This is doc/gcc.info, produced by makeinfo version 4.5 from doc/gcc.texi. INFO-DIR-SECTION Programming START-INFO-DIR-ENTRY * gcc: (gcc). The GNU Compiler Collection. END-INFO-DIR-ENTRY This file documents the use of the GNU compilers. Published by the Free Software Foundation 59 Temple Place - Suite 330 Boston, MA 02111-1307 USA Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with the Invariant Sections being "GNU General Public License" and "Funding Free Software", the Front-Cover texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in the section entitled "GNU Free Documentation License". (a) The FSF's Front-Cover Text is: A GNU Manual (b) The FSF's Back-Cover Text is: You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development.  File: gcc.info, Node: Debugging Options, Next: Optimize Options, Prev: Warning Options, Up: Invoking GCC Options for Debugging Your Program or GCC ========================================= GCC has various special options that are used for debugging either your program or GCC: `-g' Produce debugging information in the operating system's native format (stabs, COFF, XCOFF, or DWARF). GDB can work with this debugging information. On most systems that use stabs format, `-g' enables use of extra debugging information that only GDB can use; this extra information makes debugging work better in GDB but will probably make other debuggers crash or refuse to read the program. If you want to control for certain whether to generate the extra information, use `-gstabs+', `-gstabs', `-gxcoff+', `-gxcoff', `-gdwarf-1+', `-gdwarf-1', or `-gvms' (see below). Unlike most other C compilers, GCC allows you to use `-g' with `-O'. The shortcuts taken by optimized code may occasionally produce surprising results: some variables you declared may not exist at all; flow of control may briefly move where you did not expect it; some statements may not be executed because they compute constant results or their values were already at hand; some statements may execute in different places because they were moved out of loops. Nevertheless it proves possible to debug optimized output. This makes it reasonable to use the optimizer for programs that might have bugs. The following options are useful when GCC is generated with the capability for more than one debugging format. `-ggdb' Produce debugging information for use by GDB. This means to use the most expressive format available (DWARF 2, stabs, or the native format if neither of those are supported), including GDB extensions if at all possible. `-gstabs' Produce debugging information in stabs format (if that is supported), without GDB extensions. This is the format used by DBX on most BSD systems. On MIPS, Alpha and System V Release 4 systems this option produces stabs debugging output which is not understood by DBX or SDB. On System V Release 4 systems this option requires the GNU assembler. `-gstabs+' Produce debugging information in stabs format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program. `-gcoff' Produce debugging information in COFF format (if that is supported). This is the format used by SDB on most System V systems prior to System V Release 4. `-gxcoff' Produce debugging information in XCOFF format (if that is supported). This is the format used by the DBX debugger on IBM RS/6000 systems. `-gxcoff+' Produce debugging information in XCOFF format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program, and may cause assemblers other than the GNU assembler (GAS) to fail with an error. `-gdwarf' Produce debugging information in DWARF version 1 format (if that is supported). This is the format used by SDB on most System V Release 4 systems. `-gdwarf+' Produce debugging information in DWARF version 1 format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program. `-gdwarf-2' Produce debugging information in DWARF version 2 format (if that is supported). This is the format used by DBX on IRIX 6. `-gvms' Produce debugging information in VMS debug format (if that is supported). This is the format used by DEBUG on VMS systems. `-gLEVEL' `-ggdbLEVEL' `-gstabsLEVEL' `-gcoffLEVEL' `-gxcoffLEVEL' `-gvmsLEVEL' Request debugging information and also use LEVEL to specify how much information. The default level is 2. Level 1 produces minimal information, enough for making backtraces in parts of the program that you don't plan to debug. This includes descriptions of functions and external variables, but no information about local variables and no line numbers. Level 3 includes extra information, such as all the macro definitions present in the program. Some debuggers support macro expansion when you use `-g3'. Note that in order to avoid confusion between DWARF1 debug level 2, and DWARF2, neither `-gdwarf' nor `-gdwarf-2' accept a concatenated debug level. Instead use an additional `-gLEVEL' option to change the debug level for DWARF1 or DWARF2. `-p' Generate extra code to write profile information suitable for the analysis program `prof'. You must use this option when compiling the source files you want data about, and you must also use it when linking. `-pg' Generate extra code to write profile information suitable for the analysis program `gprof'. You must use this option when compiling the source files you want data about, and you must also use it when linking. `-Q' Makes the compiler print out each function name as it is compiled, and print some statistics about each