I}, @samp{N} for new or
1080 @samp{O} for old, respectively, in the first character after the line
1081 number and the colon), and whether it came from a declaration or a
1082 definition (@samp{C} or @samp{F}, respectively, in the following
1083 character). In the case of function definitions, a K&R-style list of
1084 arguments followed by their declarations is also provided, inside
1085 comments, after the declaration.
1089 Do not recognize @code{asm}, @code{inline} or @code{typeof} as a
1090 keyword, so that code can use these words as identifiers. You can use
1091 the keywords @code{__asm__}, @code{__inline__} and @code{__typeof__}
1092 instead. @option{-ansi} implies @option{-fno-asm}.
1094 In C++, this switch only affects the @code{typeof} keyword, since
1095 @code{asm} and @code{inline} are standard keywords. You may want to
1096 use the @option{-fno-gnu-keywords} flag instead, which has the same
1097 effect. In C99 mode (@option{-std=c99} or @option{-std=gnu99}), this
1098 switch only affects the @code{asm} and @code{typeof} keywords, since
1099 @code{inline} is a standard keyword in ISO C99.
1102 @itemx -fno-builtin-@var{function}
1103 @opindex fno-builtin
1104 @cindex built-in functions
1105 Don't recognize built-in functions that do not begin with
1106 @samp{__builtin_} as prefix. @xref{Other Builtins,,Other built-in
1107 functions provided by GCC}, for details of the functions affected,
1108 including those which are not built-in functions when @option{-ansi} or
1109 @option{-std} options for strict ISO C conformance are used because they
1110 do not have an ISO standard meaning.
1112 GCC normally generates special code to handle certain built-in functions
1113 more efficiently; for instance, calls to @code{alloca} may become single
1114 instructions that adjust the stack directly, and calls to @code{memcpy}
1115 may become inline copy loops. The resulting code is often both smaller
1116 and faster, but since the function calls no longer appear as such, you
1117 cannot set a breakpoint on those calls, nor can you change the behavior
1118 of the functions by linking with a different library.
1120 With the @option{-fno-builtin-@var{function}} option
1121 only the built-in function @var{function} is
1122 disabled. @var{function} must not begin with @samp{__builtin_}. If a
1123 function is named this is not built-in in this version of GCC, this
1124 option is ignored. There is no corresponding
1125 @option{-fbuiltin-@var{function}} option; if you wish to enable
1126 built-in functions selectively when using @option{-fno-builtin} or
1127 @option{-ffreestanding}, you may define macros such as:
1130 #define abs(n) __builtin_abs ((n))
1131 #define strcpy(d, s) __builtin_strcpy ((d), (s))
1136 @cindex hosted environment
1138 Assert that compilation takes place in a hosted environment. This implies
1139 @option{-fbuiltin}. A hosted environment is one in which the
1140 entire standard library is available, and in which @code{main} has a return
1141 type of @code{int}. Examples are nearly everything except a kernel.
1142 This is equivalent to @option{-fno-freestanding}.
1144 @item -ffreestanding
1145 @opindex ffreestanding
1146 @cindex hosted environment
1148 Assert that compilation takes place in a freestanding environment. This
1149 implies @option{-fno-builtin}. A freestanding environment
1150 is one in which the standard library may not exist, and program startup may
1151 not necessarily be at @code{main}. The most obvious example is an OS kernel.
1152 This is equivalent to @option{-fno-hosted}.
1154 @xref{Standards,,Language Standards Supported by GCC}, for details of
1155 freestanding and hosted environments.
1157 @item -fms-extensions
1158 @opindex fms-extensions
1159 Accept some non-standard constructs used in Microsoft header files.
1163 Support ISO C trigraphs. The @option{-ansi} option (and @option{-std}
1164 options for strict ISO C conformance) implies @option{-trigraphs}.
1166 @item -no-integrated-cpp
1167 @opindex no-integrated-cpp
1168 Performs a compilation in two passes: preprocessing and compiling. This
1169 option allows a user supplied "cc1", "cc1plus", or "cc1obj" via the
1170 @option{-B} option. The user supplied compilation step can then add in
1171 an additional preprocessing step after normal preprocessing but before
1172 compiling. The default is to use the integrated cpp (internal cpp)
1174 The semantics of this option will change if "cc1", "cc1plus", and
1175 "cc1obj" are merged.
1177 @cindex traditional C language
1178 @cindex C language, traditional
1180 @itemx -traditional-cpp
1181 @opindex traditional-cpp
1182 @opindex traditional
1183 Formerly, these options caused GCC to attempt to emulate a pre-standard
1184 C compiler. They are now only supported with the @option{-E} switch.
1185 The preprocessor continues to support a pre-standard mode. See the GNU
1186 CPP manual for details.
1188 @item -fcond-mismatch
1189 @opindex fcond-mismatch
1190 Allow conditional expressions with mismatched types in the second and
1191 third arguments. The value of such an expression is void. This option
1192 is not supported for C++.
1194 @item -funsigned-char
1195 @opindex funsigned-char
1196 Let the type @code{char} be unsigned, like @code{unsigned char}.
1198 Each kind of machine has a default for what @code{char} should
1199 be. It is either like @code{unsigned char} by default or like
1200 @code{signed char} by default.
1202 Ideally, a portable program should always use @code{signed char} or
1203 @code{unsigned char} when it depends on the signedness of an object.
1204 But many programs have been written to use plain @code{char} and
1205 expect it to be signed, or expect it to be unsigned, depending on the
1206 machines they were written for. This option, and its inverse, let you
1207 make such a program work with the opposite default.
1209 The type @code{char} is always a distinct type from each of
1210 @code{signed char} or @code{unsigned char}, even though its behavior
1211 is always just like one of those two.
1214 @opindex fsigned-char
1215 Let the type @code{char} be signed, like @code{signed char}.
1217 Note that this is equivalent to @option{-fno-unsigned-char}, which is
1218 the negative form of @option{-funsigned-char}. Likewise, the option
1219 @option{-fno-signed-char} is equivalent to @option{-funsigned-char}.
1221 @item -fsigned-bitfields
1222 @itemx -funsigned-bitfields
1223 @itemx -fno-signed-bitfields
1224 @itemx -fno-unsigned-bitfields
1225 @opindex fsigned-bitfields
1226 @opindex funsigned-bitfields
1227 @opindex fno-signed-bitfields
1228 @opindex fno-unsigned-bitfields
1229 These options control whether a bit-field is signed or unsigned, when the
1230 declaration does not use either @code{signed} or @code{unsigned}. By
1231 default, such a bit-field is signed, because this is consistent: the
1232 basic integer types such as @code{int} are signed types.
1234 @item -fwritable-strings
1235 @opindex fwritable-strings
1236 Store string constants in the writable data segment and don't uniquize
1237 them. This is for compatibility with old programs which assume they can
1238 write into string constants.
1240 Writing into string constants is a very bad idea; ``constants'' should
1244 @node C++ Dialect Options
1245 @section Options Controlling C++ Dialect
1247 @cindex compiler options, C++
1248 @cindex C++ options, command line
1249 @cindex options, C++
1250 This section describes the command-line options that are only meaningful
1251 for C++ programs; but you can also use most of the GNU compiler options
1252 regardless of what language your program is in. For example, you
1253 might compile a file @code{firstClass.C} like this:
1256 g++ -g -frepo -O -c firstClass.C
1260 In this example, only @option{-frepo} is an option meant
1261 only for C++ programs; you can use the other options with any
1262 language supported by GCC@.
1264 Here is a list of options that are @emph{only} for compiling C++ programs:
1268 @item -fabi-version=@var{n}
1269 @opindex fabi-version
1270 Use version @var{n} of the C++ ABI. Version 1 is the version of the C++
1271 ABI that first appeared in G++ 3.2. Version 0 will always be the
1272 version that conforms most closely to the C++ ABI specification.
1273 Therefore, the ABI obtained using version 0 will change as ABI bugs are
1276 The default is version 1.
1278 @item -fno-access-control
1279 @opindex fno-access-control
1280 Turn off all access checking. This switch is mainly useful for working
1281 around bugs in the access control code.
1285 Check that the pointer returned by @code{operator new} is non-null
1286 before attempting to modify the storage allocated. This check is
1287 normally unnecessary because the C++ standard specifies that
1288 @code{operator new} will only return @code{0} if it is declared
1289 @samp{throw()}, in which case the compiler will always check the
1290 return value even without this option. In all other cases, when
1291 @code{operator new} has a non-empty exception specification, memory
1292 exhaustion is signalled by throwing @code{std::bad_alloc}. See also
1293 @samp{new (nothrow)}.
1295 @item -fconserve-space
1296 @opindex fconserve-space
1297 Put uninitialized or runtime-initialized global variables into the
1298 common segment, as C does. This saves space in the executable at the
1299 cost of not diagnosing duplicate definitions. If you compile with this
1300 flag and your program mysteriously crashes after @code{main()} has
1301 completed, you may have an object that is being destroyed twice because
1302 two definitions were merged.
1304 This option is no longer useful on most targets, now that support has
1305 been added for putting variables into BSS without making them common.
1307 @item -fno-const-strings
1308 @opindex fno-const-strings
1309 Give string constants type @code{char *} instead of type @code{const
1310 char *}. By default, G++ uses type @code{const char *} as required by
1311 the standard. Even if you use @option{-fno-const-strings}, you cannot
1312 actually modify the value of a string constant, unless you also use
1313 @option{-fwritable-strings}.
1315 This option might be removed in a future release of G++. For maximum
1316 portability, you should structure your code so that it works with
1317 string constants that have type @code{const char *}.
1319 @item -fno-elide-constructors
1320 @opindex fno-elide-constructors
1321 The C++ standard allows an implementation to omit creating a temporary
1322 which is only used to initialize another object of the same type.
1323 Specifying this option disables that optimization, and forces G++ to
1324 call the copy constructor in all cases.
1326 @item -fno-enforce-eh-specs
1327 @opindex fno-enforce-eh-specs
1328 Don't check for violation of exception specifications at runtime. This
1329 option violates the C++ standard, but may be useful for reducing code
1330 size in production builds, much like defining @samp{NDEBUG}. The compiler
1331 will still optimize based on the exception specifications.
1333 @item -fexternal-templates
1334 @opindex fexternal-templates
1336 Cause @samp{#pragma interface} and @samp{implementation} to apply to
1337 template instantiation; template instances are emitted or not according
1338 to the location of the template definition. @xref{Template
1339 Instantiation}, for more information.
1341 This option is deprecated.
1343 @item -falt-external-templates
1344 @opindex falt-external-templates
1345 Similar to @option{-fexternal-templates}, but template instances are
1346 emitted or not according to the place where they are first instantiated.
1347 @xref{Template Instantiation}, for more information.
1349 This option is deprecated.
1352 @itemx -fno-for-scope
1354 @opindex fno-for-scope
1355 If @option{-ffor-scope} is specified, the scope of variables declared in
1356 a @i{for-init-statement} is limited to the @samp{for} loop itself,
1357 as specified by the C++ standard.
1358 If @option{-fno-for-scope} is specified, the scope of variables declared in
1359 a @i{for-init-statement} extends to the end of the enclosing scope,
1360 as was the case in old versions of G++, and other (traditional)
1361 implementations of C++.
1363 The default if neither flag is given to follow the standard,
1364 but to allow and give a warning for old-style code that would
1365 otherwise be invalid, or have different behavior.
1367 @item -fno-gnu-keywords
1368 @opindex fno-gnu-keywords
1369 Do not recognize @code{typeof} as a keyword, so that code can use this
1370 word as an identifier. You can use the keyword @code{__typeof__} instead.
1371 @option{-ansi} implies @option{-fno-gnu-keywords}.
1373 @item -fno-implicit-templates
1374 @opindex fno-implicit-templates
1375 Never emit code for non-inline templates which are instantiated
1376 implicitly (i.e.@: by use); only emit code for explicit instantiations.
1377 @xref{Template Instantiation}, for more information.
1379 @item -fno-implicit-inline-templates
1380 @opindex fno-implicit-inline-templates
1381 Don't emit code for implicit instantiations of inline templates, either.
1382 The default is to handle inlines differently so that compiles with and
1383 without optimization will need the same set of explicit instantiations.
1385 @item -fno-implement-inlines
1386 @opindex fno-implement-inlines
1387 To save space, do not emit out-of-line copies of inline functions
1388 controlled by @samp{#pragma implementation}. This will cause linker
1389 errors if these functions are not inlined everywhere they are called.
1391 @item -fms-extensions
1392 @opindex fms-extensions
1393 Disable pedantic warnings about constructs used in MFC, such as implicit
1394 int and getting a pointer to member function via non-standard syntax.
1396 @item -fno-nonansi-builtins
1397 @opindex fno-nonansi-builtins
1398 Disable built-in declarations of functions that are not mandated by
1399 ANSI/ISO C@. These include @code{ffs}, @code{alloca}, @code{_exit},
1400 @code{index}, @code{bzero}, @code{conjf}, and other related functions.
1402 @item -fno-operator-names
1403 @opindex fno-operator-names
1404 Do not treat the operator name keywords @code{and}, @code{bitand},
1405 @code{bitor}, @code{compl}, @code{not}, @code{or} and @code{xor} as
1406 synonyms as keywords.
1408 @item -fno-optional-diags
1409 @opindex fno-optional-diags
1410 Disable diagnostics that the standard says a compiler does not need to
1411 issue. Currently, the only such diagnostic issued by G++ is the one for
1412 a name having multiple meanings within a class.
1415 @opindex fpermissive
1416 Downgrade messages about nonconformant code from errors to warnings. By
1417 default, G++ effectively sets @option{-pedantic-errors} without
1418 @option{-pedantic}; this option reverses that. This behavior and this
1419 option are superseded by @option{-pedantic}, which works as it does for GNU C@.
1423 Enable automatic template instantiation at link time. This option also
1424 implies @option{-fno-implicit-templates}. @xref{Template
1425 Instantiation}, for more information.
1429 Disable generation of information about every class with virtual
1430 functions for use by the C++ runtime type identification features
1431 (@samp{dynamic_cast} and @samp{typeid}). If you don't use those parts
1432 of the language, you can save some space by using this flag. Note that
1433 exception handling uses the same information, but it will generate it as
1438 Emit statistics about front-end processing at the end of the compilation.
1439 This information is generally only useful to the G++ development team.
1441 @item -ftemplate-depth-@var{n}
1442 @opindex ftemplate-depth
1443 Set the maximum instantiation depth for template classes to @var{n}.
1444 A limit on the template instantiation depth is needed to detect
1445 endless recursions during template class instantiation. ANSI/ISO C++
1446 conforming programs must not rely on a maximum depth greater than 17.
1448 @item -fuse-cxa-atexit
1449 @opindex fuse-cxa-atexit
1450 Register destructors for objects with static storage duration with the
1451 @code{__cxa_atexit} function rather than the @code{atexit} function.
1452 This option is required for fully standards-compliant handling of static
1453 destructors, but will only work if your C library supports
1454 @code{__cxa_atexit}.
1458 Emit special relocations for vtables and virtual function references
1459 so that the linker can identify unused virtual functions and zero out
1460 vtable slots that refer to them. This is most useful with
1461 @option{-ffunction-sections} and @option{-Wl,--gc-sections}, in order to
1462 also discard the functions themselves.
1464 This optimization requires GNU as and GNU ld. Not all systems support
1465 this option. @option{-Wl,--gc-sections} is ignored without @option{-static}.
1469 Do not use weak symbol support, even if it is provided by the linker.
1470 By default, G++ will use weak symbols if they are available. This
1471 option exists only for testing, and should not be used by end-users;
1472 it will result in inferior code and has no benefits. This option may
1473 be removed in a future release of G++.
1477 Do not search for header files in the standard directories specific to
1478 C++, but do still search the other standard directories. (This option
1479 is used when building the C++ library.)
1482 In addition, these optimization, warning, and code generation options
1483 have meanings only for C++ programs:
1486 @item -fno-default-inline
1487 @opindex fno-default-inline
1488 Do not assume @samp{inline} for functions defined inside a class scope.
1489 @xref{Optimize Options,,Options That Control Optimization}. Note that these
1490 functions will have linkage like inline functions; they just won't be
1493 @item -Wabi @r{(C++ only)}
1495 Warn when G++ generates code that is probably not compatible with the
1496 vendor-neutral C++ ABI. Although an effort has been made to warn about
1497 all such cases, there are probably some cases that are not warned about,
1498 even though G++ is generating incompatible code. There may also be
1499 cases where warnings are emitted even though the code that is generated
1502 You should rewrite your code to avoid these warnings if you are
1503 concerned about the fact that code generated by G++ may not be binary
1504 compatible with code generated by other compilers.
1506 The known incompatibilities at this point include:
1511 Incorrect handling of tail-padding for bit-fields. G++ may attempt to
1512 pack data into the same byte as a base class. For example:
1515 struct A @{ virtual void f(); int f1 : 1; @};
1516 struct B : public A @{ int f2 : 1; @};
1520 In this case, G++ will place @code{B::f2} into the same byte
1521 as@code{A::f1}; other compilers will not. You can avoid this problem
1522 by explicitly padding @code{A} so that its size is a multiple of the
1523 byte size on your platform; that will cause G++ and other compilers to
1524 layout @code{B} identically.
1527 Incorrect handling of tail-padding for virtual bases. G++ does not use
1528 tail padding when laying out virtual bases. For example:
1531 struct A @{ virtual void f(); char c1; @};
1532 struct B @{ B(); char c2; @};
1533 struct C : public A, public virtual B @{@};
1537 In this case, G++ will not place @code{B} into the tail-padding for
1538 @code{A}; other compilers will. You can avoid this problem by
1539 explicitly padding @code{A} so that its size is a multiple of its
1540 alignment (ignoring virtual base classes); that will cause G++ and other
1541 compilers to layout @code{C} identically.
1544 Incorrect handling of bit-fields with declared widths greater than that
1545 of their underlying types, when the bit-fields appear in a union. For
1549 union U @{ int i : 4096; @};
1553 Assuming that an @code{int} does not have 4096 bits, G++ will make the
1554 union too small by the number of bits in an @code{int}.
1557 Empty classes can be placed at incorrect offsets. For example:
1567 struct C : public B, public A @{@};
1571 G++ will place the @code{A} base class of @code{C} at a nonzero offset;
1572 it should be placed at offset zero. G++ mistakenly believes that the
1573 @code{A} data member of @code{B} is already at offset zero.
1576 Names of template functions whose types involve @code{typename} or
1577 template template parameters can be mangled incorrectly.
1580 template <typename Q>
1581 void f(typename Q::X) @{@}
1583 template <template <typename> class Q>
1584 void f(typename Q<int>::X) @{@}
1588 Instantiations of these templates may be mangled incorrectly.
1592 @item -Wctor-dtor-privacy @r{(C++ only)}
1593 @opindex Wctor-dtor-privacy
1594 Warn when a class seems unusable, because all the constructors or
1595 destructors in a class are private and the class has no friends or
1596 public static member functions.
1598 @item -Wnon-virtual-dtor @r{(C++ only)}
1599 @opindex Wnon-virtual-dtor
1600 Warn when a class declares a non-virtual destructor that should probably
1601 be virtual, because it looks like the class will be used polymorphically.
1602 This warning is enabled by @option{-Wall}.
1604 @item -Wreorder @r{(C++ only)}
1606 @cindex reordering, warning
1607 @cindex warning for reordering of member initializers
1608 Warn when the order of member initializers given in the code does not
1609 match the order in which they must be executed. For instance:
1615 A(): j (0), i (1) @{ @}
1619 Here the compiler will warn that the member initializers for @samp{i}
1620 and @samp{j} will be rearranged to match the declaration order of the
1621 members. This warning is enabled by @option{-Wall}.
1624 The following @option{-W@dots{}} options are not affected by @option{-Wall}.
1627 @item -Weffc++ @r{(C++ only)}
1629 Warn about violations of the following style guidelines from Scott Meyers'
1630 @cite{Effective C++} book:
1634 Item 11: Define a copy constructor and an assignment operator for classes
1635 with dynamically allocated memory.
1638 Item 12: Prefer initialization to assignment in constructors.
1641 Item 14: Make destructors virtual in base classes.
1644 Item 15: Have @code{operator=} return a reference to @code{*this}.
1647 Item 23: Don't try to return a reference when you must return an object.
1651 and about violations of the following style guidelines from Scott Meyers'
1652 @cite{More Effective C++} book:
1656 Item 6: Distinguish between prefix and postfix forms of increment and
1657 decrement operators.
1660 Item 7: Never overload @code{&&}, @code{||}, or @code{,}.
1664 If you use this option, you should be aware that the standard library
1665 headers do not obey all of these guidelines; you can use @samp{grep -v}
1666 to filter out those warnings.
1668 @item -Wno-deprecated @r{(C++ only)}
1669 @opindex Wno-deprecated
1670 Do not warn about usage of deprecated features. @xref{Deprecated Features}.
1672 @item -Wno-non-template-friend @r{(C++ only)}
1673 @opindex Wno-non-template-friend
1674 Disable warnings when non-templatized friend functions are declared
1675 within a template. With the advent of explicit template specification
1676 support in G++, if the name of the friend is an unqualified-id (i.e.,
1677 @samp{friend foo(int)}), the C++ language specification demands that the
1678 friend declare or define an ordinary, nontemplate function. (Section
1679 14.5.3). Before G++ implemented explicit specification, unqualified-ids
1680 could be interpreted as a particular specialization of a templatized
1681 function. Because this non-conforming behavior is no longer the default
1682 behavior for G++, @option{-Wnon-template-friend} allows the compiler to
1683 check existing code for potential trouble spots, and is on by default.
1684 This new compiler behavior can be turned off with
1685 @option{-Wno-non-template-friend} which keeps the conformant compiler code
1686 but disables the helpful warning.
1688 @item -Wold-style-cast @r{(C++ only)}
1689 @opindex Wold-style-cast
1690 Warn if an old-style (C-style) cast to a non-void type is used within
1691 a C++ program. The new-style casts (@samp{static_cast},
1692 @samp{reinterpret_cast}, and @samp{const_cast}) are less vulnerable to
1693 unintended effects, and much easier to grep for.
1695 @item -Woverloaded-virtual @r{(C++ only)}
1696 @opindex Woverloaded-virtual
1697 @cindex overloaded virtual fn, warning
1698 @cindex warning for overloaded virtual fn
1699 Warn when a function declaration hides virtual functions from a
1700 base class. For example, in:
1707 struct B: public A @{
1712 the @code{A} class version of @code{f} is hidden in @code{B}, and code
1720 will fail to compile.
1722 @item -Wno-pmf-conversions @r{(C++ only)}
1723 @opindex Wno-pmf-conversions
1724 Disable the diagnostic for converting a bound pointer to member function
1727 @item -Wsign-promo @r{(C++ only)}
1728 @opindex Wsign-promo
1729 Warn when overload resolution chooses a promotion from unsigned or
1730 enumeral type to a signed type over a conversion to an unsigned type of
1731 the same size. Previous versions of G++ would try to preserve
1732 unsignedness, but the standard mandates the current behavior.
1734 @item -Wsynth @r{(C++ only)}
1736 @cindex warning for synthesized methods
1737 @cindex synthesized methods, warning
1738 Warn when G++'s synthesis behavior does not match that of cfront. For
1744 A& operator = (int);
1754 In this example, G++ will synthesize a default @samp{A& operator =
1755 (const A&);}, while cfront will use the user-defined @samp{operator =}.
1758 @node Objective-C Dialect Options
1759 @section Options Controlling Objective-C Dialect
1761 @cindex compiler options, Objective-C
1762 @cindex Objective-C options, command line
1763 @cindex options, Objective-C
1764 This section describes the command-line options that are only meaningful
1765 for Objective-C programs; but you can also use most of the GNU compiler
1766 options regardless of what language your program is in. For example,
1767 you might compile a file @code{some_class.m} like this:
1770 gcc -g -fgnu-runtime -O -c some_class.m
1774 In this example, only @option{-fgnu-runtime} is an option meant only for
1775 Objective-C programs; you can use the other options with any language
1778 Here is a list of options that are @emph{only} for compiling Objective-C
1782 @item -fconstant-string-class=@var{class-name}
1783 @opindex fconstant-string-class
1784 Use @var{class-name} as the name of the class to instantiate for each
1785 literal string specified with the syntax @code{@@"@dots{}"}. The default
1786 class name is @code{NXConstantString}.
1789 @opindex fgnu-runtime
1790 Generate object code compatible with the standard GNU Objective-C
1791 runtime. This is the default for most types of systems.
1793 @item -fnext-runtime
1794 @opindex fnext-runtime
1795 Generate output compatible with the NeXT runtime. This is the default
1796 for NeXT-based systems, including Darwin and Mac OS X@. The macro
1797 @code{__NEXT_RUNTIME__} is predefined if (and only if) this option is
1802 Dump interface declarations for all classes seen in the source file to a
1803 file named @file{@var{sourcename}.decl}.
1806 @opindex Wno-protocol
1807 If a class is declared to implement a protocol, a warning is issued for
1808 every method in the protocol that is not implemented by the class. The
1809 default behavior is to issue a warning for every method not explicitly
1810 implemented in the class, even if a method implementation is inherited
1811 from the superclass. If you use the @code{-Wno-protocol} option, then
1812 methods inherited from the superclass are considered to be implemented,
1813 and no warning is issued for them.
1817 Warn if multiple methods of different types for the same selector are
1818 found during compilation. The check is performed on the list of methods
1819 in the final stage of compilation. Additionally, a check is performed
1820 that for each selector appearing in a @code{@@selector(@dots{})}
1821 expression, a corresponding method with that selector has been found
1822 during compilation. Because these checks scan the method table only at
1823 the end of compilation, these warnings are not produced if the final
1824 stage of compilation is not reached, for example because an error is
1825 found during compilation, or because the @code{-fsyntax-only} option is
1828 @item -Wundeclared-selector
1829 @opindex Wundeclared-selector
1830 Warn if a @code{@@selector(@dots{})} expression referring to an
1831 undeclared selector is found. A selector is considered undeclared if no
1832 method with that name has been declared (explicitly, in an
1833 @code{@@interface} or @code{@@protocol} declaration, or implicitly, in
1834 an @code{@@implementation} section) before the
1835 @code{@@selector(@dots{})} expression. This option always performs its
1836 checks as soon as a @code{@@selector(@dots{})} expression is found
1837 (while @code{-Wselector} only performs its checks in the final stage of
1838 compilation), and so additionally enforces the coding style convention
1839 that methods and selectors must be declared before being used.
1841 @c not documented because only avail via -Wp
1842 @c @item -print-objc-runtime-info
1846 @node Language Independent Options
1847 @section Options to Control Diagnostic Messages Formatting
1848 @cindex options to control diagnostics formatting
1849 @cindex diagnostic messages
1850 @cindex message formatting
1852 Traditionally, diagnostic messages have been formatted irrespective of
1853 the output device's aspect (e.g.@: its width, @dots{}). The options described
1854 below can be used to control the diagnostic messages formatting
1855 algorithm, e.g.@: how many characters per line, how often source location
1856 information should be reported. Right now, only the C++ front end can
1857 honor these options. However it is expected, in the near future, that
1858 the remaining front ends would be able to digest them correctly.
1861 @item -fmessage-length=@var{n}
1862 @opindex fmessage-length
1863 Try to format error messages so that they fit on lines of about @var{n}
1864 characters. The default is 72 characters for @command{g++} and 0 for the rest of
1865 the front ends supported by GCC@. If @var{n} is zero, then no
1866 line-wrapping will be done; each error message will appear on a single
1869 @opindex fdiagnostics-show-location
1870 @item -fdiagnostics-show-location=once
1871 Only meaningful in line-wrapping mode. Instructs the diagnostic messages
1872 reporter to emit @emph{once} source location information; that is, in
1873 case the message is too long to fit on a single physical line and has to
1874 be wrapped, the source location won't be emitted (as prefix) again,
1875 over and over, in subsequent continuation lines. This is the default
1878 @item -fdiagnostics-show-location=every-line
1879 Only meaningful in line-wrapping mode. Instructs the diagnostic
1880 messages reporter to emit the same source location information (as
1881 prefix) for physical lines that result from the process of breaking
1882 a message which is too long to fit on a single line.
1886 @node Warning Options
1887 @section Options to Request or Suppress Warnings
1888 @cindex options to control warnings
1889 @cindex warning messages
1890 @cindex messages, warning
1891 @cindex suppressing warnings
1893 Warnings are diagnostic messages that report constructions which
1894 are not inherently erroneous but which are risky or suggest there
1895 may have been an error.
1897 You can request many specific warnings with options beginning @samp{-W},
1898 for example @option{-Wimplicit} to request warnings on implicit
1899 declarations. Each of these specific warning options also has a
1900 negative form beginning @samp{-Wno-} to turn off warnings;
1901 for example, @option{-Wno-implicit}. This manual lists only one of the
1902 two forms, whichever is not the default.
1904 The following options control the amount and kinds of warnings produced
1905 by GCC; for further, language-specific options also refer to
1906 @ref{C++ Dialect Options} and @ref{Objective-C Dialect Options}.
1909 @cindex syntax checking
1911 @opindex fsyntax-only
1912 Check the code for syntax errors, but don't do anything beyond that.
1916 Issue all the warnings demanded by strict ISO C and ISO C++;
1917 reject all programs that use forbidden extensions, and some other
1918 programs that do not follow ISO C and ISO C++. For ISO C, follows the
1919 version of the ISO C standard specified by any @option{-std} option used.
1921 Valid ISO C and ISO C++ programs should compile properly with or without
1922 this option (though a rare few will require @option{-ansi} or a
1923 @option{-std} option specifying the required version of ISO C)@. However,
1924 without this option, certain GNU extensions and traditional C and C++
1925 features are supported as well. With this option, they are rejected.
1927 @option{-pedantic} does not cause warning messages for use of the
1928 alternate keywords whose names begin and end with @samp{__}. Pedantic
1929 warnings are also disabled in the expression that follows
1930 @code{__extension__}. However, only system header files should use
1931 these escape routes; application programs should avoid them.
1932 @xref{Alternate Keywords}.
1934 Some users try to use @option{-pedantic} to check programs for strict ISO
1935 C conformance. They soon find that it does not do quite what they want:
1936 it finds some non-ISO practices, but not all---only those for which
1937 ISO C @emph{requires} a diagnostic, and some others for which
1938 diagnostics have been added.
1940 A feature to report any failure to conform to ISO C might be useful in
1941 some instances, but would require considerable additional work and would
1942 be quite different from @option{-pedantic}. We don't have plans to
1943 support such a feature in the near future.
1945 Where the standard specified with @option{-std} represents a GNU
1946 extended dialect of C, such as @samp{gnu89} or @samp{gnu99}, there is a
1947 corresponding @dfn{base standard}, the version of ISO C on which the GNU
1948 extended dialect is based. Warnings from @option{-pedantic} are given
1949 where they are required by the base standard. (It would not make sense
1950 for such warnings to be given only for features not in the specified GNU
1951 C dialect, since by definition the GNU dialects of C include all
1952 features the compiler supports with the given option, and there would be
1953 nothing to warn about.)
1955 @item -pedantic-errors
1956 @opindex pedantic-errors
1957 Like @option{-pedantic}, except that errors are produced rather than
1962 Inhibit all warning messages.
1966 Inhibit warning messages about the use of @samp{#import}.
1968 @item -Wchar-subscripts
1969 @opindex Wchar-subscripts
1970 Warn if an array subscript has type @code{char}. This is a common cause
1971 of error, as programmers often forget that this type is signed on some
1976 Warn whenever a comment-start sequence @samp{/*} appears in a @samp{/*}
1977 comment, or whenever a Backslash-Newline appears in a @samp{//} comment.
1981 Check calls to @code{printf} and @code{scanf}, etc., to make sure that
1982 the arguments supplied have types appropriate to the format string
1983 specified, and that the conversions specified in the format string make
1984 sense. This includes standard functions, and others specified by format
1985 attributes (@pxref{Function Attributes}), in the @code{printf},
1986 @code{scanf}, @code{strftime} and @code{strfmon} (an X/Open extension,
1987 not in the C standard) families.
1989 The formats are checked against the format features supported by GNU
1990 libc version 2.2. These include all ISO C90 and C99 features, as well
1991 as features from the Single Unix Specification and some BSD and GNU
1992 extensions. Other library implementations may not support all these
1993 features; GCC does not support warning about features that go beyond a
1994 particular library's limitations. However, if @option{-pedantic} is used
1995 with @option{-Wformat}, warnings will be given about format features not
1996 in the selected standard version (but not for @code{strfmon} formats,
1997 since those are not in any version of the C standard). @xref{C Dialect
1998 Options,,Options Controlling C Dialect}.
2000 Since @option{-Wformat} also checks for null format arguments for
2001 several functions, @option{-Wformat} also implies @option{-Wnonnull}.
2003 @option{-Wformat} is included in @option{-Wall}. For more control over some
2004 aspects of format checking, the options @option{-Wno-format-y2k},
2005 @option{-Wno-format-extra-args}, @option{-Wno-format-zero-length},
2006 @option{-Wformat-nonliteral}, @option{-Wformat-security}, and
2007 @option{-Wformat=2} are available, but are not included in @option{-Wall}.
2009 @item -Wno-format-y2k
2010 @opindex Wno-format-y2k
2011 If @option{-Wformat} is specified, do not warn about @code{strftime}
2012 formats which may yield only a two-digit year.
2014 @item -Wno-format-extra-args
2015 @opindex Wno-format-extra-args
2016 If @option{-Wformat} is specified, do not warn about excess arguments to a
2017 @code{printf} or @code{scanf} format function. The C standard specifies
2018 that such arguments are ignored.
2020 Where the unused arguments lie between used arguments that are
2021 specified with @samp{$} operand number specifications, normally
2022 warnings are still given, since the implementation could not know what
2023 type to pass to @code{va_arg} to skip the unused arguments. However,
2024 in the case of @code{scanf} formats, this option will suppress the
2025 warning if the unused arguments are all pointers, since the Single
2026 Unix Specification says that such unused arguments are allowed.
2028 @item -Wno-format-zero-length
2029 @opindex Wno-format-zero-length
2030 If @option{-Wformat} is specified, do not warn about zero-length formats.
2031 The C standard specifies that zero-length formats are allowed.
2033 @item -Wformat-nonliteral
2034 @opindex Wformat-nonliteral
2035 If @option{-Wformat} is specified, also warn if the format string is not a
2036 string literal and so cannot be checked, unless the format function
2037 takes its format arguments as a @code{va_list}.
2039 @item -Wformat-security
2040 @opindex Wformat-security
2041 If @option{-Wformat} is specified, also warn about uses of format
2042 functions that represent possible security problems. At present, this
2043 warns about calls to @code{printf} and @code{scanf} functions where the
2044 format string is not a string literal and there are no format arguments,
2045 as in @code{printf (foo);}. This may be a security hole if the format
2046 string came from untrusted input and contains @samp{%n}. (This is
2047 currently a subset of what @option{-Wformat-nonliteral} warns about, but
2048 in future warnings may be added to @option{-Wformat-security} that are not
2049 included in @option{-Wformat-nonliteral}.)
2053 Enable @option{-Wformat} plus format checks not included in
2054 @option{-Wformat}. Currently equivalent to @samp{-Wformat
2055 -Wformat-nonliteral -Wformat-security}.
2059 Enable warning about passing a null pointer for arguments marked as
2060 requiring a non-null value by the @code{nonnull} function attribute.
2062 @option{-Wnonnull} is included in @option{-Wall} and @option{-Wformat}. It
2063 can be disabled with the @option{-Wno-nonnull} option.
2065 @item -Wimplicit-int
2066 @opindex Wimplicit-int
2067 Warn when a declaration does not specify a type.
2069 @item -Wimplicit-function-declaration
2070 @itemx -Werror-implicit-function-declaration
2071 @opindex Wimplicit-function-declaration
2072 @opindex Werror-implicit-function-declaration
2073 Give a warning (or error) whenever a function is used before being
2078 Same as @option{-Wimplicit-int} and @option{-Wimplicit-function-declaration}.
2082 Warn if the type of @samp{main} is suspicious. @samp{main} should be a
2083 function with external linkage, returning int, taking either zero
2084 arguments, two, or three arguments of appropriate types.
2086 @item -Wmissing-braces
2087 @opindex Wmissing-braces
2088 Warn if an aggregate or union initializer is not fully bracketed. In
2089 the following example, the initializer for @samp{a} is not fully
2090 bracketed, but that for @samp{b} is fully bracketed.
2093 int a[2][2] = @{ 0, 1, 2, 3 @};
2094 int b[2][2] = @{ @{ 0, 1 @}, @{ 2, 3 @} @};
2098 @opindex Wparentheses
2099 Warn if parentheses are omitted in certain contexts, such
2100 as when there is an assignment in a context where a truth value
2101 is expected, or when operators are nested whose precedence people
2102 often get confused about.
2104 Also warn about constructions where there may be confusion to which
2105 @code{if} statement an @code{else} branch belongs. Here is an example of
2120 In C, every @code{else} branch belongs to the innermost possible @code{if}
2121 statement, which in this example is @code{if (b)}. This is often not
2122 what the programmer expected, as illustrated in the above example by
2123 indentation the programmer chose. When there is the potential for this
2124 confusion, GCC will issue a warning when this flag is specified.
2125 To eliminate the warning, add explicit braces around the innermost
2126 @code{if} statement so there is no way the @code{else} could belong to
2127 the enclosing @code{if}. The resulting code would look like this:
2143 @item -Wsequence-point
2144 @opindex Wsequence-point
2145 Warn about code that may have undefined semantics because of violations
2146 of sequence point rules in the C standard.
2148 The C standard defines the order in which expressions in a C program are
2149 evaluated in terms of @dfn{sequence points}, which represent a partial
2150 ordering between the execution of parts of the program: those executed
2151 before the sequence point, and those executed after it. These occur
2152 after the evaluation of a full expression (one which is not part of a
2153 larger expression), after the evaluation of the first operand of a
2154 @code{&&}, @code{||}, @code{? :} or @code{,} (comma) operator, before a
2155 function is called (but after the evaluation of its arguments and the
2156 expression denoting the called function), and in certain other places.
2157 Other than as expressed by the sequence point rules, the order of
2158 evaluation of subexpressions of an expression is not specified. All
2159 these rules describe only a partial order rather than a total order,
2160 since, for example, if two functions are called within one expression
2161 with no sequence point between them, the order in which the functions
2162 are called is not specified. However, the standards committee have
2163 ruled that function calls do not overlap.
2165 It is not specified when between sequence points modifications to the
2166 values of objects take effect. Programs whose behavior depends on this
2167 have undefined behavior; the C standard specifies that ``Between the
2168 previous and next sequence point an object shall have its stored value
2169 modified at most once by the evaluation of an expression. Furthermore,
2170 the prior value shall be read only to determine the value to be
2171 stored.''. If a program breaks these rules, the results on any
2172 particular implementation are entirely unpredictable.
2174 Examples of code with undefined behavior are @code{a = a++;}, @code{a[n]
2175 = b[n++]} and @code{a[i++] = i;}. Some more complicated cases are not
2176 diagnosed by this option, and it may give an occasional false positive
2177 result, but in general it has been found fairly effective at detecting
2178 this sort of problem in programs.
2180 The present implementation of this option only works for C programs. A
2181 future implementation may also work for C++ programs.
2183 The C standard is worded confusingly, therefore there is some debate
2184 over the precise meaning of the sequence point rules in subtle cases.
2185 Links to discussions of the problem, including proposed formal
2186 definitions, may be found on our readings page, at
2187 @w{@uref{http://gcc.gnu.org/readings.html}}.
2190 @opindex Wreturn-type
2191 Warn whenever a function is defined with a return-type that defaults to
2192 @code{int}. Also warn about any @code{return} statement with no
2193 return-value in a function whose return-type is not @code{void}.
2195 For C++, a function without return type always produces a diagnostic
2196 message, even when @option{-Wno-return-type} is specified. The only
2197 exceptions are @samp{main} and functions defined in system headers.
2201 Warn whenever a @code{switch} statement has an index of enumeral type
2202 and lacks a @code{case} for one or more of the named codes of that
2203 enumeration. (The presence of a @code{default} label prevents this
2204 warning.) @code{case} labels outside the enumeration range also
2205 provoke warnings when this option is used.
2207 @item -Wswitch-default
2208 @opindex Wswitch-switch
2209 Warn whenever a @code{switch} statement does not have a @code{default}
2213 @opindex Wswitch-enum
2214 Warn whenever a @code{switch} statement has an index of enumeral type
2215 and lacks a @code{case} for one or more of the named codes of that
2216 enumeration. @code{case} labels outside the enumeration range also
2217 provoke warnings when this option is used.
2221 Warn if any trigraphs are encountered that might change the meaning of
2222 the program (trigraphs within comments are not warned about).
2224 @item -Wunused-function
2225 @opindex Wunused-function
2226 Warn whenever a static function is declared but not defined or a
2227 non\-inline static function is unused.
2229 @item -Wunused-label
2230 @opindex Wunused-label
2231 Warn whenever a label is declared but not used.
2233 To suppress this warning use the @samp{unused} attribute
2234 (@pxref{Variable Attributes}).
2236 @item -Wunused-parameter
2237 @opindex Wunused-parameter
2238 Warn whenever a function parameter is unused aside from its declaration.
2240 To suppress this warning use the @samp{unused} attribute
2241 (@pxref{Variable Attributes}).
2243 @item -Wunused-variable
2244 @opindex Wunused-variable
2245 Warn whenever a local variable or non-constant static variable is unused
2246 aside from its declaration
2248 To suppress this warning use the @samp{unused} attribute
2249 (@pxref{Variable Attributes}).
2251 @item -Wunused-value
2252 @opindex Wunused-value
2253 Warn whenever a statement computes a result that is explicitly not used.
2255 To suppress this warning cast the expression to @samp{void}.
2259 All the above @option{-Wunused} options combined.
2261 In order to get a warning about an unused function parameter, you must
2262 either specify @samp{-Wextra -Wunused} (note that @samp{-Wall} implies
2263 @samp{-Wunused}), or separately specify @option{-Wunused-parameter}.
2265 @item -Wuninitialized
2266 @opindex Wuninitialized
2267 Warn if an automatic variable is used without first being initialized or
2268 if a variable may be clobbered by a @code{setjmp} call.
2270 These warnings are possible only in optimizing compilation,
2271 because they require data flow information that is computed only
2272 when optimizing. If you don't specify @option{-O}, you simply won't
2275 These warnings occur only for variables that are candidates for
2276 register allocation. Therefore, they do not occur for a variable that
2277 is declared @code{volatile}, or whose address is taken, or whose size
2278 is other than 1, 2, 4 or 8 bytes. Also, they do not occur for
2279 structures, unions or arrays, even when they are in registers.
2281 Note that there may be no warning about a variable that is used only
2282 to compute a value that itself is never used, because such
2283 computations may be deleted by data flow analysis before the warnings
2286 These warnings are made optional because GCC is not smart
2287 enough to see all the reasons why the code might be correct
2288 despite appearing to have an error. Here is one example of how
2309 If the value of @code{y} is always 1, 2 or 3, then @code{x} is
2310 always initialized, but GCC doesn't know this. Here is
2311 another common case:
2316 if (change_y) save_y = y, y = new_y;
2318 if (change_y) y = save_y;
2323 This has no bug because @code{save_y} is used only if it is set.
2325 @cindex @code{longjmp} warnings
2326 This option also warns when a non-volatile automatic variable might be
2327 changed by a call to @code{longjmp}. These warnings as well are possible
2328 only in optimizing compilation.
2330 The compiler sees only the calls to @code{setjmp}. It cannot know
2331 where @code{longjmp} will be called; in fact, a signal handler could
2332 call it at any point in the code. As a result, you may get a warning
2333 even when there is in fact no problem because @code{longjmp} cannot
2334 in fact be called at the place which would cause a problem.
2336 Some spurious warnings can be avoided if you declare all the functions
2337 you use that never return as @code{noreturn}. @xref{Function
2340 @item -Wunknown-pragmas
2341 @opindex Wunknown-pragmas
2342 @cindex warning for unknown pragmas
2343 @cindex unknown pragmas, warning
2344 @cindex pragmas, warning of unknown
2345 Warn when a #pragma directive is encountered which is not understood by
2346 GCC@. If this command line option is used, warnings will even be issued
2347 for unknown pragmas in system header files. This is not the case if
2348 the warnings were only enabled by the @option{-Wall} command line option.
