This is Info file gcc.info, produced by Makeinfo version 1.67 from the input file gcc.texi. This file documents the use and the internals of the GNU compiler. 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 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the sections entitled "GNU General Public License," "Funding for Free Software," and "Protect Your Freedom--Fight `Look And Feel'" are included exactly as in the original, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that the sections entitled "GNU General Public License," "Funding for Free Software," and "Protect Your Freedom--Fight `Look And Feel'", and this permission notice, may be included in translations approved by the Free Software Foundation instead of in the original English.  File: gcc.info, Node: Warning Options, Next: Debugging Options, Prev: C++ Dialect Options, Up: Invoking GCC Options to Request or Suppress Warnings ======================================= Warnings are diagnostic messages that report constructions which are not inherently erroneous but which are risky or suggest there may have been an error. You can request many specific warnings with options beginning `-W', for example `-Wimplicit' to request warnings on implicit declarations. Each of these specific warning options also has a negative form beginning `-Wno-' to turn off warnings; for example, `-Wno-implicit'. This manual lists only one of the two forms, whichever is not the default. These options control the amount and kinds of warnings produced by GNU CC: `-fsyntax-only' Check the code for syntax errors, but don't do anything beyond that. `-pedantic' Issue all the warnings demanded by strict ANSI standard C; reject all programs that use forbidden extensions. Valid ANSI standard C programs should compile properly with or without this option (though a rare few will require `-ansi'). However, without this option, certain GNU extensions and traditional C features are supported as well. With this option, they are rejected. `-pedantic' does not cause warning messages for use of the alternate keywords whose names begin and end with `__'. Pedantic warnings are also disabled in the expression that follows `__extension__'. However, only system header files should use these escape routes; application programs should avoid them. *Note Alternate Keywords::. This option is not intended to be useful; it exists only to satisfy pedants who would otherwise claim that GNU CC fails to support the ANSI standard. Some users try to use `-pedantic' to check programs for strict ANSI C conformance. They soon find that it does not do quite what they want: it finds some non-ANSI practices, but not all--only those for which ANSI C *requires* a diagnostic. A feature to report any failure to conform to ANSI C might be useful in some instances, but would require considerable additional work and would be quite different from `-pedantic'. We recommend, rather, that users take advantage of the extensions of GNU C and disregard the limitations of other compilers. Aside from certain supercomputers and obsolete small machines, there is less and less reason ever to use any other C compiler other than for bootstrapping GNU CC. `-pedantic-errors' Like `-pedantic', except that errors are produced rather than warnings. `-w' Inhibit all warning messages. `-Wno-import' Inhibit warning messages about the use of `#import'. `-Wchar-subscripts' Warn if an array subscript has type `char'. This is a common cause of error, as programmers often forget that this type is signed on some machines. `-Wcomment' Warn whenever a comment-start sequence `/*' appears in a `/*' comment, or whenever a Backslash-Newline appears in a `//' comment. `-Wformat' Check calls to `printf' and `scanf', etc., to make sure that the arguments supplied have types appropriate to the format string specified. `-Wimplicit-int' Warn when a declaration does not specify a type. `-Wimplicit-function-declarations' Warn whenever a function is used before being declared. `-Wimplicit' Same as `-Wimplicit-int' `-Wimplicit-function-declaration'. `-Wmain' Warn if the type of `main' is suspicious. `main' should be a function with external linkage, returning int, taking either zero arguments, two, or three arguments of appropriate types. `-Wparentheses' Warn if parentheses are omitted in certain contexts, such as when there is an assignment in a context where a truth value is expected, or when operators are nested whose precedence people often get confused about. Also warn about constructions where there may be confusion to which `if' statement an `else' branch belongs. Here is an example of such a case: { if (a) if (b) foo (); else bar (); } In C, every `else' branch belongs to the innermost possible `if' statement, which in this example is `if (b)'. This is often not what the programmer expected, as illustrated in the above example by indentation the programmer chose. When there is the potential for this confusion, GNU C will issue a warning when this flag is specified. To eliminate the warning, add explicit braces around the innermost `if' statement so there is no way the `else' could belong to the enclosing `if'. The resulting code would look like this: { if (a) { if (b) foo (); else bar (); } } `-Wreturn-type' Warn whenever a function is defined with a return-type that defaults to `int'. Also warn about any `return' statement with no return-value in a function whose return-type is not `void'. `-Wswitch' Warn whenever a `switch' statement has an index of enumeral type and lacks a `case' for one or more of the named codes of that enumeration. (The presence of a `default' label prevents this warning.) `case' labels outside the enumeration range also provoke warnings when this option is used. `-Wtrigraphs' Warn if any trigraphs are encountered (assuming they are enabled). `-Wunused' Warn whenever a variable is unused aside from its declaration, whenever a function is declared static but never defined, whenever a label is declared but not used, and whenever a statement computes a result that is explicitly not used. In order to get a warning about an unused function parameter, you must specify both `-W' and `-Wunused'. To suppress this warning for an expression, simply cast it to void. For unused variables and parameters, use the `unused' attribute (*note Variable Attributes::.). `-Wuninitialized' An automatic variable is used without first being initialized. These warnings are possible only in optimizing compilation, because they require data flow information that is computed only when optimizing. If you don't specify `-O', you simply won't get these warnings. These warnings occur only for variables that are candidates for register allocation. Therefore, they do not occur for a variable that is declared `volatile', or whose address is taken, or whose size is other than 1, 2, 4 or 8 bytes. Also, they do not occur for structures, unions or arrays, even when they are in registers. Note that there may be no warning about a variable that is used only to compute a value that itself is never used, because such