pass when it finishes. `-ftime-report' Makes the compiler print some statistics about the time consumed by each pass when it finishes. `-fmem-report' Makes the compiler print some statistics about permanent memory allocation when it finishes. `-fprofile-arcs' Instrument "arcs" during compilation to generate coverage data or for profile-directed block ordering. During execution the program records how many times each branch is executed and how many times it is taken. When the compiled program exits it saves this data to a file called `SOURCENAME.da' for each source file. For profile-directed block ordering, compile the program with `-fprofile-arcs' plus optimization and code generation options, generate the arc profile information by running the program on a selected workload, and then compile the program again with the same optimization and code generation options plus `-fbranch-probabilities' (*note Options that Control Optimization: Optimize Options.). The other use of `-fprofile-arcs' is for use with `gcov', when it is used with the `-ftest-coverage' option. With `-fprofile-arcs', for each function of your program GCC creates a program flow graph, then finds a spanning tree for the graph. Only arcs that are not on the spanning tree have to be instrumented: the compiler adds code to count the number of times that these arcs are executed. When an arc is the only exit or only entrance to a block, the instrumentation code can be added to the block; otherwise, a new basic block must be created to hold the instrumentation code. `-ftest-coverage' Create data files for the `gcov' code-coverage utility (*note `gcov'--a Test Coverage Program: Gcov.). The data file names begin with the name of your source file: `SOURCENAME.bb' A mapping from basic blocks to line numbers, which `gcov' uses to associate basic block execution counts with line numbers. `SOURCENAME.bbg' A list of all arcs in the program flow graph. This allows `gcov' to reconstruct the program flow graph, so that it can compute all basic block and arc execution counts from the information in the `SOURCENAME.da' file. Use `-ftest-coverage' with `-fprofile-arcs'; the latter option adds instrumentation to the program, which then writes execution counts to another data file: `SOURCENAME.da' Runtime arc execution counts, used in conjunction with the arc information in the file `SOURCENAME.bbg'. Coverage data will map better to the source files if `-ftest-coverage' is used without optimization. `-dLETTERS' Says to make debugging dumps during compilation at times specified by LETTERS. This is used for debugging the compiler. The file names for most of the dumps are made by appending a pass number and a word to the source file name (e.g. `foo.c.00.rtl' or `foo.c.01.sibling'). Here are the possible letters for use in LETTERS, and their meanings: `A' Annotate the assembler output with miscellaneous debugging information. `b' Dump after computing branch probabilities, to `FILE.14.bp'. `B' Dump after block reordering, to `FILE.29.bbro'. `c' Dump after instruction combination, to the file `FILE.16.combine'. `C' Dump after the first if conversion, to the file `FILE.17.ce'. `d' Dump after delayed branch scheduling, to `FILE.31.dbr'. `D' Dump all macro definitions, at the end of preprocessing, in addition to normal output. `e' Dump after SSA optimizations, to `FILE.04.ssa' and `FILE.07.ussa'. `E' Dump after the second if conversion, to `FILE.26.ce2'. `f' Dump after life analysis, to `FILE.15.life'. `F' Dump after purging `ADDRESSOF' codes, to `FILE.09.addressof'. `g' Dump after global register allocation, to `FILE.21.greg'. `h' Dump after finalization of EH handling code, to `FILE.02.eh'. `k' Dump after reg-to-stack conversion, to `FILE.28.stack'. `o' Dump after post-reload optimizations, to `FILE.22.postreload'. `G' Dump after GCSE, to `FILE.10.gcse'. `i' Dump after sibling call optimizations, to `FILE.01.sibling'. `j' Dump after the first jump optimization, to `FILE.03.jump'. `k' Dump after conversion from registers to stack, to `FILE.32.stack'. `l' Dump after local register allocation, to `FILE.20.lreg'. `L' Dump after loop optimization, to `FILE.11.loop'. `M' Dump after performing the machine dependent reorganisation pass, to `FILE.30.mach'. `n' Dump after register renumbering, to `FILE.25.rnreg'. `N' Dump after the register move pass, to `FILE.18.regmove'. `r' Dump after RTL generation, to `FILE.00.rtl'. `R' Dump after the second scheduling pass, to `FILE.27.sched2'. `s' Dump after CSE (including the jump optimization that sometimes follows CSE), to `FILE.08.cse'. `S' Dump after the first scheduling pass, to `FILE.19.sched'. `t' Dump after the second CSE pass (including the jump optimization that sometimes follows CSE), to `FILE.12.cse2'. `w' Dump after the second flow pass, to `FILE.23.flow2'. `X' Dump after SSA dead code elimination, to `FILE.06.ssadce'. `z' Dump after the peephole pass, to `FILE.24.peephole2'. `a' Produce all the dumps listed above. `m' Print statistics on memory usage, at the end of the run, to standard error. `p' Annotate the assembler output with a comment indicating which pattern and alternative was used. The length of each instruction is also printed. `P' Dump the RTL in the assembler output as a comment before each instruction. Also turns on `-dp' annotation. `v' For each of the other indicated dump files (except for `FILE.00.rtl'), dump a representation of the control flow graph