2350 @item -Wstrict-aliasing
2351 @opindex Wstrict-aliasing
2352 This option is only active when @option{-fstrict-aliasing} is active.
2353 It warns about code which might break the strict aliasing rules that the
2354 compiler is using for optimization. The warning does not catch all
2355 cases, but does attempt to catch the more common pitfalls. It is
2356 included in @option{-Wall}.
2360 All of the above @samp{-W} options combined. This enables all the
2361 warnings about constructions that some users consider questionable, and
2362 that are easy to avoid (or modify to prevent the warning), even in
2363 conjunction with macros. This also enables some language-specific
2364 warnings described in @ref{C++ Dialect Options} and
2365 @ref{Objective-C Dialect Options}.
2368 The following @option{-W@dots{}} options are not implied by @option{-Wall}.
2369 Some of them warn about constructions that users generally do not
2370 consider questionable, but which occasionally you might wish to check
2371 for; others warn about constructions that are necessary or hard to avoid
2372 in some cases, and there is no simple way to modify the code to suppress
2379 (This option used to be called @option{-W}. The older name is still
2380 supported, but the newer name is more descriptive.) Print extra warning
2381 messages for these events:
2385 A function can return either with or without a value. (Falling
2386 off the end of the function body is considered returning without
2387 a value.) For example, this function would evoke such a
2401 An expression-statement or the left-hand side of a comma expression
2402 contains no side effects.
2403 To suppress the warning, cast the unused expression to void.
2404 For example, an expression such as @samp{x[i,j]} will cause a warning,
2405 but @samp{x[(void)i,j]} will not.
2408 An unsigned value is compared against zero with @samp{<} or @samp{>=}.
2411 A comparison like @samp{x<=y<=z} appears; this is equivalent to
2412 @samp{(x<=y ? 1 : 0) <= z}, which is a different interpretation from
2413 that of ordinary mathematical notation.
2416 Storage-class specifiers like @code{static} are not the first things in
2417 a declaration. According to the C Standard, this usage is obsolescent.
2420 The return type of a function has a type qualifier such as @code{const}.
2421 Such a type qualifier has no effect, since the value returned by a
2422 function is not an lvalue. (But don't warn about the GNU extension of
2423 @code{volatile void} return types. That extension will be warned about
2424 if @option{-pedantic} is specified.)
2427 If @option{-Wall} or @option{-Wunused} is also specified, warn about unused
2431 A comparison between signed and unsigned values could produce an
2432 incorrect result when the signed value is converted to unsigned.
2433 (But don't warn if @option{-Wno-sign-compare} is also specified.)
2436 An aggregate has an initializer which does not initialize all members.
2437 For example, the following code would cause such a warning, because
2438 @code{x.h} would be implicitly initialized to zero:
2441 struct s @{ int f, g, h; @};
2442 struct s x = @{ 3, 4 @};
2446 A function parameter is declared without a type specifier in K&R-style
2454 An empty body occurs in an @samp{if} or @samp{else} statement.
2457 A pointer is compared against integer zero with @samp{<}, @samp{<=},
2458 @samp{>}, or @samp{>=}.
2461 A variable might be changed by @samp{longjmp} or @samp{vfork}.
2464 Any of several floating-point events that often indicate errors, such as
2465 overflow, underflow, loss of precision, etc.
2467 @item @r{(C++ only)}
2468 An enumerator and a non-enumerator both appear in a conditional expression.
2470 @item @r{(C++ only)}
2471 A non-static reference or non-static @samp{const} member appears in a
2472 class without constructors.
2474 @item @r{(C++ only)}
2475 Ambiguous virtual bases.
2477 @item @r{(C++ only)}
2478 Subscripting an array which has been declared @samp{register}.
2480 @item @r{(C++ only)}
2481 Taking the address of a variable which has been declared @samp{register}.
2483 @item @r{(C++ only)}
2484 A base class is not initialized in a derived class' copy constructor.
2487 @item -Wno-div-by-zero
2488 @opindex Wno-div-by-zero
2489 @opindex Wdiv-by-zero
2490 Do not warn about compile-time integer division by zero. Floating point
2491 division by zero is not warned about, as it can be a legitimate way of
2492 obtaining infinities and NaNs.
2494 @item -Wsystem-headers
2495 @opindex Wsystem-headers
2496 @cindex warnings from system headers
2497 @cindex system headers, warnings from
2498 Print warning messages for constructs found in system header files.
2499 Warnings from system headers are normally suppressed, on the assumption
2500 that they usually do not indicate real problems and would only make the
2501 compiler output harder to read. Using this command line option tells
2502 GCC to emit warnings from system headers as if they occurred in user
2503 code. However, note that using @option{-Wall} in conjunction with this
2504 option will @emph{not} warn about unknown pragmas in system
2505 headers---for that, @option{-Wunknown-pragmas} must also be used.
2508 @opindex Wfloat-equal
2509 Warn if floating point values are used in equality comparisons.
2511 The idea behind this is that sometimes it is convenient (for the
2512 programmer) to consider floating-point values as approximations to
2513 infinitely precise real numbers. If you are doing this, then you need
2514 to compute (by analyzing the code, or in some other way) the maximum or
2515 likely maximum error that the computation introduces, and allow for it
2516 when performing comparisons (and when producing output, but that's a
2517 different problem). In particular, instead of testing for equality, you
2518 would check to see whether the two values have ranges that overlap; and
2519 this is done with the relational operators, so equality comparisons are
2522 @item -Wtraditional @r{(C only)}
2523 @opindex Wtraditional
2524 Warn about certain constructs that behave differently in traditional and
2525 ISO C@. Also warn about ISO C constructs that have no traditional C
2526 equivalent, and/or problematic constructs which should be avoided.
2530 Macro parameters that appear within string literals in the macro body.
2531 In traditional C macro replacement takes place within string literals,
2532 but does not in ISO C@.
2535 In traditional C, some preprocessor directives did not exist.
2536 Traditional preprocessors would only consider a line to be a directive
2537 if the @samp{#} appeared in column 1 on the line. Therefore
2538 @option{-Wtraditional} warns about directives that traditional C
2539 understands but would ignore because the @samp{#} does not appear as the
2540 first character on the line. It also suggests you hide directives like
2541 @samp{#pragma} not understood by traditional C by indenting them. Some
2542 traditional implementations would not recognize @samp{#elif}, so it
2543 suggests avoiding it altogether.
2546 A function-like macro that appears without arguments.
2549 The unary plus operator.
2552 The @samp{U} integer constant suffix, or the @samp{F} or @samp{L} floating point
2553 constant suffixes. (Traditional C does support the @samp{L} suffix on integer
2554 constants.) Note, these suffixes appear in macros defined in the system
2555 headers of most modern systems, e.g.@: the @samp{_MIN}/@samp{_MAX} macros in @code{<limits.h>}.
2556 Use of these macros in user code might normally lead to spurious
2557 warnings, however gcc's integrated preprocessor has enough context to
2558 avoid warning in these cases.
2561 A function declared external in one block and then used after the end of
2565 A @code{switch} statement has an operand of type @code{long}.
2568 A non-@code{static} function declaration follows a @code{static} one.
2569 This construct is not accepted by some traditional C compilers.
2572 The ISO type of an integer constant has a different width or
2573 signedness from its traditional type. This warning is only issued if
2574 the base of the constant is ten. I.e.@: hexadecimal or octal values, which
2575 typically represent bit patterns, are not warned about.
2578 Usage of ISO string concatenation is detected.
2581 Initialization of automatic aggregates.
2584 Identifier conflicts with labels. Traditional C lacks a separate
2585 namespace for labels.
2588 Initialization of unions. If the initializer is zero, the warning is
2589 omitted. This is done under the assumption that the zero initializer in
2590 user code appears conditioned on e.g.@: @code{__STDC__} to avoid missing
2591 initializer warnings and relies on default initialization to zero in the
2595 Conversions by prototypes between fixed/floating point values and vice
2596 versa. The absence of these prototypes when compiling with traditional
2597 C would cause serious problems. This is a subset of the possible
2598 conversion warnings, for the full set use @option{-Wconversion}.
2601 Use of ISO C style function definitions. This warning intentionally is
2602 @emph{not} issued for prototype declarations or variadic functions
2603 because these ISO C features will appear in your code when using
2604 libiberty's traditional C compatibility macros, @code{PARAMS} and
2605 @code{VPARAMS}. This warning is also bypassed for nested functions
2606 because that feature is already a gcc extension and thus not relevant to
2607 traditional C compatibility.
2612 Warn if an undefined identifier is evaluated in an @samp{#if} directive.
2614 @item -Wendif-labels
2615 @opindex Wendif-labels
2616 Warn whenever an @samp{#else} or an @samp{#endif} are followed by text.
2620 Warn whenever a local variable shadows another local variable, parameter or
2621 global variable or whenever a built-in function is shadowed.
2623 @item -Wlarger-than-@var{len}
2624 @opindex Wlarger-than
2625 Warn whenever an object of larger than @var{len} bytes is defined.
2627 @item -Wpointer-arith
2628 @opindex Wpointer-arith
2629 Warn about anything that depends on the ``size of'' a function type or
2630 of @code{void}. GNU C assigns these types a size of 1, for
2631 convenience in calculations with @code{void *} pointers and pointers
2634 @item -Wbad-function-cast @r{(C only)}
2635 @opindex Wbad-function-cast
2636 Warn whenever a function call is cast to a non-matching type.
2637 For example, warn if @code{int malloc()} is cast to @code{anything *}.
2641 Warn whenever a pointer is cast so as to remove a type qualifier from
2642 the target type. For example, warn if a @code{const char *} is cast
2643 to an ordinary @code{char *}.
2646 @opindex Wcast-align
2647 Warn whenever a pointer is cast such that the required alignment of the
2648 target is increased. For example, warn if a @code{char *} is cast to
2649 an @code{int *} on machines where integers can only be accessed at
2650 two- or four-byte boundaries.
2652 @item -Wwrite-strings
2653 @opindex Wwrite-strings
2654 When compiling C, give string constants the type @code{const
2655 char[@var{length}]} so that
2656 copying the address of one into a non-@code{const} @code{char *}
2657 pointer will get a warning; when compiling C++, warn about the
2658 deprecated conversion from string constants to @code{char *}.
2659 These warnings will help you find at
2660 compile time code that can try to write into a string constant, but
2661 only if you have been very careful about using @code{const} in
2662 declarations and prototypes. Otherwise, it will just be a nuisance;
2663 this is why we did not make @option{-Wall} request these warnings.
2666 @opindex Wconversion
2667 Warn if a prototype causes a type conversion that is different from what
2668 would happen to the same argument in the absence of a prototype. This
2669 includes conversions of fixed point to floating and vice versa, and
2670 conversions changing the width or signedness of a fixed point argument
2671 except when the same as the default promotion.
2673 Also, warn if a negative integer constant expression is implicitly
2674 converted to an unsigned type. For example, warn about the assignment
2675 @code{x = -1} if @code{x} is unsigned. But do not warn about explicit
2676 casts like @code{(unsigned) -1}.
2678 @item -Wsign-compare
2679 @opindex Wsign-compare
2680 @cindex warning for comparison of signed and unsigned values
2681 @cindex comparison of signed and unsigned values, warning
2682 @cindex signed and unsigned values, comparison warning
2683 Warn when a comparison between signed and unsigned values could produce
2684 an incorrect result when the signed value is converted to unsigned.
2685 This warning is also enabled by @option{-Wextra}; to get the other warnings
2686 of @option{-Wextra} without this warning, use @samp{-Wextra -Wno-sign-compare}.
2688 @item -Waggregate-return
2689 @opindex Waggregate-return
2690 Warn if any functions that return structures or unions are defined or
2691 called. (In languages where you can return an array, this also elicits
2694 @item -Wstrict-prototypes @r{(C only)}
2695 @opindex Wstrict-prototypes
2696 Warn if a function is declared or defined without specifying the
2697 argument types. (An old-style function definition is permitted without
2698 a warning if preceded by a declaration which specifies the argument
2701 @item -Wmissing-prototypes @r{(C only)}
2702 @opindex Wmissing-prototypes
2703 Warn if a global function is defined without a previous prototype
2704 declaration. This warning is issued even if the definition itself
2705 provides a prototype. The aim is to detect global functions that fail
2706 to be declared in header files.
2708 @item -Wmissing-declarations @r{(C only)}
2709 @opindex Wmissing-declarations
2710 Warn if a global function is defined without a previous declaration.
2711 Do so even if the definition itself provides a prototype.
2712 Use this option to detect global functions that are not declared in
2715 @item -Wmissing-noreturn
2716 @opindex Wmissing-noreturn
2717 Warn about functions which might be candidates for attribute @code{noreturn}.
2718 Note these are only possible candidates, not absolute ones. Care should
2719 be taken to manually verify functions actually do not ever return before
2720 adding the @code{noreturn} attribute, otherwise subtle code generation
2721 bugs could be introduced. You will not get a warning for @code{main} in
2722 hosted C environments.
2724 @item -Wmissing-format-attribute
2725 @opindex Wmissing-format-attribute
2727 If @option{-Wformat} is enabled, also warn about functions which might be
2728 candidates for @code{format} attributes. Note these are only possible
2729 candidates, not absolute ones. GCC will guess that @code{format}
2730 attributes might be appropriate for any function that calls a function
2731 like @code{vprintf} or @code{vscanf}, but this might not always be the
2732 case, and some functions for which @code{format} attributes are
2733 appropriate may not be detected. This option has no effect unless
2734 @option{-Wformat} is enabled (possibly by @option{-Wall}).
2736 @item -Wno-multichar
2737 @opindex Wno-multichar
2739 Do not warn if a multicharacter constant (@samp{'FOOF'}) is used.
2740 Usually they indicate a typo in the user's code, as they have
2741 implementation-defined values, and should not be used in portable code.
2743 @item -Wno-deprecated-declarations
2744 @opindex Wno-deprecated-declarations
2745 Do not warn about uses of functions, variables, and types marked as
2746 deprecated by using the @code{deprecated} attribute.
2747 (@pxref{Function Attributes}, @pxref{Variable Attributes},
2748 @pxref{Type Attributes}.)
2752 Warn if a structure is given the packed attribute, but the packed
2753 attribute has no effect on the layout or size of the structure.
2754 Such structures may be mis-aligned for little benefit. For
2755 instance, in this code, the variable @code{f.x} in @code{struct bar}
2756 will be misaligned even though @code{struct bar} does not itself
2757 have the packed attribute:
2764 @} __attribute__((packed));
2774 Warn if padding is included in a structure, either to align an element
2775 of the structure or to align the whole structure. Sometimes when this
2776 happens it is possible to rearrange the fields of the structure to
2777 reduce the padding and so make the structure smaller.
2779 @item -Wredundant-decls
2780 @opindex Wredundant-decls
2781 Warn if anything is declared more than once in the same scope, even in
2782 cases where multiple declaration is valid and changes nothing.
2784 @item -Wnested-externs @r{(C only)}
2785 @opindex Wnested-externs
2786 Warn if an @code{extern} declaration is encountered within a function.
2788 @item -Wunreachable-code
2789 @opindex Wunreachable-code
2790 Warn if the compiler detects that code will never be executed.
2792 This option is intended to warn when the compiler detects that at
2793 least a whole line of source code will never be executed, because
2794 some condition is never satisfied or because it is after a
2795 procedure that never returns.
2797 It is possible for this option to produce a warning even though there
2798 are circumstances under which part of the affected line can be executed,
2799 so care should be taken when removing apparently-unreachable code.
2801 For instance, when a function is inlined, a warning may mean that the
2802 line is unreachable in only one inlined copy of the function.
2804 This option is not made part of @option{-Wall} because in a debugging
2805 version of a program there is often substantial code which checks
2806 correct functioning of the program and is, hopefully, unreachable
2807 because the program does work. Another common use of unreachable
2808 code is to provide behavior which is selectable at compile-time.
2812 Warn if a function can not be inlined and it was declared as inline.
2814 @item -Wno-invalid-offsetof @r{(C++ only)}
2815 @opindex Wno-invalid-offsetof
2816 Suppress warnings from applying the @samp{offsetof} macro to a non-POD
2817 type. According to the 1998 ISO C++ standard, applying @samp{offsetof}
2818 to a non-POD type is undefined. In existing C++ implementations,
2819 however, @samp{offsetof} typically gives meaningful results even when
2820 applied to certain kinds of non-POD types. (Such as a simple
2821 @samp{struct} that fails to be a POD type only by virtue of having a
2822 constructor.) This flag is for users who are aware that they are
2823 writing nonportable code and who have deliberately chosen to ignore the
2826 The restrictions on @samp{offsetof} may be relaxed in a future version
2827 of the C++ standard.
2830 @opindex Winvalid-pch
2831 Warn if a precompiled header (@pxref{Precompiled Headers}) is found in
2832 the search path but can't be used.
2836 @opindex Wno-long-long
2837 Warn if @samp{long long} type is used. This is default. To inhibit
2838 the warning messages, use @option{-Wno-long-long}. Flags
2839 @option{-Wlong-long} and @option{-Wno-long-long} are taken into account
2840 only when @option{-pedantic} flag is used.
2842 @item -Wdisabled-optimization
2843 @opindex Wdisabled-optimization
2844 Warn if a requested optimization pass is disabled. This warning does
2845 not generally indicate that there is anything wrong with your code; it
2846 merely indicates that GCC's optimizers were unable to handle the code
2847 effectively. Often, the problem is that your code is too big or too
2848 complex; GCC will refuse to optimize programs when the optimization
2849 itself is likely to take inordinate amounts of time.
2853 Make all warnings into errors.
2856 @node Debugging Options
2857 @section Options for Debugging Your Program or GCC
2858 @cindex options, debugging
2859 @cindex debugging information options
2861 GCC has various special options that are used for debugging
2862 either your program or GCC:
2867 Produce debugging information in the operating system's native format
2868 (stabs, COFF, XCOFF, or DWARF)@. GDB can work with this debugging
2871 On most systems that use stabs format, @option{-g} enables use of extra
2872 debugging information that only GDB can use; this extra information
2873 makes debugging work better in GDB but will probably make other debuggers
2875 refuse to read the program. If you want to control for certain whether
2876 to generate the extra information, use @option{-gstabs+}, @option{-gstabs},
2877 @option{-gxcoff+}, @option{-gxcoff}, @option{-gdwarf-1+}, @option{-gdwarf-1},
2878 or @option{-gvms} (see below).
2880 Unlike most other C compilers, GCC allows you to use @option{-g} with
2881 @option{-O}. The shortcuts taken by optimized code may occasionally
2882 produce surprising results: some variables you declared may not exist
2883 at all; flow of control may briefly move where you did not expect it;
2884 some statements may not be executed because they compute constant
2885 results or their values were already at hand; some statements may
2886 execute in different places because they were moved out of loops.
2888 Nevertheless it proves possible to debug optimized output. This makes
2889 it reasonable to use the optimizer for programs that might have bugs.
2891 The following options are useful when GCC is generated with the
2892 capability for more than one debugging format.
2896 Produce debugging information for use by GDB@. This means to use the
2897 most expressive format available (DWARF 2, stabs, or the native format
2898 if neither of those are supported), including GDB extensions if at all
2903 Produce debugging information in stabs format (if that is supported),
2904 without GDB extensions. This is the format used by DBX on most BSD
2905 systems. On MIPS, Alpha and System V Release 4 systems this option
2906 produces stabs debugging output which is not understood by DBX or SDB@.
2907 On System V Release 4 systems this option requires the GNU assembler.
2911 Produce debugging information in stabs format (if that is supported),
2912 using GNU extensions understood only by the GNU debugger (GDB)@. The
2913 use of these extensions is likely to make other debuggers crash or
2914 refuse to read the program.
2918 Produce debugging information in COFF format (if that is supported).
2919 This is the format used by SDB on most System V systems prior to
2924 Produce debugging information in XCOFF format (if that is supported).
2925 This is the format used by the DBX debugger on IBM RS/6000 systems.
2929 Produce debugging information in XCOFF format (if that is supported),
2930 using GNU extensions understood only by the GNU debugger (GDB)@. The
2931 use of these extensions is likely to make other debuggers crash or
2932 refuse to read the program, and may cause assemblers other than the GNU
2933 assembler (GAS) to fail with an error.
2937 Produce debugging information in DWARF version 1 format (if that is
2938 supported). This is the format used by SDB on most System V Release 4
2941 This option is deprecated.
2945 Produce debugging information in DWARF version 1 format (if that is
2946 supported), using GNU extensions understood only by the GNU debugger
2947 (GDB)@. The use of these extensions is likely to make other debuggers
2948 crash or refuse to read the program.
2950 This option is deprecated.
2954 Produce debugging information in DWARF version 2 format (if that is
2955 supported). This is the format used by DBX on IRIX 6.
2959 Produce debugging information in VMS debug format (if that is
2960 supported). This is the format used by DEBUG on VMS systems.
2963 @itemx -ggdb@var{level}
2964 @itemx -gstabs@var{level}
2965 @itemx -gcoff@var{level}
2966 @itemx -gxcoff@var{level}
2967 @itemx -gvms@var{level}
2968 Request debugging information and also use @var{level} to specify how
2969 much information. The default level is 2.
2971 Level 1 produces minimal information, enough for making backtraces in
2972 parts of the program that you don't plan to debug. This includes
2973 descriptions of functions and external variables, but no information
2974 about local variables and no line numbers.
2976 Level 3 includes extra information, such as all the macro definitions
2977 present in the program. Some debuggers support macro expansion when
2978 you use @option{-g3}.
2980 Note that in order to avoid confusion between DWARF1 debug level 2,
2981 and DWARF2, neither @option{-gdwarf} nor @option{-gdwarf-2} accept
2982 a concatenated debug level. Instead use an additional @option{-g@var{level}}
2983 option to change the debug level for DWARF1 or DWARF2.
2985 @item -feliminate-dwarf2-dups
2986 @opindex feliminate-dwarf2-dups
2987 Compress DWARF2 debugging information by eliminating duplicated
2988 information about each symbol. This option only makes sense when
2989 generating DWARF2 debugging information with @option{-gdwarf-2}.
2991 @cindex @command{prof}
2994 Generate extra code to write profile information suitable for the
2995 analysis program @command{prof}. You must use this option when compiling
2996 the source files you want data about, and you must also use it when
2999 @cindex @command{gprof}
3002 Generate extra code to write profile information suitable for the
3003 analysis program @command{gprof}. You must use this option when compiling
3004 the source files you want data about, and you must also use it when
3009 Makes the compiler print out each function name as it is compiled, and
3010 print some statistics about each pass when it finishes.
3013 @opindex ftime-report
3014 Makes the compiler print some statistics about the time consumed by each
3015 pass when it finishes.
3018 @opindex fmem-report
3019 Makes the compiler print some statistics about permanent memory
3020 allocation when it finishes.
3022 @item -fprofile-arcs
3023 @opindex fprofile-arcs
3024 Add code so that program flow @dfn{arcs} are instrumented. During
3025 execution the program records how many times each branch and call is
3026 executed and how many times it is taken or returns. When the compiled
3027 program exits it saves this data to a file called
3028 @file{@var{auxname}.da} for each source file. The data may be used for
3029 profile-directed optimizations (@option{-fbranch-probabilities}), or for
3030 test coverage analysis (@option{-ftest-coverage}). Each object file's
3031 @var{auxname} is generated from the name of the output file, if
3032 explicitly specified and it is not the final executable, otherwise it is
3033 the basename of the source file. In both cases any suffix is removed
3034 (e.g. @file{foo.da} for input file @file{dir/foo.c}, or
3035 @file{dir/foo.da} for output file specified as @option{-o dir/foo.o}).
3040 Compile the source files with @option{-fprofile-arcs} plus optimization
3041 and code generation options. For test coverage analysis, use the
3042 additional @option{-ftest-coverage} option. You do not need to profile
3043 every source file in a program.
3046 Link your object files with @option{-lgcov} or @option{-fprofile-arcs}
3047 (the latter implies the former).
3050 Run the program on a representative workload to generate the arc profile
3051 information. This may be repeated any number of times. You can run
3052 concurrent instances of your program, and provided that the file system
3053 supports locking, the data files will be correctly updated. Also
3054 @code{fork} calls are detected and correctly handled (double counting
3058 For profile-directed optimizations, compile the source files again with
3059 the same optimization and code generation options plus
3060 @option{-fbranch-probabilities} (@pxref{Optimize Options,,Options that
3061 Control Optimization}).
3064 For test coverage analysis, use @command{gcov} to produce human readable
3065 information from the @file{.bbg} and @file{.da} files. Refer to the
3066 @command{gcov} documentation for further information.
3070 With @option{-fprofile-arcs}, for each function of your program GCC
3071 creates a program flow graph, then finds a spanning tree for the graph.
3072 Only arcs that are not on the spanning tree have to be instrumented: the
3073 compiler adds code to count the number of times that these arcs are
3074 executed. When an arc is the only exit or only entrance to a block, the
3075 instrumentation code can be added to the block; otherwise, a new basic
3076 block must be created to hold the instrumentation code.
3079 @item -ftest-coverage
3080 @opindex ftest-coverage
3081 Produce a graph file that the @command{gcov} code-coverage utility
3082 (@pxref{Gcov,, @command{gcov}---a Test Coverage Program}) can use to
3083 show program coverage. Each source file's data file is called
3084 @file{@var{auxname}.bbg}. Refer to the @option{-fprofile-arcs} option
3085 above for a description of @var{auxname} and instructions on how to
3086 generate test coverage data. Coverage data will match the source files
3087 more closely, if you do not optimize.
3089 @item -d@var{letters}
3091 Says to make debugging dumps during compilation at times specified by
3092 @var{letters}. This is used for debugging the compiler. The file names
3093 for most of the dumps are made by appending a pass number and a word to
3094 the @var{dumpname}. @var{dumpname} is generated from the name of the
3095 output file, if explicitly specified and it is not an executable,
3096 otherwise it is the basename of the source file. In both cases any
3097 suffix is removed (e.g. @file{foo.00.rtl} or @file{foo.01.sibling}).
3098 Here are the possible letters for use in @var{letters}, and their
3104 Annotate the assembler output with miscellaneous debugging information.
3107 Dump after computing branch probabilities, to @file{@var{file}.15.bp}.
3110 Dump after block reordering, to @file{@var{file}.31.bbro}.
3113 Dump after instruction combination, to the file @file{@var{file}.21.combine}.
3116 Dump after the first if conversion, to the file @file{@var{file}.16.ce1}.
3119 Dump after delayed branch scheduling, to @file{@var{file}.36.dbr}.
3122 Dump all macro definitions, at the end of preprocessing, in addition to
3126 Dump after SSA optimizations, to @file{@var{file}.04.ssa} and
3127 @file{@var{file}.07.ussa}.
3130 Dump after the second if conversion, to @file{@var{file}.32.ce3}.
3133 Dump after life analysis, to @file{@var{file}.20.life}.
3136 Dump after purging @code{ADDRESSOF} codes, to @file{@var{file}.10.addressof}.
3139 Dump after global register allocation, to @file{@var{file}.26.greg}.
3142 Dump after finalization of EH handling code, to @file{@var{file}.02.eh}.
3145 Dump after reg-to-stack conversion, to @file{@var{file}.34.stack}.
3148 Dump after post-reload optimizations, to @file{@var{file}.27.postreload}.
3151 Dump after GCSE, to @file{@var{file}.11.gcse}.
3154 Dump after sibling call optimizations, to @file{@var{file}.01.sibling}.
3157 Dump after the first jump optimization, to @file{@var{file}.03.jump}.
3160 Dump after conversion from registers to stack, to @file{@var{file}.34.stack}.
3163 Dump after local register allocation, to @file{@var{file}.25.lreg}.
3166 Dump after loop optimization passes, to @file{@var{file}.12.loop} and
3167 @file{@var{file}.18.loop2}.
3170 Dump after performing the machine dependent reorganization pass, to
3171 @file{@var{file}.35.mach}.
3174 Dump after register renumbering, to @file{@var{file}.30.rnreg}.
3177 Dump after the register move pass, to @file{@var{file}.23.regmove}.
3180 Dump after RTL generation, to @file{@var{file}.00.rtl}.
3183 Dump after the second scheduling pass, to @file{@var{file}.33.sched2}.
3186 Dump after CSE (including the jump optimization that sometimes follows
3187 CSE), to @file{@var{file}.09.cse}.
3190 Dump after the first scheduling pass, to @file{@var{file}.24.sched}.
3193 Dump after the second CSE pass (including the jump optimization that
3194 sometimes follows CSE), to @file{@var{file}.19.cse2}.
3197 Dump after null pointer elimination pass to @file{@var{file}.08.null}.
3200 Dump after the second flow pass, to @file{@var{file}.28.flow2}.
3203 Dump after SSA dead code elimination, to @file{@var{file}.06.ssadce}.
3206 Dump after the peephole pass, to @file{@var{file}.29.peephole2}.
3209 Produce all the dumps listed above.
3212 Produce a core dump whenever an error occurs.
3215 Print statistics on memory usage, at the end of the run, to
3219 Annotate the assembler output with a comment indicating which
3220 pattern and alternative was used. The length of each instruction is
3224 Dump the RTL in the assembler output as a comment before each instruction.
3225 Also turns on @option{-dp} annotation.
3228 For each of the other indicated dump files (except for
3229 @file{@var{file}.00.rtl}), dump a representation of the control flow graph
3230 suitable for viewing with VCG to @file{@var{file}.@var{pass}.vcg}.
3233 Just generate RTL for a function instead of compiling it. Usually used
3237 Dump debugging information during parsing, to standard error.
3240 @item -fdump-unnumbered
3241 @opindex fdump-unnumbered
3242 When doing debugging dumps (see @option{-d} option above), suppress instruction
3243 numbers and line number note output. This makes it more feasible to
3244 use diff on debugging dumps for compiler invocations with different
3245 options, in particular with and without @option{-g}.
3247 @item -fdump-translation-unit @r{(C and C++ only)}
3248 @itemx -fdump-translation-unit-@var{options} @r{(C and C++ only)}
3249 @opindex fdump-translation-unit
3250 Dump a representation of the tree structure for the entire translation
3251 unit to a file. The file name is made by appending @file{.tu} to the
3252 source file name. If the @samp{-@var{options}} form is used, @var{options}
3253 controls the details of the dump as described for the
3254 @option{-fdump-tree} options.
3256 @item -fdump-class-hierarchy @r{(C++ only)}
3257 @itemx -fdump-class-hierarchy-@var{options} @r{(C++ only)}
3258 @opindex fdump-class-hierarchy
3259 Dump a representation of each class's hierarchy and virtual function
3260 table layout to a file. The file name is made by appending @file{.class}
3261 to the source file name. If the @samp{-@var{options}} form is used,
3262 @var{options} controls the details of the dump as described for the
3263 @option{-fdump-tree} options.
3265 @item -fdump-tree-@var{switch} @r{(C++ only)}
3266 @itemx -fdump-tree-@var{switch}-@var{options} @r{(C++ only)}
3268 Control the dumping at various stages of processing the intermediate
3269 language tree to a file. The file name is generated by appending a switch
3270 specific suffix to the source file name. If the @samp{-@var{options}}
3271 form is used, @var{options} is a list of @samp{-} separated options that
3272 control the details of the dump. Not all options are applicable to all
3273 dumps, those which are not meaningful will be ignored. The following
3274 options are available
3278 Print the address of each node. Usually this is not meaningful as it
3279 changes according to the environment and source file. Its primary use
3280 is for tying up a dump file with a debug environment.
3282 Inhibit dumping of members of a scope or body of a function merely
3283 because that scope has been reached. Only dump such items when they
3284 are directly reachable by some other path.
3286 Turn on all options.
3289 The following tree dumps are possible:
3292 Dump before any tree based optimization, to @file{@var{file}.original}.
3294 Dump after all tree based optimization, to @file{@var{file}.optimized}.
3296 Dump after function inlining, to @file{@var{file}.inlined}.
3299 @item -frandom-seed=@var{string}
3300 @opindex frandom-string
3301 This option provides a seed that GCC uses when it would otherwise use
3302 random numbers. At present, this is used to generate certain symbol names
3303 that have to be different in every compiled file.
3305 The @var{string} should be different for every file you compile.
3307 @item -fsched-verbose=@var{n}
3308 @opindex fsched-verbose
3309 On targets that use instruction scheduling, this option controls the
3310 amount of debugging output the scheduler prints. This information is
3311 written to standard error, unless @option{-dS} or @option{-dR} is
3312 specified, in which case it is output to the usual dump
3313 listing file, @file{.sched} or @file{.sched2} respectively. However
3314 for @var{n} greater than nine, the output is always printed to standard
3317 For @var{n} greater than zero, @option{-fsched-verbose} outputs the
3318 same information as @option{-dRS}. For @var{n} greater than one, it
3319 also output basic block probabilities, detailed ready list information
3320 and unit/insn info. For @var{n} greater than two, it includes RTL
3321 at abort point, control-flow and regions info. And for @var{n} over
3322 four, @option{-fsched-verbose} also includes dependence info.
3326 Store the usual ``temporary'' intermediate files permanently; place them
3327 in the current directory and name them based on the source file. Thus,
3328 compiling @file{foo.c} with @samp{-c -save-temps} would produce files
3329 @file{foo.i} and @file{foo.s}, as well as @file{foo.o}. This creates a
3330 preprocessed @file{foo.i} output file even though the compiler now
3331 normally uses an integrated preprocessor.
3335 Report the CPU time taken by each subprocess in the compilation
3336 sequence. For C source files, this is the compiler proper and assembler
3337 (plus the linker if linking is done). The output looks like this:
3344 The first number on each line is the ``user time,'' that is time spent
3345 executing the program itself. The second number is ``system time,''
3346 time spent executing operating system routines on behalf of the program.
3347 Both numbers are in seconds.
3349 @item -print-file-name=@var{library}
3350 @opindex print-file-name
3351 Print the full absolute name of the library file @var{library} that
3352 would be used when linking---and don't do anything else. With this
3353 option, GCC does not compile or link anything; it just prints the
3356 @item -print-multi-directory
3357 @opindex print-multi-directory
3358 Print the directory name corresponding to the multilib selected by any
3359 other switches present in the command line. This directory is supposed
3360 to exist in @env{GCC_EXEC_PREFIX}.
3362 @item -print-multi-lib
3363 @opindex print-multi-lib
3364 Print the mapping from multilib directory names to compiler switches
3365 that enable them. The directory name is separated from the switches by
3366 @samp{;}, and each switch starts with an @samp{@@} instead of the
3367 @samp{-}, without spaces between multiple switches. This is supposed to
3368 ease shell-processing.
3370 @item -print-prog-name=@var{program}
3371 @opindex print-prog-name
3372 Like @option{-print-file-name}, but searches for a program such as @samp{cpp}.
3374 @item -print-libgcc-file-name
3375 @opindex print-libgcc-file-name
3376 Same as @option{-print-file-name=libgcc.a}.
3378 This is useful when you use @option{-nostdlib} or @option{-nodefaultlibs}
3379 but you do want to link with @file{libgcc.a}. You can do
3382 gcc -nostdlib @var{files}@dots{} `gcc -print-libgcc-file-name`
3385 @item -print-search-dirs
3386 @opindex print-search-dirs
3387 Print the name of the configured installation directory and a list of
3388 program and library directories gcc will search---and don't do anything else.
3390 This is useful when gcc prints the error message
3391 @samp{installation problem, cannot exec cpp0: No such file or directory}.
3392 To resolve this you either need to put @file{cpp0} and the other compiler
3393 components where gcc expects to find them, or you can set the environment
3394 variable @env{GCC_EXEC_PREFIX} to the directory where you installed them.
3395 Don't forget the trailing '/'.
3396 @xref{Environment Variables}.
3399 @opindex dumpmachine
3400 Print the compiler's target machine (for example,
3401 @samp{i686-pc-linux-gnu})---and don't do anything else.
3404 @opindex dumpversion
3405 Print the compiler version (for example, @samp{3.0})---and don't do
3410 Print the compiler's built-in specs---and don't do anything else. (This
3411 is used when GCC itself is being built.) @xref{Spec Files}.
3413 @item -feliminate-unused-debug-types
3414 @opindex feliminate-unused-debug-types
3415 Normally, when producing DWARF2 output, GCC will emit debugging
3416 information for all types declared in a compilation
3417 unit, regardless of whether or not they are actually used
3418 in that compilation unit. Sometimes this is useful, such as
3419 if, in the debugger, you want to cast a value to a type that is
3420 not actually used in your program (but is declared). More often,
3421 however, this results in a significant amount of wasted space.
3422 With this option, GCC will avoid producing debug symbol output
3423 for types that are nowhere used in the source file being compiled.
3426 @node Optimize Options
3427 @section Options That Control Optimization
3428 @cindex optimize options
3429 @cindex options, optimization
3431 These options control various sorts of optimizations.
3433 Without any optimization option, the compiler's goal is to reduce the
3434 cost of compilation and to make debugging produce the expected
3435 results. Statements are independent: if you stop the program with a
3436 breakpoint between statements, you can then assign a new value to any
3437 variable or change the program counter to any other statement in the
3438 function and get exactly the results you would expect from the source
3441 Turning on optimization flags makes the compiler attempt to improve
3442 the performance and/or code size at the expense of compilation time
3443 and possibly the ability to debug the program.
3445 Not all optimizations are controlled directly by a flag. Only
3446 optimizations that have a flag are listed.
3453 Optimize. Optimizing compilation takes somewhat more time, and a lot
3454 more memory for a large function.
3456 With @option{-O}, the compiler tries to reduce code size and execution
3457 time, without performing any optimizations that take a great deal of
3460 @option{-O} turns on the following optimization flags:
3461 @gccoptlist{-fdefer-pop @gol
3462 -fmerge-constants @gol
3464 -floop-optimize @gol
3466 -fif-conversion @gol
3467 -fif-conversion2 @gol
3468 -fdelayed-branch @gol
3469 -fguess-branch-probability @gol
3472 @option{-O} also turns on @option{-fomit-frame-pointer} on machines
3473 where doing so does not interfere with debugging.
3477 Optimize even more. GCC performs nearly all supported optimizations
3478 that do not involve a space-speed tradeoff. The compiler does not
3479 perform loop unrolling or function inlining when you specify @option{-O2}.
3480 As compared to @option{-O}, this option increases both compilation time
3481 and the performance of the generated code.
3483 @option{-O2} turns on all optimization flags specified by @option{-O}. It
3484 also turns on the following optimization flags:
3485 @gccoptlist{-fforce-mem @gol
3486 -foptimize-sibling-calls @gol
3487 -fstrength-reduce @gol
3488 -fcse-follow-jumps -fcse-skip-blocks @gol
3489 -frerun-cse-after-loop -frerun-loop-opt @gol
3490 -fgcse -fgcse-lm -fgcse-sm @gol
3491 -fdelete-null-pointer-checks @gol
3492 -fexpensive-optimizations @gol
3494 -fschedule-insns -fschedule-insns2 @gol
3495 -fsched-interblock -fsched-spec @gol
3498 -freorder-blocks -freorder-functions @gol
3499 -fstrict-aliasing @gol
3500 -falign-functions -falign-jumps @gol
3501 -falign-loops -falign-labels}
3503 Please note the warning under @option{-fgcse} about
3504 invoking @option{-O2} on programs that use computed gotos.
3508 Optimize yet more. @option{-O3} turns on all optimizations specified by
3509 @option{-O2} and also turns on the @option{-finline-functions},
3510 @option{-funit-at-a-time} and @option{-frename-registers} options.
3514 Do not optimize. This is the default.
3518 Optimize for size. @option{-Os} enables all @option{-O2} optimizations that
3519 do not typically increase code size. It also performs further
3520 optimizations designed to reduce code size.
3522 @option{-Os} disables the following optimization flags:
3523 @gccoptlist{-falign-functions -falign-jumps -falign-loops @gol
3524 -falign-labels -freorder-blocks -fprefetch-loop-arrays}
3526 If you use multiple @option{-O} options, with or without level numbers,
3527 the last such option is the one that is effective.
3530 Options of the form @option{-f@var{flag}} specify machine-independent
3531 flags. Most flags have both positive and negative forms; the negative
3532 form of @option{-ffoo} would be @option{-fno-foo}. In the table
3533 below, only one of the forms is listed---the one you typically will
3534 use. You can figure out the other form by either removing @samp{no-}
3537 The following options control specific optimizations. They are either
3538 activated by @option{-O} options or are related to ones that are. You
3539 can use the following flags in the rare cases when ``fine-tuning'' of
3540 optimizations to be performed is desired.
3543 @item -fno-default-inline
3544 @opindex fno-default-inline
3545 Do not make member functions inline by default merely because they are
3546 defined inside the class scope (C++ only). Otherwise, when you specify
3547 @w{@option{-O}}, member functions defined inside class scope are compiled
3548 inline by default; i.e., you don't need to add @samp{inline} in front of
3549 the member function name.
3551 @item -fno-defer-pop
3552 @opindex fno-defer-pop
3553 Always pop the arguments to each function call as soon as that function
3554 returns. For machines which must pop arguments after a function call,
3555 the compiler normally lets arguments accumulate on the stack for several
3556 function calls and pops them all at once.
3558 Disabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
3562 Force memory operands to be copied into registers before doing
3563 arithmetic on them. This produces better code by making all memory
3564 references potential common subexpressions. When they are not common
3565 subexpressions, instruction combination should eliminate the separate
3568 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
3571 @opindex fforce-addr
3572 Force memory address constants to be copied into registers before
3573 doing arithmetic on them. This may produce better code just as
3574 @option{-fforce-mem} may.
3576 @item -fomit-frame-pointer
3577 @opindex fomit-frame-pointer
3578 Don't keep the frame pointer in a register for functions that
3579 don't need one. This avoids the instructions to save, set up and
3580 restore frame pointers; it also makes an extra register available
3581 in many functions. @strong{It also makes debugging impossible on
3584 On some machines, such as the VAX, this flag has no effect, because
3585 the standard calling sequence automatically handles the frame pointer
3586 and nothing is saved by pretending it doesn't exist. The
3587 machine-description macro @code{FRAME_POINTER_REQUIRED} controls
3588 whether a target machine supports this flag. @xref{Registers,,Register
3589 Usage, gccint, GNU Compiler Collection (GCC) Internals}.
3591 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
3593 @item -foptimize-sibling-calls
3594 @opindex foptimize-sibling-calls
3595 Optimize sibling and tail recursive calls.
3597 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
3601 Don't pay attention to the @code{inline} keyword. Normally this option
3602 is used to keep the compiler from expanding any functions inline.
3603 Note that if you are not optimizing, no functions can be expanded inline.
3605 @item -finline-functions
3606 @opindex finline-functions
3607 Integrate all simple functions into their callers. The compiler
3608 heuristically decides which functions are simple enough to be worth
3609 integrating in this way.
3611 If all calls to a given function are integrated, and the function is
3612 declared @code{static}, then the function is normally not output as
3613 assembler code in its own right.
3615 Enabled at level @option{-O3}.
3617 @item -finline-limit=@var{n}
3618 @opindex finline-limit
3619 By default, gcc limits the size of functions that can be inlined. This flag
3620 allows the control of this limit for functions that are explicitly marked as
3621 inline (i.e., marked with the inline keyword or defined within the class
3622 definition in c++). @var{n} is the size of functions that can be inlined in
3623 number of pseudo instructions (not counting parameter handling). The default
3624 value of @var{n} is 600.
3625 Increasing this value can result in more inlined code at
3626 the cost of compilation time and memory consumption. Decreasing usually makes
3627 the compilation faster and less code will be inlined (which presumably
3628 means slower programs). This option is particularly useful for programs that
3629 use inlining heavily such as those based on recursive templates with C++.
3631 Inlining is actually controlled by a number of parameters, which may be
3632 specified individually by using @option{--param @var{name}=@var{value}}.
3633 The @option{-finline-limit=@var{n}} option sets some of these parameters
3637 @item max-inline-insns
3639 @item max-inline-insns-single
3640 is set to @var{n}/2.
3641 @item max-inline-insns-auto
3642 is set to @var{n}/2.
3643 @item min-inline-insns
3644 is set to 130 or @var{n}/4, whichever is smaller.
3645 @item max-inline-insns-rtl
3649 Using @option{-finline-limit=600} thus results in the default settings
3650 for these parameters. See below for a documentation of the individual
3651 parameters controlling inlining.
3653 @emph{Note:} pseudo instruction represents, in this particular context, an
3654 abstract measurement of function's size. In no way, it represents a count
3655 of assembly instructions and as such its exact meaning might change from one
3656 release to an another.
3658 @item -fkeep-inline-functions
3659 @opindex fkeep-inline-functions
3660 Even if all calls to a given function are integrated, and the function
3661 is declared @code{static}, nevertheless output a separate run-time
3662 callable version of the function. This switch does not affect
3663 @code{extern inline} functions.
3665 @item -fkeep-static-consts
3666 @opindex fkeep-static-consts
3667 Emit variables declared @code{static const} when optimization isn't turned
3668 on, even if the variables aren't referenced.
3670 GCC enables this option by default. If you want to force the compiler to
3671 check if the variable was referenced, regardless of whether or not
3672 optimization is turned on, use the @option{-fno-keep-static-consts} option.
3674 @item -fmerge-constants
3675 Attempt to merge identical constants (string constants and floating point
3676 constants) across compilation units.
3678 This option is the default for optimized compilation if the assembler and
3679 linker support it. Use @option{-fno-merge-constants} to inhibit this
3682 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
3684 @item -fmerge-all-constants
3685 Attempt to merge identical constants and identical variables.
3687 This option implies @option{-fmerge-constants}. In addition to
3688 @option{-fmerge-constants} this considers e.g. even constant initialized
3689 arrays or initialized constant variables with integral or floating point
3690 types. Languages like C or C++ require each non-automatic variable to
3691 have distinct location, so using this option will result in non-conforming
3696 Use a graph coloring register allocator. Currently this option is meant
3697 for testing, so we are interested to hear about miscompilations with
3700 @item -fno-branch-count-reg
3701 @opindex fno-branch-count-reg
3702 Do not use ``decrement and branch'' instructions on a count register,
3703 but instead generate a sequence of instructions that decrement a
3704 register, compare it against zero, then branch based upon the result.
3705 This option is only meaningful on architectures that support such
3706 instructions, which include x86, PowerPC, IA-64 and S/390.
3708 The default is @option{-fbranch-count-reg}, enabled when
3709 @option{-fstrength-reduce} is enabled.
3711 @item -fno-function-cse
3712 @opindex fno-function-cse
3713 Do not put function addresses in registers; make each instruction that
3714 calls a constant function contain the function's address explicitly.
3716 This option results in less efficient code, but some strange hacks
3717 that alter the assembler output may be confused by the optimizations
3718 performed when this option is not used.
3720 The default is @option{-ffunction-cse}
3722 @item -fno-zero-initialized-in-bss
3723 @opindex fno-zero-initialized-in-bss
3724 If the target supports a BSS section, GCC by default puts variables that
3725 are initialized to zero into BSS@. This can save space in the resulting
3728 This option turns off this behavior because some programs explicitly
3729 rely on variables going to the data section. E.g., so that the
3730 resulting executable can find the beginning of that section and/or make
3731 assumptions based on that.
3733 The default is @option{-fzero-initialized-in-bss}.
3735 @item -fstrength-reduce
3736 @opindex fstrength-reduce
3737 Perform the optimizations of loop strength reduction and
3738 elimination of iteration variables.
3740 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
3742 @item -fthread-jumps
3743 @opindex fthread-jumps
3744 Perform optimizations where we check to see if a jump branches to a
3745 location where another comparison subsumed by the first is found. If
3746 so, the first branch is redirected to either the destination of the
3747 second branch or a point immediately following it, depending on whether
3748 the condition is known to be true or false.
3750 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
3752 @item -fcse-follow-jumps
3753 @opindex fcse-follow-jumps
3754 In common subexpression elimination, scan through jump instructions
3755 when the target of the jump is not reached by any other path. For
3756 example, when CSE encounters an @code{if} statement with an
3757 @code{else} clause, CSE will follow the jump when the condition
3760 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
3762 @item -fcse-skip-blocks
3763 @opindex fcse-skip-blocks
3764 This is similar to @option{-fcse-follow-jumps}, but causes CSE to
3765 follow jumps which conditionally skip over blocks. When CSE
3766 encounters a simple @code{if} statement with no else clause,
3767 @option{-fcse-skip-blocks} causes CSE to follow the jump around the
3768 body of the @code{if}.
3770 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
3772 @item -frerun-cse-after-loop
3773 @opindex frerun-cse-after-loop
3774 Re-run common subexpression elimination after loop optimizations has been
3777 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
3779 @item -frerun-loop-opt
3780 @opindex frerun-loop-opt
3781 Run the loop optimizer twice.
3783 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
3787 Perform a global common subexpression elimination pass.
3788 This pass also performs global constant and copy propagation.
3790 @emph{Note:} When compiling a program using computed gotos, a GCC
3791 extension, you may get better runtime performance if you disable
3792 the global common subexpression elimination pass by adding
3793 @option{-fno-gcse} to the command line.
3795 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
3799 When @option{-fgcse-lm} is enabled, global common subexpression elimination will
3800 attempt to move loads which are only killed by stores into themselves. This
3801 allows a loop containing a load/store sequence to be changed to a load outside
3802 the loop, and a copy/store within the loop.
3804 Enabled by default when gcse is enabled.
3808 When @option{-fgcse-sm} is enabled, A store motion pass is run after global common
3809 subexpression elimination. This pass will attempt to move stores out of loops.
3810 When used in conjunction with @option{-fgcse-lm}, loops containing a load/store sequence
3811 can be changed to a load before the loop and a store after the loop.
3813 Enabled by default when gcse is enabled.
3815 @item -floop-optimize
3816 @opindex floop-optimize
3817 Perform loop optimizations: move constant expressions out of loops, simplify
3818 exit test conditions and optionally do strength-reduction and loop unrolling as
3821 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
3823 @item -fcrossjumping
3824 @opindex crossjumping
3825 Perform cross-jumping transformation. This transformation unifies equivalent code and save code size. The
3826 resulting code may or may not perform better than without cross-jumping.
3828 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
3830 @item -fif-conversion
3831 @opindex if-conversion
3832 Attempt to transform conditional jumps into branch-less equivalents. This
3833 include use of conditional moves, min, max, set flags and abs instructions, and
3834 some tricks doable by standard arithmetics. The use of conditional execution
3835 on chips where it is available is controlled by @code{if-conversion2}.
3837 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
3839 @item -fif-conversion2
3840 @opindex if-conversion2
3841 Use conditional execution (where available) to transform conditional jumps into
3842 branch-less equivalents.
3844 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
3846 @item -fdelete-null-pointer-checks
3847 @opindex fdelete-null-pointer-checks
3848 Use global dataflow analysis to identify and eliminate useless checks
3849 for null pointers. The compiler assumes that dereferencing a null
3850 pointer would have halted the program. If a pointer is checked after
3851 it has already been dereferenced, it cannot be null.
3853 In some environments, this assumption is not true, and programs can
3854 safely dereference null pointers. Use
3855 @option{-fno-delete-null-pointer-checks} to disable this optimization
3856 for programs which depend on that behavior.
3858 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
3860 @item -fexpensive-optimizations
3861 @opindex fexpensive-optimizations
3862 Perform a number of minor optimizations that are relatively expensive.
3864 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
3866 @item -foptimize-register-move
3868 @opindex foptimize-register-move
3870 Attempt to reassign register numbers in move instructions and as
3871 operands of other simple instructions in order to maximize the amount of
3872 register tying. This is especially helpful on machines with two-operand
3875 Note @option{-fregmove} and @option{-foptimize-register-move} are the same
3878 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
3880 @item -fdelayed-branch
3881 @opindex fdelayed-branch
3882 If supported for the target machine, attempt to reorder instructions
3883 to exploit instruction slots available after delayed branch
3886 Enabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
3888 @item -fschedule-insns
3889 @opindex fschedule-insns
3890 If supported for the target machine, attempt to reorder instructions to
3891 eliminate execution stalls due to required data being unavailable. This
3892 helps machines that have slow floating point or memory load instructions
3893 by allowing other instructions to be issued until the result of the load
3894 or floating point instruction is required.
3896 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
3898 @item -fschedule-insns2
3899 @opindex fschedule-insns2
3900 Similar to @option{-fschedule-insns}, but requests an additional pass of
3901 instruction scheduling after register allocation has been done. This is
3902 especially useful on machines with a relatively small number of
3903 registers and where memory load instructions take more than one cycle.
3905 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
3907 @item -fno-sched-interblock
3908 @opindex fno-sched-interblock
3909 Don't schedule instructions across basic blocks. This is normally
3910 enabled by default when scheduling before register allocation, i.e.@:
3911 with @option{-fschedule-insns} or at @option{-O2} or higher.
3913 @item -fno-sched-spec
3914 @opindex fno-sched-spec
3915 Don't allow speculative motion of non-load instructions. This is normally
3916 enabled by default when scheduling before register allocation, i.e.@:
3917 with @option{-fschedule-insns} or at @option{-O2} or higher.
3919 @item -fsched-spec-load
3920 @opindex fsched-spec-load
3921 Allow speculative motion of some load instructions. This only makes
3922 sense when scheduling before register allocation, i.e.@: with
3923 @option{-fschedule-insns} or at @option{-O2} or higher.
3925 @item -fsched-spec-load-dangerous
3926 @opindex fsched-spec-load-dangerous
3927 Allow speculative motion of more load instructions. This only makes
3928 sense when scheduling before register allocation, i.e.@: with
3929 @option{-fschedule-insns} or at @option{-O2} or higher.
3931 @item -fsched2-use-superblocks
3932 @opindex fsched2-use-superblocks
3933 When schedulilng after register allocation, do use superblock scheduling
3934 algorithm. Superblock scheduling allows motion across basic block boundaries
3935 resulting on faster schedules. This option is experimental, as not all machine
3936 descriptions used by GCC model the CPU closely enough to avoid unreliable
3937 results from the algorithm.
3939 This only makes sense when scheduling after register allocation, i.e.@: with
3940 @option{-fschedule-insns2} or at @option{-O2} or higher.
3942 @item -fsched2-use-traces
3943 @opindex fsched2-use-traces
3944 Use @option{-fsched2-use-superblocks} algorithm when scheduling after register
3945 allocation and additionally perform code duplication in order to increase the
3946 size of superblocks using tracer pass. See @option{-ftracer} for details on
3949 This mode should produce faster but significantly longer programs. Also
3950 without @code{-fbranch-probabilities} the traces constructed may not match the
3951 reality and hurt the performance. This only makes
3952 sense when scheduling after register allocation, i.e.@: with
3953 @option{-fschedule-insns2} or at @option{-O2} or higher.
3955 @item -fcaller-saves
3956 @opindex fcaller-saves
3957 Enable values to be allocated in registers that will be clobbered by
3958 function calls, by emitting extra instructions to save and restore the
3959 registers around such calls. Such allocation is done only when it
3960 seems to result in better code than would otherwise be produced.
3962 This option is always enabled by default on certain machines, usually
3963 those which have no call-preserved registers to use instead.
3965 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
3967 @item -fmove-all-movables
3968 @opindex fmove-all-movables
3969 Forces all invariant computations in loops to be moved
3972 @item -freduce-all-givs
3973 @opindex freduce-all-givs
3974 Forces all general-induction variables in loops to be
3977 @emph{Note:} When compiling programs written in Fortran,
3978 @option{-fmove-all-movables} and @option{-freduce-all-givs} are enabled
3979 by default when you use the optimizer.
3981 These options may generate better or worse code; results are highly
3982 dependent on the structure of loops within the source code.
3984 These two options are intended to be removed someday, once
3985 they have helped determine the efficacy of various
3986 approaches to improving loop optimizations.
3988 Please let us (@w{@email{gcc@@gcc.gnu.org}} and @w{@email{fortran@@gnu.org}})
3989 know how use of these options affects
3990 the performance of your production code.
3991 We're very interested in code that runs @emph{slower}
3992 when these options are @emph{enabled}.
3995 @itemx -fno-peephole2
3996 @opindex fno-peephole
3997 @opindex fno-peephole2
3998 Disable any machine-specific peephole optimizations. The difference
3999 between @option{-fno-peephole} and @option{-fno-peephole2} is in how they
4000 are implemented in the compiler; some targets use one, some use the
4001 other, a few use both.
4003 @option{-fpeephole} is enabled by default.
4004 @option{-fpeephole2} enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4006 @item -fno-guess-branch-probability
4007 @opindex fno-guess-branch-probability
4008 Do not guess branch probabilities using a randomized model.
4010 Sometimes gcc will opt to use a randomized model to guess branch
4011 probabilities, when none are available from either profiling feedback
4012 (@option{-fprofile-arcs}) or @samp{__builtin_expect}. This means that
4013 different runs of the compiler on the same program may produce different
4016 In a hard real-time system, people don't want different runs of the
4017 compiler to produce code that has different behavior; minimizing
4018 non-determinism is of paramount import. This switch allows users to
4019 reduce non-determinism, possibly at the expense of inferior
4022 The default is @option{-fguess-branch-probability} at levels
4023 @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4025 @item -freorder-blocks
4026 @opindex freorder-blocks
4027 Reorder basic blocks in the compiled function in order to reduce number of
4028 taken branches and improve code locality.
4030 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4032 @item -freorder-functions
4033 @opindex freorder-functions
4034 Reorder basic blocks in the compiled function in order to reduce number of
4035 taken branches and improve code locality. This is implemented by using special
4036 subsections @code{text.hot} for most frequently executed functions and
4037 @code{text.unlikely} for unlikely executed functions. Reordering is done by
4038 the linker so object file format must support named sections and linker must
4039 place them in a reasonable way.
4041 Also profile feedback must be available in to make this option effective. See
4042 @option{-fprofile-arcs} for details.
4044 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4046 @item -fstrict-aliasing
4047 @opindex fstrict-aliasing
4048 Allows the compiler to assume the strictest aliasing rules applicable to
4049 the language being compiled. For C (and C++), this activates
4050 optimizations based on the type of expressions. In particular, an
4051 object of one type is assumed never to reside at the same address as an
4052 object of a different type, unless the types are almost the same. For
4053 example, an @code{unsigned int} can alias an @code{int}, but not a
4054 @code{void*} or a @code{double}. A character type may alias any other
4057 Pay special attention to code like this:
4070 The practice of reading from a different union member than the one most
4071 recently written to (called ``type-punning'') is common. Even with
4072 @option{-fstrict-aliasing}, type-punning is allowed, provided the memory
4073 is accessed through the union type. So, the code above will work as
4074 expected. However, this code might not:
4085 Every language that wishes to perform language-specific alias analysis
4086 should define a function that computes, given an @code{tree}
4087 node, an alias set for the node. Nodes in different alias sets are not
4088 allowed to alias. For an example, see the C front-end function
4089 @code{c_get_alias_set}.
4091 Enabled at levels @option{-O2}, @option{-O3}, @option{-Os}.
4093 @item -falign-functions
4094 @itemx -falign-functions=@var{n}
4095 @opindex falign-functions
4096 Align the start of functions to the next power-of-two greater than
4097 @var{n}, skipping up to @var{n} bytes. For instance,
4098 @option{-falign-functions=32} aligns functions to the next 32-byte
4099 boundary, but @option{-falign-functions=24} would align to the next
4100 32-byte boundary only if this can be done by skipping 23 bytes or less.
4102 @option{-fno-align-functions} and @option{-falign-functions=1} are
4103 equivalent and mean that functions will not be aligned.
4105 Some assemblers only support this flag when @var{n} is a power of two;
4106 in that case, it is rounded up.
4108 If @var{n} is not specified, use a machine-dependent default.
4110 Enabled at levels @option{-O2}, @option{-O3}.
4112 @item -falign-labels
4113 @itemx -falign-labels=@var{n}
4114 @opindex falign-labels
4115 Align all branch targets to a power-of-two boundary, skipping up to
4116 @var{n} bytes like @option{-falign-functions}. This option can easily
4117 make code slower, because it must insert dummy operations for when the
4118 branch target is reached in the usual flow of the code.
4120 If @option{-falign-loops} or @option{-falign-jumps} are applicable and
4121 are greater than this value, then their values are used instead.
4123 If @var{n} is not specified, use a machine-dependent default which is
4124 very likely to be @samp{1}, meaning no alignment.
4126 Enabled at levels @option{-O2}, @option{-O3}.
4129 @itemx -falign-loops=@var{n}
4130 @opindex falign-loops
4131 Align loops to a power-of-two boundary, skipping up to @var{n} bytes
4132 like @option{-falign-functions}. The hope is that the loop will be
4133 executed many times, which will make up for any execution of the dummy
4136 If @var{n} is not specified, use a machine-dependent default.
4138 Enabled at levels @option{-O2}, @option{-O3}.
4141 @itemx -falign-jumps=@var{n}
4142 @opindex falign-jumps
4143 Align branch targets to a power-of-two boundary, for branch targets
4144 where the targets can only be reached by jumping, skipping up to @var{n}
4145 bytes like @option{-falign-functions}. In this case, no dummy operations
4148 If @var{n} is not specified, use a machine-dependent default.
4150 Enabled at levels @option{-O2}, @option{-O3}.
4152 @item -frename-registers
4153 @opindex frename-registers
4154 Attempt to avoid false dependencies in scheduled code by making use
4155 of registers left over after register allocation. This optimization
4156 will most benefit processors with lots of registers. It can, however,
4157 make debugging impossible, since variables will no longer stay in
4158 a ``home register''.
4160 Enabled at levels @option{-O3}.
4162 @item -fno-cprop-registers
4163 @opindex fno-cprop-registers
4164 After register allocation and post-register allocation instruction splitting,
4165 we perform a copy-propagation pass to try to reduce scheduling dependencies
4166 and occasionally eliminate the copy.
4168 Disabled at levels @option{-O}, @option{-O2}, @option{-O3}, @option{-Os}.
4172 The following options control compiler behavior regarding floating
4173 point arithmetic. These options trade off between speed and
4174 correctness. All must be specifically enabled.
4178 @opindex ffloat-store
4179 Do not store floating point variables in registers, and inhibit other
4180 options that might change whether a floating point value is taken from a
4183 @cindex floating point precision
4184 This option prevents undesirable excess precision on machines such as
4185 the 68000 where the floating registers (of the 68881) keep more
4186 precision than a @code{double} is supposed to have. Similarly for the
4187 x86 architecture. For most programs, the excess precision does only
4188 good, but a few programs rely on the precise definition of IEEE floating
4189 point. Use @option{-ffloat-store} for such programs, after modifying
4190 them to store all pertinent intermediate computations into variables.
4194 Sets @option{-fno-math-errno}, @option{-funsafe-math-optimizations}, @*
4195 @option{-fno-trapping-math}, @option{-ffinite-math-only} and @*
4196 @option{-fno-signaling-nans}.
4198 This option causes the preprocessor macro @code{__FAST_MATH__} to be defined.
4200 This option should never be turned on by any @option{-O} option since
4201 it can result in incorrect output for programs which depend on
4202 an exact implementation of IEEE or ISO rules/specifications for
4205 @item -fno-math-errno
4206 @opindex fno-math-errno
4207 Do not set ERRNO after calling math functions that are executed
4208 with a single instruction, e.g., sqrt. A program that relies on
4209 IEEE exceptions for math error handling may want to use this flag
4210 for speed while maintaining IEEE arithmetic compatibility.
4212 This option should never be turned on by any @option{-O} option since
4213 it can result in incorrect output for programs which depend on
4214 an exact implementation of IEEE or ISO rules/specifications for
4217 The default is @option{-fmath-errno}.
4219 @item -funsafe-math-optimizations
4220 @opindex funsafe-math-optimizations
4221 Allow optimizations for floating-point arithmetic that (a) assume
4222 that arguments and results are valid and (b) may violate IEEE or
4223 ANSI standards. When used at link-time, it may include libraries
4224 or startup files that change the default FPU control word or other
4225 similar optimizations.
4227 This option should never be turned on by any @option{-O} option since
4228 it can result in incorrect output for programs which depend on
4229 an exact implementation of IEEE or ISO rules/specifications for
4232 The default is @option{-fno-unsafe-math-optimizations}.
4234 @item -ffinite-math-only
4235 @opindex ffinite-math-only
4236 Allow optimizations for floating-point arithmetic that assume
4237 that arguments and results are not NaNs or +-Infs.
4239 This option should never be turned on by any @option{-O} option since
4240 it can result in incorrect output for programs which depend on
4241 an exact implementation of IEEE or ISO rules/specifications.
4243 The default is @option{-fno-finite-math-only}.
4245 @item -fno-trapping-math
4246 @opindex fno-trapping-math
4247 Compile code assuming that floating-point operations cannot generate
4248 user-visible traps. These traps include division by zero, overflow,
4249 underflow, inexact result and invalid operation. This option implies
4250 @option{-fno-signaling-nans}. Setting this option may allow faster
4251 code if one relies on ``non-stop'' IEEE arithmetic, for example.
4253 This option should never be turned on by any @option{-O} option since
4254 it can result in incorrect output for programs which depend on
4255 an exact implementation of IEEE or ISO rules/specifications for
4258 The default is @option{-ftrapping-math}.
4260 @item -fsignaling-nans
4261 @opindex fsignaling-nans
4262 Compile code assuming that IEEE signaling NaNs may generate user-visible
4263 traps during floating-point operations. Setting this option disables
4264 optimizations that may change the number of exceptions visible with
4265 signaling NaNs. This option implies @option{-ftrapping-math}.
4267 This option causes the preprocessor macro @code{__SUPPORT_SNAN__} to
4270 The default is @option{-fno-signaling-nans}.
4272 This option is experimental and does not currently guarantee to
4273 disable all GCC optimizations that affect signaling NaN behavior.
4275 @item -fsingle-precision-constant
4276 @opindex fsingle-precision-constant
4277 Treat floating point constant as single precision constant instead of
4278 implicitly converting it to double precision constant.
4283 The following options control optimizations that may improve
4284 performance, but are not enabled by any @option{-O} options. This
4285 section includes experimental options that may produce broken code.
4288 @item -fbranch-probabilities
4289 @opindex fbranch-probabilities
4290 After running a program compiled with @option{-fprofile-arcs}
4291 (@pxref{Debugging Options,, Options for Debugging Your Program or
4292 @command{gcc}}), you can compile it a second time using
4293 @option{-fbranch-probabilities}, to improve optimizations based on
4294 the number of times each branch was taken. When the program
4295 compiled with @option{-fprofile-arcs} exits it saves arc execution
4296 counts to a file called @file{@var{sourcename}.da} for each source
4297 file The information in this data file is very dependent on the
4298 structure of the generated code, so you must use the same source code
4299 and the same optimization options for both compilations.
4301 With @option{-fbranch-probabilities}, GCC puts a
4302 @samp{REG_BR_PROB} note on each @samp{JUMP_INSN} and @samp{CALL_INSN}.
4303 These can be used to improve optimization. Currently, they are only
4304 used in one place: in @file{reorg.c}, instead of guessing which path a
4305 branch is mostly to take, the @samp{REG_BR_PROB} values are used to
4306 exactly determine which path is taken more often.
4310 Use a graph coloring register allocator. Currently this option is meant
4311 for testing, so we are interested to hear about miscompilations with
4316 Perform tail duplication to enlarge superblock size. This transformation
4317 simplifies the control flow of the function allowing other optimizations to do
4320 @item -funit-at-a-time
4321 @opindex funit-at-a-time
4322 Parse the whole compilation unit before starting to produce code. This allows some
4323 extra optimizations to take place but consumes more memory.
4325 @item -funroll-loops
4326 @opindex funroll-loops
4327 Unroll loops whose number of iterations can be determined at compile time or
4328 upon entry to the loop. @option{-funroll-loops} implies
4329 @option{-frerun-cse-after-loop}. It also turns on complete loop peeling
4330 (i.e. complete removal of loops with small constant number of iterations).
4331 This option makes code larger, and may or may not make it run faster.
4333 @item -funroll-all-loops
4334 @opindex funroll-all-loops
4335 Unroll all loops, even if their number of iterations is uncertain when
4336 the loop is entered. This usually makes programs run more slowly.
4337 @option{-funroll-all-loops} implies the same options as
4338 @option{-funroll-loops}.
4341 @opindex fpeel-loops
4342 Peels the loops for that there is enough information that they do not
4343 roll much (from profile feedback). It also turns on complete loop peeling
4344 (i.e. complete removal of loops with small constant number of iterations).
4346 @item -funswitch-loops
4347 @opindex funswitch-loops
4348 Move branches with loop invariant conditions out of the loop, with duplicates
4349 of the loop on both branches (modified according to result of the condition).
4351 @item -fold-unroll-loops
4352 @opindex fold-unroll-loops
4353 Unroll loops whose number of iterations can be determined at compile
4354 time or upon entry to the loop, using the old loop unroller whose loop
4355 recognition is based on notes from frontend. @option{-fold-unroll-loops} implies
4356 both @option{-fstrength-reduce} and @option{-frerun-cse-after-loop}. This
4357 option makes code larger, and may or may not make it run faster.
4359 @item -fold-unroll-all-loops
4360 @opindex fold-unroll-all-loops
4361 Unroll all loops, even if their number of iterations is uncertain when
4362 the loop is entered. This is done using the old loop unroller whose loop
4363 recognition is based on notes from frontend. This usually makes programs run more slowly.
4364 @option{-fold-unroll-all-loops} implies the same options as
4365 @option{-fold-unroll-loops}.
4367 @item -funswitch-loops
4368 @opindex funswitch-loops
4369 Move branches with loop invariant conditions out of the loop, with duplicates
4370 of the loop on both branches (modified according to result of the condition).
4372 @item -funswitch-loops
4373 @opindex funswitch-loops
4374 Move branches with loop invariant conditions out of the loop, with duplicates
4375 of the loop on both branches (modified according to result of the condition).
4377 @item -fprefetch-loop-arrays
4378 @opindex fprefetch-loop-arrays
4379 If supported by the target machine, generate instructions to prefetch
4380 memory to improve the performance of loops that access large arrays.
4382 Disabled at level @option{-Os}.
4384 @item -ffunction-sections
4385 @itemx -fdata-sections
4386 @opindex ffunction-sections
4387 @opindex fdata-sections
4388 Place each function or data item into its own section in the output
4389 file if the target supports arbitrary sections. The name of the
4390 function or the name of the data item determines the section's name
4393 Use these options on systems where the linker can perform optimizations
4394 to improve locality of reference in the instruction space. Most systems
4395 using the ELF object format and SPARC processors running Solaris 2 have
4396 linkers with such optimizations. AIX may have these optimizations in
4399 Only use these options when there are significant benefits from doing
4400 so. When you specify these options, the assembler and linker will
4401 create larger object and executable files and will also be slower.
4402 You will not be able to use @code{gprof} on all systems if you
4403 specify this option and you may have problems with debugging if
4404 you specify both this option and @option{-g}.
4408 Perform optimizations in static single assignment form. Each function's
4409 flow graph is translated into SSA form, optimizations are performed, and
4410 the flow graph is translated back from SSA form. Users should not
4411 specify this option, since it is not yet ready for production use.
4415 Perform Sparse Conditional Constant Propagation in SSA form. Requires
4416 @option{-fssa}. Like @option{-fssa}, this is an experimental feature.
4420 Perform aggressive dead-code elimination in SSA form. Requires @option{-fssa}.
4421 Like @option{-fssa}, this is an experimental feature.
4426 @item --param @var{name}=@var{value}
4428 In some places, GCC uses various constants to control the amount of
4429 optimization that is done. For example, GCC will not inline functions
4430 that contain more that a certain number of instructions. You can
4431 control some of these constants on the command-line using the
4432 @option{--param} option.
4434 In each case, the @var{value} is an integer. The allowable choices for
4435 @var{name} are given in the following table:
4438 @item max-crossjump-edges
4439 The maximum number of incoming edges to consider for crossjumping.
4440 The algorithm used by @option{-fcrossjumping} is @math{O(N^2)} in
4441 the number of edges incoming to each block. Increasing values mean
4442 more aggressive optimization, making the compile time increase with
4443 probably small improvement in executable size.
4445 @item max-delay-slot-insn-search
4446 The maximum number of instructions to consider when looking for an
4447 instruction to fill a delay slot. If more than this arbitrary number of
4448 instructions is searched, the time savings from filling the delay slot
4449 will be minimal so stop searching. Increasing values mean more
4450 aggressive optimization, making the compile time increase with probably
4451 small improvement in executable run time.
4453 @item max-delay-slot-live-search
4454 When trying to fill delay slots, the maximum number of instructions to
4455 consider when searching for a block with valid live register
4456 information. Increasing this arbitrarily chosen value means more
4457 aggressive optimization, increasing the compile time. This parameter
4458 should be removed when the delay slot code is rewritten to maintain the
4461 @item max-gcse-memory
4462 The approximate maximum amount of memory that will be allocated in
4463 order to perform the global common subexpression elimination
4464 optimization. If more memory than specified is required, the
4465 optimization will not be done.
4467 @item max-gcse-passes
4468 The maximum number of passes of GCSE to run.
4470 @item max-pending-list-length
4471 The maximum number of pending dependencies scheduling will allow
4472 before flushing the current state and starting over. Large functions
4473 with few branches or calls can create excessively large lists which
4474 needlessly consume memory and resources.
4476 @item max-inline-insns-single
4477 Several parameters control the tree inliner used in gcc.
4478 This number sets the maximum number of instructions (counted in gcc's
4479 internal representation) in a single function that the tree inliner
4480 will consider for inlining. This only affects functions declared
4481 inline and methods implemented in a class declaration (C++).
4482 The default value is 300.
4484 @item max-inline-insns-auto
4485 When you use @option{-finline-functions} (included in @option{-O3}),
4486 a lot of functions that would otherwise not be considered for inlining
4487 by the compiler will be investigated. To those functions, a different
4488 (more restrictive) limit compared to functions declared inline can
4490 The default value is 300.
4492 @item max-inline-insns
4493 The tree inliner does decrease the allowable size for single functions
4494 to be inlined after we already inlined the number of instructions
4495 given here by repeated inlining. This number should be a factor of
4496 two or more larger than the single function limit.
4497 Higher numbers result in better runtime performance, but incur higher
4498 compile-time resource (CPU time, memory) requirements and result in
4499 larger binaries. Very high values are not advisable, as too large
4500 binaries may adversely affect runtime performance.
4501 The default value is 600.
4503 @item max-inline-slope
4504 After exceeding the maximum number of inlined instructions by repeated
4505 inlining, a linear function is used to decrease the allowable size
4506 for single functions. The slope of that function is the negative
4507 reciprocal of the number specified here.
4508 The default value is 32.
4510 @item min-inline-insns
4511 The repeated inlining is throttled more and more by the linear function
4512 after exceeding the limit. To avoid too much throttling, a minimum for
4513 this function is specified here to allow repeated inlining for very small
4514 functions even when a lot of repeated inlining already has been done.
4515 The default value is 130.
4517 @item max-inline-insns-rtl
4518 For languages that use the RTL inliner (this happens at a later stage
4519 than tree inlining), you can set the maximum allowable size (counted
4520 in RTL instructions) for the RTL inliner with this parameter.
4521 The default value is 600.
4524 @item max-unrolled-insns
4525 The maximum number of instructions that a loop should have if that loop
4526 is unrolled, and if the loop is unrolled, it determines how many times
4527 the loop code is unrolled.
4529 @item max-average-unrolled-insns
4530 The maximum number of instructions biased by probabilities of their execution
4531 that a loop should have if that loop is unrolled, and if the loop is unrolled,
4532 it determines how many times the loop code is unrolled.
4534 @item max-unroll-times
4535 The maximum number of unrollings of a single loop.
4537 @item max-peeled-insns
4538 The maximum number of instructions that a loop should have if that loop
4539 is peeled, and if the loop is peeled, it determines how many times
4540 the loop code is peeled.
4542 @item max-peel-times
4543 The maximum number of peelings of a single loop.
4545 @item max-completely-peeled-insns
4546 The maximum number of insns of a completely peeled loop.
4548 @item max-completely-peel-times
4549 The maximum number of iterations of a loop to be suitable for complete peeling.
4551 @item max-unswitch-insns
4552 The maximum number of insns of an unswitched loop.
4554 @item max-unswitch-level
4555 The maximum number of branches unswitched in a single loop.
4557 @item hot-bb-count-fraction
4558 Select fraction of the maximal count of repetitions of basic block in program
4559 given basic block needs to have to be considered hot.
4561 @item hot-bb-frequency-fraction
4562 Select fraction of the maximal frequency of executions of basic block in
4563 function given basic block needs to have to be considered hot
4565 @item tracer-dynamic-coverage
4566 @itemx tracer-dynamic-coverage-feedback
4568 This value is used to limit superblock formation once the given percentage of
4569 executed instructions is covered. This limits unnecessary code size
4572 The @option{tracer-dynamic-coverage-feedback} is used only when profile
4573 feedback is available. The real profiles (as opposed to statically estimated
4574 ones) are much less balanced allowing the threshold to be larger value.
4576 @item tracer-max-code-growth
4577 Stop tail duplication once code growth has reached given percentage. This is
4578 rather hokey argument, as most of the duplicates will be eliminated later in
4579 cross jumping, so it may be set to much higher values than is the desired code
4582 @item tracer-min-branch-ratio
4584 Stop reverse growth when the reverse probability of best edge is less than this
4585 threshold (in percent).
4587 @item tracer-min-branch-ratio
4588 @itemx tracer-min-branch-ratio-feedback
4590 Stop forward growth if the best edge do have probability lower than this
4593 Similarly to @option{tracer-dynamic-coverage} two values are present, one for
4594 compilation for profile feedback and one for compilation without. The value
4595 for compilation with profile feedback needs to be more conservative (higher) in
4596 order to make tracer effective.
4598 @item ggc-min-expand
4600 GCC uses a garbage collector to manage its own memory allocation. This
4601 parameter specifies the minimum percentage by which the garbage
4602 collector's heap should be allowed to expand between collections.
4603 Tuning this may improve compilation speed; it has no effect on code
4606 The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when
4607 RAM >= 1GB. If @code{getrlimit} is available, the notion of "RAM" is
4608 the smallest of actual RAM, RLIMIT_RSS, RLIMIT_DATA and RLIMIT_AS. If
4609 GCC is not able to calculate RAM on a particular platform, the lower
4610 bound of 30% is used. Setting this parameter and
4611 @option{ggc-min-heapsize} to zero causes a full collection to occur at
4612 every opportunity. This is extremely slow, but can be useful for
4615 @item ggc-min-heapsize
4617 Minimum size of the garbage collector's heap before it begins bothering
4618 to collect garbage. The first collection occurs after the heap expands
4619 by @option{ggc-min-expand}% beyond @option{ggc-min-heapsize}. Again,
4620 tuning this may improve compilation speed, and has no effect on code
4623 The default is RAM/8, with a lower bound of 4096 (four megabytes) and an
4624 upper bound of 131072 (128 megabytes). If @code{getrlimit} is
4625 available, the notion of "RAM" is the smallest of actual RAM,
4626 RLIMIT_RSS, RLIMIT_DATA and RLIMIT_AS. If GCC is not able to calculate
4627 RAM on a particular platform, the lower bound is used. Setting this
4628 parameter very large effectively disables garbage collection. Setting
4629 this parameter and @option{ggc-min-expand} to zero causes a full
4630 collection to occur at every opportunity.
4632 @item reorder-blocks-duplicate
4633 @itemx reorder-blocks-duplicate-feedback
4635 Used by basic block reordering pass to decide whether to use unconditional
4636 branch or duplicate the code on it's destination. Code is duplicated when it's
4637 estimated size is smaller than this value multiplied by the estimated size of
4638 unconditional jump in the hot spots of the program.
4640 The @option{reorder-block-duplicate-feedback} is used only when profile
4641 feedback is available and may be set to higher values than
4642 @option{reorder-block-duplicate} since information about the hot spots is more
4647 @node Preprocessor Options
4648 @section Options Controlling the Preprocessor
4649 @cindex preprocessor options
4650 @cindex options, preprocessor
4652 These options control the C preprocessor, which is run on each C source
4653 file before actual compilation.
4655 If you use the @option{-E} option, nothing is done except preprocessing.
4656 Some of these options make sense only together with @option{-E} because
4657 they cause the preprocessor output to be unsuitable for actual
4662 You can use @option{-Wp,@var{option}} to bypass the compiler driver
4663 and pass @var{option} directly through to the preprocessor. If
4664 @var{option} contains commas, it is split into multiple options at the
4665 commas. However, many options are modified, translated or interpreted
4666 by the compiler driver before being passed to the preprocessor, and
4667 @option{-Wp} forcibly bypasses this phase. The preprocessor's direct
4668 interface is undocumented and subject to change, so whenever possible
4669 you should avoid using @option{-Wp} and let the driver handle the
4672 @item -Xpreprocessor @var{option}
4673 @opindex preprocessor
4674 Pass @var{option} as an option to the preprocessor. You can use this to
4675 supply system-specific preprocessor options which GCC does not know how to
4678 If you want to pass an option that takes an argument, you must use
4679 @option{-Xpreprocessor} twice, once for the option and once for the argument.
4682 @include cppopts.texi
4684 @node Assembler Options
4685 @section Passing Options to the Assembler
4687 @c prevent bad page break with this line
4688 You can pass options to the assembler.
4691 @item -Wa,@var{option}
4693 Pass @var{option} as an option to the assembler. If @var{option}
4694 contains commas, it is split into multiple options at the commas.
4696 @item -Xassembler @var{option}
4698 Pass @var{option} as an option to the assembler. You can use this to
4699 supply system-specific assembler options which GCC does not know how to
4702 If you want to pass an option that takes an argument, you must use
4703 @option{-Xassembler} twice, once for the option and once for the argument.
4708 @section Options for Linking
4709 @cindex link options
4710 @cindex options, linking
4712 These options come into play when the compiler links object files into
4713 an executable output file. They are meaningless if the compiler is
4714 not doing a link step.
4718 @item @var{object-file-name}
4719 A file name that does not end in a special recognized suffix is
4720 considered to name an object file or library. (Object files are
4721 distinguished from libraries by the linker according to the file
4722 contents.) If linking is done, these object files are used as input
4731 If any of these options is used, then the linker is not run, and
4732 object file names should not be used as arguments. @xref{Overall
4736 @item -l@var{library}
4737 @itemx -l @var{library}
4739 Search the library named @var{library} when linking. (The second
4740 alternative with the library as a separate argument is only for
4741 POSIX compliance and is not recommended.)
4743 It makes a difference where in the command you write this option; the
4744 linker searches and processes libraries and object files in the order they
4745 are specified. Thus, @samp{foo.o -lz bar.o} searches library @samp{z}
4746 after file @file{foo.o} but before @file{bar.o}. If @file{bar.o} refers
4747 to functions in @samp{z}, those functions may not be loaded.
4749 The linker searches a standard list of directories for the library,
4750 which is actually a file named @file{lib@var{library}.a}. The linker
4751 then uses this file as if it had been specified precisely by name.
4753 The directories searched include several standard system directories
4754 plus any that you specify with @option{-L}.
4756 Normally the files found this way are library files---archive files
4757 whose members are object files. The linker handles an archive file by
4758 scanning through it for members which define symbols that have so far
4759 been referenced but not defined. But if the file that is found is an
4760 ordinary object file, it is linked in the usual fashion. The only
4761 difference between using an @option{-l} option and specifying a file name
4762 is that @option{-l} surrounds @var{library} with @samp{lib} and @samp{.a}
4763 and searches several directories.
4767 You need this special case of the @option{-l} option in order to
4768 link an Objective-C program.
4771 @opindex nostartfiles
4772 Do not use the standard system startup files when linking.
4773 The standard system libraries are used normally, unless @option{-nostdlib}
4774 or @option{-nodefaultlibs} is used.
4776 @item -nodefaultlibs
4777 @opindex nodefaultlibs
4778 Do not use the standard system libraries when linking.
4779 Only the libraries you specify will be passed to the linker.
4780 The standard startup files are used normally, unless @option{-nostartfiles}
4781 is used. The compiler may generate calls to memcmp, memset, and memcpy
4782 for System V (and ISO C) environments or to bcopy and bzero for
4783 BSD environments. These entries are usually resolved by entries in
4784 libc. These entry points should be supplied through some other
4785 mechanism when this option is specified.
4789 Do not use the standard system startup files or libraries when linking.
4790 No startup files and only the libraries you specify will be passed to
4791 the linker. The compiler may generate calls to memcmp, memset, and memcpy
4792 for System V (and ISO C) environments or to bcopy and bzero for
4793 BSD environments. These entries are usually resolved by entries in
4794 libc. These entry points should be supplied through some other
4795 mechanism when this option is specified.
4797 @cindex @option{-lgcc}, use with @option{-nostdlib}
4798 @cindex @option{-nostdlib} and unresolved references
4799 @cindex unresolved references and @option{-nostdlib}
4800 @cindex @option{-lgcc}, use with @option{-nodefaultlibs}
4801 @cindex @option{-nodefaultlibs} and unresolved references
4802 @cindex unresolved references and @option{-nodefaultlibs}
4803 One of the standard libraries bypassed by @option{-nostdlib} and
4804 @option{-nodefaultlibs} is @file{libgcc.a}, a library of internal subroutines
4805 that GCC uses to overcome shortcomings of particular machines, or special
4806 needs for some languages.
4807 (@xref{Interface,,Interfacing to GCC Output,gccint,GNU Compiler
4808 Collection (GCC) Internals},
4809 for more discussion of @file{libgcc.a}.)
4810 In most cases, you need @file{libgcc.a} even when you want to avoid
4811 other standard libraries. In other words, when you specify @option{-nostdlib}
4812 or @option{-nodefaultlibs} you should usually specify @option{-lgcc} as well.
4813 This ensures that you have no unresolved references to internal GCC
4814 library subroutines. (For example, @samp{__main}, used to ensure C++
4815 constructors will be called; @pxref{Collect2,,@code{collect2}, gccint,
4816 GNU Compiler Collection (GCC) Internals}.)
4820 Produce a position independent executable on targets which support it.
4821 For predictable results, you must also specify the same set of options
4822 that were used to generate code (@option{-fpie}, @option{-fPIE},
4823 or model suboptions) when you specify this option.
4827 Remove all symbol table and relocation information from the executable.
4831 On systems that support dynamic linking, this prevents linking with the shared
4832 libraries. On other systems, this option has no effect.
4836 Produce a shared object which can then be linked with other objects to
4837 form an executable. Not all systems support this option. For predictable
4838 results, you must also specify the same set of options that were used to
4839 generate code (@option{-fpic}, @option{-fPIC}, or model suboptions)
4840 when you specify this option.@footnote{On some systems, @samp{gcc -shared}
4841 needs to build supplementary stub code for constructors to work. On
4842 multi-libbed systems, @samp{gcc -shared} must select the correct support
4843 libraries to link against. Failing to supply the correct flags may lead
4844 to subtle defects. Supplying them in cases where they are not necessary
4847 @item -shared-libgcc
4848 @itemx -static-libgcc
4849 @opindex shared-libgcc
4850 @opindex static-libgcc
4851 On systems that provide @file{libgcc} as a shared library, these options
4852 force the use of either the shared or static version respectively.
4853 If no shared version of @file{libgcc} was built when the compiler was
4854 configured, these options have no effect.
4856 There are several situations in which an application should use the
4857 shared @file{libgcc} instead of the static version. The most common
4858 of these is when the application wishes to throw and catch exceptions
4859 across different shared libraries. In that case, each of the libraries
4860 as well as the application itself should use the shared @file{libgcc}.
4862 Therefore, the G++ and GCJ drivers automatically add
4863 @option{-shared-libgcc} whenever you build a shared library or a main
4864 executable, because C++ and Java programs typically use exceptions, so
4865 this is the right thing to do.
4867 If, instead, you use the GCC driver to create shared libraries, you may
4868 find that they will not always be linked with the shared @file{libgcc}.
4869 If GCC finds, at its configuration time, that you have a GNU linker that
4870 does not support option @option{--eh-frame-hdr}, it will link the shared
4871 version of @file{libgcc} into shared libraries by default. Otherwise,
4872 it will take advantage of the linker and optimize away the linking with
4873 the shared version of @file{libgcc}, linking with the static version of
4874 libgcc by default. This allows exceptions to propagate through such
4875 shared libraries, without incurring relocation costs at library load
4878 However, if a library or main executable is supposed to throw or catch
4879 exceptions, you must link it using the G++ or GCJ driver, as appropriate
4880 for the languages used in the program, or using the option
4881 @option{-shared-libgcc}, such that it is linked with the shared
4886 Bind references to global symbols when building a shared object. Warn
4887 about any unresolved references (unless overridden by the link editor
4888 option @samp{-Xlinker -z -Xlinker defs}). Only a few systems support
4891 @item -Xlinker @var{option}
4893 Pass @var{option} as an option to the linker. You can use this to
4894 supply system-specific linker options which GCC does not know how to
4897 If you want to pass an option that takes an argument, you must use
4898 @option{-Xlinker} twice, once for the option and once for the argument.