suitable for viewing with VCG to `FILE.PASS.vcg'. `x' Just generate RTL for a function instead of compiling it. Usually used with `r'. `y' Dump debugging information during parsing, to standard error. `-fdump-unnumbered' When doing debugging dumps (see `-d' option above), suppress instruction numbers and line number note output. This makes it more feasible to use diff on debugging dumps for compiler invocations with different options, in particular with and without `-g'. `-fdump-translation-unit (C and C++ only)' `-fdump-translation-unit-OPTIONS (C and C++ only)' Dump a representation of the tree structure for the entire translation unit to a file. The file name is made by appending `.tu' to the source file name. If the `-OPTIONS' form is used, OPTIONS controls the details of the dump as described for the `-fdump-tree' options. `-fdump-class-hierarchy (C++ only)' `-fdump-class-hierarchy-OPTIONS (C++ only)' Dump a representation of each class's hierarchy and virtual function table layout to a file. The file name is made by appending `.class' to the source file name. If the `-OPTIONS' form is used, OPTIONS controls the details of the dump as described for the `-fdump-tree' options. `-fdump-tree-SWITCH (C++ only)' `-fdump-tree-SWITCH-OPTIONS (C++ only)' Control the dumping at various stages of processing the intermediate language tree to a file. The file name is generated by appending a switch specific suffix to the source file name. If the `-OPTIONS' form is used, OPTIONS is a list of `-' separated options that control the details of the dump. Not all options are applicable to all dumps, those which are not meaningful will be ignored. The following options are available `address' Print the address of each node. Usually this is not meaningful as it changes according to the environment and source file. Its primary use is for tying up a dump file with a debug environment. `slim' Inhibit dumping of members of a scope or body of a function merely because that scope has been reached. Only dump such items when they are directly reachable by some other path. `all' Turn on all options. The following tree dumps are possible: `original' Dump before any tree based optimization, to `FILE.original'. `optimized' Dump after all tree based optimization, to `FILE.optimized'. `inlined' Dump after function inlining, to `FILE.inlined'. `-fsched-verbose=N' On targets that use instruction scheduling, this option controls the amount of debugging output the scheduler prints. This information is written to standard error, unless `-dS' or `-dR' is specified, in which case it is output to the usual dump listing file, `.sched' or `.sched2' respectively. However for N greater than nine, the output is always printed to standard error. For N greater than zero, `-fsched-verbose' outputs the same information as `-dRS'. For N greater than one, it also output basic block probabilities, detailed ready list information and unit/insn info. For N greater than two, it includes RTL at abort point, control-flow and regions info. And for N over four, `-fsched-verbose' also includes dependence info. `-fpretend-float' When running a cross-compiler, pretend that the target machine uses the same floating point format as the host machine. This causes incorrect output of the actual floating constants, but the actual instruction sequence will probably be the same as GCC would make when running on the target machine. `-save-temps' Store the usual "temporary" intermediate files permanently; place them in the current directory and name them based on the source file. Thus, compiling `foo.c' with `-c -save-temps' would produce files `foo.i' and `foo.s', as well as `foo.o'. This creates a preprocessed `foo.i' output file even though the compiler now normally uses an integrated preprocessor. `-time' Report the CPU time taken by each subprocess in the compilation sequence. For C source files, this is the compiler proper and assembler (plus the linker if linking is done). The output looks like this: # cc1 0.12 0.01 # as 0.00 0.01 The first number on each line is the "user time," that is time spent executing the program itself. The second number is "system time," time spent executing operating system routines on behalf of the program. Both numbers are in seconds. `-print-file-name=LIBRARY' Print the full absolute name of the library file LIBRARY that would be used when linking--and don't do anything else. With this option, GCC does not compile or link anything; it just prints the file name. `-print-multi-directory' Print the directory name corresponding to the multilib selected by any other switches present in the command line. This directory is supposed to exist in `GCC_EXEC_PREFIX'. `-print-multi-lib' Print the mapping from multilib directory names to compiler switches that enable them. The directory name is separated from the switches by `;', and each switch starts with an `@' instead of the `-', without spaces between multiple switches. This is supposed to ease shell-processing. `-print-prog-name=PROGRAM' Like `-print-file-name', but searches for a program such as `cpp'. `-print-libgcc-file-name' Same as `-print-file-name=libgcc.a'. This is useful when you use `-nostdlib' or `-nodefaultlibs' but you do want to link with `libgcc.a'. You can do gcc -nostdlib FILES... `gcc -print-libgcc-file-name` `-print-search-dirs' Print the name of the configured installation directory and a list of program and library directories gcc will search--and don't do anything else. This is useful when gcc prints the error message `installation problem, cannot exec cpp0: No such file or directory'. To resolve this you either need to put `cpp0' and the other compiler components where gcc expects to find them, or you can set the environment variable `GCC_EXEC_PREFIX' to the directory where you installed them. Don't forget the trailing '/'. *Note Environment Variables::. `-dumpmachine' Print the compiler's target machine (for example, `i686-pc-linux-gnu')--and don't do anything else. `-dumpversion' Print the compiler version (for example, `3.0')--and don't do anything else. `-dumpspecs' Print the compiler's built-in specs--and don't do anything else. (This is used when GCC itself is being built.) *Note Spec Files::.  File: gcc.info, Node: Optimize Options, Next: Preprocessor Options, Prev: Debugging Options, Up: Invoking GCC Options That Control Optimization ================================= These options control various sorts of optimizations: `-O' `-O1' Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large function. Without `-O', the compiler's goal is to reduce the cost of compilation and to make debugging produce the expected results. Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the program counter to any other statement in the function and get exactly the results you would expect from the source code. With `-O', the compiler tries to reduce code size and execution time, without performing any optimizations that take a great deal of compilation time. `-O2' Optimize even more. GCC performs nearly all supported optimizations that do not involve a space-speed tradeoff. The compiler does not perform loop unrolling or function inlining when you specify `-O2'. As compared to `-O', this option increases both compilation time and the performance of the generated code. `-O2' turns on all optional optimizations except for loop unrolling, function inlining, and register renaming. It also turns on the `-fforce-mem' option on all machines and frame pointer elimination on machines where doing so does not interfere with debugging. Please note the warning under `-fgcse' about invoking `-O2' on programs that use computed gotos. `-O3' Optimize yet more. `-O3' turns on all optimizations specified by `-O2' and also turns on the `-finline-functions' and `-frename-registers' options. `-O0' Do not optimize. `-Os' Optimize for size. `-Os' enables all `-O2' optimizations that do not typically increase code size. It also performs further optimizations designed to reduce code size. If you use multiple `-O' options, with or without level numbers, the last such option is the one that is effective. Options of the form `-fFLAG' specify machine-independent flags. Most flags have both positive and negative forms; the negative form of `-ffoo' would be `-fno-foo'. In the table below, only one of the forms is listed--the one which is not the default. You can figure out the other form by either removing `no-' or adding it. `-ffloat-store' Do not store floating point variables in registers, and inhibit other options that might change whether a floating point value is taken from a register or memory. This option prevents undesirable excess precision on machines such as the 68000 where the floating registers (of the 68881) keep more precision than a `double' is supposed to have. Similarly for the x86 architecture. For most programs, the excess precision does only good, but a few programs rely on the precise definition of IEEE floating point. Use `-ffloat-store' for such programs, after modifying them to store all pertinent intermediate computations into variables. `-fno-default-inline' Do not make member functions inline by default merely because they are defined inside the class scope (C++ only). Otherwise, when you specify `-O', member functions defined inside class scope are compiled inline by default; i.e., you don't need to add `inline' in front of the member function name. `-fno-defer-pop' Always pop the arguments to each function call as soon as that function returns. For machines which must pop arguments after a function call, the compiler normally lets arguments accumulate on the stack for several function calls and pops them all at once. `-fforce-mem' Force memory operands to be copied into registers before doing arithmetic on them. This produces better code by making all memory references potential common subexpressions. When they are not common subexpressions, instruction combination should eliminate the separate register-load. The `-O2' option turns on this option. `-fforce-addr' Force memory address constants to be copied into registers before doing arithmetic on them. This may produce better code just as `-fforce-mem' may. `-fomit-frame-pointer' Don't keep the frame pointer in a register for functions that don't need one. This avoids the instructions to save, set up and restore frame pointers; it also makes an extra register available in many functions. *It also makes debugging impossible on some machines.