4899 For example, to pass @option{-assert definitions}, you must write
4900 @samp{-Xlinker -assert -Xlinker definitions}. It does not work to write
4901 @option{-Xlinker "-assert definitions"}, because this passes the entire
4902 string as a single argument, which is not what the linker expects.
4904 @item -Wl,@var{option}
4906 Pass @var{option} as an option to the linker. If @var{option} contains
4907 commas, it is split into multiple options at the commas.
4909 @item -u @var{symbol}
4911 Pretend the symbol @var{symbol} is undefined, to force linking of
4912 library modules to define it. You can use @option{-u} multiple times with
4913 different symbols to force loading of additional library modules.
4916 @node Directory Options
4917 @section Options for Directory Search
4918 @cindex directory options
4919 @cindex options, directory search
4922 These options specify directories to search for header files, for
4923 libraries and for parts of the compiler:
4928 Add the directory @var{dir} to the head of the list of directories to be
4929 searched for header files. This can be used to override a system header
4930 file, substituting your own version, since these directories are
4931 searched before the system header file directories. However, you should
4932 not use this option to add directories that contain vendor-supplied
4933 system header files (use @option{-isystem} for that). If you use more than
4934 one @option{-I} option, the directories are scanned in left-to-right
4935 order; the standard system directories come after.
4937 If a standard system include directory, or a directory specified with
4938 @option{-isystem}, is also specified with @option{-I}, the @option{-I}
4939 option will be ignored. The directory will still be searched but as a
4940 system directory at its normal position in the system include chain.
4941 This is to ensure that GCC's procedure to fix buggy system headers and
4942 the ordering for the include_next directive are not inadvertently changed.
4943 If you really need to change the search order for system directories,
4944 use the @option{-nostdinc} and/or @option{-isystem} options.
4948 Any directories you specify with @option{-I} options before the @option{-I-}
4949 option are searched only for the case of @samp{#include "@var{file}"};
4950 they are not searched for @samp{#include <@var{file}>}.
4952 If additional directories are specified with @option{-I} options after
4953 the @option{-I-}, these directories are searched for all @samp{#include}
4954 directives. (Ordinarily @emph{all} @option{-I} directories are used
4957 In addition, the @option{-I-} option inhibits the use of the current
4958 directory (where the current input file came from) as the first search
4959 directory for @samp{#include "@var{file}"}. There is no way to
4960 override this effect of @option{-I-}. With @option{-I.} you can specify
4961 searching the directory which was current when the compiler was
4962 invoked. That is not exactly the same as what the preprocessor does
4963 by default, but it is often satisfactory.
4965 @option{-I-} does not inhibit the use of the standard system directories
4966 for header files. Thus, @option{-I-} and @option{-nostdinc} are
4971 Add directory @var{dir} to the list of directories to be searched
4974 @item -B@var{prefix}
4976 This option specifies where to find the executables, libraries,
4977 include files, and data files of the compiler itself.
4979 The compiler driver program runs one or more of the subprograms
4980 @file{cpp}, @file{cc1}, @file{as} and @file{ld}. It tries
4981 @var{prefix} as a prefix for each program it tries to run, both with and
4982 without @samp{@var{machine}/@var{version}/} (@pxref{Target Options}).
4984 For each subprogram to be run, the compiler driver first tries the
4985 @option{-B} prefix, if any. If that name is not found, or if @option{-B}
4986 was not specified, the driver tries two standard prefixes, which are
4987 @file{/usr/lib/gcc/} and @file{/usr/local/lib/gcc-lib/}. If neither of
4988 those results in a file name that is found, the unmodified program
4989 name is searched for using the directories specified in your
4990 @env{PATH} environment variable.
4992 The compiler will check to see if the path provided by the @option{-B}
4993 refers to a directory, and if necessary it will add a directory
4994 separator character at the end of the path.
4996 @option{-B} prefixes that effectively specify directory names also apply
4997 to libraries in the linker, because the compiler translates these
4998 options into @option{-L} options for the linker. They also apply to
4999 includes files in the preprocessor, because the compiler translates these
5000 options into @option{-isystem} options for the preprocessor. In this case,
5001 the compiler appends @samp{include} to the prefix.
5003 The run-time support file @file{libgcc.a} can also be searched for using
5004 the @option{-B} prefix, if needed. If it is not found there, the two
5005 standard prefixes above are tried, and that is all. The file is left
5006 out of the link if it is not found by those means.
5008 Another way to specify a prefix much like the @option{-B} prefix is to use
5009 the environment variable @env{GCC_EXEC_PREFIX}. @xref{Environment
5012 As a special kludge, if the path provided by @option{-B} is
5013 @file{[dir/]stage@var{N}/}, where @var{N} is a number in the range 0 to
5014 9, then it will be replaced by @file{[dir/]include}. This is to help
5015 with boot-strapping the compiler.
5017 @item -specs=@var{file}
5019 Process @var{file} after the compiler reads in the standard @file{specs}
5020 file, in order to override the defaults that the @file{gcc} driver
5021 program uses when determining what switches to pass to @file{cc1},
5022 @file{cc1plus}, @file{as}, @file{ld}, etc. More than one
5023 @option{-specs=@var{file}} can be specified on the command line, and they
5024 are processed in order, from left to right.
5030 @section Specifying subprocesses and the switches to pass to them
5033 @command{gcc} is a driver program. It performs its job by invoking a
5034 sequence of other programs to do the work of compiling, assembling and
5035 linking. GCC interprets its command-line parameters and uses these to
5036 deduce which programs it should invoke, and which command-line options
5037 it ought to place on their command lines. This behavior is controlled
5038 by @dfn{spec strings}. In most cases there is one spec string for each
5039 program that GCC can invoke, but a few programs have multiple spec
5040 strings to control their behavior. The spec strings built into GCC can
5041 be overridden by using the @option{-specs=} command-line switch to specify
5044 @dfn{Spec files} are plaintext files that are used to construct spec
5045 strings. They consist of a sequence of directives separated by blank
5046 lines. The type of directive is determined by the first non-whitespace
5047 character on the line and it can be one of the following:
5050 @item %@var{command}
5051 Issues a @var{command} to the spec file processor. The commands that can
5055 @item %include <@var{file}>
5057 Search for @var{file} and insert its text at the current point in the
5060 @item %include_noerr <@var{file}>
5061 @cindex %include_noerr
5062 Just like @samp{%include}, but do not generate an error message if the include
5063 file cannot be found.
5065 @item %rename @var{old_name} @var{new_name}
5067 Rename the spec string @var{old_name} to @var{new_name}.
5071 @item *[@var{spec_name}]:
5072 This tells the compiler to create, override or delete the named spec
5073 string. All lines after this directive up to the next directive or
5074 blank line are considered to be the text for the spec string. If this
5075 results in an empty string then the spec will be deleted. (Or, if the
5076 spec did not exist, then nothing will happened.) Otherwise, if the spec
5077 does not currently exist a new spec will be created. If the spec does
5078 exist then its contents will be overridden by the text of this
5079 directive, unless the first character of that text is the @samp{+}
5080 character, in which case the text will be appended to the spec.
5082 @item [@var{suffix}]:
5083 Creates a new @samp{[@var{suffix}] spec} pair. All lines after this directive
5084 and up to the next directive or blank line are considered to make up the
5085 spec string for the indicated suffix. When the compiler encounters an
5086 input file with the named suffix, it will processes the spec string in
5087 order to work out how to compile that file. For example:
5094 This says that any input file whose name ends in @samp{.ZZ} should be
5095 passed to the program @samp{z-compile}, which should be invoked with the
5096 command-line switch @option{-input} and with the result of performing the
5097 @samp{%i} substitution. (See below.)
5099 As an alternative to providing a spec string, the text that follows a
5100 suffix directive can be one of the following:
5103 @item @@@var{language}
5104 This says that the suffix is an alias for a known @var{language}. This is
5105 similar to using the @option{-x} command-line switch to GCC to specify a
5106 language explicitly. For example:
5113 Says that .ZZ files are, in fact, C++ source files.
5116 This causes an error messages saying:
5119 @var{name} compiler not installed on this system.
5123 GCC already has an extensive list of suffixes built into it.
5124 This directive will add an entry to the end of the list of suffixes, but
5125 since the list is searched from the end backwards, it is effectively
5126 possible to override earlier entries using this technique.
5130 GCC has the following spec strings built into it. Spec files can
5131 override these strings or create their own. Note that individual
5132 targets can also add their own spec strings to this list.
5135 asm Options to pass to the assembler
5136 asm_final Options to pass to the assembler post-processor
5137 cpp Options to pass to the C preprocessor
5138 cc1 Options to pass to the C compiler
5139 cc1plus Options to pass to the C++ compiler
5140 endfile Object files to include at the end of the link
5141 link Options to pass to the linker
5142 lib Libraries to include on the command line to the linker
5143 libgcc Decides which GCC support library to pass to the linker
5144 linker Sets the name of the linker
5145 predefines Defines to be passed to the C preprocessor
5146 signed_char Defines to pass to CPP to say whether @code{char} is signed
5148 startfile Object files to include at the start of the link
5151 Here is a small example of a spec file:
5157 --start-group -lgcc -lc -leval1 --end-group %(old_lib)
5160 This example renames the spec called @samp{lib} to @samp{old_lib} and
5161 then overrides the previous definition of @samp{lib} with a new one.
5162 The new definition adds in some extra command-line options before
5163 including the text of the old definition.
5165 @dfn{Spec strings} are a list of command-line options to be passed to their
5166 corresponding program. In addition, the spec strings can contain
5167 @samp{%}-prefixed sequences to substitute variable text or to
5168 conditionally insert text into the command line. Using these constructs
5169 it is possible to generate quite complex command lines.
5171 Here is a table of all defined @samp{%}-sequences for spec
5172 strings. Note that spaces are not generated automatically around the
5173 results of expanding these sequences. Therefore you can concatenate them
5174 together or combine them with constant text in a single argument.
5178 Substitute one @samp{%} into the program name or argument.
5181 Substitute the name of the input file being processed.
5184 Substitute the basename of the input file being processed.
5185 This is the substring up to (and not including) the last period
5186 and not including the directory.
5189 This is the same as @samp{%b}, but include the file suffix (text after
5193 Marks the argument containing or following the @samp{%d} as a
5194 temporary file name, so that that file will be deleted if GCC exits
5195 successfully. Unlike @samp{%g}, this contributes no text to the
5198 @item %g@var{suffix}
5199 Substitute a file name that has suffix @var{suffix} and is chosen
5200 once per compilation, and mark the argument in the same way as
5201 @samp{%d}. To reduce exposure to denial-of-service attacks, the file
5202 name is now chosen in a way that is hard to predict even when previously
5203 chosen file names are known. For example, @samp{%g.s @dots{} %g.o @dots{} %g.s}
5204 might turn into @samp{ccUVUUAU.s ccXYAXZ12.o ccUVUUAU.s}. @var{suffix} matches
5205 the regexp @samp{[.A-Za-z]*} or the special string @samp{%O}, which is
5206 treated exactly as if @samp{%O} had been preprocessed. Previously, @samp{%g}
5207 was simply substituted with a file name chosen once per compilation,
5208 without regard to any appended suffix (which was therefore treated
5209 just like ordinary text), making such attacks more likely to succeed.
5211 @item %u@var{suffix}
5212 Like @samp{%g}, but generates a new temporary file name even if
5213 @samp{%u@var{suffix}} was already seen.
5215 @item %U@var{suffix}
5216 Substitutes the last file name generated with @samp{%u@var{suffix}}, generating a
5217 new one if there is no such last file name. In the absence of any
5218 @samp{%u@var{suffix}}, this is just like @samp{%g@var{suffix}}, except they don't share
5219 the same suffix @emph{space}, so @samp{%g.s @dots{} %U.s @dots{} %g.s @dots{} %U.s}
5220 would involve the generation of two distinct file names, one
5221 for each @samp{%g.s} and another for each @samp{%U.s}. Previously, @samp{%U} was
5222 simply substituted with a file name chosen for the previous @samp{%u},
5223 without regard to any appended suffix.
5225 @item %j@var{suffix}
5226 Substitutes the name of the @code{HOST_BIT_BUCKET}, if any, and if it is
5227 writable, and if save-temps is off; otherwise, substitute the name
5228 of a temporary file, just like @samp{%u}. This temporary file is not
5229 meant for communication between processes, but rather as a junk
5232 @item %|@var{suffix}
5233 @itemx %m@var{suffix}
5234 Like @samp{%g}, except if @option{-pipe} is in effect. In that case
5235 @samp{%|} substitutes a single dash and @samp{%m} substitutes nothing at
5236 all. These are the two most common ways to instruct a program that it
5237 should read from standard input or write to standard output. If you
5238 need something more elaborate you can use an @samp{%@{pipe:@code{X}@}}
5239 construct: see for example @file{f/lang-specs.h}.
5241 @item %.@var{SUFFIX}
5242 Substitutes @var{.SUFFIX} for the suffixes of a matched switch's args
5243 when it is subsequently output with @samp{%*}. @var{SUFFIX} is
5244 terminated by the next space or %.
5247 Marks the argument containing or following the @samp{%w} as the
5248 designated output file of this compilation. This puts the argument
5249 into the sequence of arguments that @samp{%o} will substitute later.
5252 Substitutes the names of all the output files, with spaces
5253 automatically placed around them. You should write spaces
5254 around the @samp{%o} as well or the results are undefined.
5255 @samp{%o} is for use in the specs for running the linker.
5256 Input files whose names have no recognized suffix are not compiled
5257 at all, but they are included among the output files, so they will
5261 Substitutes the suffix for object files. Note that this is
5262 handled specially when it immediately follows @samp{%g, %u, or %U},
5263 because of the need for those to form complete file names. The
5264 handling is such that @samp{%O} is treated exactly as if it had already
5265 been substituted, except that @samp{%g, %u, and %U} do not currently
5266 support additional @var{suffix} characters following @samp{%O} as they would
5267 following, for example, @samp{.o}.
5270 Substitutes the standard macro predefinitions for the
5271 current target machine. Use this when running @code{cpp}.
5274 Like @samp{%p}, but puts @samp{__} before and after the name of each
5275 predefined macro, except for macros that start with @samp{__} or with
5276 @samp{_@var{L}}, where @var{L} is an uppercase letter. This is for ISO
5280 Substitute any of @option{-iprefix} (made from @env{GCC_EXEC_PREFIX}),
5281 @option{-isysroot} (made from @env{TARGET_SYSTEM_ROOT}), and
5282 @option{-isystem} (made from @env{COMPILER_PATH} and @option{-B} options)
5286 Current argument is the name of a library or startup file of some sort.
5287 Search for that file in a standard list of directories and substitute
5288 the full name found.
5291 Print @var{str} as an error message. @var{str} is terminated by a newline.
5292 Use this when inconsistent options are detected.
5295 Substitute the contents of spec string @var{name} at this point.
5298 Like @samp{%(@dots{})} but put @samp{__} around @option{-D} arguments.
5300 @item %x@{@var{option}@}
5301 Accumulate an option for @samp{%X}.
5304 Output the accumulated linker options specified by @option{-Wl} or a @samp{%x}
5308 Output the accumulated assembler options specified by @option{-Wa}.
5311 Output the accumulated preprocessor options specified by @option{-Wp}.
5314 Process the @code{asm} spec. This is used to compute the
5315 switches to be passed to the assembler.
5318 Process the @code{asm_final} spec. This is a spec string for
5319 passing switches to an assembler post-processor, if such a program is
5323 Process the @code{link} spec. This is the spec for computing the
5324 command line passed to the linker. Typically it will make use of the
5325 @samp{%L %G %S %D and %E} sequences.
5328 Dump out a @option{-L} option for each directory that GCC believes might
5329 contain startup files. If the target supports multilibs then the
5330 current multilib directory will be prepended to each of these paths.
5333 Output the multilib directory with directory separators replaced with
5334 @samp{_}. If multilib directories are not set, or the multilib directory is
5335 @file{.} then this option emits nothing.
5338 Process the @code{lib} spec. This is a spec string for deciding which
5339 libraries should be included on the command line to the linker.
5342 Process the @code{libgcc} spec. This is a spec string for deciding
5343 which GCC support library should be included on the command line to the linker.
5346 Process the @code{startfile} spec. This is a spec for deciding which
5347 object files should be the first ones passed to the linker. Typically
5348 this might be a file named @file{crt0.o}.
5351 Process the @code{endfile} spec. This is a spec string that specifies
5352 the last object files that will be passed to the linker.
5355 Process the @code{cpp} spec. This is used to construct the arguments
5356 to be passed to the C preprocessor.
5359 Process the @code{signed_char} spec. This is intended to be used
5360 to tell cpp whether a char is signed. It typically has the definition:
5362 %@{funsigned-char:-D__CHAR_UNSIGNED__@}
5366 Process the @code{cc1} spec. This is used to construct the options to be
5367 passed to the actual C compiler (@samp{cc1}).
5370 Process the @code{cc1plus} spec. This is used to construct the options to be
5371 passed to the actual C++ compiler (@samp{cc1plus}).
5374 Substitute the variable part of a matched option. See below.
5375 Note that each comma in the substituted string is replaced by
5379 Remove all occurrences of @code{-S} from the command line. Note---this
5380 command is position dependent. @samp{%} commands in the spec string
5381 before this one will see @code{-S}, @samp{%} commands in the spec string
5382 after this one will not.
5384 @item %:@var{function}(@var{args})
5385 Call the named function @var{function}, passing it @var{args}.
5386 @var{args} is first processed as a nested spec string, then split
5387 into an argument vector in the usual fashion. The function returns
5388 a string which is processed as if it had appeared literally as part
5389 of the current spec.
5391 The following built-in spec functions are provided:
5394 @item @code{if-exists}
5395 The @code{if-exists} spec function takes one argument, an absolute
5396 pathname to a file. If the file exists, @code{if-exists} returns the
5397 pathname. Here is a small example of its usage:
5401 crt0%O%s %:if-exists(crti%O%s) crtbegin%O%s
5404 @item @code{if-exists-else}
5405 The @code{if-exists-else} spec function is similar to the @code{if-exists}
5406 spec function, except that it takes two arguments. The first argument is
5407 an absolute pathname to a file. If the file exists, @code{if-exists-else}
5408 returns the pathname. If it does not exist, it returns the second argument.
5409 This way, @code{if-exists-else} can be used to select one file or another,
5410 based on the existence of the first. Here is a small example of its usage:
5414 crt0%O%s %:if-exists(crti%O%s) \
5415 %:if-exists-else(crtbeginT%O%s crtbegin%O%s)
5420 Substitutes the @code{-S} switch, if that switch was given to GCC@.
5421 If that switch was not specified, this substitutes nothing. Note that
5422 the leading dash is omitted when specifying this option, and it is
5423 automatically inserted if the substitution is performed. Thus the spec
5424 string @samp{%@{foo@}} would match the command-line option @option{-foo}
5425 and would output the command line option @option{-foo}.
5427 @item %W@{@code{S}@}
5428 Like %@{@code{S}@} but mark last argument supplied within as a file to be
5431 @item %@{@code{S}*@}
5432 Substitutes all the switches specified to GCC whose names start
5433 with @code{-S}, but which also take an argument. This is used for
5434 switches like @option{-o}, @option{-D}, @option{-I}, etc.
5435 GCC considers @option{-o foo} as being
5436 one switch whose names starts with @samp{o}. %@{o*@} would substitute this
5437 text, including the space. Thus two arguments would be generated.
5439 @item %@{@code{S}*&@code{T}*@}
5440 Like %@{@code{S}*@}, but preserve order of @code{S} and @code{T} options
5441 (the order of @code{S} and @code{T} in the spec is not significant).
5442 There can be any number of ampersand-separated variables; for each the
5443 wild card is optional. Useful for CPP as @samp{%@{D*&U*&A*@}}.
5445 @item %@{@code{S}:@code{X}@}
5446 Substitutes @code{X}, if the @samp{-S} switch was given to GCC@.
5448 @item %@{!@code{S}:@code{X}@}
5449 Substitutes @code{X}, if the @samp{-S} switch was @emph{not} given to GCC@.
5451 @item %@{@code{S}*:@code{X}@}
5452 Substitutes @code{X} if one or more switches whose names start with
5453 @code{-S} are specified to GCC@. Normally @code{X} is substituted only
5454 once, no matter how many such switches appeared. However, if @code{%*}
5455 appears somewhere in @code{X}, then @code{X} will be substituted once
5456 for each matching switch, with the @code{%*} replaced by the part of
5457 that switch that matched the @code{*}.
5459 @item %@{.@code{S}:@code{X}@}
5460 Substitutes @code{X}, if processing a file with suffix @code{S}.
5462 @item %@{!.@code{S}:@code{X}@}
5463 Substitutes @code{X}, if @emph{not} processing a file with suffix @code{S}.
5465 @item %@{@code{S}|@code{P}:@code{X}@}
5466 Substitutes @code{X} if either @code{-S} or @code{-P} was given to GCC@.
5467 This may be combined with @samp{!}, @samp{.}, and @code{*} sequences as well,
5468 although they have a stronger binding than the @samp{|}. If @code{%*}
5469 appears in @code{X}, all of the alternatives must be starred, and only
5470 the first matching alternative is substituted.
5472 For example, a spec string like this:
5475 %@{.c:-foo@} %@{!.c:-bar@} %@{.c|d:-baz@} %@{!.c|d:-boggle@}
5478 will output the following command-line options from the following input
5479 command-line options:
5484 -d fred.c -foo -baz -boggle
5485 -d jim.d -bar -baz -boggle
5488 @item %@{S:X; T:Y; :D@}
5490 If @code{S} was given to GCC, substitues @code{X}; else if @code{T} was
5491 given to GCC, substitues @code{Y}; else substitutes @code{D}. There can
5492 be as many clauses as you need. This may be combined with @code{.},
5493 @code{!}, @code{|}, and @code{*} as needed.
5498 The conditional text @code{X} in a %@{@code{S}:@code{X}@} or similar
5499 construct may contain other nested @samp{%} constructs or spaces, or
5500 even newlines. They are processed as usual, as described above.
5501 Trailing white space in @code{X} is ignored. White space may also
5502 appear anywhere on the left side of the colon in these constructs,
5503 except between @code{.} or @code{*} and the corresponding word.
5505 The @option{-O}, @option{-f}, @option{-m}, and @option{-W} switches are
5506 handled specifically in these constructs. If another value of
5507 @option{-O} or the negated form of a @option{-f}, @option{-m}, or
5508 @option{-W} switch is found later in the command line, the earlier
5509 switch value is ignored, except with @{@code{S}*@} where @code{S} is
5510 just one letter, which passes all matching options.
5512 The character @samp{|} at the beginning of the predicate text is used to
5513 indicate that a command should be piped to the following command, but
5514 only if @option{-pipe} is specified.
5516 It is built into GCC which switches take arguments and which do not.
5517 (You might think it would be useful to generalize this to allow each
5518 compiler's spec to say which switches take arguments. But this cannot
5519 be done in a consistent fashion. GCC cannot even decide which input
5520 files have been specified without knowing which switches take arguments,
5521 and it must know which input files to compile in order to tell which
5524 GCC also knows implicitly that arguments starting in @option{-l} are to be
5525 treated as compiler output files, and passed to the linker in their
5526 proper position among the other output files.
5528 @c man begin OPTIONS
5530 @node Target Options
5531 @section Specifying Target Machine and Compiler Version
5532 @cindex target options
5533 @cindex cross compiling
5534 @cindex specifying machine version
5535 @cindex specifying compiler version and target machine
5536 @cindex compiler version, specifying
5537 @cindex target machine, specifying
5539 The usual way to run GCC is to run the executable called @file{gcc}, or
5540 @file{<machine>-gcc} when cross-compiling, or
5541 @file{<machine>-gcc-<version>} to run a version other than the one that
5542 was installed last. Sometimes this is inconvenient, so GCC provides
5543 options that will switch to another cross-compiler or version.
5546 @item -b @var{machine}
5548 The argument @var{machine} specifies the target machine for compilation.
5550 The value to use for @var{machine} is the same as was specified as the
5551 machine type when configuring GCC as a cross-compiler. For
5552 example, if a cross-compiler was configured with @samp{configure
5553 i386v}, meaning to compile for an 80386 running System V, then you
5554 would specify @option{-b i386v} to run that cross compiler.
5556 @item -V @var{version}
5558 The argument @var{version} specifies which version of GCC to run.
5559 This is useful when multiple versions are installed. For example,
5560 @var{version} might be @samp{2.0}, meaning to run GCC version 2.0.
5563 The @option{-V} and @option{-b} options work by running the
5564 @file{<machine>-gcc-<version>} executable, so there's no real reason to
5565 use them if you can just run that directly.
5567 @node Submodel Options
5568 @section Hardware Models and Configurations
5569 @cindex submodel options
5570 @cindex specifying hardware config
5571 @cindex hardware models and configurations, specifying
5572 @cindex machine dependent options
5574 Earlier we discussed the standard option @option{-b} which chooses among
5575 different installed compilers for completely different target
5576 machines, such as VAX vs.@: 68000 vs.@: 80386.
5578 In addition, each of these target machine types can have its own
5579 special options, starting with @samp{-m}, to choose among various
5580 hardware models or configurations---for example, 68010 vs 68020,
5581 floating coprocessor or none. A single installed version of the
5582 compiler can compile for any model or configuration, according to the
5585 Some configurations of the compiler also support additional special
5586 options, usually for compatibility with other compilers on the same
5589 These options are defined by the macro @code{TARGET_SWITCHES} in the
5590 machine description. The default for the options is also defined by
5591 that macro, which enables you to change the defaults.
5603 * RS/6000 and PowerPC Options::
5607 * i386 and x86-64 Options::
5609 * Intel 960 Options::
5610 * DEC Alpha Options::
5611 * DEC Alpha/VMS Options::
5614 * System V Options::
5615 * TMS320C3x/C4x Options::
5623 * S/390 and zSeries Options::
5627 * Xstormy16 Options::
5632 @node M680x0 Options
5633 @subsection M680x0 Options
5634 @cindex M680x0 options
5636 These are the @samp{-m} options defined for the 68000 series. The default
5637 values for these options depends on which style of 68000 was selected when
5638 the compiler was configured; the defaults for the most common choices are
5646 Generate output for a 68000. This is the default
5647 when the compiler is configured for 68000-based systems.
5649 Use this option for microcontrollers with a 68000 or EC000 core,
5650 including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356.
5656 Generate output for a 68020. This is the default
5657 when the compiler is configured for 68020-based systems.
5661 Generate output containing 68881 instructions for floating point.
5662 This is the default for most 68020 systems unless @option{--nfp} was
5663 specified when the compiler was configured.
5667 Generate output for a 68030. This is the default when the compiler is
5668 configured for 68030-based systems.
5672 Generate output for a 68040. This is the default when the compiler is
5673 configured for 68040-based systems.
5675 This option inhibits the use of 68881/68882 instructions that have to be
5676 emulated by software on the 68040. Use this option if your 68040 does not
5677 have code to emulate those instructions.
5681 Generate output for a 68060. This is the default when the compiler is
5682 configured for 68060-based systems.
5684 This option inhibits the use of 68020 and 68881/68882 instructions that
5685 have to be emulated by software on the 68060. Use this option if your 68060
5686 does not have code to emulate those instructions.
5690 Generate output for a CPU32. This is the default
5691 when the compiler is configured for CPU32-based systems.
5693 Use this option for microcontrollers with a
5694 CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334,
5695 68336, 68340, 68341, 68349 and 68360.
5699 Generate output for a 520X ``coldfire'' family cpu. This is the default
5700 when the compiler is configured for 520X-based systems.
5702 Use this option for microcontroller with a 5200 core, including
5703 the MCF5202, MCF5203, MCF5204 and MCF5202.
5708 Generate output for a 68040, without using any of the new instructions.
5709 This results in code which can run relatively efficiently on either a
5710 68020/68881 or a 68030 or a 68040. The generated code does use the
5711 68881 instructions that are emulated on the 68040.
5715 Generate output for a 68060, without using any of the new instructions.
5716 This results in code which can run relatively efficiently on either a
5717 68020/68881 or a 68030 or a 68040. The generated code does use the
5718 68881 instructions that are emulated on the 68060.
5721 @opindex msoft-float
5722 Generate output containing library calls for floating point.
5723 @strong{Warning:} the requisite libraries are not available for all m68k
5724 targets. Normally the facilities of the machine's usual C compiler are
5725 used, but this can't be done directly in cross-compilation. You must
5726 make your own arrangements to provide suitable library functions for
5727 cross-compilation. The embedded targets @samp{m68k-*-aout} and
5728 @samp{m68k-*-coff} do provide software floating point support.
5732 Consider type @code{int} to be 16 bits wide, like @code{short int}.
5735 @opindex mnobitfield
5736 Do not use the bit-field instructions. The @option{-m68000}, @option{-mcpu32}
5737 and @option{-m5200} options imply @w{@option{-mnobitfield}}.
5741 Do use the bit-field instructions. The @option{-m68020} option implies
5742 @option{-mbitfield}. This is the default if you use a configuration
5743 designed for a 68020.
5747 Use a different function-calling convention, in which functions
5748 that take a fixed number of arguments return with the @code{rtd}
5749 instruction, which pops their arguments while returning. This
5750 saves one instruction in the caller since there is no need to pop
5751 the arguments there.
5753 This calling convention is incompatible with the one normally
5754 used on Unix, so you cannot use it if you need to call libraries
5755 compiled with the Unix compiler.
5757 Also, you must provide function prototypes for all functions that
5758 take variable numbers of arguments (including @code{printf});
5759 otherwise incorrect code will be generated for calls to those
5762 In addition, seriously incorrect code will result if you call a
5763 function with too many arguments. (Normally, extra arguments are
5764 harmlessly ignored.)
5766 The @code{rtd} instruction is supported by the 68010, 68020, 68030,
5767 68040, 68060 and CPU32 processors, but not by the 68000 or 5200.
5770 @itemx -mno-align-int
5772 @opindex mno-align-int
5773 Control whether GCC aligns @code{int}, @code{long}, @code{long long},
5774 @code{float}, @code{double}, and @code{long double} variables on a 32-bit
5775 boundary (@option{-malign-int}) or a 16-bit boundary (@option{-mno-align-int}).
5776 Aligning variables on 32-bit boundaries produces code that runs somewhat
5777 faster on processors with 32-bit busses at the expense of more memory.
5779 @strong{Warning:} if you use the @option{-malign-int} switch, GCC will
5780 align structures containing the above types differently than
5781 most published application binary interface specifications for the m68k.
5785 Use the pc-relative addressing mode of the 68000 directly, instead of
5786 using a global offset table. At present, this option implies @option{-fpic},
5787 allowing at most a 16-bit offset for pc-relative addressing. @option{-fPIC} is
5788 not presently supported with @option{-mpcrel}, though this could be supported for
5789 68020 and higher processors.
5791 @item -mno-strict-align
5792 @itemx -mstrict-align
5793 @opindex mno-strict-align
5794 @opindex mstrict-align
5795 Do not (do) assume that unaligned memory references will be handled by
5800 @node M68hc1x Options
5801 @subsection M68hc1x Options
5802 @cindex M68hc1x options
5804 These are the @samp{-m} options defined for the 68hc11 and 68hc12
5805 microcontrollers. The default values for these options depends on
5806 which style of microcontroller was selected when the compiler was configured;
5807 the defaults for the most common choices are given below.
5814 Generate output for a 68HC11. This is the default
5815 when the compiler is configured for 68HC11-based systems.
5821 Generate output for a 68HC12. This is the default
5822 when the compiler is configured for 68HC12-based systems.
5828 Generate output for a 68HCS12.
5831 @opindex mauto-incdec
5832 Enable the use of 68HC12 pre and post auto-increment and auto-decrement
5839 Enable the use of 68HC12 min and max instructions.
5842 @itemx -mno-long-calls
5843 @opindex mlong-calls
5844 @opindex mno-long-calls
5845 Treat all calls as being far away (near). If calls are assumed to be
5846 far away, the compiler will use the @code{call} instruction to
5847 call a function and the @code{rtc} instruction for returning.
5851 Consider type @code{int} to be 16 bits wide, like @code{short int}.
5853 @item -msoft-reg-count=@var{count}
5854 @opindex msoft-reg-count
5855 Specify the number of pseudo-soft registers which are used for the
5856 code generation. The maximum number is 32. Using more pseudo-soft
5857 register may or may not result in better code depending on the program.
5858 The default is 4 for 68HC11 and 2 for 68HC12.
5863 @subsection VAX Options
5866 These @samp{-m} options are defined for the VAX:
5871 Do not output certain jump instructions (@code{aobleq} and so on)
5872 that the Unix assembler for the VAX cannot handle across long
5877 Do output those jump instructions, on the assumption that you
5878 will assemble with the GNU assembler.
5882 Output code for g-format floating point numbers instead of d-format.
5886 @subsection SPARC Options
5887 @cindex SPARC options
5889 These @samp{-m} switches are supported on the SPARC:
5894 @opindex mno-app-regs
5896 Specify @option{-mapp-regs} to generate output using the global registers
5897 2 through 4, which the SPARC SVR4 ABI reserves for applications. This
5900 To be fully SVR4 ABI compliant at the cost of some performance loss,
5901 specify @option{-mno-app-regs}. You should compile libraries and system
5902 software with this option.
5907 @opindex mhard-float
5908 Generate output containing floating point instructions. This is the
5914 @opindex msoft-float
5915 Generate output containing library calls for floating point.
5916 @strong{Warning:} the requisite libraries are not available for all SPARC
5917 targets. Normally the facilities of the machine's usual C compiler are
5918 used, but this cannot be done directly in cross-compilation. You must make
5919 your own arrangements to provide suitable library functions for
5920 cross-compilation. The embedded targets @samp{sparc-*-aout} and
5921 @samp{sparclite-*-*} do provide software floating point support.
5923 @option{-msoft-float} changes the calling convention in the output file;
5924 therefore, it is only useful if you compile @emph{all} of a program with
5925 this option. In particular, you need to compile @file{libgcc.a}, the
5926 library that comes with GCC, with @option{-msoft-float} in order for
5929 @item -mhard-quad-float
5930 @opindex mhard-quad-float
5931 Generate output containing quad-word (long double) floating point
5934 @item -msoft-quad-float
5935 @opindex msoft-quad-float
5936 Generate output containing library calls for quad-word (long double)
5937 floating point instructions. The functions called are those specified
5938 in the SPARC ABI@. This is the default.
5940 As of this writing, there are no sparc implementations that have hardware
5941 support for the quad-word floating point instructions. They all invoke
5942 a trap handler for one of these instructions, and then the trap handler
5943 emulates the effect of the instruction. Because of the trap handler overhead,
5944 this is much slower than calling the ABI library routines. Thus the
5945 @option{-msoft-quad-float} option is the default.
5951 With @option{-mflat}, the compiler does not generate save/restore instructions
5952 and will use a ``flat'' or single register window calling convention.
5953 This model uses %i7 as the frame pointer and is compatible with the normal
5954 register window model. Code from either may be intermixed.
5955 The local registers and the input registers (0--5) are still treated as
5956 ``call saved'' registers and will be saved on the stack as necessary.
5958 With @option{-mno-flat} (the default), the compiler emits save/restore
5959 instructions (except for leaf functions) and is the normal mode of operation.
5961 @item -mno-unaligned-doubles
5962 @itemx -munaligned-doubles
5963 @opindex mno-unaligned-doubles
5964 @opindex munaligned-doubles
5965 Assume that doubles have 8 byte alignment. This is the default.
5967 With @option{-munaligned-doubles}, GCC assumes that doubles have 8 byte
5968 alignment only if they are contained in another type, or if they have an
5969 absolute address. Otherwise, it assumes they have 4 byte alignment.
5970 Specifying this option avoids some rare compatibility problems with code
5971 generated by other compilers. It is not the default because it results
5972 in a performance loss, especially for floating point code.
5974 @item -mno-faster-structs
5975 @itemx -mfaster-structs
5976 @opindex mno-faster-structs
5977 @opindex mfaster-structs
5978 With @option{-mfaster-structs}, the compiler assumes that structures
5979 should have 8 byte alignment. This enables the use of pairs of
5980 @code{ldd} and @code{std} instructions for copies in structure
5981 assignment, in place of twice as many @code{ld} and @code{st} pairs.
5982 However, the use of this changed alignment directly violates the SPARC
5983 ABI@. Thus, it's intended only for use on targets where the developer
5984 acknowledges that their resulting code will not be directly in line with
5985 the rules of the ABI@.
5991 These two options select variations on the SPARC architecture.
5993 By default (unless specifically configured for the Fujitsu SPARClite),
5994 GCC generates code for the v7 variant of the SPARC architecture.
5996 @option{-mv8} will give you SPARC v8 code. The only difference from v7
5997 code is that the compiler emits the integer multiply and integer
5998 divide instructions which exist in SPARC v8 but not in SPARC v7.
6000 @option{-msparclite} will give you SPARClite code. This adds the integer
6001 multiply, integer divide step and scan (@code{ffs}) instructions which
6002 exist in SPARClite but not in SPARC v7.
6004 These options are deprecated and will be deleted in a future GCC release.
6005 They have been replaced with @option{-mcpu=xxx}.
6010 @opindex msupersparc
6011 These two options select the processor for which the code is optimized.
6013 With @option{-mcypress} (the default), the compiler optimizes code for the
6014 Cypress CY7C602 chip, as used in the SPARCStation/SPARCServer 3xx series.
6015 This is also appropriate for the older SPARCStation 1, 2, IPX etc.
6017 With @option{-msupersparc} the compiler optimizes code for the SuperSPARC cpu, as
6018 used in the SPARCStation 10, 1000 and 2000 series. This flag also enables use
6019 of the full SPARC v8 instruction set.
6021 These options are deprecated and will be deleted in a future GCC release.
6022 They have been replaced with @option{-mcpu=xxx}.
6024 @item -mcpu=@var{cpu_type}
6026 Set the instruction set, register set, and instruction scheduling parameters
6027 for machine type @var{cpu_type}. Supported values for @var{cpu_type} are
6028 @samp{v7}, @samp{cypress}, @samp{v8}, @samp{supersparc}, @samp{sparclite},
6029 @samp{hypersparc}, @samp{sparclite86x}, @samp{f930}, @samp{f934},
6030 @samp{sparclet}, @samp{tsc701}, @samp{v9}, @samp{ultrasparc}, and
6033 Default instruction scheduling parameters are used for values that select
6034 an architecture and not an implementation. These are @samp{v7}, @samp{v8},
6035 @samp{sparclite}, @samp{sparclet}, @samp{v9}.
6037 Here is a list of each supported architecture and their supported
6042 v8: supersparc, hypersparc
6043 sparclite: f930, f934, sparclite86x
6045 v9: ultrasparc, ultrasparc3
6048 @item -mtune=@var{cpu_type}
6050 Set the instruction scheduling parameters for machine type
6051 @var{cpu_type}, but do not set the instruction set or register set that the
6052 option @option{-mcpu=@var{cpu_type}} would.
6054 The same values for @option{-mcpu=@var{cpu_type}} can be used for
6055 @option{-mtune=@var{cpu_type}}, but the only useful values are those
6056 that select a particular cpu implementation. Those are @samp{cypress},
6057 @samp{supersparc}, @samp{hypersparc}, @samp{f930}, @samp{f934},
6058 @samp{sparclite86x}, @samp{tsc701}, @samp{ultrasparc}, and
6063 These @samp{-m} switches are supported in addition to the above
6064 on the SPARCLET processor.
6067 @item -mlittle-endian
6068 @opindex mlittle-endian
6069 Generate code for a processor running in little-endian mode.
6073 Treat register @code{%g0} as a normal register.
6074 GCC will continue to clobber it as necessary but will not assume
6075 it always reads as 0.
6077 @item -mbroken-saverestore
6078 @opindex mbroken-saverestore
6079 Generate code that does not use non-trivial forms of the @code{save} and
6080 @code{restore} instructions. Early versions of the SPARCLET processor do
6081 not correctly handle @code{save} and @code{restore} instructions used with
6082 arguments. They correctly handle them used without arguments. A @code{save}
6083 instruction used without arguments increments the current window pointer
6084 but does not allocate a new stack frame. It is assumed that the window
6085 overflow trap handler will properly handle this case as will interrupt
6089 These @samp{-m} switches are supported in addition to the above
6090 on SPARC V9 processors in 64-bit environments.
6093 @item -mlittle-endian
6094 @opindex mlittle-endian
6095 Generate code for a processor running in little-endian mode.
6101 Generate code for a 32-bit or 64-bit environment.
6102 The 32-bit environment sets int, long and pointer to 32 bits.
6103 The 64-bit environment sets int to 32 bits and long and pointer
6106 @item -mcmodel=medlow
6107 @opindex mcmodel=medlow
6108 Generate code for the Medium/Low code model: the program must be linked
6109 in the low 32 bits of the address space. Pointers are 64 bits.
6110 Programs can be statically or dynamically linked.
6112 @item -mcmodel=medmid
6113 @opindex mcmodel=medmid
6114 Generate code for the Medium/Middle code model: the program must be linked
6115 in the low 44 bits of the address space, the text segment must be less than
6116 2G bytes, and data segment must be within 2G of the text segment.
6117 Pointers are 64 bits.
6119 @item -mcmodel=medany
6120 @opindex mcmodel=medany
6121 Generate code for the Medium/Anywhere code model: the program may be linked
6122 anywhere in the address space, the text segment must be less than
6123 2G bytes, and data segment must be within 2G of the text segment.
6124 Pointers are 64 bits.
6126 @item -mcmodel=embmedany
6127 @opindex mcmodel=embmedany
6128 Generate code for the Medium/Anywhere code model for embedded systems:
6129 assume a 32-bit text and a 32-bit data segment, both starting anywhere
6130 (determined at link time). Register %g4 points to the base of the
6131 data segment. Pointers are still 64 bits.
6132 Programs are statically linked, PIC is not supported.
6135 @itemx -mno-stack-bias
6136 @opindex mstack-bias
6137 @opindex mno-stack-bias
6138 With @option{-mstack-bias}, GCC assumes that the stack pointer, and
6139 frame pointer if present, are offset by @minus{}2047 which must be added back
6140 when making stack frame references.
6141 Otherwise, assume no such offset is present.
6145 @subsection ARM Options
6148 These @samp{-m} options are defined for Advanced RISC Machines (ARM)
6153 @opindex mapcs-frame
6154 Generate a stack frame that is compliant with the ARM Procedure Call
6155 Standard for all functions, even if this is not strictly necessary for
6156 correct execution of the code. Specifying @option{-fomit-frame-pointer}
6157 with this option will cause the stack frames not to be generated for
6158 leaf functions. The default is @option{-mno-apcs-frame}.
6162 This is a synonym for @option{-mapcs-frame}.
6166 Generate code for a processor running with a 26-bit program counter,
6167 and conforming to the function calling standards for the APCS 26-bit
6168 option. This option replaces the @option{-m2} and @option{-m3} options
6169 of previous releases of the compiler.
6173 Generate code for a processor running with a 32-bit program counter,
6174 and conforming to the function calling standards for the APCS 32-bit
6175 option. This option replaces the @option{-m6} option of previous releases
6179 @c not currently implemented
6180 @item -mapcs-stack-check
6181 @opindex mapcs-stack-check
6182 Generate code to check the amount of stack space available upon entry to
6183 every function (that actually uses some stack space). If there is
6184 insufficient space available then either the function
6185 @samp{__rt_stkovf_split_small} or @samp{__rt_stkovf_split_big} will be
6186 called, depending upon the amount of stack space required. The run time
6187 system is required to provide these functions. The default is
6188 @option{-mno-apcs-stack-check}, since this produces smaller code.