* On some machines, such as the VAX, this flag has no effect, because the standard calling sequence automatically handles the frame pointer and nothing is saved by pretending it doesn't exist. The machine-description macro `FRAME_POINTER_REQUIRED' controls whether a target machine supports this flag. *Note Register Usage: (gccint)Registers. `-foptimize-sibling-calls' Optimize sibling and tail recursive calls. `-ftrapv' This option generates traps for signed overflow on addition, subtraction, multiplication operations. `-fno-inline' Don't pay attention to the `inline' keyword. Normally this option is used to keep the compiler from expanding any functions inline. Note that if you are not optimizing, no functions can be expanded inline. `-finline-functions' Integrate all simple functions into their callers. The compiler heuristically decides which functions are simple enough to be worth integrating in this way. If all calls to a given function are integrated, and the function is declared `static', then the function is normally not output as assembler code in its own right. `-finline-limit=N' By default, gcc limits the size of functions that can be inlined. This flag allows the control of this limit for functions that are explicitly marked as inline (ie marked with the inline keyword or defined within the class definition in c++). N is the size of functions that can be inlined in number of pseudo instructions (not counting parameter handling). The default value of N is 600. Increasing this value can result in more inlined code at the cost of compilation time and memory consumption. Decreasing usually makes the compilation faster and less code will be inlined (which presumably means slower programs). This option is particularly useful for programs that use inlining heavily such as those based on recursive templates with C++. _Note:_ pseudo instruction represents, in this particular context, an abstract measurement of function's size. In no way, it represents a count of assembly instructions and as such its exact meaning might change from one release to an another. `-fkeep-inline-functions' Even if all calls to a given function are integrated, and the function is declared `static', nevertheless output a separate run-time callable version of the function. This switch does not affect `extern inline' functions. `-fkeep-static-consts' Emit variables declared `static const' when optimization isn't turned on, even if the variables aren't referenced. GCC enables this option by default. If you want to force the compiler to check if the variable was referenced, regardless of whether or not optimization is turned on, use the `-fno-keep-static-consts' option. `-fmerge-constants' Attempt to merge identical constants (string constants and floating point constants) accross compilation units. This option is default for optimized compilation if assembler and linker support it. Use `-fno-merge-constants' to inhibit this behavior. `-fmerge-all-constants' Attempt to merge identical constants and identical variables. This option implies `-fmerge-constants'. In addition to `-fmerge-constants' this considers e.g. even constant initialized arrays or initialized constant variables with integral or floating point types. Languages like C or C++ require each non-automatic variable to have distinct location, so using this option will result in non-conforming behavior. `-fno-branch-count-reg' Do not use "decrement and branch" instructions on a count register, but instead generate a sequence of instructions that decrement a register, compare it against zero, then branch based upon the result. This option is only meaningful on architectures that support such instructions, which include x86, PowerPC, IA-64 and S/390. `-fno-function-cse' Do not put function addresses in registers; make each instruction that calls a constant function contain the function's address explicitly. This option results in less efficient code, but some strange hacks that alter the assembler output may be confused by the optimizations performed when this option is not used. `-ffast-math' Sets `-fno-math-errno', `-funsafe-math-optimizations', and `-fno-trapping-math'. This option causes the preprocessor macro `__FAST_MATH__' to be defined. This option should never be turned on by any `-O' option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math functions. `-fno-math-errno' Do not set ERRNO after calling math functions that are executed with a single instruction, e.g., sqrt. A program that relies on IEEE exceptions for math error handling may want to use this flag for speed while maintaining IEEE arithmetic compatibility. This option should never be turned on by any `-O' option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math functions. The default is `-fmath-errno'. `-funsafe-math-optimizations' Allow optimizations for floating-point arithmetic that (a) assume that arguments and results are valid and (b) may violate IEEE or ANSI standards. When used at link-time, it may include libraries or startup files that change the default FPU control word or other similar optimizations. This option should never be turned on by any `-O' option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math functions. The default is `-fno-unsafe-math-optimizations'. `-fno-trapping-math' Compile code assuming that floating-point operations cannot generate user-visible traps. Setting this option may allow faster code if one relies on "non-stop" IEEE arithmetic, for example. This option should never be turned on by any `-O' option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math functions. The default is `-ftrapping-math'. `-fbounds-check' For front-ends that support it, generate additional code to check that indices used to access arrays are within the declared range. This is currenly only supported