6190 @c not currently implemented
6192 @opindex mapcs-float
6193 Pass floating point arguments using the float point registers. This is
6194 one of the variants of the APCS@. This option is recommended if the
6195 target hardware has a floating point unit or if a lot of floating point
6196 arithmetic is going to be performed by the code. The default is
6197 @option{-mno-apcs-float}, since integer only code is slightly increased in
6198 size if @option{-mapcs-float} is used.
6200 @c not currently implemented
6201 @item -mapcs-reentrant
6202 @opindex mapcs-reentrant
6203 Generate reentrant, position independent code. The default is
6204 @option{-mno-apcs-reentrant}.
6207 @item -mthumb-interwork
6208 @opindex mthumb-interwork
6209 Generate code which supports calling between the ARM and Thumb
6210 instruction sets. Without this option the two instruction sets cannot
6211 be reliably used inside one program. The default is
6212 @option{-mno-thumb-interwork}, since slightly larger code is generated
6213 when @option{-mthumb-interwork} is specified.
6215 @item -mno-sched-prolog
6216 @opindex mno-sched-prolog
6217 Prevent the reordering of instructions in the function prolog, or the
6218 merging of those instruction with the instructions in the function's
6219 body. This means that all functions will start with a recognizable set
6220 of instructions (or in fact one of a choice from a small set of
6221 different function prologues), and this information can be used to
6222 locate the start if functions inside an executable piece of code. The
6223 default is @option{-msched-prolog}.
6226 @opindex mhard-float
6227 Generate output containing floating point instructions. This is the
6231 @opindex msoft-float
6232 Generate output containing library calls for floating point.
6233 @strong{Warning:} the requisite libraries are not available for all ARM
6234 targets. Normally the facilities of the machine's usual C compiler are
6235 used, but this cannot be done directly in cross-compilation. You must make
6236 your own arrangements to provide suitable library functions for
6239 @option{-msoft-float} changes the calling convention in the output file;
6240 therefore, it is only useful if you compile @emph{all} of a program with
6241 this option. In particular, you need to compile @file{libgcc.a}, the
6242 library that comes with GCC, with @option{-msoft-float} in order for
6245 @item -mlittle-endian
6246 @opindex mlittle-endian
6247 Generate code for a processor running in little-endian mode. This is
6248 the default for all standard configurations.
6251 @opindex mbig-endian
6252 Generate code for a processor running in big-endian mode; the default is
6253 to compile code for a little-endian processor.
6255 @item -mwords-little-endian
6256 @opindex mwords-little-endian
6257 This option only applies when generating code for big-endian processors.
6258 Generate code for a little-endian word order but a big-endian byte
6259 order. That is, a byte order of the form @samp{32107654}. Note: this
6260 option should only be used if you require compatibility with code for
6261 big-endian ARM processors generated by versions of the compiler prior to
6264 @item -malignment-traps
6265 @opindex malignment-traps
6266 Generate code that will not trap if the MMU has alignment traps enabled.
6267 On ARM architectures prior to ARMv4, there were no instructions to
6268 access half-word objects stored in memory. However, when reading from
6269 memory a feature of the ARM architecture allows a word load to be used,
6270 even if the address is unaligned, and the processor core will rotate the
6271 data as it is being loaded. This option tells the compiler that such
6272 misaligned accesses will cause a MMU trap and that it should instead
6273 synthesize the access as a series of byte accesses. The compiler can
6274 still use word accesses to load half-word data if it knows that the
6275 address is aligned to a word boundary.
6277 This option is ignored when compiling for ARM architecture 4 or later,
6278 since these processors have instructions to directly access half-word
6281 @item -mno-alignment-traps
6282 @opindex mno-alignment-traps
6283 Generate code that assumes that the MMU will not trap unaligned
6284 accesses. This produces better code when the target instruction set
6285 does not have half-word memory operations (i.e.@: implementations prior to
6288 Note that you cannot use this option to access unaligned word objects,
6289 since the processor will only fetch one 32-bit aligned object from
6292 The default setting for most targets is @option{-mno-alignment-traps}, since
6293 this produces better code when there are no half-word memory
6294 instructions available.
6296 @item -mshort-load-bytes
6297 @itemx -mno-short-load-words
6298 @opindex mshort-load-bytes
6299 @opindex mno-short-load-words
6300 These are deprecated aliases for @option{-malignment-traps}.
6302 @item -mno-short-load-bytes
6303 @itemx -mshort-load-words
6304 @opindex mno-short-load-bytes
6305 @opindex mshort-load-words
6306 This are deprecated aliases for @option{-mno-alignment-traps}.
6308 @item -mcpu=@var{name}
6310 This specifies the name of the target ARM processor. GCC uses this name
6311 to determine what kind of instructions it can emit when generating
6312 assembly code. Permissible names are: @samp{arm2}, @samp{arm250},
6313 @samp{arm3}, @samp{arm6}, @samp{arm60}, @samp{arm600}, @samp{arm610},
6314 @samp{arm620}, @samp{arm7}, @samp{arm7m}, @samp{arm7d}, @samp{arm7dm},
6315 @samp{arm7di}, @samp{arm7dmi}, @samp{arm70}, @samp{arm700},
6316 @samp{arm700i}, @samp{arm710}, @samp{arm710c}, @samp{arm7100},
6317 @samp{arm7500}, @samp{arm7500fe}, @samp{arm7tdmi}, @samp{arm8},
6318 @samp{strongarm}, @samp{strongarm110}, @samp{strongarm1100},
6319 @samp{arm8}, @samp{arm810}, @samp{arm9}, @samp{arm9e}, @samp{arm920},
6320 @samp{arm920t}, @samp{arm940t}, @samp{arm9tdmi}, @samp{arm10tdmi},
6321 @samp{arm1020t}, @samp{xscale}, @samp{ep9312}.
6323 @itemx -mtune=@var{name}
6325 This option is very similar to the @option{-mcpu=} option, except that
6326 instead of specifying the actual target processor type, and hence
6327 restricting which instructions can be used, it specifies that GCC should
6328 tune the performance of the code as if the target were of the type
6329 specified in this option, but still choosing the instructions that it
6330 will generate based on the cpu specified by a @option{-mcpu=} option.
6331 For some ARM implementations better performance can be obtained by using
6334 @item -march=@var{name}
6336 This specifies the name of the target ARM architecture. GCC uses this
6337 name to determine what kind of instructions it can emit when generating
6338 assembly code. This option can be used in conjunction with or instead
6339 of the @option{-mcpu=} option. Permissible names are: @samp{armv2},
6340 @samp{armv2a}, @samp{armv3}, @samp{armv3m}, @samp{armv4}, @samp{armv4t},
6341 @samp{armv5}, @samp{armv5t}, @samp{armv5te}, @samp{ep9312}.
6343 @item -mfpe=@var{number}
6344 @itemx -mfp=@var{number}
6347 This specifies the version of the floating point emulation available on
6348 the target. Permissible values are 2 and 3. @option{-mfp=} is a synonym
6349 for @option{-mfpe=}, for compatibility with older versions of GCC@.
6351 @item -mstructure-size-boundary=@var{n}
6352 @opindex mstructure-size-boundary
6353 The size of all structures and unions will be rounded up to a multiple
6354 of the number of bits set by this option. Permissible values are 8 and
6355 32. The default value varies for different toolchains. For the COFF
6356 targeted toolchain the default value is 8. Specifying the larger number
6357 can produce faster, more efficient code, but can also increase the size
6358 of the program. The two values are potentially incompatible. Code
6359 compiled with one value cannot necessarily expect to work with code or
6360 libraries compiled with the other value, if they exchange information
6361 using structures or unions.
6363 @item -mabort-on-noreturn
6364 @opindex mabort-on-noreturn
6365 Generate a call to the function @code{abort} at the end of a
6366 @code{noreturn} function. It will be executed if the function tries to
6370 @itemx -mno-long-calls
6371 @opindex mlong-calls
6372 @opindex mno-long-calls
6373 Tells the compiler to perform function calls by first loading the
6374 address of the function into a register and then performing a subroutine
6375 call on this register. This switch is needed if the target function
6376 will lie outside of the 64 megabyte addressing range of the offset based
6377 version of subroutine call instruction.
6379 Even if this switch is enabled, not all function calls will be turned
6380 into long calls. The heuristic is that static functions, functions
6381 which have the @samp{short-call} attribute, functions that are inside
6382 the scope of a @samp{#pragma no_long_calls} directive and functions whose
6383 definitions have already been compiled within the current compilation
6384 unit, will not be turned into long calls. The exception to this rule is
6385 that weak function definitions, functions with the @samp{long-call}
6386 attribute or the @samp{section} attribute, and functions that are within
6387 the scope of a @samp{#pragma long_calls} directive, will always be
6388 turned into long calls.
6390 This feature is not enabled by default. Specifying
6391 @option{-mno-long-calls} will restore the default behavior, as will
6392 placing the function calls within the scope of a @samp{#pragma
6393 long_calls_off} directive. Note these switches have no effect on how
6394 the compiler generates code to handle function calls via function
6397 @item -mnop-fun-dllimport
6398 @opindex mnop-fun-dllimport
6399 Disable support for the @code{dllimport} attribute.
6401 @item -msingle-pic-base
6402 @opindex msingle-pic-base
6403 Treat the register used for PIC addressing as read-only, rather than
6404 loading it in the prologue for each function. The run-time system is
6405 responsible for initializing this register with an appropriate value
6406 before execution begins.
6408 @item -mpic-register=@var{reg}
6409 @opindex mpic-register
6410 Specify the register to be used for PIC addressing. The default is R10
6411 unless stack-checking is enabled, when R9 is used.
6413 @item -mcirrus-fix-invalid-insns
6414 @opindex -mcirrus-fix-invalid-insns
6415 @opindex -mno-cirrus-fix-invalid-insns
6416 Insert NOPs into the instruction stream to in order to work around
6417 problems with invalid Maverick instruction combinations. This option
6418 is only valid if the @option{-mcpu=ep9312} option has been used to
6419 enable generation of instructions for the Cirrus Maverick floating
6420 point co-processor. This option is not enabled by default, since the
6421 problem is only present in older Maverick implementations. The default
6422 can be re-enabled by use of the @option{-mno-cirrus-fix-invalid-insns}
6425 @item -mpoke-function-name
6426 @opindex mpoke-function-name
6427 Write the name of each function into the text section, directly
6428 preceding the function prologue. The generated code is similar to this:
6432 .ascii "arm_poke_function_name", 0
6435 .word 0xff000000 + (t1 - t0)
6436 arm_poke_function_name
6438 stmfd sp!, @{fp, ip, lr, pc@}
6442 When performing a stack backtrace, code can inspect the value of
6443 @code{pc} stored at @code{fp + 0}. If the trace function then looks at
6444 location @code{pc - 12} and the top 8 bits are set, then we know that
6445 there is a function name embedded immediately preceding this location
6446 and has length @code{((pc[-3]) & 0xff000000)}.
6450 Generate code for the 16-bit Thumb instruction set. The default is to
6451 use the 32-bit ARM instruction set.
6454 @opindex mtpcs-frame
6455 Generate a stack frame that is compliant with the Thumb Procedure Call
6456 Standard for all non-leaf functions. (A leaf function is one that does
6457 not call any other functions.) The default is @option{-mno-tpcs-frame}.
6459 @item -mtpcs-leaf-frame
6460 @opindex mtpcs-leaf-frame
6461 Generate a stack frame that is compliant with the Thumb Procedure Call
6462 Standard for all leaf functions. (A leaf function is one that does
6463 not call any other functions.) The default is @option{-mno-apcs-leaf-frame}.
6465 @item -mcallee-super-interworking
6466 @opindex mcallee-super-interworking
6467 Gives all externally visible functions in the file being compiled an ARM
6468 instruction set header which switches to Thumb mode before executing the
6469 rest of the function. This allows these functions to be called from
6470 non-interworking code.
6472 @item -mcaller-super-interworking
6473 @opindex mcaller-super-interworking
6474 Allows calls via function pointers (including virtual functions) to
6475 execute correctly regardless of whether the target code has been
6476 compiled for interworking or not. There is a small overhead in the cost
6477 of executing a function pointer if this option is enabled.
6481 @node MN10200 Options
6482 @subsection MN10200 Options
6483 @cindex MN10200 options
6485 These @option{-m} options are defined for Matsushita MN10200 architectures:
6490 Indicate to the linker that it should perform a relaxation optimization pass
6491 to shorten branches, calls and absolute memory addresses. This option only
6492 has an effect when used on the command line for the final link step.
6494 This option makes symbolic debugging impossible.
6497 @node MN10300 Options
6498 @subsection MN10300 Options
6499 @cindex MN10300 options
6501 These @option{-m} options are defined for Matsushita MN10300 architectures:
6506 Generate code to avoid bugs in the multiply instructions for the MN10300
6507 processors. This is the default.
6510 @opindex mno-mult-bug
6511 Do not generate code to avoid bugs in the multiply instructions for the
6516 Generate code which uses features specific to the AM33 processor.
6520 Do not generate code which uses features specific to the AM33 processor. This
6525 Do not link in the C run-time initialization object file.
6529 Indicate to the linker that it should perform a relaxation optimization pass
6530 to shorten branches, calls and absolute memory addresses. This option only
6531 has an effect when used on the command line for the final link step.
6533 This option makes symbolic debugging impossible.
6537 @node M32R/D Options
6538 @subsection M32R/D Options
6539 @cindex M32R/D options
6541 These @option{-m} options are defined for Mitsubishi M32R/D architectures:
6546 Generate code for the M32R/X@.
6550 Generate code for the M32R@. This is the default.
6552 @item -mcode-model=small
6553 @opindex mcode-model=small
6554 Assume all objects live in the lower 16MB of memory (so that their addresses
6555 can be loaded with the @code{ld24} instruction), and assume all subroutines
6556 are reachable with the @code{bl} instruction.
6557 This is the default.
6559 The addressability of a particular object can be set with the
6560 @code{model} attribute.
6562 @item -mcode-model=medium
6563 @opindex mcode-model=medium
6564 Assume objects may be anywhere in the 32-bit address space (the compiler
6565 will generate @code{seth/add3} instructions to load their addresses), and
6566 assume all subroutines are reachable with the @code{bl} instruction.
6568 @item -mcode-model=large
6569 @opindex mcode-model=large
6570 Assume objects may be anywhere in the 32-bit address space (the compiler
6571 will generate @code{seth/add3} instructions to load their addresses), and
6572 assume subroutines may not be reachable with the @code{bl} instruction
6573 (the compiler will generate the much slower @code{seth/add3/jl}
6574 instruction sequence).
6577 @opindex msdata=none
6578 Disable use of the small data area. Variables will be put into
6579 one of @samp{.data}, @samp{bss}, or @samp{.rodata} (unless the
6580 @code{section} attribute has been specified).
6581 This is the default.
6583 The small data area consists of sections @samp{.sdata} and @samp{.sbss}.
6584 Objects may be explicitly put in the small data area with the
6585 @code{section} attribute using one of these sections.
6588 @opindex msdata=sdata
6589 Put small global and static data in the small data area, but do not
6590 generate special code to reference them.
6594 Put small global and static data in the small data area, and generate
6595 special instructions to reference them.
6599 @cindex smaller data references
6600 Put global and static objects less than or equal to @var{num} bytes
6601 into the small data or bss sections instead of the normal data or bss
6602 sections. The default value of @var{num} is 8.
6603 The @option{-msdata} option must be set to one of @samp{sdata} or @samp{use}
6604 for this option to have any effect.
6606 All modules should be compiled with the same @option{-G @var{num}} value.
6607 Compiling with different values of @var{num} may or may not work; if it
6608 doesn't the linker will give an error message---incorrect code will not be
6614 @subsection M88K Options
6615 @cindex M88k options
6617 These @samp{-m} options are defined for Motorola 88k architectures:
6622 Generate code that works well on both the m88100 and the
6627 Generate code that works best for the m88100, but that also
6632 Generate code that works best for the m88110, and may not run
6637 Obsolete option to be removed from the next revision.
6640 @item -midentify-revision
6641 @opindex midentify-revision
6642 @cindex identifying source, compiler (88k)
6643 Include an @code{ident} directive in the assembler output recording the
6644 source file name, compiler name and version, timestamp, and compilation
6647 @item -mno-underscores
6648 @opindex mno-underscores
6649 @cindex underscores, avoiding (88k)
6650 In assembler output, emit symbol names without adding an underscore
6651 character at the beginning of each name. The default is to use an
6652 underscore as prefix on each name.
6654 @item -mocs-debug-info
6655 @itemx -mno-ocs-debug-info
6656 @opindex mocs-debug-info
6657 @opindex mno-ocs-debug-info
6659 @cindex debugging, 88k OCS
6660 Include (or omit) additional debugging information (about registers used
6661 in each stack frame) as specified in the 88open Object Compatibility
6662 Standard, ``OCS''@. This extra information allows debugging of code that
6663 has had the frame pointer eliminated. The default for SVr4 and Delta 88
6664 SVr3.2 is to include this information; other 88k configurations omit this
6665 information by default.
6667 @item -mocs-frame-position
6668 @opindex mocs-frame-position
6669 @cindex register positions in frame (88k)
6670 When emitting COFF debugging information for automatic variables and
6671 parameters stored on the stack, use the offset from the canonical frame
6672 address, which is the stack pointer (register 31) on entry to the
6673 function. The SVr4 and Delta88 SVr3.2, and BCS configurations use
6674 @option{-mocs-frame-position}; other 88k configurations have the default
6675 @option{-mno-ocs-frame-position}.
6677 @item -mno-ocs-frame-position
6678 @opindex mno-ocs-frame-position
6679 @cindex register positions in frame (88k)
6680 When emitting COFF debugging information for automatic variables and
6681 parameters stored on the stack, use the offset from the frame pointer
6682 register (register 30). When this option is in effect, the frame
6683 pointer is not eliminated when debugging information is selected by the
6686 @item -moptimize-arg-area
6687 @opindex moptimize-arg-area
6688 @cindex arguments in frame (88k)
6689 Save space by reorganizing the stack frame. This option generates code
6690 that does not agree with the 88open specifications, but uses less
6693 @itemx -mno-optimize-arg-area
6694 @opindex mno-optimize-arg-area
6695 Do not reorganize the stack frame to save space. This is the default.
6696 The generated conforms to the specification, but uses more memory.
6698 @item -mshort-data-@var{num}
6699 @opindex mshort-data
6700 @cindex smaller data references (88k)
6701 @cindex r0-relative references (88k)
6702 Generate smaller data references by making them relative to @code{r0},
6703 which allows loading a value using a single instruction (rather than the
6704 usual two). You control which data references are affected by
6705 specifying @var{num} with this option. For example, if you specify
6706 @option{-mshort-data-512}, then the data references affected are those
6707 involving displacements of less than 512 bytes.
6708 @option{-mshort-data-@var{num}} is not effective for @var{num} greater
6711 @item -mserialize-volatile
6712 @opindex mserialize-volatile
6713 @itemx -mno-serialize-volatile
6714 @opindex mno-serialize-volatile
6715 @cindex sequential consistency on 88k
6716 Do, or don't, generate code to guarantee sequential consistency
6717 of volatile memory references. By default, consistency is
6720 The order of memory references made by the MC88110 processor does
6721 not always match the order of the instructions requesting those
6722 references. In particular, a load instruction may execute before
6723 a preceding store instruction. Such reordering violates
6724 sequential consistency of volatile memory references, when there
6725 are multiple processors. When consistency must be guaranteed,
6726 GCC generates special instructions, as needed, to force
6727 execution in the proper order.
6729 The MC88100 processor does not reorder memory references and so
6730 always provides sequential consistency. However, by default, GCC
6731 generates the special instructions to guarantee consistency
6732 even when you use @option{-m88100}, so that the code may be run on an
6733 MC88110 processor. If you intend to run your code only on the
6734 MC88100 processor, you may use @option{-mno-serialize-volatile}.
6736 The extra code generated to guarantee consistency may affect the
6737 performance of your application. If you know that you can safely
6738 forgo this guarantee, you may use @option{-mno-serialize-volatile}.
6744 @cindex assembler syntax, 88k
6746 Turn on (@option{-msvr4}) or off (@option{-msvr3}) compiler extensions
6747 related to System V release 4 (SVr4). This controls the following:
6751 Which variant of the assembler syntax to emit.
6753 @option{-msvr4} makes the C preprocessor recognize @samp{#pragma weak}
6754 that is used on System V release 4.
6756 @option{-msvr4} makes GCC issue additional declaration directives used in
6760 @option{-msvr4} is the default for the m88k-motorola-sysv4 configuration.
6761 @option{-msvr3} is the default for all other m88k configurations.
6763 @item -mversion-03.00
6764 @opindex mversion-03.00
6765 This option is obsolete, and is ignored.
6766 @c ??? which asm syntax better for GAS? option there too?
6768 @item -mno-check-zero-division
6769 @itemx -mcheck-zero-division
6770 @opindex mno-check-zero-division
6771 @opindex mcheck-zero-division
6772 @cindex zero division on 88k
6773 Do, or don't, generate code to guarantee that integer division by
6774 zero will be detected. By default, detection is guaranteed.
6776 Some models of the MC88100 processor fail to trap upon integer
6777 division by zero under certain conditions. By default, when
6778 compiling code that might be run on such a processor, GCC
6779 generates code that explicitly checks for zero-valued divisors
6780 and traps with exception number 503 when one is detected. Use of
6781 @option{-mno-check-zero-division} suppresses such checking for code
6782 generated to run on an MC88100 processor.
6784 GCC assumes that the MC88110 processor correctly detects all instances
6785 of integer division by zero. When @option{-m88110} is specified, no
6786 explicit checks for zero-valued divisors are generated, and both
6787 @option{-mcheck-zero-division} and @option{-mno-check-zero-division} are
6790 @item -muse-div-instruction
6791 @opindex muse-div-instruction
6792 @cindex divide instruction, 88k
6793 Use the div instruction for signed integer division on the
6794 MC88100 processor. By default, the div instruction is not used.
6796 On the MC88100 processor the signed integer division instruction
6797 div) traps to the operating system on a negative operand. The
6798 operating system transparently completes the operation, but at a
6799 large cost in execution time. By default, when compiling code
6800 that might be run on an MC88100 processor, GCC emulates signed
6801 integer division using the unsigned integer division instruction
6802 divu), thereby avoiding the large penalty of a trap to the
6803 operating system. Such emulation has its own, smaller, execution
6804 cost in both time and space. To the extent that your code's
6805 important signed integer division operations are performed on two
6806 nonnegative operands, it may be desirable to use the div
6807 instruction directly.
6809 On the MC88110 processor the div instruction (also known as the
6810 divs instruction) processes negative operands without trapping to
6811 the operating system. When @option{-m88110} is specified,
6812 @option{-muse-div-instruction} is ignored, and the div instruction is used
6813 for signed integer division.
6815 Note that the result of dividing @code{INT_MIN} by @minus{}1 is undefined. In
6816 particular, the behavior of such a division with and without
6817 @option{-muse-div-instruction} may differ.
6819 @item -mtrap-large-shift
6820 @itemx -mhandle-large-shift
6821 @opindex mtrap-large-shift
6822 @opindex mhandle-large-shift
6823 @cindex bit shift overflow (88k)
6824 @cindex large bit shifts (88k)
6825 Include code to detect bit-shifts of more than 31 bits; respectively,
6826 trap such shifts or emit code to handle them properly. By default GCC
6827 makes no special provision for large bit shifts.
6829 @item -mwarn-passed-structs
6830 @opindex mwarn-passed-structs
6831 @cindex structure passing (88k)
6832 Warn when a function passes a struct as an argument or result.
6833 Structure-passing conventions have changed during the evolution of the C
6834 language, and are often the source of portability problems. By default,
6835 GCC issues no such warning.
6838 @c break page here to avoid unsightly interparagraph stretch.
6842 @node RS/6000 and PowerPC Options
6843 @subsection IBM RS/6000 and PowerPC Options
6844 @cindex RS/6000 and PowerPC Options
6845 @cindex IBM RS/6000 and PowerPC Options
6847 These @samp{-m} options are defined for the IBM RS/6000 and PowerPC:
6855 @itemx -mpowerpc-gpopt
6856 @itemx -mno-powerpc-gpopt
6857 @itemx -mpowerpc-gfxopt
6858 @itemx -mno-powerpc-gfxopt
6860 @itemx -mno-powerpc64
6866 @opindex mno-powerpc
6867 @opindex mpowerpc-gpopt
6868 @opindex mno-powerpc-gpopt
6869 @opindex mpowerpc-gfxopt
6870 @opindex mno-powerpc-gfxopt
6872 @opindex mno-powerpc64
6873 GCC supports two related instruction set architectures for the
6874 RS/6000 and PowerPC@. The @dfn{POWER} instruction set are those
6875 instructions supported by the @samp{rios} chip set used in the original
6876 RS/6000 systems and the @dfn{PowerPC} instruction set is the
6877 architecture of the Motorola MPC5xx, MPC6xx, MPC8xx microprocessors, and
6878 the IBM 4xx microprocessors.
6880 Neither architecture is a subset of the other. However there is a
6881 large common subset of instructions supported by both. An MQ
6882 register is included in processors supporting the POWER architecture.
6884 You use these options to specify which instructions are available on the
6885 processor you are using. The default value of these options is
6886 determined when configuring GCC@. Specifying the
6887 @option{-mcpu=@var{cpu_type}} overrides the specification of these
6888 options. We recommend you use the @option{-mcpu=@var{cpu_type}} option
6889 rather than the options listed above.
6891 The @option{-mpower} option allows GCC to generate instructions that
6892 are found only in the POWER architecture and to use the MQ register.
6893 Specifying @option{-mpower2} implies @option{-power} and also allows GCC
6894 to generate instructions that are present in the POWER2 architecture but
6895 not the original POWER architecture.
6897 The @option{-mpowerpc} option allows GCC to generate instructions that
6898 are found only in the 32-bit subset of the PowerPC architecture.
6899 Specifying @option{-mpowerpc-gpopt} implies @option{-mpowerpc} and also allows
6900 GCC to use the optional PowerPC architecture instructions in the
6901 General Purpose group, including floating-point square root. Specifying
6902 @option{-mpowerpc-gfxopt} implies @option{-mpowerpc} and also allows GCC to
6903 use the optional PowerPC architecture instructions in the Graphics
6904 group, including floating-point select.
6906 The @option{-mpowerpc64} option allows GCC to generate the additional
6907 64-bit instructions that are found in the full PowerPC64 architecture
6908 and to treat GPRs as 64-bit, doubleword quantities. GCC defaults to
6909 @option{-mno-powerpc64}.
6911 If you specify both @option{-mno-power} and @option{-mno-powerpc}, GCC
6912 will use only the instructions in the common subset of both
6913 architectures plus some special AIX common-mode calls, and will not use
6914 the MQ register. Specifying both @option{-mpower} and @option{-mpowerpc}
6915 permits GCC to use any instruction from either architecture and to
6916 allow use of the MQ register; specify this for the Motorola MPC601.
6918 @item -mnew-mnemonics
6919 @itemx -mold-mnemonics
6920 @opindex mnew-mnemonics
6921 @opindex mold-mnemonics
6922 Select which mnemonics to use in the generated assembler code. With
6923 @option{-mnew-mnemonics}, GCC uses the assembler mnemonics defined for
6924 the PowerPC architecture. With @option{-mold-mnemonics} it uses the
6925 assembler mnemonics defined for the POWER architecture. Instructions
6926 defined in only one architecture have only one mnemonic; GCC uses that
6927 mnemonic irrespective of which of these options is specified.
6929 GCC defaults to the mnemonics appropriate for the architecture in
6930 use. Specifying @option{-mcpu=@var{cpu_type}} sometimes overrides the
6931 value of these option. Unless you are building a cross-compiler, you
6932 should normally not specify either @option{-mnew-mnemonics} or
6933 @option{-mold-mnemonics}, but should instead accept the default.
6935 @item -mcpu=@var{cpu_type}
6937 Set architecture type, register usage, choice of mnemonics, and
6938 instruction scheduling parameters for machine type @var{cpu_type}.
6939 Supported values for @var{cpu_type} are @samp{rios}, @samp{rios1},
6940 @samp{rsc}, @samp{rios2}, @samp{rs64a}, @samp{601}, @samp{602},
6941 @samp{603}, @samp{603e}, @samp{604}, @samp{604e}, @samp{620},
6942 @samp{630}, @samp{740}, @samp{7400}, @samp{7450}, @samp{750},
6943 @samp{power}, @samp{power2}, @samp{powerpc}, @samp{403}, @samp{505},
6944 @samp{801}, @samp{821}, @samp{823}, and @samp{860} and @samp{common}.
6946 @option{-mcpu=common} selects a completely generic processor. Code
6947 generated under this option will run on any POWER or PowerPC processor.
6948 GCC will use only the instructions in the common subset of both
6949 architectures, and will not use the MQ register. GCC assumes a generic
6950 processor model for scheduling purposes.
6952 @option{-mcpu=power}, @option{-mcpu=power2}, @option{-mcpu=powerpc}, and
6953 @option{-mcpu=powerpc64} specify generic POWER, POWER2, pure 32-bit
6954 PowerPC (i.e., not MPC601), and 64-bit PowerPC architecture machine
6955 types, with an appropriate, generic processor model assumed for
6956 scheduling purposes.
6958 The other options specify a specific processor. Code generated under
6959 those options will run best on that processor, and may not run at all on
6962 The @option{-mcpu} options automatically enable or disable other
6963 @option{-m} options as follows:
6967 @option{-mno-power}, @option{-mno-powerpc}
6974 @option{-mpower}, @option{-mno-powerpc}, @option{-mno-new-mnemonics}
6989 @option{-mno-power}, @option{-mpowerpc}, @option{-mnew-mnemonics}
6992 @option{-mpower}, @option{-mpowerpc}, @option{-mnew-mnemonics}
6997 @option{-mno-power}, @option{-mpowerpc}, @option{-mnew-mnemonics}, @option{-msoft-float}
7000 @item -mtune=@var{cpu_type}
7002 Set the instruction scheduling parameters for machine type
7003 @var{cpu_type}, but do not set the architecture type, register usage, or
7004 choice of mnemonics, as @option{-mcpu=@var{cpu_type}} would. The same
7005 values for @var{cpu_type} are used for @option{-mtune} as for
7006 @option{-mcpu}. If both are specified, the code generated will use the
7007 architecture, registers, and mnemonics set by @option{-mcpu}, but the
7008 scheduling parameters set by @option{-mtune}.
7013 @opindex mno-altivec
7014 These switches enable or disable the use of built-in functions that
7015 allow access to the AltiVec instruction set. You may also need to set
7016 @option{-mabi=altivec} to adjust the current ABI with AltiVec ABI
7021 Extend the current ABI with SPE ABI extensions. This does not change
7022 the default ABI, instead it adds the SPE ABI extensions to the current
7026 @opindex mabi=no-spe
7027 Disable Booke SPE ABI extensions for the current ABI.
7029 @item -misel=@var{yes/no}
7032 This switch enables or disables the generation of ISEL instructions.
7034 @item -mspe=@var{yes/no}
7037 This switch enables or disables the generation of SPE simd
7040 @item -mfloat-gprs=@var{yes/no}
7042 @opindex mfloat-gprs
7043 This switch enables or disables the generation of floating point
7044 operations on the general purpose registers for architectures that
7045 support it. This option is currently only available on the MPC8540.
7048 @itemx -mno-fp-in-toc
7049 @itemx -mno-sum-in-toc
7050 @itemx -mminimal-toc
7052 @opindex mno-fp-in-toc
7053 @opindex mno-sum-in-toc
7054 @opindex mminimal-toc
7055 Modify generation of the TOC (Table Of Contents), which is created for
7056 every executable file. The @option{-mfull-toc} option is selected by
7057 default. In that case, GCC will allocate at least one TOC entry for
7058 each unique non-automatic variable reference in your program. GCC
7059 will also place floating-point constants in the TOC@. However, only
7060 16,384 entries are available in the TOC@.
7062 If you receive a linker error message that saying you have overflowed
7063 the available TOC space, you can reduce the amount of TOC space used
7064 with the @option{-mno-fp-in-toc} and @option{-mno-sum-in-toc} options.
7065 @option{-mno-fp-in-toc} prevents GCC from putting floating-point
7066 constants in the TOC and @option{-mno-sum-in-toc} forces GCC to
7067 generate code to calculate the sum of an address and a constant at
7068 run-time instead of putting that sum into the TOC@. You may specify one
7069 or both of these options. Each causes GCC to produce very slightly
7070 slower and larger code at the expense of conserving TOC space.
7072 If you still run out of space in the TOC even when you specify both of
7073 these options, specify @option{-mminimal-toc} instead. This option causes
7074 GCC to make only one TOC entry for every file. When you specify this
7075 option, GCC will produce code that is slower and larger but which
7076 uses extremely little TOC space. You may wish to use this option
7077 only on files that contain less frequently executed code.
7083 Enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit
7084 @code{long} type, and the infrastructure needed to support them.
7085 Specifying @option{-maix64} implies @option{-mpowerpc64} and
7086 @option{-mpowerpc}, while @option{-maix32} disables the 64-bit ABI and
7087 implies @option{-mno-powerpc64}. GCC defaults to @option{-maix32}.
7092 @opindex mno-xl-call
7093 On AIX, pass floating-point arguments to prototyped functions beyond the
7094 register save area (RSA) on the stack in addition to argument FPRs. The
7095 AIX calling convention was extended but not initially documented to
7096 handle an obscure K&R C case of calling a function that takes the
7097 address of its arguments with fewer arguments than declared. AIX XL
7098 compilers access floating point arguments which do not fit in the
7099 RSA from the stack when a subroutine is compiled without
7100 optimization. Because always storing floating-point arguments on the
7101 stack is inefficient and rarely needed, this option is not enabled by
7102 default and only is necessary when calling subroutines compiled by AIX
7103 XL compilers without optimization.
7107 Support @dfn{IBM RS/6000 SP} @dfn{Parallel Environment} (PE)@. Link an
7108 application written to use message passing with special startup code to
7109 enable the application to run. The system must have PE installed in the
7110 standard location (@file{/usr/lpp/ppe.poe/}), or the @file{specs} file
7111 must be overridden with the @option{-specs=} option to specify the
7112 appropriate directory location. The Parallel Environment does not
7113 support threads, so the @option{-mpe} option and the @option{-pthread}
7114 option are incompatible.
7116 @item -malign-natural
7117 @itemx -malign-power
7118 @opindex malign-natural
7119 @opindex malign-power
7120 On AIX, Darwin, and 64-bit PowerPC Linux, the option
7121 @option{-malign-natural} overrides the ABI-defined alignment of larger
7122 types, such as floating-point doubles, on their natural size-based boundary.
7123 The option @option{-malign-power} instructs GCC to follow the ABI-specified
7124 alignment rules. GCC defaults to the standard alignment defined in the ABI.
7128 @opindex msoft-float
7129 @opindex mhard-float
7130 Generate code that does not use (uses) the floating-point register set.
7131 Software floating point emulation is provided if you use the
7132 @option{-msoft-float} option, and pass the option to GCC when linking.
7135 @itemx -mno-multiple
7137 @opindex mno-multiple
7138 Generate code that uses (does not use) the load multiple word
7139 instructions and the store multiple word instructions. These
7140 instructions are generated by default on POWER systems, and not
7141 generated on PowerPC systems. Do not use @option{-mmultiple} on little
7142 endian PowerPC systems, since those instructions do not work when the
7143 processor is in little endian mode. The exceptions are PPC740 and
7144 PPC750 which permit the instructions usage in little endian mode.
7150 Generate code that uses (does not use) the load string instructions
7151 and the store string word instructions to save multiple registers and
7152 do small block moves. These instructions are generated by default on
7153 POWER systems, and not generated on PowerPC systems. Do not use
7154 @option{-mstring} on little endian PowerPC systems, since those
7155 instructions do not work when the processor is in little endian mode.
7156 The exceptions are PPC740 and PPC750 which permit the instructions
7157 usage in little endian mode.
7163 Generate code that uses (does not use) the load or store instructions
7164 that update the base register to the address of the calculated memory
7165 location. These instructions are generated by default. If you use
7166 @option{-mno-update}, there is a small window between the time that the
7167 stack pointer is updated and the address of the previous frame is
7168 stored, which means code that walks the stack frame across interrupts or
7169 signals may get corrupted data.
7172 @itemx -mno-fused-madd
7173 @opindex mfused-madd
7174 @opindex mno-fused-madd
7175 Generate code that uses (does not use) the floating point multiply and
7176 accumulate instructions. These instructions are generated by default if
7177 hardware floating is used.
7179 @item -mno-bit-align
7181 @opindex mno-bit-align
7183 On System V.4 and embedded PowerPC systems do not (do) force structures
7184 and unions that contain bit-fields to be aligned to the base type of the
7187 For example, by default a structure containing nothing but 8
7188 @code{unsigned} bit-fields of length 1 would be aligned to a 4 byte
7189 boundary and have a size of 4 bytes. By using @option{-mno-bit-align},
7190 the structure would be aligned to a 1 byte boundary and be one byte in
7193 @item -mno-strict-align
7194 @itemx -mstrict-align
7195 @opindex mno-strict-align
7196 @opindex mstrict-align
7197 On System V.4 and embedded PowerPC systems do not (do) assume that
7198 unaligned memory references will be handled by the system.
7201 @itemx -mno-relocatable
7202 @opindex mrelocatable
7203 @opindex mno-relocatable
7204 On embedded PowerPC systems generate code that allows (does not allow)
7205 the program to be relocated to a different address at runtime. If you
7206 use @option{-mrelocatable} on any module, all objects linked together must
7207 be compiled with @option{-mrelocatable} or @option{-mrelocatable-lib}.
7209 @item -mrelocatable-lib
7210 @itemx -mno-relocatable-lib
7211 @opindex mrelocatable-lib
7212 @opindex mno-relocatable-lib
7213 On embedded PowerPC systems generate code that allows (does not allow)
7214 the program to be relocated to a different address at runtime. Modules
7215 compiled with @option{-mrelocatable-lib} can be linked with either modules
7216 compiled without @option{-mrelocatable} and @option{-mrelocatable-lib} or
7217 with modules compiled with the @option{-mrelocatable} options.
7223 On System V.4 and embedded PowerPC systems do not (do) assume that
7224 register 2 contains a pointer to a global area pointing to the addresses
7225 used in the program.
7228 @itemx -mlittle-endian
7230 @opindex mlittle-endian
7231 On System V.4 and embedded PowerPC systems compile code for the
7232 processor in little endian mode. The @option{-mlittle-endian} option is
7233 the same as @option{-mlittle}.
7238 @opindex mbig-endian
7239 On System V.4 and embedded PowerPC systems compile code for the
7240 processor in big endian mode. The @option{-mbig-endian} option is
7241 the same as @option{-mbig}.
7243 @item -mdynamic-no-pic
7244 @opindex mdynamic-no-pic
7245 On Darwin and Mac OS X systems, compile code so that it is not
7246 relocatable, but that its external references are relocatable. The
7247 resulting code is suitable for applications, but not shared
7252 On System V.4 and embedded PowerPC systems compile code using calling
7253 conventions that adheres to the March 1995 draft of the System V
7254 Application Binary Interface, PowerPC processor supplement. This is the
7255 default unless you configured GCC using @samp{powerpc-*-eabiaix}.
7257 @item -mcall-sysv-eabi
7258 @opindex mcall-sysv-eabi
7259 Specify both @option{-mcall-sysv} and @option{-meabi} options.
7261 @item -mcall-sysv-noeabi
7262 @opindex mcall-sysv-noeabi
7263 Specify both @option{-mcall-sysv} and @option{-mno-eabi} options.
7265 @item -mcall-solaris
7266 @opindex mcall-solaris
7267 On System V.4 and embedded PowerPC systems compile code for the Solaris
7271 @opindex mcall-linux
7272 On System V.4 and embedded PowerPC systems compile code for the
7273 Linux-based GNU system.
7277 On System V.4 and embedded PowerPC systems compile code for the
7278 Hurd-based GNU system.
7281 @opindex mcall-netbsd
7282 On System V.4 and embedded PowerPC systems compile code for the
7283 NetBSD operating system.
7285 @item -maix-struct-return
7286 @opindex maix-struct-return
7287 Return all structures in memory (as specified by the AIX ABI)@.
7289 @item -msvr4-struct-return
7290 @opindex msvr4-struct-return
7291 Return structures smaller than 8 bytes in registers (as specified by the
7295 @opindex mabi=altivec
7296 Extend the current ABI with AltiVec ABI extensions. This does not
7297 change the default ABI, instead it adds the AltiVec ABI extensions to
7300 @item -mabi=no-altivec
7301 @opindex mabi=no-altivec
7302 Disable AltiVec ABI extensions for the current ABI.
7305 @itemx -mno-prototype
7307 @opindex mno-prototype
7308 On System V.4 and embedded PowerPC systems assume that all calls to
7309 variable argument functions are properly prototyped. Otherwise, the
7310 compiler must insert an instruction before every non prototyped call to
7311 set or clear bit 6 of the condition code register (@var{CR}) to
7312 indicate whether floating point values were passed in the floating point
7313 registers in case the function takes a variable arguments. With
7314 @option{-mprototype}, only calls to prototyped variable argument functions
7315 will set or clear the bit.
7319 On embedded PowerPC systems, assume that the startup module is called
7320 @file{sim-crt0.o} and that the standard C libraries are @file{libsim.a} and
7321 @file{libc.a}. This is the default for @samp{powerpc-*-eabisim}.
7326 On embedded PowerPC systems, assume that the startup module is called
7327 @file{crt0.o} and the standard C libraries are @file{libmvme.a} and
7332 On embedded PowerPC systems, assume that the startup module is called
7333 @file{crt0.o} and the standard C libraries are @file{libads.a} and
7337 @opindex myellowknife
7338 On embedded PowerPC systems, assume that the startup module is called
7339 @file{crt0.o} and the standard C libraries are @file{libyk.a} and
7344 On System V.4 and embedded PowerPC systems, specify that you are
7345 compiling for a VxWorks system.
7349 Specify that you are compiling for the WindISS simulation environment.
7353 On embedded PowerPC systems, set the @var{PPC_EMB} bit in the ELF flags
7354 header to indicate that @samp{eabi} extended relocations are used.
7360 On System V.4 and embedded PowerPC systems do (do not) adhere to the
7361 Embedded Applications Binary Interface (eabi) which is a set of
7362 modifications to the System V.4 specifications. Selecting @option{-meabi}
7363 means that the stack is aligned to an 8 byte boundary, a function
7364 @code{__eabi} is called to from @code{main} to set up the eabi
7365 environment, and the @option{-msdata} option can use both @code{r2} and
7366 @code{r13} to point to two separate small data areas. Selecting
7367 @option{-mno-eabi} means that the stack is aligned to a 16 byte boundary,
7368 do not call an initialization function from @code{main}, and the
7369 @option{-msdata} option will only use @code{r13} to point to a single
7370 small data area. The @option{-meabi} option is on by default if you
7371 configured GCC using one of the @samp{powerpc*-*-eabi*} options.
7374 @opindex msdata=eabi
7375 On System V.4 and embedded PowerPC systems, put small initialized
7376 @code{const} global and static data in the @samp{.sdata2} section, which
7377 is pointed to by register @code{r2}. Put small initialized
7378 non-@code{const} global and static data in the @samp{.sdata} section,
7379 which is pointed to by register @code{r13}. Put small uninitialized
7380 global and static data in the @samp{.sbss} section, which is adjacent to
7381 the @samp{.sdata} section. The @option{-msdata=eabi} option is
7382 incompatible with the @option{-mrelocatable} option. The
7383 @option{-msdata=eabi} option also sets the @option{-memb} option.
7386 @opindex msdata=sysv
7387 On System V.4 and embedded PowerPC systems, put small global and static
7388 data in the @samp{.sdata} section, which is pointed to by register
7389 @code{r13}. Put small uninitialized global and static data in the
7390 @samp{.sbss} section, which is adjacent to the @samp{.sdata} section.
7391 The @option{-msdata=sysv} option is incompatible with the
7392 @option{-mrelocatable} option.
7394 @item -msdata=default
7396 @opindex msdata=default
7398 On System V.4 and embedded PowerPC systems, if @option{-meabi} is used,
7399 compile code the same as @option{-msdata=eabi}, otherwise compile code the
7400 same as @option{-msdata=sysv}.