by the Java and Fortran 77 front-ends, where this option defaults to true and false respectively. The following options control specific optimizations. The `-O2' option turns on all of these optimizations except `-funroll-loops' and `-funroll-all-loops'. On most machines, the `-O' option turns on the `-fthread-jumps' and `-fdelayed-branch' options, but specific machines may handle it differently. You can use the following flags in the rare cases when "fine-tuning" of optimizations to be performed is desired. Not all of the optimizations performed by GCC have `-f' options to control them. `-fstrength-reduce' Perform the optimizations of loop strength reduction and elimination of iteration variables. `-fthread-jumps' Perform optimizations where we check to see if a jump branches to a location where another comparison subsumed by the first is found. If so, the first branch is redirected to either the destination of the second branch or a point immediately following it, depending on whether the condition is known to be true or false. `-fcse-follow-jumps' In common subexpression elimination, scan through jump instructions when the target of the jump is not reached by any other path. For example, when CSE encounters an `if' statement with an `else' clause, CSE will follow the jump when the condition tested is false. `-fcse-skip-blocks' This is similar to `-fcse-follow-jumps', but causes CSE to follow jumps which conditionally skip over blocks. When CSE encounters a simple `if' statement with no else clause, `-fcse-skip-blocks' causes CSE to follow the jump around the body of the `if'. `-frerun-cse-after-loop' Re-run common subexpression elimination after loop optimizations has been performed. `-frerun-loop-opt' Run the loop optimizer twice. `-fgcse' Perform a global common subexpression elimination pass. This pass also performs global constant and copy propagation. _Note:_ When compiling a program using computed gotos, a GCC extension, you may get better runtime performance if you disable the global common subexpression elmination pass by adding `-fno-gcse' to the command line. `-fgcse-lm' When `-fgcse-lm' is enabled, global common subexpression elimination will attempt to move loads which are only killed by stores into themselves. This allows a loop containing a load/store sequence to be changed to a load outside the loop, and a copy/store within the loop. `-fgcse-sm' When `-fgcse-sm' is enabled, A store motion pass is run after global common subexpression elimination. This pass will attempt to move stores out of loops. When used in conjunction with `-fgcse-lm', loops containing a load/store sequence can be changed to a load before the loop and a store after the loop. `-fdelete-null-pointer-checks' Use global dataflow analysis to identify and eliminate useless checks for null pointers. The compiler assumes that dereferencing a null pointer would have halted the program. If a pointer is checked after it has already been dereferenced, it cannot be null. In some environments, this assumption is not true, and programs can safely dereference null pointers. Use `-fno-delete-null-pointer-checks' to disable this optimization for programs which depend on that behavior. `-fexpensive-optimizations' Perform a number of minor optimizations that are relatively expensive. `-foptimize-register-move' `-fregmove' Attempt to reassign register numbers in move instructions and as operands of other simple instructions in order to maximize the amount of register tying. This is especially helpful on machines with two-operand instructions. GCC enables this optimization by default with `-O2' or higher. Note `-fregmove' and `-foptimize-register-move' are the same optimization. `-fdelayed-branch' If supported for the target machine, attempt to reorder instructions to exploit instruction slots available after delayed branch instructions. `-fschedule-insns' If supported for the target machine, attempt to reorder instructions to eliminate execution stalls due to required data being unavailable. This helps machines that have slow floating point or memory load instructions by allowing other instructions to be issued until the result of the load or floating point instruction is required. `-fschedule-insns2' Similar to `-fschedule-insns', but requests an additional pass of instruction scheduling after register allocation has been done. This is especially useful on machines with a relatively small number of registers and where memory load instructions take more than one cycle. `-fno-sched-interblock' Don't schedule instructions across basic blocks. This is normally enabled by default when scheduling before register allocation, i.e. with `-fschedule-insns' or at `-O2' or higher. `-fno-sched-spec' Don't allow speculative motion of non-load instructions. This is normally enabled by default when scheduling before register allocation, i.e. with `-fschedule-insns' or at `-O2' or higher. `-fsched-spec-load' Allow speculative motion of some load instructions. This only makes sense when scheduling before register allocation, i.e. with `-fschedule-insns' or at `-O2' or higher. `-fsched-spec-load-dangerous' Allow speculative motion of more load instructions. This only makes sense when scheduling before register allocation, i.e. with `-fschedule-insns' or at `-O2' or higher. `-ffunction-sections' `-fdata-sections' Place each function or data item into its own section in the output file if the