7403 @opindex msdata-data
7404 On System V.4 and embedded PowerPC systems, put small global and static
7405 data in the @samp{.sdata} section. Put small uninitialized global and
7406 static data in the @samp{.sbss} section. Do not use register @code{r13}
7407 to address small data however. This is the default behavior unless
7408 other @option{-msdata} options are used.
7412 @opindex msdata=none
7414 On embedded PowerPC systems, put all initialized global and static data
7415 in the @samp{.data} section, and all uninitialized data in the
7416 @samp{.bss} section.
7420 @cindex smaller data references (PowerPC)
7421 @cindex .sdata/.sdata2 references (PowerPC)
7422 On embedded PowerPC systems, put global and static items less than or
7423 equal to @var{num} bytes into the small data or bss sections instead of
7424 the normal data or bss section. By default, @var{num} is 8. The
7425 @option{-G @var{num}} switch is also passed to the linker.
7426 All modules should be compiled with the same @option{-G @var{num}} value.
7429 @itemx -mno-regnames
7431 @opindex mno-regnames
7432 On System V.4 and embedded PowerPC systems do (do not) emit register
7433 names in the assembly language output using symbolic forms.
7436 @itemx -mno-longcall
7438 @opindex mno-longcall
7439 Default to making all function calls via pointers, so that functions
7440 which reside further than 64 megabytes (67,108,864 bytes) from the
7441 current location can be called. This setting can be overridden by the
7442 @code{shortcall} function attribute, or by @code{#pragma longcall(0)}.
7444 Some linkers are capable of detecting out-of-range calls and generating
7445 glue code on the fly. On these systems, long calls are unnecessary and
7446 generate slower code. As of this writing, the AIX linker can do this,
7447 as can the GNU linker for PowerPC/64. It is planned to add this feature
7448 to the GNU linker for 32-bit PowerPC systems as well.
7450 In the future, we may cause GCC to ignore all longcall specifications
7451 when the linker is known to generate glue.
7455 Adds support for multithreading with the @dfn{pthreads} library.
7456 This option sets flags for both the preprocessor and linker.
7460 @node Darwin Options
7461 @subsection Darwin Options
7462 @cindex Darwin options
7464 These options are defined for all architectures running the Darwin operating
7465 system. They are useful for compatibility with other Mac OS compilers.
7470 Loads all members of static archive libraries.
7471 See man ld(1) for more information.
7473 @item -arch_errors_fatal
7474 @opindex arch_errors_fatal
7475 Cause the errors having to do with files that have the wrong architecture
7479 @opindex bind_at_load
7480 Causes the output file to be marked such that the dynamic linker will
7481 bind all undefined references when the file is loaded or launched.
7485 Produce a Mach-o bundle format file.
7486 See man ld(1) for more information.
7488 @item -bundle_loader @var{executable}
7489 @opindex bundle_loader
7490 This specifies the @var{executable} that will be loading the build
7491 output file being linked. See man ld(1) for more information.
7493 @item -allowable_client @var{client_name}
7497 @item -compatibility_version
7498 @item -current_version
7499 @item -dependency-file
7501 @item -dylinker_install_name
7504 @item -exported_symbols_list
7506 @item -flat_namespace
7507 @item -force_cpusubtype_ALL
7508 @item -force_flat_namespace
7509 @item -headerpad_max_install_names
7513 @item -keep_private_externs
7515 @item -multiply_defined
7516 @item -multiply_defined_unused
7520 @item -noseglinkedit
7521 @item -pagezero_size
7523 @item -prebind_all_twolevel_modules
7524 @item -private_bundle
7525 @item -read_only_relocs
7527 @item -sectobjectsymbols
7531 @item -sectobjectsymbols
7533 @item -seg_addr_table
7534 @item -seg_addr_table_filename
7537 @item -segs_read_only_addr
7538 @item -segs_read_write_addr
7539 @item -single_module
7543 @item -twolevel_namespace
7546 @item -unexported_symbols_list
7547 @item -weak_reference_mismatches
7550 @opindex allowable_client
7552 @opindex client_name
7553 @opindex compatibility_version
7554 @opindex current_version
7555 @opindex dependency-file
7557 @opindex dylinker_install_name
7560 @opindex exported_symbols_list
7562 @opindex flat_namespace
7563 @opindex force_cpusubtype_ALL
7564 @opindex force_flat_namespace
7565 @opindex headerpad_max_install_names
7568 @opindex install_name
7569 @opindex keep_private_externs
7570 @opindex multi_module
7571 @opindex multiply_defined
7572 @opindex multiply_defined_unused
7574 @opindex nomultidefs
7576 @opindex noseglinkedit
7577 @opindex pagezero_size
7579 @opindex prebind_all_twolevel_modules
7580 @opindex private_bundle
7581 @opindex read_only_relocs
7583 @opindex sectobjectsymbols
7587 @opindex sectobjectsymbols
7589 @opindex seg_addr_table
7590 @opindex seg_addr_table_filename
7591 @opindex seglinkedit
7593 @opindex segs_read_only_addr
7594 @opindex segs_read_write_addr
7595 @opindex single_module
7597 @opindex sub_library
7598 @opindex sub_umbrella
7599 @opindex twolevel_namespace
7602 @opindex unexported_symbols_list
7603 @opindex weak_reference_mismatches
7604 @opindex whatsloaded
7606 This options are available for Darwin linker. Darwin linker man page
7607 describes them in detail.
7612 @subsection IBM RT Options
7614 @cindex IBM RT options
7616 These @samp{-m} options are defined for the IBM RT PC:
7620 @opindex min-line-mul
7621 Use an in-line code sequence for integer multiplies. This is the
7624 @item -mcall-lib-mul
7625 @opindex mcall-lib-mul
7626 Call @code{lmul$$} for integer multiples.
7628 @item -mfull-fp-blocks
7629 @opindex mfull-fp-blocks
7630 Generate full-size floating point data blocks, including the minimum
7631 amount of scratch space recommended by IBM@. This is the default.
7633 @item -mminimum-fp-blocks
7634 @opindex mminimum-fp-blocks
7635 Do not include extra scratch space in floating point data blocks. This
7636 results in smaller code, but slower execution, since scratch space must
7637 be allocated dynamically.
7639 @cindex @file{stdarg.h} and RT PC
7640 @item -mfp-arg-in-fpregs
7641 @opindex mfp-arg-in-fpregs
7642 Use a calling sequence incompatible with the IBM calling convention in
7643 which floating point arguments are passed in floating point registers.
7644 Note that @code{stdarg.h} will not work with floating point operands
7645 if this option is specified.
7647 @item -mfp-arg-in-gregs
7648 @opindex mfp-arg-in-gregs
7649 Use the normal calling convention for floating point arguments. This is
7652 @item -mhc-struct-return
7653 @opindex mhc-struct-return
7654 Return structures of more than one word in memory, rather than in a
7655 register. This provides compatibility with the MetaWare HighC (hc)
7656 compiler. Use the option @option{-fpcc-struct-return} for compatibility
7657 with the Portable C Compiler (pcc).
7659 @item -mnohc-struct-return
7660 @opindex mnohc-struct-return
7661 Return some structures of more than one word in registers, when
7662 convenient. This is the default. For compatibility with the
7663 IBM-supplied compilers, use the option @option{-fpcc-struct-return} or the
7664 option @option{-mhc-struct-return}.
7668 @subsection MIPS Options
7669 @cindex MIPS options
7671 These @samp{-m} options are defined for the MIPS family of computers:
7675 @item -march=@var{arch}
7677 Generate code that will run on @var{arch}, which can be the name of a
7678 generic MIPS ISA, or the name of a particular processor.
7680 @samp{mips1}, @samp{mips2}, @samp{mips3}, @samp{mips4},
7681 @samp{mips32}, @samp{mips32r2}, and @samp{mips64}.
7682 The processor names are:
7683 @samp{4kc}, @samp{4kp}, @samp{5kc}, @samp{20kc},
7685 @samp{r2000}, @samp{r3000}, @samp{r3900}, @samp{r4000}, @samp{r4400},
7686 @samp{r4600}, @samp{r4650}, @samp{r6000}, @samp{r8000},
7689 @samp{vr4100}, @samp{vr4300}, and @samp{vr5000}.
7690 The special value @samp{from-abi} selects the
7691 most compatible architecture for the selected ABI (that is,
7692 @samp{mips1} for 32-bit ABIs and @samp{mips3} for 64-bit ABIs)@.
7694 In processor names, a final @samp{000} can be abbreviated as @samp{k}
7695 (for example, @samp{-march=r2k}). Prefixes are optional, and
7696 @samp{vr} may be written @samp{r}.
7698 GCC defines two macros based on the value of this option. The first
7699 is @samp{_MIPS_ARCH}, which gives the name of target architecture, as
7700 a string. The second has the form @samp{_MIPS_ARCH_@var{foo}},
7701 where @var{foo} is the capitalized value of @samp{_MIPS_ARCH}@.
7702 For example, @samp{-march=r2000} will set @samp{_MIPS_ARCH}
7703 to @samp{"r2000"} and define the macro @samp{_MIPS_ARCH_R2000}.
7705 Note that the @samp{_MIPS_ARCH} macro uses the processor names given
7706 above. In other words, it will have the full prefix and will not
7707 abbreviate @samp{000} as @samp{k}. In the case of @samp{from-abi},
7708 the macro names the resolved architecture (either @samp{"mips1"} or
7709 @samp{"mips3"}). It names the default architecture when no
7710 @option{-march} option is given.
7712 @item -mtune=@var{arch}
7714 Optimize for @var{arch}. Among other things, this option controls
7715 the way instructions are scheduled, and the perceived cost of arithmetic
7716 operations. The list of @var{arch} values is the same as for
7719 When this option is not used, GCC will optimize for the processor
7720 specified by @option{-march}. By using @option{-march} and
7721 @option{-mtune} together, it is possible to generate code that will
7722 run on a family of processors, but optimize the code for one
7723 particular member of that family.
7725 @samp{-mtune} defines the macros @samp{_MIPS_TUNE} and
7726 @samp{_MIPS_TUNE_@var{foo}}, which work in the same way as the
7727 @samp{-march} ones described above.
7731 Equivalent to @samp{-march=mips1}.
7735 Equivalent to @samp{-march=mips2}.
7739 Equivalent to @samp{-march=mips3}.
7743 Equivalent to @samp{-march=mips4}.
7747 Equivalent to @samp{-march=mips32}.
7751 Equivalent to @samp{-march=mips32r2}.
7755 Equivalent to @samp{-march=mips64}.
7758 @itemx -mno-fused-madd
7759 @opindex mfused-madd
7760 @opindex mno-fused-madd
7761 Generate code that uses (does not use) the floating point multiply and
7762 accumulate instructions, when they are available. These instructions
7763 are generated by default if they are available, but this may be
7764 undesirable if the extra precision causes problems or on certain chips
7765 in the mode where denormals are rounded to zero where denormals
7766 generated by multiply and accumulate instructions cause exceptions
7771 Assume that floating point registers are 32 bits wide.
7775 Assume that floating point registers are 64 bits wide.
7779 Assume that general purpose registers are 32 bits wide.
7783 Assume that general purpose registers are 64 bits wide.
7787 Force int and long types to be 64 bits wide. See @option{-mlong32} for an
7788 explanation of the default, and the width of pointers.
7792 Force long types to be 64 bits wide. See @option{-mlong32} for an
7793 explanation of the default, and the width of pointers.
7797 Force long, int, and pointer types to be 32 bits wide.
7799 The default size of ints, longs and pointers depends on the ABI@. All
7800 the supported ABIs use 32-bit ints. The n64 ABI uses 64-bit longs, as
7801 does the 64-bit Cygnus EABI; the others use 32-bit longs. Pointers
7802 are the same size as longs, or the same size as integer registers,
7803 whichever is smaller.
7817 Generate code for the given ABI@.
7819 Note that there are two embedded ABIs: @option{-mabi=eabi}
7820 selects the one defined by Cygnus while @option{-meabi=meabi}
7821 selects the one defined by MIPS@. Both these ABIs have
7822 32-bit and 64-bit variants. Normally, GCC will generate
7823 64-bit code when you select a 64-bit architecture, but you
7824 can use @option{-mgp32} to get 32-bit code instead.
7826 @item -mabi-fake-default
7827 @opindex mabi-fake-default
7828 You don't want to know what this option does. No, really. I mean
7829 it. Move on to the next option.
7831 What? You're still here? Oh, well@enddots{} Ok, here's the deal. GCC
7832 wants the default set of options to get the root of the multilib tree,
7833 and the shared library SONAMEs without any multilib-indicating
7834 suffixes. This is not convenience for @samp{mips64-linux-gnu}, since
7835 we want to default to the N32 ABI, while still being binary-compatible
7836 with @samp{mips-linux-gnu} if you stick to the O32 ABI@. Being
7837 binary-compatible means shared libraries should have the same SONAMEs,
7838 and libraries should live in the same location. Having O32 libraries
7839 in a sub-directory named say @file{o32} is not acceptable.
7841 So we trick GCC into believing that O32 is the default ABI, except
7842 that we override the default with some internal command-line
7843 processing magic. Problem is, if we stopped at that, and you then
7844 created a multilib-aware package that used the output of @command{gcc
7845 -print-multi-lib} to decide which multilibs to build, and how, and
7846 you'd find yourself in an awkward situation when you found out that
7847 some of the options listed ended up mapping to the same multilib, and
7848 none of your libraries was actually built for the multilib that
7849 @option{-print-multi-lib} claims to be the default. So we added this
7850 option that disables the default switcher, falling back to GCC's
7851 original notion of the default library. Confused yet?
7853 For short: don't ever use this option, unless you find it in the list
7854 of additional options to be used when building for multilibs, in the
7855 output of @option{gcc -print-multi-lib}.
7859 Generate code for the MIPS assembler, and invoke @file{mips-tfile} to
7860 add normal debug information. This is the default for all
7861 platforms except for the OSF/1 reference platform, using the OSF/rose
7862 object format. If the either of the @option{-gstabs} or @option{-gstabs+}
7863 switches are used, the @file{mips-tfile} program will encapsulate the
7864 stabs within MIPS ECOFF@.
7868 Generate code for the GNU assembler. This is the default on the OSF/1
7869 reference platform, using the OSF/rose object format. Also, this is
7870 the default if the configure option @option{--with-gnu-as} is used.
7872 @item -msplit-addresses
7873 @itemx -mno-split-addresses
7874 @opindex msplit-addresses
7875 @opindex mno-split-addresses
7876 Generate code to load the high and low parts of address constants separately.
7877 This allows GCC to optimize away redundant loads of the high order
7878 bits of addresses. This optimization requires GNU as and GNU ld.
7879 This optimization is enabled by default for some embedded targets where
7880 GNU as and GNU ld are standard.
7886 The @option{-mrnames} switch says to output code using the MIPS software
7887 names for the registers, instead of the hardware names (ie, @var{a0}
7888 instead of @var{$4}). The only known assembler that supports this option
7889 is the Algorithmics assembler.
7895 The @option{-mgpopt} switch says to write all of the data declarations
7896 before the instructions in the text section, this allows the MIPS
7897 assembler to generate one word memory references instead of using two
7898 words for short global or static data items. This is on by default if
7899 optimization is selected.
7905 The @option{-mmemcpy} switch makes all block moves call the appropriate
7906 string function (@samp{memcpy} or @samp{bcopy}) instead of possibly
7907 generating inline code.
7910 @itemx -mno-mips-tfile
7911 @opindex mmips-tfile
7912 @opindex mno-mips-tfile
7913 The @option{-mno-mips-tfile} switch causes the compiler not
7914 postprocess the object file with the @file{mips-tfile} program,
7915 after the MIPS assembler has generated it to add debug support. If
7916 @file{mips-tfile} is not run, then no local variables will be
7917 available to the debugger. In addition, @file{stage2} and
7918 @file{stage3} objects will have the temporary file names passed to the
7919 assembler embedded in the object file, which means the objects will
7920 not compare the same. The @option{-mno-mips-tfile} switch should only
7921 be used when there are bugs in the @file{mips-tfile} program that
7922 prevents compilation.
7925 @opindex msoft-float
7926 Generate output containing library calls for floating point.
7927 @strong{Warning:} the requisite libraries are not part of GCC@.
7928 Normally the facilities of the machine's usual C compiler are used, but
7929 this can't be done directly in cross-compilation. You must make your
7930 own arrangements to provide suitable library functions for
7934 @opindex mhard-float
7935 Generate output containing floating point instructions. This is the
7936 default if you use the unmodified sources.
7939 @itemx -mno-abicalls
7941 @opindex mno-abicalls
7942 Emit (or do not emit) the pseudo operations @samp{.abicalls},
7943 @samp{.cpload}, and @samp{.cprestore} that some System V.4 ports use for
7944 position independent code.
7947 @itemx -mno-long-calls
7948 @opindex mlong-calls
7949 @opindex mno-long-calls
7950 Do all calls with the @samp{JALR} instruction, which requires
7951 loading up a function's address into a register before the call.
7952 You need to use this switch, if you call outside of the current
7953 512 megabyte segment to functions that are not through pointers.
7956 @itemx -mno-half-pic
7958 @opindex mno-half-pic
7959 Put pointers to extern references into the data section and load them
7960 up, rather than put the references in the text section.
7962 @item -membedded-pic
7963 @itemx -mno-embedded-pic
7964 @opindex membedded-pic
7965 @opindex mno-embedded-pic
7966 Generate PIC code suitable for some embedded systems. All calls are
7967 made using PC relative address, and all data is addressed using the $gp
7968 register. No more than 65536 bytes of global data may be used. This
7969 requires GNU as and GNU ld which do most of the work. This currently
7970 only works on targets which use ECOFF; it does not work with ELF@.
7972 @item -membedded-data
7973 @itemx -mno-embedded-data
7974 @opindex membedded-data
7975 @opindex mno-embedded-data
7976 Allocate variables to the read-only data section first if possible, then
7977 next in the small data section if possible, otherwise in data. This gives
7978 slightly slower code than the default, but reduces the amount of RAM required
7979 when executing, and thus may be preferred for some embedded systems.
7981 @item -muninit-const-in-rodata
7982 @itemx -mno-uninit-const-in-rodata
7983 @opindex muninit-const-in-rodata
7984 @opindex mno-uninit-const-in-rodata
7985 When used together with @option{-membedded-data}, it will always store uninitialized
7986 const variables in the read-only data section.
7988 @item -msingle-float
7989 @itemx -mdouble-float
7990 @opindex msingle-float
7991 @opindex mdouble-float
7992 The @option{-msingle-float} switch tells gcc to assume that the floating
7993 point coprocessor only supports single precision operations, as on the
7994 @samp{r4650} chip. The @option{-mdouble-float} switch permits gcc to use
7995 double precision operations. This is the default.
8001 Permit use of the @samp{mad}, @samp{madu} and @samp{mul} instructions,
8002 as on the @samp{r4650} chip.
8006 Turns on @option{-msingle-float}, @option{-mmad}, and, at least for now,
8007 @option{-mcpu=r4650}.
8013 Enable 16-bit instructions.
8017 Use the entry and exit pseudo ops. This option can only be used with
8022 Compile code for the processor in little endian mode.
8023 The requisite libraries are assumed to exist.
8027 Compile code for the processor in big endian mode.
8028 The requisite libraries are assumed to exist.
8032 @cindex smaller data references (MIPS)
8033 @cindex gp-relative references (MIPS)
8034 Put global and static items less than or equal to @var{num} bytes into
8035 the small data or bss sections instead of the normal data or bss
8036 section. This allows the assembler to emit one word memory reference
8037 instructions based on the global pointer (@var{gp} or @var{$28}),
8038 instead of the normal two words used. By default, @var{num} is 8 when
8039 the MIPS assembler is used, and 0 when the GNU assembler is used. The
8040 @option{-G @var{num}} switch is also passed to the assembler and linker.
8041 All modules should be compiled with the same @option{-G @var{num}}
8046 Tell the MIPS assembler to not run its preprocessor over user
8047 assembler files (with a @samp{.s} suffix) when assembling them.
8051 Pass an option to gas which will cause nops to be inserted if
8052 the read of the destination register of an mfhi or mflo instruction
8053 occurs in the following two instructions.
8057 Do not include the default crt0.
8059 @item -mflush-func=@var{func}
8060 @itemx -mno-flush-func
8061 @opindex mflush-func
8062 Specifies the function to call to flush the I and D caches, or to not
8063 call any such function. If called, the function must take the same
8064 arguments as the common @code{_flush_func()}, that is, the address of the
8065 memory range for which the cache is being flushed, the size of the
8066 memory range, and the number 3 (to flush both caches). The default
8067 depends on the target gcc was configured for, but commonly is either
8068 @samp{_flush_func} or @samp{__cpu_flush}.
8070 @item -mbranch-likely
8071 @itemx -mno-branch-likely
8072 @opindex mbranch-likely
8073 @opindex mno-branch-likely
8074 Enable or disable use of Branch Likely instructions, regardless of the
8075 default for the selected architecture. By default, Branch Likely
8076 instructions may be generated if they are supported by the selected
8077 architecture. An exception is for the MIPS32 and MIPS64 architectures
8078 and processors which implement those architectures; for those, Branch
8079 Likely instructions will not be generated by default because the MIPS32
8080 and MIPS64 architectures specifically deprecate their use.
8083 @node i386 and x86-64 Options
8084 @subsection Intel 386 and AMD x86-64 Options
8085 @cindex i386 Options
8086 @cindex x86-64 Options
8087 @cindex Intel 386 Options
8088 @cindex AMD x86-64 Options
8090 These @samp{-m} options are defined for the i386 and x86-64 family of
8094 @item -mtune=@var{cpu-type}
8096 Tune to @var{cpu-type} everything applicable about the generated code, except
8097 for the ABI and the set of available instructions. The choices for
8098 @var{cpu-type} are @samp{i386}, @samp{i486}, @samp{i586}, @samp{i686},
8099 @samp{pentium}, @samp{pentium-mmx}, @samp{pentiumpro}, @samp{pentium2},
8100 @samp{pentium3}, @samp{pentium4}, @samp{k6}, @samp{k6-2}, @samp{k6-3},
8101 @samp{athlon}, @samp{athlon-tbird}, @samp{athlon-4}, @samp{athlon-xp},
8102 @samp{athlon-mp}, @samp{winchip-c6}, @samp{winchip2}, @samp{k8}, @samp{c3}
8105 While picking a specific @var{cpu-type} will schedule things appropriately
8106 for that particular chip, the compiler will not generate any code that
8107 does not run on the i386 without the @option{-march=@var{cpu-type}} option
8108 being used. @samp{i586} is equivalent to @samp{pentium} and @samp{i686}
8109 is equivalent to @samp{pentiumpro}. @samp{k6} and @samp{athlon} are the
8110 AMD chips as opposed to the Intel ones.
8112 @item -march=@var{cpu-type}
8114 Generate instructions for the machine type @var{cpu-type}. The choices
8115 for @var{cpu-type} are the same as for @option{-mtune}. Moreover,
8116 specifying @option{-march=@var{cpu-type}} implies @option{-mtune=@var{cpu-type}}.
8118 @item -mcpu=@var{cpu-type}
8120 A deprecated synonym for @option{-mtune}.
8129 @opindex mpentiumpro
8130 These options are synonyms for @option{-mtune=i386}, @option{-mtune=i486},
8131 @option{-mtune=pentium}, and @option{-mtune=pentiumpro} respectively.
8132 These synonyms are deprecated.
8134 @item -mfpmath=@var{unit}
8136 generate floating point arithmetics for selected unit @var{unit}. the choices
8141 Use the standard 387 floating point coprocessor present majority of chips and
8142 emulated otherwise. Code compiled with this option will run almost everywhere.
8143 The temporary results are computed in 80bit precision instead of precision
8144 specified by the type resulting in slightly different results compared to most
8145 of other chips. See @option{-ffloat-store} for more detailed description.
8147 This is the default choice for i386 compiler.
8150 Use scalar floating point instructions present in the SSE instruction set.
8151 This instruction set is supported by Pentium3 and newer chips, in the AMD line
8152 by Athlon-4, Athlon-xp and Athlon-mp chips. The earlier version of SSE
8153 instruction set supports only single precision arithmetics, thus the double and
8154 extended precision arithmetics is still done using 387. Later version, present
8155 only in Pentium4 and the future AMD x86-64 chips supports double precision
8158 For i387 you need to use @option{-march=@var{cpu-type}}, @option{-msse} or
8159 @option{-msse2} switches to enable SSE extensions and make this option
8160 effective. For x86-64 compiler, these extensions are enabled by default.
8162 The resulting code should be considerably faster in majority of cases and avoid
8163 the numerical instability problems of 387 code, but may break some existing
8164 code that expects temporaries to be 80bit.
8166 This is the default choice for x86-64 compiler.
8169 Attempt to utilize both instruction sets at once. This effectively double the
8170 amount of available registers and on chips with separate execution units for
8171 387 and SSE the execution resources too. Use this option with care, as it is
8172 still experimental, because gcc register allocator does not model separate
8173 functional units well resulting in instable performance.
8176 @item -masm=@var{dialect}
8177 @opindex masm=@var{dialect}
8178 Output asm instructions using selected @var{dialect}. Supported choices are
8179 @samp{intel} or @samp{att} (the default one).
8184 @opindex mno-ieee-fp
8185 Control whether or not the compiler uses IEEE floating point
8186 comparisons. These handle correctly the case where the result of a
8187 comparison is unordered.
8190 @opindex msoft-float
8191 Generate output containing library calls for floating point.
8192 @strong{Warning:} the requisite libraries are not part of GCC@.
8193 Normally the facilities of the machine's usual C compiler are used, but
8194 this can't be done directly in cross-compilation. You must make your
8195 own arrangements to provide suitable library functions for
8198 On machines where a function returns floating point results in the 80387
8199 register stack, some floating point opcodes may be emitted even if
8200 @option{-msoft-float} is used.
8202 @item -mno-fp-ret-in-387
8203 @opindex mno-fp-ret-in-387
8204 Do not use the FPU registers for return values of functions.
8206 The usual calling convention has functions return values of types
8207 @code{float} and @code{double} in an FPU register, even if there
8208 is no FPU@. The idea is that the operating system should emulate
8211 The option @option{-mno-fp-ret-in-387} causes such values to be returned
8212 in ordinary CPU registers instead.
8214 @item -mno-fancy-math-387
8215 @opindex mno-fancy-math-387
8216 Some 387 emulators do not support the @code{sin}, @code{cos} and
8217 @code{sqrt} instructions for the 387. Specify this option to avoid
8218 generating those instructions. This option is the default on FreeBSD,
8219 OpenBSD and NetBSD@. This option is overridden when @option{-march}
8220 indicates that the target cpu will always have an FPU and so the
8221 instruction will not need emulation. As of revision 2.6.1, these
8222 instructions are not generated unless you also use the
8223 @option{-funsafe-math-optimizations} switch.
8225 @item -malign-double
8226 @itemx -mno-align-double
8227 @opindex malign-double
8228 @opindex mno-align-double
8229 Control whether GCC aligns @code{double}, @code{long double}, and
8230 @code{long long} variables on a two word boundary or a one word
8231 boundary. Aligning @code{double} variables on a two word boundary will
8232 produce code that runs somewhat faster on a @samp{Pentium} at the
8233 expense of more memory.
8235 @strong{Warning:} if you use the @option{-malign-double} switch,
8236 structures containing the above types will be aligned differently than
8237 the published application binary interface specifications for the 386
8238 and will not be binary compatible with structures in code compiled
8239 without that switch.
8241 @item -m128bit-long-double
8242 @opindex m128bit-long-double
8243 Control the size of @code{long double} type. i386 application binary interface
8244 specify the size to be 12 bytes, while modern architectures (Pentium and newer)
8245 prefer @code{long double} aligned to 8 or 16 byte boundary. This is
8246 impossible to reach with 12 byte long doubles in the array accesses.
8248 @strong{Warning:} if you use the @option{-m128bit-long-double} switch, the
8249 structures and arrays containing @code{long double} will change their size as
8250 well as function calling convention for function taking @code{long double}
8253 @item -m96bit-long-double
8254 @opindex m96bit-long-double
8255 Set the size of @code{long double} to 96 bits as required by the i386
8256 application binary interface. This is the default.
8259 @itemx -mno-svr3-shlib
8260 @opindex msvr3-shlib
8261 @opindex mno-svr3-shlib
8262 Control whether GCC places uninitialized local variables into the
8263 @code{bss} or @code{data} segments. @option{-msvr3-shlib} places them
8264 into @code{bss}. These options are meaningful only on System V Release 3.
8268 Use a different function-calling convention, in which functions that
8269 take a fixed number of arguments return with the @code{ret} @var{num}
8270 instruction, which pops their arguments while returning. This saves one
8271 instruction in the caller since there is no need to pop the arguments
8274 You can specify that an individual function is called with this calling
8275 sequence with the function attribute @samp{stdcall}. You can also
8276 override the @option{-mrtd} option by using the function attribute
8277 @samp{cdecl}. @xref{Function Attributes}.
8279 @strong{Warning:} this calling convention is incompatible with the one
8280 normally used on Unix, so you cannot use it if you need to call
8281 libraries compiled with the Unix compiler.
8283 Also, you must provide function prototypes for all functions that
8284 take variable numbers of arguments (including @code{printf});
8285 otherwise incorrect code will be generated for calls to those
8288 In addition, seriously incorrect code will result if you call a
8289 function with too many arguments. (Normally, extra arguments are
8290 harmlessly ignored.)
8292 @item -mregparm=@var{num}
8294 Control how many registers are used to pass integer arguments. By
8295 default, no registers are used to pass arguments, and at most 3
8296 registers can be used. You can control this behavior for a specific
8297 function by using the function attribute @samp{regparm}.
8298 @xref{Function Attributes}.
8300 @strong{Warning:} if you use this switch, and
8301 @var{num} is nonzero, then you must build all modules with the same
8302 value, including any libraries. This includes the system libraries and
8305 @item -mpreferred-stack-boundary=@var{num}
8306 @opindex mpreferred-stack-boundary
8307 Attempt to keep the stack boundary aligned to a 2 raised to @var{num}
8308 byte boundary. If @option{-mpreferred-stack-boundary} is not specified,
8309 the default is 4 (16 bytes or 128 bits), except when optimizing for code
8310 size (@option{-Os}), in which case the default is the minimum correct
8311 alignment (4 bytes for x86, and 8 bytes for x86-64).
8313 On Pentium and PentiumPro, @code{double} and @code{long double} values
8314 should be aligned to an 8 byte boundary (see @option{-malign-double}) or
8315 suffer significant run time performance penalties. On Pentium III, the
8316 Streaming SIMD Extension (SSE) data type @code{__m128} suffers similar
8317 penalties if it is not 16 byte aligned.
8319 To ensure proper alignment of this values on the stack, the stack boundary
8320 must be as aligned as that required by any value stored on the stack.
8321 Further, every function must be generated such that it keeps the stack
8322 aligned. Thus calling a function compiled with a higher preferred
8323 stack boundary from a function compiled with a lower preferred stack
8324 boundary will most likely misalign the stack. It is recommended that
8325 libraries that use callbacks always use the default setting.
8327 This extra alignment does consume extra stack space, and generally
8328 increases code size. Code that is sensitive to stack space usage, such
8329 as embedded systems and operating system kernels, may want to reduce the
8330 preferred alignment to @option{-mpreferred-stack-boundary=2}.
8346 These switches enable or disable the use of built-in functions that allow
8347 direct access to the MMX, SSE and 3Dnow extensions of the instruction set.
8349 @xref{X86 Built-in Functions}, for details of the functions enabled
8350 and disabled by these switches.
8352 To have SSE/SSE2 instructions generated automatically from floating-point
8353 code, see @option{-mfpmath=sse}.
8356 @itemx -mno-push-args
8358 @opindex mno-push-args
8359 Use PUSH operations to store outgoing parameters. This method is shorter
8360 and usually equally fast as method using SUB/MOV operations and is enabled
8361 by default. In some cases disabling it may improve performance because of
8362 improved scheduling and reduced dependencies.
8364 @item -maccumulate-outgoing-args
8365 @opindex maccumulate-outgoing-args
8366 If enabled, the maximum amount of space required for outgoing arguments will be
8367 computed in the function prologue. This is faster on most modern CPUs
8368 because of reduced dependencies, improved scheduling and reduced stack usage
8369 when preferred stack boundary is not equal to 2. The drawback is a notable
8370 increase in code size. This switch implies @option{-mno-push-args}.
8374 Support thread-safe exception handling on @samp{Mingw32}. Code that relies
8375 on thread-safe exception handling must compile and link all code with the
8376 @option{-mthreads} option. When compiling, @option{-mthreads} defines
8377 @option{-D_MT}; when linking, it links in a special thread helper library
8378 @option{-lmingwthrd} which cleans up per thread exception handling data.
8380 @item -mno-align-stringops
8381 @opindex mno-align-stringops
8382 Do not align destination of inlined string operations. This switch reduces
8383 code size and improves performance in case the destination is already aligned,
8384 but gcc don't know about it.
8386 @item -minline-all-stringops
8387 @opindex minline-all-stringops
8388 By default GCC inlines string operations only when destination is known to be
8389 aligned at least to 4 byte boundary. This enables more inlining, increase code
8390 size, but may improve performance of code that depends on fast memcpy, strlen
8391 and memset for short lengths.
8393 @item -momit-leaf-frame-pointer
8394 @opindex momit-leaf-frame-pointer
8395 Don't keep the frame pointer in a register for leaf functions. This
8396 avoids the instructions to save, set up and restore frame pointers and
8397 makes an extra register available in leaf functions. The option
8398 @option{-fomit-frame-pointer} removes the frame pointer for all functions
8399 which might make debugging harder.
8402 These @samp{-m} switches are supported in addition to the above
8403 on AMD x86-64 processors in 64-bit environments.
8410 Generate code for a 32-bit or 64-bit environment.
8411 The 32-bit environment sets int, long and pointer to 32 bits and
8412 generates code that runs on any i386 system.
8413 The 64-bit environment sets int to 32 bits and long and pointer
8414 to 64 bits and generates code for AMD's x86-64 architecture.
8417 @opindex no-red-zone
8418 Do not use a so called red zone for x86-64 code. The red zone is mandated
8419 by the x86-64 ABI, it is a 128-byte area beyond the location of the
8420 stack pointer that will not be modified by signal or interrupt handlers
8421 and therefore can be used for temporary data without adjusting the stack
8422 pointer. The flag @option{-mno-red-zone} disables this red zone.
8424 @item -mcmodel=small
8425 @opindex mcmodel=small
8426 Generate code for the small code model: the program and its symbols must
8427 be linked in the lower 2 GB of the address space. Pointers are 64 bits.
8428 Programs can be statically or dynamically linked. This is the default
8431 @item -mcmodel=kernel
8432 @opindex mcmodel=kernel
8433 Generate code for the kernel code model. The kernel runs in the
8434 negative 2 GB of the address space.
8435 This model has to be used for Linux kernel code.
8437 @item -mcmodel=medium
8438 @opindex mcmodel=medium
8439 Generate code for the medium model: The program is linked in the lower 2
8440 GB of the address space but symbols can be located anywhere in the
8441 address space. Programs can be statically or dynamically linked, but
8442 building of shared libraries are not supported with the medium model.
8444 @item -mcmodel=large
8445 @opindex mcmodel=large
8446 Generate code for the large model: This model makes no assumptions
8447 about addresses and sizes of sections. Currently GCC does not implement
8452 @subsection HPPA Options
8453 @cindex HPPA Options
8455 These @samp{-m} options are defined for the HPPA family of computers:
8458 @item -march=@var{architecture-type}
8460 Generate code for the specified architecture. The choices for
8461 @var{architecture-type} are @samp{1.0} for PA 1.0, @samp{1.1} for PA
8462 1.1, and @samp{2.0} for PA 2.0 processors. Refer to
8463 @file{/usr/lib/sched.models} on an HP-UX system to determine the proper
8464 architecture option for your machine. Code compiled for lower numbered
8465 architectures will run on higher numbered architectures, but not the
8468 PA 2.0 support currently requires gas snapshot 19990413 or later. The
8469 next release of binutils (current is 2.9.1) will probably contain PA 2.0
8473 @itemx -mpa-risc-1-1
8474 @itemx -mpa-risc-2-0
8475 @opindex mpa-risc-1-0
8476 @opindex mpa-risc-1-1
8477 @opindex mpa-risc-2-0
8478 Synonyms for @option{-march=1.0}, @option{-march=1.1}, and @option{-march=2.0} respectively.
8481 @opindex mbig-switch
8482 Generate code suitable for big switch tables. Use this option only if
8483 the assembler/linker complain about out of range branches within a switch
8486 @item -mjump-in-delay
8487 @opindex mjump-in-delay
8488 Fill delay slots of function calls with unconditional jump instructions
8489 by modifying the return pointer for the function call to be the target
8490 of the conditional jump.
8492 @item -mdisable-fpregs
8493 @opindex mdisable-fpregs
8494 Prevent floating point registers from being used in any manner. This is
8495 necessary for compiling kernels which perform lazy context switching of
8496 floating point registers. If you use this option and attempt to perform
8497 floating point operations, the compiler will abort.
8499 @item -mdisable-indexing
8500 @opindex mdisable-indexing
8501 Prevent the compiler from using indexing address modes. This avoids some
8502 rather obscure problems when compiling MIG generated code under MACH@.
8504 @item -mno-space-regs
8505 @opindex mno-space-regs
8506 Generate code that assumes the target has no space registers. This allows
8507 GCC to generate faster indirect calls and use unscaled index address modes.
8509 Such code is suitable for level 0 PA systems and kernels.
8511 @item -mfast-indirect-calls
8512 @opindex mfast-indirect-calls
8513 Generate code that assumes calls never cross space boundaries. This
8514 allows GCC to emit code which performs faster indirect calls.
8516 This option will not work in the presence of shared libraries or nested
8519 @item -mlong-load-store
8520 @opindex mlong-load-store
8521 Generate 3-instruction load and store sequences as sometimes required by
8522 the HP-UX 10 linker. This is equivalent to the @samp{+k} option to
8525 @item -mportable-runtime
8526 @opindex mportable-runtime
8527 Use the portable calling conventions proposed by HP for ELF systems.
8531 Enable the use of assembler directives only GAS understands.
8533 @item -mschedule=@var{cpu-type}
8535 Schedule code according to the constraints for the machine type
8536 @var{cpu-type}. The choices for @var{cpu-type} are @samp{700}
8537 @samp{7100}, @samp{7100LC}, @samp{7200}, @samp{7300} and @samp{8000}. Refer
8538 to @file{/usr/lib/sched.models} on an HP-UX system to determine the
8539 proper scheduling option for your machine. The default scheduling is
8543 @opindex mlinker-opt
8544 Enable the optimization pass in the HP-UX linker. Note this makes symbolic
8545 debugging impossible. It also triggers a bug in the HP-UX 8 and HP-UX 9
8546 linkers in which they give bogus error messages when linking some programs.
8549 @opindex msoft-float
8550 Generate output containing library calls for floating point.
8551 @strong{Warning:} the requisite libraries are not available for all HPPA
8552 targets. Normally the facilities of the machine's usual C compiler are
8553 used, but this cannot be done directly in cross-compilation. You must make
8554 your own arrangements to provide suitable library functions for
8555 cross-compilation. The embedded target @samp{hppa1.1-*-pro}
8556 does provide software floating point support.
8558 @option{-msoft-float} changes the calling convention in the output file;
8559 therefore, it is only useful if you compile @emph{all} of a program with
8560 this option. In particular, you need to compile @file{libgcc.a}, the
8561 library that comes with GCC, with @option{-msoft-float} in order for
8566 Generate the predefine, @code{_SIO}, for server IO. The default is
8567 @option{-mwsio}. This generates the predefines, @code{__hp9000s700},
8568 @code{__hp9000s700__} and @code{_WSIO}, for workstation IO. These
8569 options are available under HP-UX and HI-UX.
8573 Use GNU ld specific options. This passes @option{-shared} to ld when
8574 building a shared library. It is the default when GCC is configured,
8575 explicitly or implicitly, with the GNU linker. This option does not
8576 have any affect on which ld is called, it only changes what parameters
8577 are passed to that ld. The ld that is called is determined by the
8578 @option{--with-ld} configure option, gcc's program search path, and
8579 finally by the user's @env{PATH}. The linker used by GCC can be printed
8580 using @samp{which `gcc -print-prog-name=ld`}.
8584 Use HP ld specific options. This passes @option{-b} to ld when building
8585 a shared library and passes @option{+Accept TypeMismatch} to ld on all
8586 links. It is the default when GCC is configured, explicitly or
8587 implicitly, with the HP linker. This option does not have any affect on
8588 which ld is called, it only changes what parameters are passed to that
8589 ld. The ld that is called is determined by the @option{--with-ld}
8590 configure option, gcc's program search path, and finally by the user's
8591 @env{PATH}. The linker used by GCC can be printed using @samp{which
8592 `gcc -print-prog-name=ld`}.
8595 @opindex mno-long-calls
8596 Generate code that uses long call sequences. This ensures that a call
8597 is always able to reach linker generated stubs. The default is to generate
8598 long calls only when the distance from the call site to the beginning
8599 of the function or translation unit, as the case may be, exceeds a
8600 predefined limit set by the branch type being used. The limits for
8601 normal calls are 7,600,000 and 240,000 bytes, respectively for the
8602 PA 2.0 and PA 1.X architectures. Sibcalls are always limited at
8605 Distances are measured from the beginning of functions when using the
8606 @option{-ffunction-sections} option, or when using the @option{-mgas}
8607 and @option{-mno-portable-runtime} options together under HP-UX with
8610 It is normally not desirable to use this option as it will degrade
8611 performance. However, it may be useful in large applications,
8612 particularly when partial linking is used to build the application.
8614 The types of long calls used depends on the capabilities of the
8615 assembler and linker, and the type of code being generated. The
8616 impact on systems that support long absolute calls, and long pic
8617 symbol-difference or pc-relative calls should be relatively small.
8618 However, an indirect call is used on 32-bit ELF systems in pic code
8619 and it is quite long.
8623 Suppress the generation of link options to search libdld.sl when the
8624 @option{-static} option is specified on HP-UX 10 and later.
8628 The HP-UX implementation of setlocale in libc has a dependency on
8629 libdld.sl. There isn't an archive version of libdld.sl. Thus,
8630 when the @option{-static} option is specified, special link options
8631 are needed to resolve this dependency.
8633 On HP-UX 10 and later, the GCC driver adds the necessary options to
8634 link with libdld.sl when the @option{-static} option is specified.
8635 This causes the resulting binary to be dynamic. On the 64-bit port,
8636 the linkers generate dynamic binaries by default in any case. The
8637 @option{-nolibdld} option can be used to prevent the GCC driver from
8638 adding these link options.
8642 Add support for multithreading with the @dfn{dce thread} library
8643 under HP-UX. This option sets flags for both the preprocessor and
8647 @node Intel 960 Options
8648 @subsection Intel 960 Options
8650 These @samp{-m} options are defined for the Intel 960 implementations:
8653 @item -m@var{cpu-type}
8661 Assume the defaults for the machine type @var{cpu-type} for some of
8662 the other options, including instruction scheduling, floating point
8663 support, and addressing modes. The choices for @var{cpu-type} are
8664 @samp{ka}, @samp{kb}, @samp{mc}, @samp{ca}, @samp{cf},
8665 @samp{sa}, and @samp{sb}.
8672 @opindex msoft-float
8673 The @option{-mnumerics} option indicates that the processor does support
8674 floating-point instructions. The @option{-msoft-float} option indicates
8675 that floating-point support should not be assumed.
8677 @item -mleaf-procedures
8678 @itemx -mno-leaf-procedures
8679 @opindex mleaf-procedures
8680 @opindex mno-leaf-procedures
8681 Do (or do not) attempt to alter leaf procedures to be callable with the
8682 @code{bal} instruction as well as @code{call}. This will result in more
8683 efficient code for explicit calls when the @code{bal} instruction can be
8684 substituted by the assembler or linker, but less efficient code in other
8685 cases, such as calls via function pointers, or using a linker that doesn't
8686 support this optimization.
8689 @itemx -mno-tail-call
8691 @opindex mno-tail-call
8692 Do (or do not) make additional attempts (beyond those of the
8693 machine-independent portions of the compiler) to optimize tail-recursive
8694 calls into branches. You may not want to do this because the detection of
8695 cases where this is not valid is not totally complete. The default is
8696 @option{-mno-tail-call}.
8698 @item -mcomplex-addr
8699 @itemx -mno-complex-addr
8700 @opindex mcomplex-addr
8701 @opindex mno-complex-addr
8702 Assume (or do not assume) that the use of a complex addressing mode is a
8703 win on this implementation of the i960. Complex addressing modes may not
8704 be worthwhile on the K-series, but they definitely are on the C-series.
8705 The default is currently @option{-mcomplex-addr} for all processors except
8709 @itemx -mno-code-align
8710 @opindex mcode-align
8711 @opindex mno-code-align
8712 Align code to 8-byte boundaries for faster fetching (or don't bother).
8713 Currently turned on by default for C-series implementations only.
8716 @item -mclean-linkage
8717 @itemx -mno-clean-linkage
8718 @opindex mclean-linkage
8719 @opindex mno-clean-linkage
8720 These options are not fully implemented.
8724 @itemx -mic2.0-compat
8725 @itemx -mic3.0-compat
8727 @opindex mic2.0-compat
8728 @opindex mic3.0-compat
8729 Enable compatibility with iC960 v2.0 or v3.0.
8733 @opindex masm-compat
8735 Enable compatibility with the iC960 assembler.
8737 @item -mstrict-align
8738 @itemx -mno-strict-align
8739 @opindex mstrict-align
8740 @opindex mno-strict-align
8741 Do not permit (do permit) unaligned accesses.
8745 Enable structure-alignment compatibility with Intel's gcc release version
8746 1.3 (based on gcc 1.37). This option implies @option{-mstrict-align}.
8748 @item -mlong-double-64
8749 @opindex mlong-double-64
8750 Implement type @samp{long double} as 64-bit floating point numbers.
8751 Without the option @samp{long double} is implemented by 80-bit
8752 floating point numbers. The only reason we have it because there is
8753 no 128-bit @samp{long double} support in @samp{fp-bit.c} yet. So it
8754 is only useful for people using soft-float targets. Otherwise, we
8755 should recommend against use of it.
8759 @node DEC Alpha Options
8760 @subsection DEC Alpha Options
8762 These @samp{-m} options are defined for the DEC Alpha implementations:
8765 @item -mno-soft-float
8767 @opindex mno-soft-float
8768 @opindex msoft-float
8769 Use (do not use) the hardware floating-point instructions for
8770 floating-point operations. When @option{-msoft-float} is specified,
8771 functions in @file{libgcc.a} will be used to perform floating-point
8772 operations. Unless they are replaced by routines that emulate the
8773 floating-point operations, or compiled in such a way as to call such
8774 emulations routines, these routines will issue floating-point
8775 operations. If you are compiling for an Alpha without floating-point
8776 operations, you must ensure that the library is built so as not to call
8779 Note that Alpha implementations without floating-point operations are
8780 required to have floating-point registers.
8785 @opindex mno-fp-regs
8786 Generate code that uses (does not use) the floating-point register set.
8787 @option{-mno-fp-regs} implies @option{-msoft-float}. If the floating-point
8788 register set is not used, floating point operands are passed in integer
8789 registers as if they were integers and floating-point results are passed
8790 in @code{$0} instead of @code{$f0}. This is a non-standard calling sequence,
8791 so any function with a floating-point argument or return value called by code
8792 compiled with @option{-mno-fp-regs} must also be compiled with that
8795 A typical use of this option is building a kernel that does not use,
8796 and hence need not save and restore, any floating-point registers.
8800 The Alpha architecture implements floating-point hardware optimized for
8801 maximum performance. It is mostly compliant with the IEEE floating
8802 point standard. However, for full compliance, software assistance is
8803 required. This option generates code fully IEEE compliant code
8804 @emph{except} that the @var{inexact-flag} is not maintained (see below).
8805 If this option is turned on, the preprocessor macro @code{_IEEE_FP} is
8806 defined during compilation. The resulting code is less efficient but is
8807 able to correctly support denormalized numbers and exceptional IEEE
8808 values such as not-a-number and plus/minus infinity. Other Alpha
8809 compilers call this option @option{-ieee_with_no_inexact}.
8811 @item -mieee-with-inexact
8812 @opindex mieee-with-inexact
8813 This is like @option{-mieee} except the generated code also maintains
8814 the IEEE @var{inexact-flag}. Turning on this option causes the
8815 generated code to implement fully-compliant IEEE math. In addition to
8816 @code{_IEEE_FP}, @code{_IEEE_FP_EXACT} is defined as a preprocessor
8817 macro. On some Alpha implementations the resulting code may execute
8818 significantly slower than the code generated by default. Since there is
8819 very little code that depends on the @var{inexact-flag}, you should
8820 normally not specify this option. Other Alpha compilers call this
8821 option @option{-ieee_with_inexact}.
8823 @item -mfp-trap-mode=@var{trap-mode}
8824 @opindex mfp-trap-mode
8825 This option controls what floating-point related traps are enabled.
8826 Other Alpha compilers call this option @option{-fptm @var{trap-mode}}.
8827 The trap mode can be set to one of four values:
8831 This is the default (normal) setting. The only traps that are enabled
8832 are the ones that cannot be disabled in software (e.g., division by zero
8836 In addition to the traps enabled by @samp{n}, underflow traps are enabled
8840 Like @samp{su}, but the instructions are marked to be safe for software
8841 completion (see Alpha architecture manual for details).
8844 Like @samp{su}, but inexact traps are enabled as well.
8847 @item -mfp-rounding-mode=@var{rounding-mode}
8848 @opindex mfp-rounding-mode
8849 Selects the IEEE rounding mode. Other Alpha compilers call this option
8850 @option{-fprm @var{rounding-mode}}. The @var{rounding-mode} can be one
8855 Normal IEEE rounding mode. Floating point numbers are rounded towards
8856 the nearest machine number or towards the even machine number in case
8860 Round towards minus infinity.
8863 Chopped rounding mode. Floating point numbers are rounded towards zero.
8866 Dynamic rounding mode. A field in the floating point control register
8867 (@var{fpcr}, see Alpha architecture reference manual) controls the
8868 rounding mode in effect. The C library initializes this register for
8869 rounding towards plus infinity. Thus, unless your program modifies the
8870 @var{fpcr}, @samp{d} corresponds to round towards plus infinity.
8873 @item -mtrap-precision=@var{trap-precision}
8874 @opindex mtrap-precision
8875 In the Alpha architecture, floating point traps are imprecise. This
8876 means without software assistance it is impossible to recover from a
8877 floating trap and program execution normally needs to be terminated.
8878 GCC can generate code that can assist operating system trap handlers
8879 in determining the exact location that caused a floating point trap.
8880 Depending on the requirements of an application, different levels of
8881 precisions can be selected:
8885 Program precision. This option is the default and means a trap handler
8886 can only identify which program caused a floating point exception.
8889 Function precision. The trap handler can determine the function that
8890 caused a floating point exception.
8893 Instruction precision. The trap handler can determine the exact
8894 instruction that caused a floating point exception.
8897 Other Alpha compilers provide the equivalent options called
8898 @option{-scope_safe} and @option{-resumption_safe}.
8900 @item -mieee-conformant
8901 @opindex mieee-conformant
8902 This option marks the generated code as IEEE conformant. You must not
8903 use this option unless you also specify @option{-mtrap-precision=i} and either
8904 @option{-mfp-trap-mode=su} or @option{-mfp-trap-mode=sui}. Its only effect
8905 is to emit the line @samp{.eflag 48} in the function prologue of the
8906 generated assembly file. Under DEC Unix, this has the effect that
8907 IEEE-conformant math library routines will be linked in.
8909 @item -mbuild-constants
8910 @opindex mbuild-constants
8911 Normally GCC examines a 32- or 64-bit integer constant to
8912 see if it can construct it from smaller constants in two or three
8913 instructions. If it cannot, it will output the constant as a literal and
8914 generate code to load it from the data segment at runtime.
8916 Use this option to require GCC to construct @emph{all} integer constants
8917 using code, even if it takes more instructions (the maximum is six).
8919 You would typically use this option to build a shared library dynamic
8920 loader. Itself a shared library, it must relocate itself in memory
8921 before it can find the variables and constants in its own data segment.
8927 Select whether to generate code to be assembled by the vendor-supplied
8928 assembler (@option{-malpha-as}) or by the GNU assembler @option{-mgas}.
8946 Indicate whether GCC should generate code to use the optional BWX,
8947 CIX, FIX and MAX instruction sets. The default is to use the instruction
8948 sets supported by the CPU type specified via @option{-mcpu=} option or that
8949 of the CPU on which GCC was built if none was specified.
8954 @opindex mfloat-ieee
8955 Generate code that uses (does not use) VAX F and G floating point
8956 arithmetic instead of IEEE single and double precision.
8958 @item -mexplicit-relocs
8959 @itemx -mno-explicit-relocs
8960 @opindex mexplicit-relocs
8961 @opindex mno-explicit-relocs
8962 Older Alpha assemblers provided no way to generate symbol relocations
8963 except via assembler macros. Use of these macros does not allow
8964 optimal instruction scheduling. GNU binutils as of version 2.12
8965 supports a new syntax that allows the compiler to explicitly mark
8966 which relocations should apply to which instructions. This option
8967 is mostly useful for debugging, as GCC detects the capabilities of
8968 the assembler when it is built and sets the default accordingly.
8972 @opindex msmall-data
8973 @opindex mlarge-data
8974 When @option{-mexplicit-relocs} is in effect, static data is
8975 accessed via @dfn{gp-relative} relocations. When @option{-msmall-data}
8976 is used, objects 8 bytes long or smaller are placed in a @dfn{small data area}
8977 (the @code{.sdata} and @code{.sbss} sections) and are accessed via
8978 16-bit relocations off of the @code{$gp} register. This limits the
8979 size of the small data area to 64KB, but allows the variables to be
8980 directly accessed via a single instruction.
8982 The default is @option{-mlarge-data}. With this option the data area
8983 is limited to just below 2GB. Programs that require more than 2GB of
8984 data must use @code{malloc} or @code{mmap} to allocate the data in the
8985 heap instead of in the program's data segment.
8987 When generating code for shared libraries, @option{-fpic} implies
8988 @option{-msmall-data} and @option{-fPIC} implies @option{-mlarge-data}.
8992 @opindex msmall-text
8993 @opindex mlarge-text
8994 When @option{-msmall-text} is used, the compiler assumes that the
8995 code of the entire program (or shared library) fits in 4MB, and is
8996 thus reachable with a branch instruction. When @option{-msmall-data}
8997 is used, the compiler can assume that all local symbols share the
8998 same @code{$gp} value, and thus reduce the number of instructions
8999 required for a function call from 4 to 1.
9001 The default is @option{-mlarge-text}.
9003 @item -mcpu=@var{cpu_type}
9005 Set the instruction set and instruction scheduling parameters for
9006 machine type @var{cpu_type}. You can specify either the @samp{EV}
9007 style name or the corresponding chip number. GCC supports scheduling
9008 parameters for the EV4, EV5 and EV6 family of processors and will
9009 choose the default values for the instruction set from the processor
9010 you specify. If you do not specify a processor type, GCC will default
9011 to the processor on which the compiler was built.
9013 Supported values for @var{cpu_type} are
9019 Schedules as an EV4 and has no instruction set extensions.
9023 Schedules as an EV5 and has no instruction set extensions.
9027 Schedules as an EV5 and supports the BWX extension.
9032 Schedules as an EV5 and supports the BWX and MAX extensions.
9036 Schedules as an EV6 and supports the BWX, FIX, and MAX extensions.
9040 Schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.
9043 @item -mtune=@var{cpu_type}
9045 Set only the instruction scheduling parameters for machine type
9046 @var{cpu_type}. The instruction set is not changed.
9048 @item -mmemory-latency=@var{time}
9049 @opindex mmemory-latency
9050 Sets the latency the scheduler should assume for typical memory
9051 references as seen by the application. This number is highly
9052 dependent on the memory access patterns used by the application
9053 and the size of the external cache on the machine.
9055 Valid options for @var{time} are
9059 A decimal number representing clock cycles.
9065 The compiler contains estimates of the number of clock cycles for
9066 ``typical'' EV4 & EV5 hardware for the Level 1, 2 & 3 caches
9067 (also called Dcache, Scache, and Bcache), as well as to main memory.
9068 Note that L3 is only valid for EV5.
9073 @node DEC Alpha/VMS Options
9074 @subsection DEC Alpha/VMS Options
9076 These @samp{-m} options are defined for the DEC Alpha/VMS implementations:
9079 @item -mvms-return-codes
9080 @opindex mvms-return-codes
9081 Return VMS condition codes from main. The default is to return POSIX
9082 style condition (e.g.@ error) codes.
9085 @node H8/300 Options
9086 @subsection H8/300 Options
9088 These @samp{-m} options are defined for the H8/300 implementations:
9093 Shorten some address references at link time, when possible; uses the
9094 linker option @option{-relax}. @xref{H8/300,, @code{ld} and the H8/300,
9095 ld.info, Using ld}, for a fuller description.
9099 Generate code for the H8/300H@.
9103 Generate code for the H8S@.
9107 Generate code for the H8S and H8/300H in the normal mode. This switch
9108 must be used either with -mh or -ms.
9112 Generate code for the H8S/2600. This switch must be used with @option{-ms}.
9116 Make @code{int} data 32 bits by default.
9120 On the H8/300H and H8S, use the same alignment rules as for the H8/300.
9121 The default for the H8/300H and H8S is to align longs and floats on 4
9123 @option{-malign-300} causes them to be aligned on 2 byte boundaries.
9124 This option has no effect on the H8/300.
9128 @subsection SH Options
9130 These @samp{-m} options are defined for the SH implementations:
9135 Generate code for the SH1.
9139 Generate code for the SH2.
9142 Generate code for the SH2e.
9146 Generate code for the SH3.
9150 Generate code for the SH3e.
9154 Generate code for the SH4 without a floating-point unit.
9156 @item -m4-single-only
9157 @opindex m4-single-only
9158 Generate code for the SH4 with a floating-point unit that only
9159 supports single-precision arithmetic.
9163 Generate code for the SH4 assuming the floating-point unit is in
9164 single-precision mode by default.
9168 Generate code for the SH4.
9172 Compile code for the processor in big endian mode.
9176 Compile code for the processor in little endian mode.
9180 Align doubles at 64-bit boundaries. Note that this changes the calling
9181 conventions, and thus some functions from the standard C library will
9182 not work unless you recompile it first with @option{-mdalign}.
9186 Shorten some address references at link time, when possible; uses the
9187 linker option @option{-relax}.
9191 Use 32-bit offsets in @code{switch} tables. The default is to use
9196 Enable the use of the instruction @code{fmovd}.
9200 Comply with the calling conventions defined by Hitachi.
9204 Mark the @code{MAC} register as call-clobbered, even if
9205 @option{-mhitachi} is given.
9209 Increase IEEE-compliance of floating-point code.
9213 Dump instruction size and location in the assembly code.
9217 This option is deprecated. It pads structures to multiple of 4 bytes,
9218 which is incompatible with the SH ABI@.
9222 Optimize for space instead of speed. Implied by @option{-Os}.
9226 When generating position-independent code, emit function calls using
9227 the Global Offset Table instead of the Procedure Linkage Table.
9231 Generate a library function call to invalidate instruction cache
9232 entries, after fixing up a trampoline. This library function call
9233 doesn't assume it can write to the whole memory address space. This
9234 is the default when the target is @code{sh-*-linux*}.
9237 @node System V Options
9238 @subsection Options for System V
9240 These additional options are available on System V Release 4 for
9241 compatibility with other compilers on those systems:
9246 Create a shared object.
9247 It is recommended that @option{-symbolic} or @option{-shared} be used instead.
9251 Identify the versions of each tool used by the compiler, in a
9252 @code{.ident} assembler directive in the output.
9256 Refrain from adding @code{.ident} directives to the output file (this is
9259 @item -YP,@var{dirs}
9261 Search the directories @var{dirs}, and no others, for libraries
9262 specified with @option{-l}.
9266 Look in the directory @var{dir} to find the M4 preprocessor.
9267 The assembler uses this option.
9268 @c This is supposed to go with a -Yd for predefined M4 macro files, but
9269 @c the generic assembler that comes with Solaris takes just -Ym.
9272 @node TMS320C3x/C4x Options
9273 @subsection TMS320C3x/C4x Options
9274 @cindex TMS320C3x/C4x Options
9276 These @samp{-m} options are defined for TMS320C3x/C4x implementations:
9280 @item -mcpu=@var{cpu_type}
9282 Set the instruction set, register set, and instruction scheduling
9283 parameters for machine type @var{cpu_type}. Supported values for
9284 @var{cpu_type} are @samp{c30}, @samp{c31}, @samp{c32}, @samp{c40}, and
9285 @samp{c44}. The default is @samp{c40} to generate code for the
9290 @itemx -msmall-memory
9292 @opindex mbig-memory
9294 @opindex msmall-memory
9296 Generates code for the big or small memory model. The small memory
9297 model assumed that all data fits into one 64K word page. At run-time
9298 the data page (DP) register must be set to point to the 64K page
9299 containing the .bss and .data program sections. The big memory model is
9300 the default and requires reloading of the DP register for every direct
9307 Allow (disallow) allocation of general integer operands into the block
9314 Enable (disable) generation of code using decrement and branch,
9315 DBcond(D), instructions. This is enabled by default for the C4x. To be
9316 on the safe side, this is disabled for the C3x, since the maximum
9317 iteration count on the C3x is @math{2^{23} + 1} (but who iterates loops more than
9318 @math{2^{23}} times on the C3x?). Note that GCC will try to reverse a loop so
9319 that it can utilize the decrement and branch instruction, but will give
9320 up if there is more than one memory reference in the loop. Thus a loop
9321 where the loop counter is decremented can generate slightly more
9322 efficient code, in cases where the RPTB instruction cannot be utilized.
9324 @item -mdp-isr-reload
9326 @opindex mdp-isr-reload
9328 Force the DP register to be saved on entry to an interrupt service
9329 routine (ISR), reloaded to point to the data section, and restored on
9330 exit from the ISR@. This should not be required unless someone has
9331 violated the small memory model by modifying the DP register, say within
9338 For the C3x use the 24-bit MPYI instruction for integer multiplies
9339 instead of a library call to guarantee 32-bit results. Note that if one
9340 of the operands is a constant, then the multiplication will be performed
9341 using shifts and adds. If the @option{-mmpyi} option is not specified for the C3x,
9342 then squaring operations are performed inline instead of a library call.
9345 @itemx -mno-fast-fix
9347 @opindex mno-fast-fix
9348 The C3x/C4x FIX instruction to convert a floating point value to an
9349 integer value chooses the nearest integer less than or equal to the
9350 floating point value rather than to the nearest integer. Thus if the
9351 floating point number is negative, the result will be incorrectly
9352 truncated an additional code is necessary to detect and correct this
9353 case. This option can be used to disable generation of the additional
9354 code required to correct the result.
9360 Enable (disable) generation of repeat block sequences using the RPTB
9361 instruction for zero overhead looping. The RPTB construct is only used
9362 for innermost loops that do not call functions or jump across the loop
9363 boundaries. There is no advantage having nested RPTB loops due to the
9364 overhead required to save and restore the RC, RS, and RE registers.
9365 This is enabled by default with @option{-O2}.
9367 @item -mrpts=@var{count}
9371 Enable (disable) the use of the single instruction repeat instruction
9372 RPTS@. If a repeat block contains a single instruction, and the loop
9373 count can be guaranteed to be less than the value @var{count}, GCC will
9374 emit a RPTS instruction instead of a RPTB@. If no value is specified,
9375 then a RPTS will be emitted even if the loop count cannot be determined
9376 at compile time. Note that the repeated instruction following RPTS does
9377 not have to be reloaded from memory each iteration, thus freeing up the
9378 CPU buses for operands. However, since interrupts are blocked by this
9379 instruction, it is disabled by default.
9381 @item -mloop-unsigned
9382 @itemx -mno-loop-unsigned
9383 @opindex mloop-unsigned
9384 @opindex mno-loop-unsigned
9385 The maximum iteration count when using RPTS and RPTB (and DB on the C40)
9386 is @math{2^{31} + 1} since these instructions test if the iteration count is
9387 negative to terminate the loop. If the iteration count is unsigned
9388 there is a possibility than the @math{2^{31} + 1} maximum iteration count may be
9389 exceeded. This switch allows an unsigned iteration count.
9393 Try to emit an assembler syntax that the TI assembler (asm30) is happy
9394 with. This also enforces compatibility with the API employed by the TI
9395 C3x C compiler. For example, long doubles are passed as structures
9396 rather than in floating point registers.
9402 Generate code that uses registers (stack) for passing arguments to functions.
9403 By default, arguments are passed in registers where possible rather
9404 than by pushing arguments on to the stack.
9406 @item -mparallel-insns
9407 @itemx -mno-parallel-insns
9408 @opindex mparallel-insns
9409 @opindex mno-parallel-insns
9410 Allow the generation of parallel instructions. This is enabled by
9411 default with @option{-O2}.
9413 @item -mparallel-mpy
9414 @itemx -mno-parallel-mpy
9415 @opindex mparallel-mpy
9416 @opindex mno-parallel-mpy
9417 Allow the generation of MPY||ADD and MPY||SUB parallel instructions,
9418 provided @option{-mparallel-insns} is also specified. These instructions have
9419 tight register constraints which can pessimize the code generation
9425 @subsection V850 Options
9426 @cindex V850 Options
9428 These @samp{-m} options are defined for V850 implementations:
9432 @itemx -mno-long-calls
9433 @opindex mlong-calls
9434 @opindex mno-long-calls
9435 Treat all calls as being far away (near). If calls are assumed to be
9436 far away, the compiler will always load the functions address up into a
9437 register, and call indirect through the pointer.
9443 Do not optimize (do optimize) basic blocks that use the same index
9444 pointer 4 or more times to copy pointer into the @code{ep} register, and
9445 use the shorter @code{sld} and @code{sst} instructions. The @option{-mep}
9446 option is on by default if you optimize.
9448 @item -mno-prolog-function
9449 @itemx -mprolog-function
9450 @opindex mno-prolog-function
9451 @opindex mprolog-function
9452 Do not use (do use) external functions to save and restore registers at
9453 the prolog and epilog of a function. The external functions are slower,
9454 but use less code space if more than one function saves the same number
9455 of registers. The @option{-mprolog-function} option is on by default if
9460 Try to make the code as small as possible. At present, this just turns
9461 on the @option{-mep} and @option{-mprolog-function} options.
9465 Put static or global variables whose size is @var{n} bytes or less into
9466 the tiny data area that register @code{ep} points to. The tiny data
9467 area can hold up to 256 bytes in total (128 bytes for byte references).
9471 Put static or global variables whose size is @var{n} bytes or less into
9472 the small data area that register @code{gp} points to. The small data
9473 area can hold up to 64 kilobytes.
9477 Put static or global variables whose size is @var{n} bytes or less into
9478 the first 32 kilobytes of memory.
9482 Specify that the target processor is the V850.
9485 @opindex mbig-switch
9486 Generate code suitable for big switch tables. Use this option only if
9487 the assembler/linker complain about out of range branches within a switch
9492 This option will cause r2 and r5 to be used in the code generated by
9493 the compiler. This setting is the default.
9496 @opindex -mno-app-regs
9497 This option will cause r2 and r5 to be treated as fixed registers.
9501 Specify that the target processor is the V850E. The preprocessor
9502 constant @samp{__v850e__} will be defined if this option is used.
9504 If neither @option{-mv850} nor @option{-mv850e} are defined
9505 then a default target processor will be chosen and the relevant
9506 @samp{__v850*__} preprocessor constant will be defined.
9508 The preprocessor constants @samp{__v850} and @samp{__v851__} are always
9509 defined, regardless of which processor variant is the target.
9511 @item -mdisable-callt
9512 @opindex -mdisable-callt
9513 This option will suppress generation of the CALLT instruction for the
9514 v850e flavors of the v850 architecture. The default is
9515 @option{-mno-disable-callt} which allows the CALLT instruction to be used.
9520 @subsection ARC Options
9523 These options are defined for ARC implementations:
9528 Compile code for little endian mode. This is the default.
9532 Compile code for big endian mode.
9535 @opindex mmangle-cpu
9536 Prepend the name of the cpu to all public symbol names.
9537 In multiple-processor systems, there are many ARC variants with different
9538 instruction and register set characteristics. This flag prevents code
9539 compiled for one cpu to be linked with code compiled for another.
9540 No facility exists for handling variants that are ``almost identical''.
9541 This is an all or nothing option.
9543 @item -mcpu=@var{cpu}
9545 Compile code for ARC variant @var{cpu}.
9546 Which variants are supported depend on the configuration.
9547 All variants support @option{-mcpu=base}, this is the default.
9549 @item -mtext=@var{text-section}
9550 @itemx -mdata=@var{data-section}
9551 @itemx -mrodata=@var{readonly-data-section}
9555 Put functions, data, and readonly data in @var{text-section},
9556 @var{data-section}, and @var{readonly-data-section} respectively
9557 by default. This can be overridden with the @code{section} attribute.
9558 @xref{Variable Attributes}.
9563 @subsection NS32K Options
9564 @cindex NS32K options
9566 These are the @samp{-m} options defined for the 32000 series. The default
9567 values for these options depends on which style of 32000 was selected when
9568 the compiler was configured; the defaults for the most common choices are
9576 Generate output for a 32032. This is the default
9577 when the compiler is configured for 32032 and 32016 based systems.
9583 Generate output for a 32332. This is the default
9584 when the compiler is configured for 32332-based systems.
9590 Generate output for a 32532. This is the default
9591 when the compiler is configured for 32532-based systems.
9595 Generate output containing 32081 instructions for floating point.
9596 This is the default for all systems.
9600 Generate output containing 32381 instructions for floating point. This
9601 also implies @option{-m32081}. The 32381 is only compatible with the 32332
9602 and 32532 cpus. This is the default for the pc532-netbsd configuration.
9606 Try and generate multiply-add floating point instructions @code{polyF}
9607 and @code{dotF}. This option is only available if the @option{-m32381}
9608 option is in effect. Using these instructions requires changes to
9609 register allocation which generally has a negative impact on
9610 performance. This option should only be enabled when compiling code
9611 particularly likely to make heavy use of multiply-add instructions.
9614 @opindex mnomulti-add
9615 Do not try and generate multiply-add floating point instructions
9616 @code{polyF} and @code{dotF}. This is the default on all platforms.
9619 @opindex msoft-float
9620 Generate output containing library calls for floating point.
9621 @strong{Warning:} the requisite libraries may not be available.
9623 @item -mieee-compare
9624 @itemx -mno-ieee-compare
9625 @opindex mieee-compare
9626 @opindex mno-ieee-compare
9627 Control whether or not the compiler uses IEEE floating point
9628 comparisons. These handle correctly the case where the result of a
9629 comparison is unordered.
9630 @strong{Warning:} the requisite kernel support may not be available.
9633 @opindex mnobitfield
9634 Do not use the bit-field instructions. On some machines it is faster to
9635 use shifting and masking operations. This is the default for the pc532.
9639 Do use the bit-field instructions. This is the default for all platforms
9644 Use a different function-calling convention, in which functions
9645 that take a fixed number of arguments return pop their
9646 arguments on return with the @code{ret} instruction.
9648 This calling convention is incompatible with the one normally
9649 used on Unix, so you cannot use it if you need to call libraries
9650 compiled with the Unix compiler.
9652 Also, you must provide function prototypes for all functions that
9653 take variable numbers of arguments (including @code{printf});
9654 otherwise incorrect code will be generated for calls to those
9657 In addition, seriously incorrect code will result if you call a
9658 function with too many arguments. (Normally, extra arguments are
9659 harmlessly ignored.)
9661 This option takes its name from the 680x0 @code{rtd} instruction.
9666 Use a different function-calling convention where the first two arguments
9667 are passed in registers.
9669 This calling convention is incompatible with the one normally
9670 used on Unix, so you cannot use it if you need to call libraries
9671 compiled with the Unix compiler.
9674 @opindex mnoregparam
9675 Do not pass any arguments in registers. This is the default for all
9680 It is OK to use the sb as an index register which is always loaded with
9681 zero. This is the default for the pc532-netbsd target.
9685 The sb register is not available for use or has not been initialized to
9686 zero by the run time system. This is the default for all targets except
9687 the pc532-netbsd. It is also implied whenever @option{-mhimem} or
9688 @option{-fpic} is set.
9692 Many ns32000 series addressing modes use displacements of up to 512MB@.
9693 If an address is above 512MB then displacements from zero can not be used.
9694 This option causes code to be generated which can be loaded above 512MB@.
9695 This may be useful for operating systems or ROM code.
9699 Assume code will be loaded in the first 512MB of virtual address space.
9700 This is the default for all platforms.
9706 @subsection AVR Options
9709 These options are defined for AVR implementations:
9712 @item -mmcu=@var{mcu}
9714 Specify ATMEL AVR instruction set or MCU type.
9716 Instruction set avr1 is for the minimal AVR core, not supported by the C
9717 compiler, only for assembler programs (MCU types: at90s1200, attiny10,
9718 attiny11, attiny12, attiny15, attiny28).
9720 Instruction set avr2 (default) is for the classic AVR core with up to
9721 8K program memory space (MCU types: at90s2313, at90s2323, attiny22,
9722 at90s2333, at90s2343, at90s4414, at90s4433, at90s4434, at90s8515,
9723 at90c8534, at90s8535).
9725 Instruction set avr3 is for the classic AVR core with up to 128K program
9726 memory space (MCU types: atmega103, atmega603, at43usb320, at76c711).
9728 Instruction set avr4 is for the enhanced AVR core with up to 8K program
9729 memory space (MCU types: atmega8, atmega83, atmega85).
9731 Instruction set avr5 is for the enhanced AVR core with up to 128K program
9732 memory space (MCU types: atmega16, atmega161, atmega163, atmega32, atmega323,
9733 atmega64, atmega128, at43usb355, at94k).
9737 Output instruction sizes to the asm file.
9739 @item -minit-stack=@var{N}
9740 @opindex minit-stack
9741 Specify the initial stack address, which may be a symbol or numeric value,
9742 @samp{__stack} is the default.
9744 @item -mno-interrupts
9745 @opindex mno-interrupts
9746 Generated code is not compatible with hardware interrupts.
9747 Code size will be smaller.
9749 @item -mcall-prologues
9750 @opindex mcall-prologues
9751 Functions prologues/epilogues expanded as call to appropriate
9752 subroutines. Code size will be smaller.
9754 @item -mno-tablejump
9755 @opindex mno-tablejump
9756 Do not generate tablejump insns which sometimes increase code size.
9759 @opindex mtiny-stack
9760 Change only the low 8 bits of the stack pointer.
9764 @subsection MCore Options
9765 @cindex MCore options
9767 These are the @samp{-m} options defined for the Motorola M*Core
9775 @opindex mno-hardlit
9776 Inline constants into the code stream if it can be done in two
9777 instructions or less.
9783 Use the divide instruction. (Enabled by default).
9785 @item -mrelax-immediate
9786 @itemx -mno-relax-immediate
9787 @opindex mrelax-immediate
9788 @opindex mno-relax-immediate
9789 Allow arbitrary sized immediates in bit operations.
9791 @item -mwide-bitfields
9792 @itemx -mno-wide-bitfields
9793 @opindex mwide-bitfields
9794 @opindex mno-wide-bitfields
9795 Always treat bit-fields as int-sized.
9797 @item -m4byte-functions
9798 @itemx -mno-4byte-functions
9799 @opindex m4byte-functions
9800 @opindex mno-4byte-functions
9801 Force all functions to be aligned to a four byte boundary.
9803 @item -mcallgraph-data
9804 @itemx -mno-callgraph-data
9805 @opindex mcallgraph-data
9806 @opindex mno-callgraph-data
9807 Emit callgraph information.
9810 @itemx -mno-slow-bytes
9811 @opindex mslow-bytes
9812 @opindex mno-slow-bytes
9813 Prefer word access when reading byte quantities.
9815 @item -mlittle-endian
9817 @opindex mlittle-endian
9818 @opindex mbig-endian
9819 Generate code for a little endian target.
9825 Generate code for the 210 processor.
9829 @subsection IA-64 Options
9830 @cindex IA-64 Options
9832 These are the @samp{-m} options defined for the Intel IA-64 architecture.
9836 @opindex mbig-endian
9837 Generate code for a big endian target. This is the default for HP-UX@.
9839 @item -mlittle-endian
9840 @opindex mlittle-endian
9841 Generate code for a little endian target. This is the default for AIX5
9848 Generate (or don't) code for the GNU assembler. This is the default.
9849 @c Also, this is the default if the configure option @option{--with-gnu-as}
9856 Generate (or don't) code for the GNU linker. This is the default.
9857 @c Also, this is the default if the configure option @option{--with-gnu-ld}
9862 Generate code that does not use a global pointer register. The result
9863 is not position independent code, and violates the IA-64 ABI@.
9865 @item -mvolatile-asm-stop
9866 @itemx -mno-volatile-asm-stop
9867 @opindex mvolatile-asm-stop
9868 @opindex mno-volatile-asm-stop
9869 Generate (or don't) a stop bit immediately before and after volatile asm
9874 Generate code that works around Itanium B step errata.
9876 @item -mregister-names
9877 @itemx -mno-register-names
9878 @opindex mregister-names
9879 @opindex mno-register-names
9880 Generate (or don't) @samp{in}, @samp{loc}, and @samp{out} register names for
9881 the stacked registers. This may make assembler output more readable.
9887 Disable (or enable) optimizations that use the small data section. This may
9888 be useful for working around optimizer bugs.
9891 @opindex mconstant-gp
9892 Generate code that uses a single constant global pointer value. This is
9893 useful when compiling kernel code.
9897 Generate code that is self-relocatable. This implies @option{-mconstant-gp}.
9898 This is useful when compiling firmware code.
9900 @item -minline-float-divide-min-latency
9901 @opindex minline-float-divide-min-latency
9902 Generate code for inline divides of floating point values
9903 using the minimum latency algorithm.
9905 @item -minline-float-divide-max-throughput
9906 @opindex minline-float-divide-max-throughput
9907 Generate code for inline divides of floating point values
9908 using the maximum throughput algorithm.
9910 @item -minline-int-divide-min-latency
9911 @opindex minline-int-divide-min-latency
9912 Generate code for inline divides of integer values
9913 using the minimum latency algorithm.
9915 @item -minline-int-divide-max-throughput
9916 @opindex minline-int-divide-max-throughput
9917 Generate code for inline divides of integer values
9918 using the maximum throughput algorithm.
9920 @item -mno-dwarf2-asm
9922 @opindex mno-dwarf2-asm
9923 @opindex mdwarf2-asm
9924 Don't (or do) generate assembler code for the DWARF2 line number debugging
9925 info. This may be useful when not using the GNU assembler.
9927 @item -mfixed-range=@var{register-range}
9928 @opindex mfixed-range
9929 Generate code treating the given register range as fixed registers.
9930 A fixed register is one that the register allocator can not use. This is
9931 useful when compiling kernel code. A register range is specified as
9932 two registers separated by a dash. Multiple register ranges can be
9933 specified separated by a comma.
9935 @item -mearly-stop-bits
9936 @itemx -mno-early-stop-bits
9937 @opindex mearly-stop-bits
9938 @opindex mno-early-stop-bits
9939 Allow stop bits to be placed earlier than immediately preceding the
9940 instruction that triggered the stop bit. This can improve instruction
9941 scheduling, but does not always do so.
9945 @subsection D30V Options
9946 @cindex D30V Options
9948 These @samp{-m} options are defined for D30V implementations:
9953 Link the @samp{.text}, @samp{.data}, @samp{.bss}, @samp{.strings},
9954 @samp{.rodata}, @samp{.rodata1}, @samp{.data1} sections into external
9955 memory, which starts at location @code{0x80000000}.
9959 Same as the @option{-mextmem} switch.
9963 Link the @samp{.text} section into onchip text memory, which starts at
9964 location @code{0x0}. Also link @samp{.data}, @samp{.bss},
9965 @samp{.strings}, @samp{.rodata}, @samp{.rodata1}, @samp{.data1} sections
9966 into onchip data memory, which starts at location @code{0x20000000}.
9968 @item -mno-asm-optimize
9969 @itemx -masm-optimize
9970 @opindex mno-asm-optimize
9971 @opindex masm-optimize
9972 Disable (enable) passing @option{-O} to the assembler when optimizing.
9973 The assembler uses the @option{-O} option to automatically parallelize
9974 adjacent short instructions where possible.
9976 @item -mbranch-cost=@var{n}
9977 @opindex mbranch-cost
9978 Increase the internal costs of branches to @var{n}. Higher costs means
9979 that the compiler will issue more instructions to avoid doing a branch.
9982 @item -mcond-exec=@var{n}
9984 Specify the maximum number of conditionally executed instructions that
9985 replace a branch. The default is 4.
9988 @node S/390 and zSeries Options
9989 @subsection S/390 and zSeries Options
9990 @cindex S/390 and zSeries Options
9992 These are the @samp{-m} options defined for the S/390 and zSeries architecture.
9997 @opindex mhard-float
9998 @opindex msoft-float
9999 Use (do not use) the hardware floating-point instructions and registers
10000 for floating-point operations. When @option{-msoft-float} is specified,
10001 functions in @file{libgcc.a} will be used to perform floating-point
10002 operations. When @option{-mhard-float} is specified, the compiler
10003 generates IEEE floating-point instructions. This is the default.
10006 @itemx -mno-backchain
10007 @opindex mbackchain
10008 @opindex mno-backchain
10009 Generate (or do not generate) code which maintains an explicit
10010 backchain within the stack frame that points to the caller's frame.
10011 This is currently needed to allow debugging. The default is to
10012 generate the backchain.
10015 @itemx -mno-small-exec
10016 @opindex msmall-exec
10017 @opindex mno-small-exec
10018 Generate (or do not generate) code using the @code{bras} instruction
10019 to do subroutine calls.
10020 This only works reliably if the total executable size does not
10021 exceed 64k. The default is to use the @code{basr} instruction instead,
10022 which does not have this limitation.
10028 When @option{-m31} is specified, generate code compliant to the
10029 Linux for S/390 ABI@. When @option{-m64} is specified, generate
10030 code compliant to the Linux for zSeries ABI@. This allows GCC in
10031 particular to generate 64-bit instructions. For the @samp{s390}
10032 targets, the default is @option{-m31}, while the @samp{s390x}
10033 targets default to @option{-m64}.
10039 When @option{-mzarch} is specified, generate code using the
10040 instructions available on z/Architecture.
10041 When @option{-mesa} is specified, generate code using the
10042 instructions available on ESA/390. Note that @option{-mesa} is
10043 not possible with @option{-m64}.
10044 For the @samp{s390} targets, the default is @option{-mesa},
10045 while the @samp{s390x} targets default to @option{-mzarch}.
10051 Generate (or do not generate) code using the @code{mvcle} instruction
10052 to perform block moves. When @option{-mno-mvcle} is specified,
10053 use a @code{mvc} loop instead. This is the default.
10059 Print (or do not print) additional debug information when compiling.
10060 The default is to not print debug information.
10062 @item -march=@var{arch}
10064 Generate code that will run on @var{arch}, which is the name of system
10065 representing a certain processor type. Possible values for
10066 @var{cpu-type} are @samp{g5}, @samp{g6} and @samp{z900}.
10068 @item -mtune=@var{arch}
10070 Tune to @var{cpu-type} everything applicable about the generated code,
10071 except for the ABI and the set of available instructions.
10072 The list of @var{arch} values is the same as for @option{-march}.
10077 @subsection CRIS Options
10078 @cindex CRIS Options
10080 These options are defined specifically for the CRIS ports.
10083 @item -march=@var{architecture-type}
10084 @itemx -mcpu=@var{architecture-type}
10087 Generate code for the specified architecture. The choices for
10088 @var{architecture-type} are @samp{v3}, @samp{v8} and @samp{v10} for
10089 respectively ETRAX@w{ }4, ETRAX@w{ }100, and ETRAX@w{ }100@w{ }LX.
10090 Default is @samp{v0} except for cris-axis-linux-gnu, where the default is
10093 @item -mtune=@var{architecture-type}
10095 Tune to @var{architecture-type} everything applicable about the generated
10096 code, except for the ABI and the set of available instructions. The
10097 choices for @var{architecture-type} are the same as for
10098 @option{-march=@var{architecture-type}}.
10100 @item -mmax-stack-frame=@var{n}
10101 @opindex mmax-stack-frame
10102 Warn when the stack frame of a function exceeds @var{n} bytes.
10104 @item -melinux-stacksize=@var{n}
10105 @opindex melinux-stacksize
10106 Only available with the @samp{cris-axis-aout} target. Arranges for
10107 indications in the program to the kernel loader that the stack of the
10108 program should be set to @var{n} bytes.
10114 The options @option{-metrax4} and @option{-metrax100} are synonyms for
10115 @option{-march=v3} and @option{-march=v8} respectively.
10119 Enable CRIS-specific verbose debug-related information in the assembly
10120 code. This option also has the effect to turn off the @samp{#NO_APP}
10121 formatted-code indicator to the assembler at the beginning of the
10126 Do not use condition-code results from previous instruction; always emit
10127 compare and test instructions before use of condition codes.
10129 @item -mno-side-effects
10130 @opindex mno-side-effects
10131 Do not emit instructions with side-effects in addressing modes other than
10134 @item -mstack-align
10135 @itemx -mno-stack-align
10136 @itemx -mdata-align
10137 @itemx -mno-data-align
10138 @itemx -mconst-align
10139 @itemx -mno-const-align
10140 @opindex mstack-align
10141 @opindex mno-stack-align
10142 @opindex mdata-align
10143 @opindex mno-data-align
10144 @opindex mconst-align
10145 @opindex mno-const-align
10146 These options (no-options) arranges (eliminate arrangements) for the
10147 stack-frame, individual data and constants to be aligned for the maximum
10148 single data access size for the chosen CPU model. The default is to
10149 arrange for 32-bit alignment. ABI details such as structure layout are
10150 not affected by these options.
10158 Similar to the stack- data- and const-align options above, these options
10159 arrange for stack-frame, writable data and constants to all be 32-bit,
10160 16-bit or 8-bit aligned. The default is 32-bit alignment.
10162 @item -mno-prologue-epilogue
10163 @itemx -mprologue-epilogue
10164 @opindex mno-prologue-epilogue
10165 @opindex mprologue-epilogue
10166 With @option{-mno-prologue-epilogue}, the normal function prologue and
10167 epilogue that sets up the stack-frame are omitted and no return
10168 instructions or return sequences are generated in the code. Use this
10169 option only together with visual inspection of the compiled code: no
10170 warnings or errors are generated when call-saved registers must be saved,
10171 or storage for local variable needs to be allocated.
10175 @opindex mno-gotplt
10177 With @option{-fpic} and @option{-fPIC}, don't generate (do generate)
10178 instruction sequences that load addresses for functions from the PLT part
10179 of the GOT rather than (traditional on other architectures) calls to the
10180 PLT. The default is @option{-mgotplt}.
10184 Legacy no-op option only recognized with the cris-axis-aout target.
10188 Legacy no-op option only recognized with the cris-axis-elf and
10189 cris-axis-linux-gnu targets.
10193 Only recognized with the cris-axis-aout target, where it selects a
10194 GNU/linux-like multilib, include files and instruction set for
10195 @option{-march=v8}.
10199 Legacy no-op option only recognized with the cris-axis-linux-gnu target.
10203 This option, recognized for the cris-axis-aout and cris-axis-elf arranges
10204 to link with input-output functions from a simulator library. Code,
10205 initialized data and zero-initialized data are allocated consecutively.
10209 Like @option{-sim}, but pass linker options to locate initialized data at
10210 0x40000000 and zero-initialized data at 0x80000000.
10214 @subsection MMIX Options
10215 @cindex MMIX Options
10217 These options are defined for the MMIX:
10221 @itemx -mno-libfuncs
10223 @opindex mno-libfuncs
10224 Specify that intrinsic library functions are being compiled, passing all
10225 values in registers, no matter the size.
10228 @itemx -mno-epsilon
10230 @opindex mno-epsilon
10231 Generate floating-point comparison instructions that compare with respect
10232 to the @code{rE} epsilon register.
10234 @item -mabi=mmixware
10236 @opindex mabi-mmixware
10238 Generate code that passes function parameters and return values that (in
10239 the called function) are seen as registers @code{$0} and up, as opposed to
10240 the GNU ABI which uses global registers @code{$231} and up.