target supports arbitrary sections. The name of the function or the name of the data item determines the section's name in the output file. Use these options on systems where the linker can perform optimizations to improve locality of reference in the instruction space. HPPA processors running HP-UX and Sparc processors running Solaris 2 have linkers with such optimizations. Other systems using the ELF object format as well as AIX may have these optimizations in the future. Only use these options when there are significant benefits from doing so. When you specify these options, the assembler and linker will create larger object and executable files and will also be slower. You will not be able to use `gprof' on all systems if you specify this option and you may have problems with debugging if you specify both this option and `-g'. `-fcaller-saves' Enable values to be allocated in registers that will be clobbered by function calls, by emitting extra instructions to save and restore the registers around such calls. Such allocation is done only when it seems to result in better code than would otherwise be produced. This option is always enabled by default on certain machines, usually those which have no call-preserved registers to use instead. For all machines, optimization level 2 and higher enables this flag by default. `-funroll-loops' Unroll loops whose number of iterations can be determined at compile time or upon entry to the loop. `-funroll-loops' implies both `-fstrength-reduce' and `-frerun-cse-after-loop'. This option makes code larger, and may or may not make it run faster. `-funroll-all-loops' Unroll all loops, even if their number of iterations is uncertain when the loop is entered. This usually makes programs run more slowly. `-funroll-all-loops' implies the same options as `-funroll-loops', `-fprefetch-loop-arrays' If supported by the target machine, generate instructions to prefetch memory to improve the performance of loops that access large arrays. `-fmove-all-movables' Forces all invariant computations in loops to be moved outside the loop. `-freduce-all-givs' Forces all general-induction variables in loops to be strength-reduced. _Note:_ When compiling programs written in Fortran, `-fmove-all-movables' and `-freduce-all-givs' are enabled by default when you use the optimizer. These options may generate better or worse code; results are highly dependent on the structure of loops within the source code. These two options are intended to be removed someday, once they have helped determine the efficacy of various approaches to improving loop optimizations. Please let us ( and ) know how use of these options affects the performance of your production code. We're very interested in code that runs _slower_ when these options are _enabled_. `-fno-peephole' `-fno-peephole2' Disable any machine-specific peephole optimizations. The difference between `-fno-peephole' and `-fno-peephole2' is in how they are implemented in the compiler; some targets use one, some use the other, a few use both. `-fbranch-probabilities' After running a program compiled with `-fprofile-arcs' (*note Options for Debugging Your Program or `gcc': Debugging Options.), you can compile it a second time using `-fbranch-probabilities', to improve optimizations based on the number of times each branch was taken. When the program compiled with `-fprofile-arcs' exits it saves arc execution counts to a file called `SOURCENAME.da' for each source file The information in this data file is very dependent on the structure of the generated code, so you must use the same source code and the same optimization options for both compilations. With `-fbranch-probabilities', GCC puts a `REG_EXEC_COUNT' note on the first instruction of each basic block, and a `REG_BR_PROB' note on each `JUMP_INSN' and `CALL_INSN'. These can be used to improve optimization. Currently, they are only used in one place: in `reorg.c', instead of guessing which path a branch is mostly to take, the `REG_BR_PROB' values are used to exactly determine which path is taken more often. `-fno-guess-branch-probability' Do not guess branch probabilities using a randomized model. Sometimes gcc will opt to use a randomized model to guess branch probabilities, when none are available from either profiling feedback (`-fprofile-arcs') or `__builtin_expect'. This means that different runs of the compiler on the same program may produce different object code. In a hard real-time system, people don't want different runs of the compiler to produce code that has different behavior; minimizing non-determinism is of paramount import. This switch allows users to reduce non-determinism, possibly at the expense of inferior optimization. `-fstrict-aliasing' Allows the compiler to assume the strictest aliasing rules applicable to the language being compiled. For C (and C++), this activates optimizations based on the type of expressions. In particular, an object of one type is assumed never to reside at the same address as an object of a different type, unless the types are almost the same. For example, an `unsigned int' can alias an `int', but not a `void*' or a `double'. A character type may alias any other type. Pay special attention to code like this: union a_union { int i; double d; }; int f() { a_union t; t.d = 3.0; return t.i; } The practice of reading from a different union member than the one most recently written to (called "type-punning") is common. Even with `-fstrict-aliasing', type-punning is allowed, provided the memory is accessed through the union type. So, the code above will work as expected. However, this code might not: int f() { a_union t; int* ip; t.d = 3.0; ip = &t.i; return *ip; } Every language that wishes to perform language-specific alias analysis should define a function that computes, given an `tree' node, an alias set for the node. Nodes in different alias sets are not allowed to alias. For an example, see the C front-end function `c_get_alias_set'. `-falign-functions' `-falign-functions=N' Align the start of functions to the next power-of-two greater than N, skipping up to N bytes. For instance, `-falign-functions=32' aligns functions to the next 32-byte boundary, but `-falign-functions=24' would align to the next 32-byte boundary only if this can be done by skipping 23 bytes or less. `-fno-align-functions' and `-falign-functions=1' are equivalent and mean that functions will not be aligned. Some assemblers only support this flag when N is a power of two; in that case, it is rounded up. If N is not specified, use a machine-dependent default. `-falign-labels' `-falign-labels=N' Align all branch targets to a power-of-two boundary, skipping up to N bytes like `-falign-functions'. This option can easily make code slower, because it must insert dummy operations for when the branch target is reached in the usual flow of the code. If `-falign-loops' or `-falign-jumps' are applicable and are greater than this value, then their values are used instead. If N is not specified, use a machine-dependent default which is very likely to be `1', meaning no alignment. `-falign-loops' `-falign-loops=N' Align loops to a power-of-two boundary, skipping up to N bytes like `-falign-functions'. The hope is that the loop will be executed many times, which will make up for any execution of the dummy operations. If N is not specified, use a machine-dependent default. `-falign-jumps' `-falign-jumps=N' Align branch targets to a power-of-two boundary, for branch targets where the targets can only be reached by jumping, skipping up to N bytes like `-falign-functions'. In this case, no dummy operations need be executed. If N is not specified, use a machine-dependent default. `-fssa' Perform optimizations in static single assignment form. Each function's flow graph is translated into SSA form, optimizations are performed, and the flow graph is translated back from SSA form. Users should not specify this option, since it is not yet ready for production use. `-fssa-ccp' Perform Sparse Conditional Constant Propagation in SSA form. Requires `-fssa'. Like `-fssa', this is an experimental feature. `-fssa-dce' Perform aggressive dead-code elimination in SSA form. Requires `-fssa'. Like `-fssa', this is an experimental feature. `-fsingle-precision-constant' Treat floating point constant as single precision constant instead of implicitly converting it to double precision constant. `-frename-registers' Attempt to avoid false dependencies in scheduled code by making use of registers left over after register allocation. This optimization will most benefit processors with lots of registers. It can, however, make debugging impossible, since variables will no longer stay in a "home register". `-fno-cprop-registers' After register allocation and post-register allocation instruction splitting, we perform a copy-propagation pass to try to reduce scheduling dependencies and occasionally eliminate the copy. `--param NAME=VALUE' In some places, GCC uses various constants to control the amount of optimization that is done. For example, GCC will not inline functions that contain more that a certain number of instructions. You can control some of these constants on the command-line using the `--param' option. In each case, the VALUE is an integer. The allowable choices for NAME are given in the following table: `max-delay-slot-insn-search' The maximum number of instructions to consider when looking for an instruction to fill a delay slot. If more than this arbitrary number of instructions is searched, the time savings from filling the delay slot will be minimal so stop searching. Increasing values mean more aggressive optimization, making the compile time increase with probably small improvement in executable run time. `max-delay-slot-live-search' When trying to fill delay slots, the maximum number of instructions to consider when searching for a block with valid live register information. Increasing this arbitrarily chosen value means more aggressive optimization, increasing the compile time. This parameter should be removed when the delay slot code is rewritten to maintain the control-flow graph. `max-gcse-memory' The approximate maximum amount of memory that will be allocated in order to perform the global common subexpression elimination optimization. If more memory than specified is required, the optimization will not be done. `max-gcse-passes' The maximum number of passes of GCSE to run. `max-pending-list-length' The maximum number of pending dependencies scheduling will allow before flushing the current state and starting over. Large functions with few branches or calls can create excessively large lists which needlessly consume memory and resources. `max-inline-insns' If an function contains more than this many instructions, it will not be inlined. This option is precisely equivalent to `-finline-limit'.