10242 @item -mzero-extend
10243 @itemx -mno-zero-extend
10244 @opindex mzero-extend
10245 @opindex mno-zero-extend
10246 When reading data from memory in sizes shorter than 64 bits, use (do not
10247 use) zero-extending load instructions by default, rather than
10248 sign-extending ones.
10251 @itemx -mno-knuthdiv
10253 @opindex mno-knuthdiv
10254 Make the result of a division yielding a remainder have the same sign as
10255 the divisor. With the default, @option{-mno-knuthdiv}, the sign of the
10256 remainder follows the sign of the dividend. Both methods are
10257 arithmetically valid, the latter being almost exclusively used.
10259 @item -mtoplevel-symbols
10260 @itemx -mno-toplevel-symbols
10261 @opindex mtoplevel-symbols
10262 @opindex mno-toplevel-symbols
10263 Prepend (do not prepend) a @samp{:} to all global symbols, so the assembly
10264 code can be used with the @code{PREFIX} assembly directive.
10268 Generate an executable in the ELF format, rather than the default
10269 @samp{mmo} format used by the @command{mmix} simulator.
10271 @item -mbranch-predict
10272 @itemx -mno-branch-predict
10273 @opindex mbranch-predict
10274 @opindex mno-branch-predict
10275 Use (do not use) the probable-branch instructions, when static branch
10276 prediction indicates a probable branch.
10278 @item -mbase-addresses
10279 @itemx -mno-base-addresses
10280 @opindex mbase-addresses
10281 @opindex mno-base-addresses
10282 Generate (do not generate) code that uses @emph{base addresses}. Using a
10283 base address automatically generates a request (handled by the assembler
10284 and the linker) for a constant to be set up in a global register. The
10285 register is used for one or more base address requests within the range 0
10286 to 255 from the value held in the register. The generally leads to short
10287 and fast code, but the number of different data items that can be
10288 addressed is limited. This means that a program that uses lots of static
10289 data may require @option{-mno-base-addresses}.
10291 @item -msingle-exit
10292 @itemx -mno-single-exit
10293 @opindex msingle-exit
10294 @opindex mno-single-exit
10295 Force (do not force) generated code to have a single exit point in each
10299 @node PDP-11 Options
10300 @subsection PDP-11 Options
10301 @cindex PDP-11 Options
10303 These options are defined for the PDP-11:
10308 Use hardware FPP floating point. This is the default. (FIS floating
10309 point on the PDP-11/40 is not supported.)
10312 @opindex msoft-float
10313 Do not use hardware floating point.
10317 Return floating-point results in ac0 (fr0 in Unix assembler syntax).
10321 Return floating-point results in memory. This is the default.
10325 Generate code for a PDP-11/40.
10329 Generate code for a PDP-11/45. This is the default.
10333 Generate code for a PDP-11/10.
10335 @item -mbcopy-builtin
10336 @opindex bcopy-builtin
10337 Use inline @code{movstrhi} patterns for copying memory. This is the
10342 Do not use inline @code{movstrhi} patterns for copying memory.
10348 Use 16-bit @code{int}. This is the default.
10354 Use 32-bit @code{int}.
10357 @itemx -mno-float32
10359 @opindex mno-float32
10360 Use 64-bit @code{float}. This is the default.
10365 @opindex mno-float64
10366 Use 32-bit @code{float}.
10370 Use @code{abshi2} pattern. This is the default.
10374 Do not use @code{abshi2} pattern.
10376 @item -mbranch-expensive
10377 @opindex mbranch-expensive
10378 Pretend that branches are expensive. This is for experimenting with
10379 code generation only.
10381 @item -mbranch-cheap
10382 @opindex mbranch-cheap
10383 Do not pretend that branches are expensive. This is the default.
10387 Generate code for a system with split I&D.
10391 Generate code for a system without split I&D. This is the default.
10395 Use Unix assembler syntax. This is the default when configured for
10396 @samp{pdp11-*-bsd}.
10400 Use DEC assembler syntax. This is the default when configured for any
10401 PDP-11 target other than @samp{pdp11-*-bsd}.
10404 @node Xstormy16 Options
10405 @subsection Xstormy16 Options
10406 @cindex Xstormy16 Options
10408 These options are defined for Xstormy16:
10413 Choose startup files and linker script suitable for the simulator.
10417 @subsection FRV Options
10418 @cindex FRV Options
10424 Only use the first 32 general purpose registers.
10429 Use all 64 general purpose registers.
10434 Use only the first 32 floating point registers.
10439 Use all 64 floating point registers
10442 @opindex mhard-float
10444 Use hardware instructions for floating point operations.
10447 @opindex msoft-float
10449 Use library routines for floating point operations.
10454 Dynamically allocate condition code registers.
10459 Do not try to dynamically allocate condition code registers, only
10460 use @code{icc0} and @code{fcc0}.
10465 Change ABI to use double word insns.
10470 Do not use double word instructions.
10475 Use floating point double instructions.
10478 @opindex mno-double
10480 Do not use floating point double instructions.
10485 Use media instructions.
10490 Do not use media instructions.
10495 Use multiply and add/subtract instructions.
10498 @opindex mno-muladd
10500 Do not use multiply and add/subtract instructions.
10502 @item -mlibrary-pic
10503 @opindex mlibrary-pic
10505 Enable PIC support for building libraries
10510 Use only the first four media accumulator registers.
10515 Use all eight media accumulator registers.
10520 Pack VLIW instructions.
10525 Do not pack VLIW instructions.
10528 @opindex mno-eflags
10530 Do not mark ABI switches in e_flags.
10533 @opindex mcond-move
10535 Enable the use of conditional-move instructions (default).
10537 This switch is mainly for debugging the compiler and will likely be removed
10538 in a future version.
10540 @item -mno-cond-move
10541 @opindex mno-cond-move
10543 Disable the use of conditional-move instructions.
10545 This switch is mainly for debugging the compiler and will likely be removed
10546 in a future version.
10551 Enable the use of conditional set instructions (default).
10553 This switch is mainly for debugging the compiler and will likely be removed
10554 in a future version.
10559 Disable the use of conditional set instructions.
10561 This switch is mainly for debugging the compiler and will likely be removed
10562 in a future version.
10565 @opindex mcond-exec
10567 Enable the use of conditional execution (default).
10569 This switch is mainly for debugging the compiler and will likely be removed
10570 in a future version.
10572 @item -mno-cond-exec
10573 @opindex mno-cond-exec
10575 Disable the use of conditional execution.
10577 This switch is mainly for debugging the compiler and will likely be removed
10578 in a future version.
10580 @item -mvliw-branch
10581 @opindex mvliw-branch
10583 Run a pass to pack branches into VLIW instructions (default).
10585 This switch is mainly for debugging the compiler and will likely be removed
10586 in a future version.
10588 @item -mno-vliw-branch
10589 @opindex mno-vliw-branch
10591 Do not run a pass to pack branches into VLIW instructions.
10593 This switch is mainly for debugging the compiler and will likely be removed
10594 in a future version.
10596 @item -mmulti-cond-exec
10597 @opindex mmulti-cond-exec
10599 Enable optimization of @code{&&} and @code{||} in conditional execution
10602 This switch is mainly for debugging the compiler and will likely be removed
10603 in a future version.
10605 @item -mno-multi-cond-exec
10606 @opindex mno-multi-cond-exec
10608 Disable optimization of @code{&&} and @code{||} in conditional execution.
10610 This switch is mainly for debugging the compiler and will likely be removed
10611 in a future version.
10613 @item -mnested-cond-exec
10614 @opindex mnested-cond-exec
10616 Enable nested conditional execution optimizations (default).
10618 This switch is mainly for debugging the compiler and will likely be removed
10619 in a future version.
10621 @item -mno-nested-cond-exec
10622 @opindex mno-nested-cond-exec
10624 Disable nested conditional execution optimizations.
10626 This switch is mainly for debugging the compiler and will likely be removed
10627 in a future version.
10629 @item -mtomcat-stats
10630 @opindex mtomcat-stats
10632 Cause gas to print out tomcat statistics.
10634 @item -mcpu=@var{cpu}
10637 Select the processor type for which to generate code. Possible values are
10638 @samp{simple}, @samp{tomcat}, @samp{fr500}, @samp{fr400}, @samp{fr300},
10643 @node Xtensa Options
10644 @subsection Xtensa Options
10645 @cindex Xtensa Options
10647 The Xtensa architecture is designed to support many different
10648 configurations. The compiler's default options can be set to match a
10649 particular Xtensa configuration by copying a configuration file into the
10650 GCC sources when building GCC@. The options below may be used to
10651 override the default options.
10655 @itemx -mlittle-endian
10656 @opindex mbig-endian
10657 @opindex mlittle-endian
10658 Specify big-endian or little-endian byte ordering for the target Xtensa
10662 @itemx -mno-density
10664 @opindex mno-density
10665 Enable or disable use of the optional Xtensa code density instructions.
10668 @itemx -mno-const16
10670 @opindex mno-const16
10671 Enable or disable use of @code{CONST16} instructions for loading
10672 constant values. The @code{CONST16} instruction is currently not a
10673 standard option from Tensilica. When enabled, @code{CONST16}
10674 instructions are always used in place of the standard @code{L32R}
10675 instructions. The use of @code{CONST16} is enabled by default only if
10676 the @code{L32R} instruction is not available.
10682 Enable or disable use of the Xtensa @code{ABS} instruction for absolute
10689 Enable or disable use of the Xtensa @code{ADDX} and @code{SUBX}
10696 Enable or disable use of the Xtensa MAC16 option. When enabled, GCC
10697 will generate MAC16 instructions from standard C code, with the
10698 limitation that it will use neither the MR register file nor any
10699 instruction that operates on the MR registers. When this option is
10700 disabled, GCC will translate 16-bit multiply/accumulate operations to a
10701 combination of core instructions and library calls, depending on whether
10702 any other multiplier options are enabled.
10708 Enable or disable use of the 16-bit integer multiplier option. When
10709 enabled, the compiler will generate 16-bit multiply instructions for
10710 multiplications of 16 bits or smaller in standard C code. When this
10711 option is disabled, the compiler will either use 32-bit multiply or
10712 MAC16 instructions if they are available or generate library calls to
10713 perform the multiply operations using shifts and adds.
10719 Enable or disable use of the 32-bit integer multiplier option. When
10720 enabled, the compiler will generate 32-bit multiply instructions for
10721 multiplications of 32 bits or smaller in standard C code. When this
10722 option is disabled, the compiler will generate library calls to perform
10723 the multiply operations using either shifts and adds or 16-bit multiply
10724 instructions if they are available.
10730 Enable or disable use of the optional normalization shift amount
10731 (@code{NSA}) instructions to implement the built-in @code{ffs} function.
10736 @opindex mno-minmax
10737 Enable or disable use of the optional minimum and maximum value
10744 Enable or disable use of the optional sign extend (@code{SEXT})
10748 @itemx -mno-booleans
10750 @opindex mno-booleans
10751 Enable or disable support for the boolean register file used by Xtensa
10752 coprocessors. This is not typically useful by itself but may be
10753 required for other options that make use of the boolean registers (e.g.,
10754 the floating-point option).
10757 @itemx -msoft-float
10758 @opindex mhard-float
10759 @opindex msoft-float
10760 Enable or disable use of the floating-point option. When enabled, GCC
10761 generates floating-point instructions for 32-bit @code{float}
10762 operations. When this option is disabled, GCC generates library calls
10763 to emulate 32-bit floating-point operations using integer instructions.
10764 Regardless of this option, 64-bit @code{double} operations are always
10765 emulated with calls to library functions.
10768 @itemx -mno-fused-madd
10769 @opindex mfused-madd
10770 @opindex mno-fused-madd
10771 Enable or disable use of fused multiply/add and multiply/subtract
10772 instructions in the floating-point option. This has no effect if the
10773 floating-point option is not also enabled. Disabling fused multiply/add
10774 and multiply/subtract instructions forces the compiler to use separate
10775 instructions for the multiply and add/subtract operations. This may be
10776 desirable in some cases where strict IEEE 754-compliant results are
10777 required: the fused multiply add/subtract instructions do not round the
10778 intermediate result, thereby producing results with @emph{more} bits of
10779 precision than specified by the IEEE standard. Disabling fused multiply
10780 add/subtract instructions also ensures that the program output is not
10781 sensitive to the compiler's ability to combine multiply and add/subtract
10784 @item -mtext-section-literals
10785 @itemx -mno-text-section-literals
10786 @opindex mtext-section-literals
10787 @opindex mno-text-section-literals
10788 Control the treatment of literal pools. The default is
10789 @option{-mno-text-section-literals}, which places literals in a separate
10790 section in the output file. This allows the literal pool to be placed
10791 in a data RAM/ROM, and it also allows the linker to combine literal
10792 pools from separate object files to remove redundant literals and
10793 improve code size. With @option{-mtext-section-literals}, the literals
10794 are interspersed in the text section in order to keep them as close as
10795 possible to their references. This may be necessary for large assembly
10798 @item -mtarget-align
10799 @itemx -mno-target-align
10800 @opindex mtarget-align
10801 @opindex mno-target-align
10802 When this option is enabled, GCC instructs the assembler to
10803 automatically align instructions to reduce branch penalties at the
10804 expense of some code density. The assembler attempts to widen density
10805 instructions to align branch targets and the instructions following call
10806 instructions. If there are not enough preceding safe density
10807 instructions to align a target, no widening will be performed. The
10808 default is @option{-mtarget-align}. These options do not affect the
10809 treatment of auto-aligned instructions like @code{LOOP}, which the
10810 assembler will always align, either by widening density instructions or
10811 by inserting no-op instructions.
10814 @itemx -mno-longcalls
10815 @opindex mlongcalls
10816 @opindex mno-longcalls
10817 When this option is enabled, GCC instructs the assembler to translate
10818 direct calls to indirect calls unless it can determine that the target
10819 of a direct call is in the range allowed by the call instruction. This
10820 translation typically occurs for calls to functions in other source
10821 files. Specifically, the assembler translates a direct @code{CALL}
10822 instruction into an @code{L32R} followed by a @code{CALLX} instruction.
10823 The default is @option{-mno-longcalls}. This option should be used in
10824 programs where the call target can potentially be out of range. This
10825 option is implemented in the assembler, not the compiler, so the
10826 assembly code generated by GCC will still show direct call
10827 instructions---look at the disassembled object code to see the actual
10828 instructions. Note that the assembler will use an indirect call for
10829 every cross-file call, not just those that really will be out of range.
10832 @node Code Gen Options
10833 @section Options for Code Generation Conventions
10834 @cindex code generation conventions
10835 @cindex options, code generation
10836 @cindex run-time options
10838 These machine-independent options control the interface conventions
10839 used in code generation.
10841 Most of them have both positive and negative forms; the negative form
10842 of @option{-ffoo} would be @option{-fno-foo}. In the table below, only
10843 one of the forms is listed---the one which is not the default. You
10844 can figure out the other form by either removing @samp{no-} or adding
10848 @item -fbounds-check
10849 @opindex fbounds-check
10850 For front-ends that support it, generate additional code to check that
10851 indices used to access arrays are within the declared range. This is
10852 currently only supported by the Java and Fortran 77 front-ends, where
10853 this option defaults to true and false respectively.
10857 This option generates traps for signed overflow on addition, subtraction,
10858 multiplication operations.
10862 This option instructs the compiler to assume that signed arithmetic
10863 overflow of addition, subtraction and multiplication wraps around
10864 using twos-complement representation. This flag enables some optimzations
10865 and disables other. This option is enabled by default for the Java
10866 front-end, as required by the Java language specification.
10869 @opindex fexceptions
10870 Enable exception handling. Generates extra code needed to propagate
10871 exceptions. For some targets, this implies GCC will generate frame
10872 unwind information for all functions, which can produce significant data
10873 size overhead, although it does not affect execution. If you do not
10874 specify this option, GCC will enable it by default for languages like
10875 C++ which normally require exception handling, and disable it for
10876 languages like C that do not normally require it. However, you may need
10877 to enable this option when compiling C code that needs to interoperate
10878 properly with exception handlers written in C++. You may also wish to
10879 disable this option if you are compiling older C++ programs that don't
10880 use exception handling.
10882 @item -fnon-call-exceptions
10883 @opindex fnon-call-exceptions
10884 Generate code that allows trapping instructions to throw exceptions.
10885 Note that this requires platform-specific runtime support that does
10886 not exist everywhere. Moreover, it only allows @emph{trapping}
10887 instructions to throw exceptions, i.e.@: memory references or floating
10888 point instructions. It does not allow exceptions to be thrown from
10889 arbitrary signal handlers such as @code{SIGALRM}.
10891 @item -funwind-tables
10892 @opindex funwind-tables
10893 Similar to @option{-fexceptions}, except that it will just generate any needed
10894 static data, but will not affect the generated code in any other way.
10895 You will normally not enable this option; instead, a language processor
10896 that needs this handling would enable it on your behalf.
10898 @item -fasynchronous-unwind-tables
10899 @opindex funwind-tables
10900 Generate unwind table in dwarf2 format, if supported by target machine. The
10901 table is exact at each instruction boundary, so it can be used for stack
10902 unwinding from asynchronous events (such as debugger or garbage collector).
10904 @item -fpcc-struct-return
10905 @opindex fpcc-struct-return
10906 Return ``short'' @code{struct} and @code{union} values in memory like
10907 longer ones, rather than in registers. This convention is less
10908 efficient, but it has the advantage of allowing intercallability between
10909 GCC-compiled files and files compiled with other compilers, particularly
10910 the Portable C Compiler (pcc).
10912 The precise convention for returning structures in memory depends
10913 on the target configuration macros.
10915 Short structures and unions are those whose size and alignment match
10916 that of some integer type.
10918 @strong{Warning:} code compiled with the @option{-fpcc-struct-return}
10919 switch is not binary compatible with code compiled with the
10920 @option{-freg-struct-return} switch.
10921 Use it to conform to a non-default application binary interface.
10923 @item -freg-struct-return
10924 @opindex freg-struct-return
10925 Return @code{struct} and @code{union} values in registers when possible.
10926 This is more efficient for small structures than
10927 @option{-fpcc-struct-return}.
10929 If you specify neither @option{-fpcc-struct-return} nor
10930 @option{-freg-struct-return}, GCC defaults to whichever convention is
10931 standard for the target. If there is no standard convention, GCC
10932 defaults to @option{-fpcc-struct-return}, except on targets where GCC is
10933 the principal compiler. In those cases, we can choose the standard, and
10934 we chose the more efficient register return alternative.
10936 @strong{Warning:} code compiled with the @option{-freg-struct-return}
10937 switch is not binary compatible with code compiled with the
10938 @option{-fpcc-struct-return} switch.
10939 Use it to conform to a non-default application binary interface.
10941 @item -fshort-enums
10942 @opindex fshort-enums
10943 Allocate to an @code{enum} type only as many bytes as it needs for the
10944 declared range of possible values. Specifically, the @code{enum} type
10945 will be equivalent to the smallest integer type which has enough room.
10947 @strong{Warning:} the @option{-fshort-enums} switch causes GCC to generate
10948 code that is not binary compatible with code generated without that switch.
10949 Use it to conform to a non-default application binary interface.
10951 @item -fshort-double
10952 @opindex fshort-double
10953 Use the same size for @code{double} as for @code{float}.
10955 @strong{Warning:} the @option{-fshort-double} switch causes GCC to generate
10956 code that is not binary compatible with code generated without that switch.
10957 Use it to conform to a non-default application binary interface.
10959 @item -fshort-wchar
10960 @opindex fshort-wchar
10961 Override the underlying type for @samp{wchar_t} to be @samp{short
10962 unsigned int} instead of the default for the target. This option is
10963 useful for building programs to run under WINE@.
10965 @strong{Warning:} the @option{-fshort-wchar} switch causes GCC to generate
10966 code that is not binary compatible with code generated without that switch.
10967 Use it to conform to a non-default application binary interface.
10969 @item -fshared-data
10970 @opindex fshared-data
10971 Requests that the data and non-@code{const} variables of this
10972 compilation be shared data rather than private data. The distinction
10973 makes sense only on certain operating systems, where shared data is
10974 shared between processes running the same program, while private data
10975 exists in one copy per process.
10978 @opindex fno-common
10979 In C, allocate even uninitialized global variables in the data section of the
10980 object file, rather than generating them as common blocks. This has the
10981 effect that if the same variable is declared (without @code{extern}) in
10982 two different compilations, you will get an error when you link them.
10983 The only reason this might be useful is if you wish to verify that the
10984 program will work on other systems which always work this way.
10988 Ignore the @samp{#ident} directive.
10990 @item -fno-gnu-linker
10991 @opindex fno-gnu-linker
10992 Do not output global initializations (such as C++ constructors and
10993 destructors) in the form used by the GNU linker (on systems where the GNU
10994 linker is the standard method of handling them). Use this option when
10995 you want to use a non-GNU linker, which also requires using the
10996 @command{collect2} program to make sure the system linker includes
10997 constructors and destructors. (@command{collect2} is included in the GCC
10998 distribution.) For systems which @emph{must} use @command{collect2}, the
10999 compiler driver @command{gcc} is configured to do this automatically.
11001 @item -finhibit-size-directive
11002 @opindex finhibit-size-directive
11003 Don't output a @code{.size} assembler directive, or anything else that
11004 would cause trouble if the function is split in the middle, and the
11005 two halves are placed at locations far apart in memory. This option is
11006 used when compiling @file{crtstuff.c}; you should not need to use it
11009 @item -fverbose-asm
11010 @opindex fverbose-asm
11011 Put extra commentary information in the generated assembly code to
11012 make it more readable. This option is generally only of use to those
11013 who actually need to read the generated assembly code (perhaps while
11014 debugging the compiler itself).
11016 @option{-fno-verbose-asm}, the default, causes the
11017 extra information to be omitted and is useful when comparing two assembler
11022 @cindex global offset table
11024 Generate position-independent code (PIC) suitable for use in a shared
11025 library, if supported for the target machine. Such code accesses all
11026 constant addresses through a global offset table (GOT)@. The dynamic
11027 loader resolves the GOT entries when the program starts (the dynamic
11028 loader is not part of GCC; it is part of the operating system). If
11029 the GOT size for the linked executable exceeds a machine-specific
11030 maximum size, you get an error message from the linker indicating that
11031 @option{-fpic} does not work; in that case, recompile with @option{-fPIC}
11032 instead. (These maximums are 16k on the m88k, 8k on the SPARC, and 32k
11033 on the m68k and RS/6000. The 386 has no such limit.)
11035 Position-independent code requires special support, and therefore works
11036 only on certain machines. For the 386, GCC supports PIC for System V
11037 but not for the Sun 386i. Code generated for the IBM RS/6000 is always
11038 position-independent.
11042 If supported for the target machine, emit position-independent code,
11043 suitable for dynamic linking and avoiding any limit on the size of the
11044 global offset table. This option makes a difference on the m68k, m88k,
11047 Position-independent code requires special support, and therefore works
11048 only on certain machines.
11054 These options are similar to @option{-fpic} and @option{-fPIC}, but
11055 generated position independent code can be only linked into executables.
11056 Usually these options are used when @option{-pie} GCC option will be
11057 used during linking.
11059 @item -ffixed-@var{reg}
11061 Treat the register named @var{reg} as a fixed register; generated code
11062 should never refer to it (except perhaps as a stack pointer, frame
11063 pointer or in some other fixed role).
11065 @var{reg} must be the name of a register. The register names accepted
11066 are machine-specific and are defined in the @code{REGISTER_NAMES}
11067 macro in the machine description macro file.
11069 This flag does not have a negative form, because it specifies a
11072 @item -fcall-used-@var{reg}
11073 @opindex fcall-used
11074 Treat the register named @var{reg} as an allocable register that is
11075 clobbered by function calls. It may be allocated for temporaries or
11076 variables that do not live across a call. Functions compiled this way
11077 will not save and restore the register @var{reg}.
11079 It is an error to used this flag with the frame pointer or stack pointer.
11080 Use of this flag for other registers that have fixed pervasive roles in
11081 the machine's execution model will produce disastrous results.
11083 This flag does not have a negative form, because it specifies a
11086 @item -fcall-saved-@var{reg}
11087 @opindex fcall-saved
11088 Treat the register named @var{reg} as an allocable register saved by
11089 functions. It may be allocated even for temporaries or variables that
11090 live across a call. Functions compiled this way will save and restore
11091 the register @var{reg} if they use it.
11093 It is an error to used this flag with the frame pointer or stack pointer.
11094 Use of this flag for other registers that have fixed pervasive roles in
11095 the machine's execution model will produce disastrous results.
11097 A different sort of disaster will result from the use of this flag for
11098 a register in which function values may be returned.
11100 This flag does not have a negative form, because it specifies a
11103 @item -fpack-struct
11104 @opindex fpack-struct
11105 Pack all structure members together without holes.
11107 @strong{Warning:} the @option{-fpack-struct} switch causes GCC to generate
11108 code that is not binary compatible with code generated without that switch.
11109 Additionally, it makes the code suboptimal.
11110 Use it to conform to a non-default application binary interface.
11112 @item -finstrument-functions
11113 @opindex finstrument-functions
11114 Generate instrumentation calls for entry and exit to functions. Just
11115 after function entry and just before function exit, the following
11116 profiling functions will be called with the address of the current
11117 function and its call site. (On some platforms,
11118 @code{__builtin_return_address} does not work beyond the current
11119 function, so the call site information may not be available to the
11120 profiling functions otherwise.)
11123 void __cyg_profile_func_enter (void *this_fn,
11125 void __cyg_profile_func_exit (void *this_fn,
11129 The first argument is the address of the start of the current function,
11130 which may be looked up exactly in the symbol table.
11132 This instrumentation is also done for functions expanded inline in other
11133 functions. The profiling calls will indicate where, conceptually, the
11134 inline function is entered and exited. This means that addressable
11135 versions of such functions must be available. If all your uses of a
11136 function are expanded inline, this may mean an additional expansion of
11137 code size. If you use @samp{extern inline} in your C code, an
11138 addressable version of such functions must be provided. (This is
11139 normally the case anyways, but if you get lucky and the optimizer always
11140 expands the functions inline, you might have gotten away without
11141 providing static copies.)
11143 A function may be given the attribute @code{no_instrument_function}, in
11144 which case this instrumentation will not be done. This can be used, for
11145 example, for the profiling functions listed above, high-priority
11146 interrupt routines, and any functions from which the profiling functions
11147 cannot safely be called (perhaps signal handlers, if the profiling
11148 routines generate output or allocate memory).
11150 @item -fstack-check
11151 @opindex fstack-check
11152 Generate code to verify that you do not go beyond the boundary of the
11153 stack. You should specify this flag if you are running in an
11154 environment with multiple threads, but only rarely need to specify it in
11155 a single-threaded environment since stack overflow is automatically
11156 detected on nearly all systems if there is only one stack.
11158 Note that this switch does not actually cause checking to be done; the
11159 operating system must do that. The switch causes generation of code
11160 to ensure that the operating system sees the stack being extended.
11162 @item -fstack-limit-register=@var{reg}
11163 @itemx -fstack-limit-symbol=@var{sym}
11164 @itemx -fno-stack-limit
11165 @opindex fstack-limit-register
11166 @opindex fstack-limit-symbol
11167 @opindex fno-stack-limit
11168 Generate code to ensure that the stack does not grow beyond a certain value,
11169 either the value of a register or the address of a symbol. If the stack
11170 would grow beyond the value, a signal is raised. For most targets,
11171 the signal is raised before the stack overruns the boundary, so
11172 it is possible to catch the signal without taking special precautions.
11174 For instance, if the stack starts at absolute address @samp{0x80000000}
11175 and grows downwards, you can use the flags
11176 @option{-fstack-limit-symbol=__stack_limit} and
11177 @option{-Wl,--defsym,__stack_limit=0x7ffe0000} to enforce a stack limit
11178 of 128KB@. Note that this may only work with the GNU linker.
11180 @cindex aliasing of parameters
11181 @cindex parameters, aliased
11182 @item -fargument-alias
11183 @itemx -fargument-noalias
11184 @itemx -fargument-noalias-global
11185 @opindex fargument-alias
11186 @opindex fargument-noalias
11187 @opindex fargument-noalias-global
11188 Specify the possible relationships among parameters and between
11189 parameters and global data.
11191 @option{-fargument-alias} specifies that arguments (parameters) may
11192 alias each other and may alias global storage.@*
11193 @option{-fargument-noalias} specifies that arguments do not alias
11194 each other, but may alias global storage.@*
11195 @option{-fargument-noalias-global} specifies that arguments do not
11196 alias each other and do not alias global storage.
11198 Each language will automatically use whatever option is required by
11199 the language standard. You should not need to use these options yourself.
11201 @item -fleading-underscore
11202 @opindex fleading-underscore
11203 This option and its counterpart, @option{-fno-leading-underscore}, forcibly
11204 change the way C symbols are represented in the object file. One use
11205 is to help link with legacy assembly code.
11207 @strong{Warning:} the @option{-fleading-underscore} switch causes GCC to
11208 generate code that is not binary compatible with code generated without that
11209 switch. Use it to conform to a non-default application binary interface.
11210 Not all targets provide complete support for this switch.
11212 @item -ftls-model=@var{model}
11213 Alter the thread-local storage model to be used (@pxref{Thread-Local}).
11214 The @var{model} argument should be one of @code{global-dynamic},
11215 @code{local-dynamic}, @code{initial-exec} or @code{local-exec}.
11217 The default without @option{-fpic} is @code{initial-exec}; with
11218 @option{-fpic} the default is @code{global-dynamic}.
11223 @node Environment Variables
11224 @section Environment Variables Affecting GCC
11225 @cindex environment variables
11227 @c man begin ENVIRONMENT
11228 This section describes several environment variables that affect how GCC
11229 operates. Some of them work by specifying directories or prefixes to use
11230 when searching for various kinds of files. Some are used to specify other
11231 aspects of the compilation environment.
11233 Note that you can also specify places to search using options such as
11234 @option{-B}, @option{-I} and @option{-L} (@pxref{Directory Options}). These
11235 take precedence over places specified using environment variables, which
11236 in turn take precedence over those specified by the configuration of GCC@.
11237 @xref{Driver,, Controlling the Compilation Driver @file{gcc}, gccint,
11238 GNU Compiler Collection (GCC) Internals}.
11243 @c @itemx LC_COLLATE
11245 @c @itemx LC_MONETARY
11246 @c @itemx LC_NUMERIC
11251 @c @findex LC_COLLATE
11252 @findex LC_MESSAGES
11253 @c @findex LC_MONETARY
11254 @c @findex LC_NUMERIC
11258 These environment variables control the way that GCC uses
11259 localization information that allow GCC to work with different
11260 national conventions. GCC inspects the locale categories
11261 @env{LC_CTYPE} and @env{LC_MESSAGES} if it has been configured to do
11262 so. These locale categories can be set to any value supported by your
11263 installation. A typical value is @samp{en_UK} for English in the United
11266 The @env{LC_CTYPE} environment variable specifies character
11267 classification. GCC uses it to determine the character boundaries in
11268 a string; this is needed for some multibyte encodings that contain quote
11269 and escape characters that would otherwise be interpreted as a string
11272 The @env{LC_MESSAGES} environment variable specifies the language to
11273 use in diagnostic messages.
11275 If the @env{LC_ALL} environment variable is set, it overrides the value
11276 of @env{LC_CTYPE} and @env{LC_MESSAGES}; otherwise, @env{LC_CTYPE}
11277 and @env{LC_MESSAGES} default to the value of the @env{LANG}
11278 environment variable. If none of these variables are set, GCC
11279 defaults to traditional C English behavior.
11283 If @env{TMPDIR} is set, it specifies the directory to use for temporary
11284 files. GCC uses temporary files to hold the output of one stage of
11285 compilation which is to be used as input to the next stage: for example,
11286 the output of the preprocessor, which is the input to the compiler
11289 @item GCC_EXEC_PREFIX
11290 @findex GCC_EXEC_PREFIX
11291 If @env{GCC_EXEC_PREFIX} is set, it specifies a prefix to use in the
11292 names of the subprograms executed by the compiler. No slash is added
11293 when this prefix is combined with the name of a subprogram, but you can
11294 specify a prefix that ends with a slash if you wish.
11296 If @env{GCC_EXEC_PREFIX} is not set, GCC will attempt to figure out
11297 an appropriate prefix to use based on the pathname it was invoked with.
11299 If GCC cannot find the subprogram using the specified prefix, it
11300 tries looking in the usual places for the subprogram.
11302 The default value of @env{GCC_EXEC_PREFIX} is
11303 @file{@var{prefix}/lib/gcc-lib/} where @var{prefix} is the value
11304 of @code{prefix} when you ran the @file{configure} script.
11306 Other prefixes specified with @option{-B} take precedence over this prefix.
11308 This prefix is also used for finding files such as @file{crt0.o} that are
11311 In addition, the prefix is used in an unusual way in finding the
11312 directories to search for header files. For each of the standard
11313 directories whose name normally begins with @samp{/usr/local/lib/gcc-lib}
11314 (more precisely, with the value of @env{GCC_INCLUDE_DIR}), GCC tries
11315 replacing that beginning with the specified prefix to produce an
11316 alternate directory name. Thus, with @option{-Bfoo/}, GCC will search
11317 @file{foo/bar} where it would normally search @file{/usr/local/lib/bar}.
11318 These alternate directories are searched first; the standard directories
11321 @item COMPILER_PATH
11322 @findex COMPILER_PATH
11323 The value of @env{COMPILER_PATH} is a colon-separated list of
11324 directories, much like @env{PATH}. GCC tries the directories thus
11325 specified when searching for subprograms, if it can't find the
11326 subprograms using @env{GCC_EXEC_PREFIX}.
11329 @findex LIBRARY_PATH
11330 The value of @env{LIBRARY_PATH} is a colon-separated list of
11331 directories, much like @env{PATH}. When configured as a native compiler,
11332 GCC tries the directories thus specified when searching for special
11333 linker files, if it can't find them using @env{GCC_EXEC_PREFIX}. Linking
11334 using GCC also uses these directories when searching for ordinary
11335 libraries for the @option{-l} option (but directories specified with
11336 @option{-L} come first).
11340 @cindex locale definition
11341 This variable is used to pass locale information to the compiler. One way in
11342 which this information is used is to determine the character set to be used
11343 when character literals, string literals and comments are parsed in C and C++.
11344 When the compiler is configured to allow multibyte characters,
11345 the following values for @env{LANG} are recognized:
11349 Recognize JIS characters.
11351 Recognize SJIS characters.
11353 Recognize EUCJP characters.
11356 If @env{LANG} is not defined, or if it has some other value, then the
11357 compiler will use mblen and mbtowc as defined by the default locale to
11358 recognize and translate multibyte characters.
11362 Some additional environments variables affect the behavior of the
11365 @include cppenv.texi
11369 @node Precompiled Headers
11370 @section Using Precompiled Headers
11371 @cindex precompiled headers
11372 @cindex speed of compilation
11374 Often large projects have many header files that are included in every
11375 source file. The time the compiler takes to process these header files
11376 over and over again can account for nearly all of the time required to
11377 build the project. To make builds faster, GCC allows users to
11378 `precompile' a header file; then, if builds can use the precompiled
11379 header file they will be much faster.
11381 To create a precompiled header file, simply compile it as you would any
11382 other file, if necessary using the @option{-x} option to make the driver
11383 treat it as a C or C++ header file. You will probably want to use a
11384 tool like @command{make} to keep the precompiled header up-to-date when
11385 the headers it contains change.
11387 A precompiled header file will be searched for when @code{#include} is
11388 seen in the compilation. As it searches for the included file
11389 (@pxref{Search Path,,Search Path,cpp.info,The C Preprocessor}) the
11390 compiler looks for a precompiled header in each directory just before it
11391 looks for the include file in that directory. The name searched for is
11392 the name specified in the @code{#include} with @samp{.gch} appended. If
11393 the precompiled header file can't be used, it is ignored.
11395 For instance, if you have @code{#include "all.h"}, and you have
11396 @file{all.h.gch} in the same directory as @file{all.h}, then the
11397 precompiled header file will be used if possible, and the original
11398 header will be used otherwise.
11400 Alternatively, you might decide to put the precompiled header file in a
11401 directory and use @option{-I} to ensure that directory is searched
11402 before (or instead of) the directory containing the original header.
11403 Then, if you want to check that the precompiled header file is always
11404 used, you can put a file of the same name as the original header in this
11405 directory containing an @code{#error} command.
11407 This also works with @option{-include}. So yet another way to use
11408 precompiled headers, good for projects not designed with precompiled
11409 header files in mind, is to simply take most of the header files used by
11410 a project, include them from another header file, precompile that header
11411 file, and @option{-include} the precompiled header. If the header files
11412 have guards against multiple inclusion, they will be skipped because
11413 they've already been included (in the precompiled header).
11415 If you need to precompile the same header file for different
11416 languages, targets, or compiler options, you can instead make a
11417 @emph{directory} named like @file{all.h.gch}, and put each precompiled
11418 header in the directory. (It doesn't matter what you call the files
11419 in the directory, every precompiled header in the directory will be
11420 considered.) The first precompiled header encountered in the
11421 directory that is valid for this compilation will be used; they're
11422 searched in no particular order.
11424 There are many other possibilities, limited only by your imagination,
11425 good sense, and the constraints of your build system.
11427 A precompiled header file can be used only when these conditions apply:
11431 Only one precompiled header can be used in a particular compilation.
11433 A precompiled header can't be used once the first C token is seen. You
11434 can have preprocessor directives before a precompiled header; you can
11435 even include a precompiled header from inside another header, so long as
11436 there are no C tokens before the @code{#include}.
11438 The precompiled header file must be produced for the same language as
11439 the current compilation. You can't use a C precompiled header for a C++
11442 The precompiled header file must be produced by the same compiler
11443 version and configuration as the current compilation is using.
11444 The easiest way to guarantee this is to use the same compiler binary
11445 for creating and using precompiled headers.
11447 Any macros defined before the precompiled header (including with
11448 @option{-D}) must either be defined in the same way as when the
11449 precompiled header was generated, or must not affect the precompiled
11450 header, which usually means that the they don't appear in the
11451 precompiled header at all.
11453 Certain command-line options must be defined in the same way as when the
11454 precompiled header was generated. At present, it's not clear which
11455 options are safe to change and which are not; the safest choice is to
11456 use exactly the same options when generating and using the precompiled
11460 For all of these but the last, the compiler will automatically ignore
11461 the precompiled header if the conditions aren't met. For the last item,
11462 some option changes will cause the precompiled header to be rejected,
11463 but not all incompatible option combinations have yet been found. If
11464 you find a new incompatible combination, please consider filing a bug
11465 report, see @ref{Bugs}.
11467 @node Running Protoize
11468 @section Running Protoize
11470 The program @code{protoize} is an optional part of GCC@. You can use
11471 it to add prototypes to a program, thus converting the program to ISO
11472 C in one respect. The companion program @code{unprotoize} does the
11473 reverse: it removes argument types from any prototypes that are found.
11475 When you run these programs, you must specify a set of source files as
11476 command line arguments. The conversion programs start out by compiling
11477 these files to see what functions they define. The information gathered
11478 about a file @var{foo} is saved in a file named @file{@var{foo}.X}.
11480 After scanning comes actual conversion. The specified files are all
11481 eligible to be converted; any files they include (whether sources or
11482 just headers) are eligible as well.
11484 But not all the eligible files are converted. By default,
11485 @code{protoize} and @code{unprotoize} convert only source and header
11486 files in the current directory. You can specify additional directories
11487 whose files should be converted with the @option{-d @var{directory}}
11488 option. You can also specify particular files to exclude with the
11489 @option{-x @var{file}} option. A file is converted if it is eligible, its
11490 directory name matches one of the specified directory names, and its
11491 name within the directory has not been excluded.
11493 Basic conversion with @code{protoize} consists of rewriting most
11494 function definitions and function declarations to specify the types of
11495 the arguments. The only ones not rewritten are those for varargs
11498 @code{protoize} optionally inserts prototype declarations at the
11499 beginning of the source file, to make them available for any calls that
11500 precede the function's definition. Or it can insert prototype
11501 declarations with block scope in the blocks where undeclared functions
11504 Basic conversion with @code{unprotoize} consists of rewriting most
11505 function declarations to remove any argument types, and rewriting
11506 function definitions to the old-style pre-ISO form.
11508 Both conversion programs print a warning for any function declaration or
11509 definition that they can't convert. You can suppress these warnings
11512 The output from @code{protoize} or @code{unprotoize} replaces the
11513 original source file. The original file is renamed to a name ending
11514 with @samp{.save} (for DOS, the saved filename ends in @samp{.sav}
11515 without the original @samp{.c} suffix). If the @samp{.save} (@samp{.sav}
11516 for DOS) file already exists, then the source file is simply discarded.
11518 @code{protoize} and @code{unprotoize} both depend on GCC itself to
11519 scan the program and collect information about the functions it uses.
11520 So neither of these programs will work until GCC is installed.
11522 Here is a table of the options you can use with @code{protoize} and
11523 @code{unprotoize}. Each option works with both programs unless
11527 @item -B @var{directory}
11528 Look for the file @file{SYSCALLS.c.X} in @var{directory}, instead of the
11529 usual directory (normally @file{/usr/local/lib}). This file contains
11530 prototype information about standard system functions. This option
11531 applies only to @code{protoize}.
11533 @item -c @var{compilation-options}
11534 Use @var{compilation-options} as the options when running @command{gcc} to
11535 produce the @samp{.X} files. The special option @option{-aux-info} is
11536 always passed in addition, to tell @command{gcc} to write a @samp{.X} file.
11538 Note that the compilation options must be given as a single argument to
11539 @code{protoize} or @code{unprotoize}. If you want to specify several
11540 @command{gcc} options, you must quote the entire set of compilation options
11541 to make them a single word in the shell.
11543 There are certain @command{gcc} arguments that you cannot use, because they
11544 would produce the wrong kind of output. These include @option{-g},
11545 @option{-O}, @option{-c}, @option{-S}, and @option{-o} If you include these in
11546 the @var{compilation-options}, they are ignored.
11549 Rename files to end in @samp{.C} (@samp{.cc} for DOS-based file
11550 systems) instead of @samp{.c}. This is convenient if you are converting
11551 a C program to C++. This option applies only to @code{protoize}.
11554 Add explicit global declarations. This means inserting explicit
11555 declarations at the beginning of each source file for each function
11556 that is called in the file and was not declared. These declarations
11557 precede the first function definition that contains a call to an
11558 undeclared function. This option applies only to @code{protoize}.
11560 @item -i @var{string}
11561 Indent old-style parameter declarations with the string @var{string}.
11562 This option applies only to @code{protoize}.
11564 @code{unprotoize} converts prototyped function definitions to old-style
11565 function definitions, where the arguments are declared between the
11566 argument list and the initial @samp{@{}. By default, @code{unprotoize}
11567 uses five spaces as the indentation. If you want to indent with just
11568 one space instead, use @option{-i " "}.
11571 Keep the @samp{.X} files. Normally, they are deleted after conversion
11575 Add explicit local declarations. @code{protoize} with @option{-l} inserts
11576 a prototype declaration for each function in each block which calls the
11577 function without any declaration. This option applies only to
11581 Make no real changes. This mode just prints information about the conversions
11582 that would have been done without @option{-n}.
11585 Make no @samp{.save} files. The original files are simply deleted.
11586 Use this option with caution.
11588 @item -p @var{program}
11589 Use the program @var{program} as the compiler. Normally, the name
11590 @file{gcc} is used.
11593 Work quietly. Most warnings are suppressed.
11596 Print the version number, just like @option{-v} for @command{gcc}.
11599 If you need special compiler options to compile one of your program's
11600 source files, then you should generate that file's @samp{.X} file
11601 specially, by running @command{gcc} on that source file with the
11602 appropriate options and the option @option{-aux-info}. Then run
11603 @code{protoize} on the entire set of files. @code{protoize} will use
11604 the existing @samp{.X} file because it is newer than the source file.
11608 gcc -Dfoo=bar file1.c -aux-info file1.X
11613 You need to include the special files along with the rest in the
11614 @code{protoize} command, even though their @samp{.X} files already
11615 exist, because otherwise they won't get converted.
11617 @xref{Protoize Caveats}, for more information on how to use
11618 @code{protoize} successfully.