computations may be deleted by data flow analysis before the warnings are printed. These warnings are made optional because GNU CC is not smart enough to see all the reasons why the code might be correct despite appearing to have an error. Here is one example of how this can happen: { int x; switch (y) { case 1: x = 1; break; case 2: x = 4; break; case 3: x = 5; } foo (x); } If the value of `y' is always 1, 2 or 3, then `x' is always initialized, but GNU CC doesn't know this. Here is another common case: { int save_y; if (change_y) save_y = y, y = new_y; ... if (change_y) y = save_y; } This has no bug because `save_y' is used only if it is set. Some spurious warnings can be avoided if you declare all the functions you use that never return as `noreturn'. *Note Function Attributes::. `-Wreorder (C++ only)' Warn when the order of member initializers given in the code does not match the order in which they must be executed. For instance: struct A { int i; int j; A(): j (0), i (1) { } }; Here the compiler will warn that the member initializers for `i' and `j' will be rearranged to match the declaration order of the members. `-Wtemplate-debugging' When using templates in a C++ program, warn if debugging is not yet fully available (C++ only). `-Wall' All of the above `-W' options combined. This enables all the warnings about constructions that some users consider questionable, and that are easy to avoid (or modify to prevent the warning), even in conjunction with macros. The following `-W...' options are not implied by `-Wall'. Some of them warn about constructions that users generally do not consider questionable, but which occasionally you might wish to check for; others warn about constructions that are necessary or hard to avoid in some cases, and there is no simple way to modify the code to suppress the warning. `-W' Print extra warning messages for these events: * A nonvolatile automatic variable might be changed by a call to `longjmp'. These warnings as well are possible only in optimizing compilation. The compiler sees only the calls to `setjmp'. It cannot know where `longjmp' will be called; in fact, a signal handler could call it at any point in the code. As a result, you may get a warning even when there is in fact no problem because `longjmp' cannot in fact be called at the place which would cause a problem. * A function can return either with or without a value. (Falling off the end of the function body is considered returning without a value.) For example, this function would evoke such a warning: foo (a) { if (a > 0) return a; } * An expression-statement or the left-hand side of a comma expression contains no side effects. To suppress the warning, cast the unused expression to void. For example, an expression such as `x[i,j]' will cause a warning, but `x[(void)i,j]' will not. * An unsigned value is compared against zero with `<' or `<='. * A comparison like `x<=y<=z' appears; this is equivalent to `(x<=y ? 1 : 0) <= z', which is a different interpretation from that of ordinary mathematical notation. * Storage-class specifiers like `static' are not the first things in a declaration. According to the C Standard, this usage is obsolescent. * If `-Wall' or `-Wunused' is also specified, warn about unused arguments. * A comparison between signed and unsigned values could produce an incorrect result when the signed value is converted to unsigned. (But do not warn if `-Wno-sign-compare' is also specified.) * An aggregate has a partly bracketed initializer. For example, the following code would evoke such a warning, because braces are missing around the initializer for `x.h': struct s { int f, g; }; struct t { struct s h; int i; }; struct t x = { 1, 2, 3 }; `-Wtraditional' Warn about certain constructs that behave differently in traditional and ANSI C. * Macro arguments occurring within string constants in the macro body. These would substitute the argument in traditional C, but are part of the constant in ANSI C. * A function declared external in one block and then used after the end of the block. * A `switch' statement has an operand of type `long'. `-Wundef' Warn if an undefined identifier is evaluated in an `#if' directive. `-Wshadow' Warn whenever a local variable shadows another local variable. `-Wid-clash-LEN' Warn whenever two distinct identifiers match in the first LEN characters. This may help you prepare a program that will compile with certain obsolete, brain-damaged compilers. `-Wlarger-than-LEN' Warn whenever an object of larger than LEN bytes is defined. `-Wpointer-arith' Warn about anything that depends on the "size of" a function type or of `void'. GNU C assigns these types a size of 1, for convenience in calculations with `void *' pointers and pointers to functions. `-Wbad-function-cast' Warn whenever a function call is cast to a non-matching type. For example, warn if `int malloc()' is cast to `anything *'. `-Wcast-qual' Warn whenever a pointer is cast so as to remove a type qualifier from the target type. For example, warn if a `const char *' is cast to an ordinary `char *'. `-Wcast-align' Warn whenever a pointer is cast such that the required alignment of the target is increased. For example, warn if a `char *' is cast to an `int *' on machines where integers can only be accessed at two- or four-byte boundaries. `-Wwrite-strings' Give string constants the type `const char[LENGTH]' so that copying the address of one into a non-`const' `char *' pointer will get a warning. These warnings will help you find at compile time code that can try to write into a string constant, but only if you have been very careful about using `const' in declarations and prototypes. Otherwise, it will just be a nuisance; this is why we did not make `-Wall' request these warnings. `-Wconversion' Warn if a prototype causes a type conversion that is different from what would happen to the same argument in the absence of a prototype. This includes conversions of fixed point to floating and vice versa, and conversions changing the width or signedness of a fixed point argument except when the same as the default promotion. Also, warn if a negative integer constant expression is implicitly converted to an unsigned type. For example, warn about the assignment `x = -1' if `x' is unsigned. But do not warn about explicit casts like `(unsigned) -1'. `-Wsign-compare' Warn when a comparison between signed and unsigned values could produce an incorrect result when the signed value is converted to unsigned. This warning is also enabled by `-W'; to get the other warnings of `-W' without this warning, use `-W -Wno-sign-compare'. `-Waggregate-return' Warn if any functions that return structures or unions are defined or called. (In languages where you can return an array, this also elicits a warning.) `-Wstrict-prototypes' Warn if a function is declared or defined without specifying the argument types. (An old-style function definition is permitted without a warning if preceded by a declaration which specifies the argument types.) `-Wmissing-prototypes' Warn if a global function is defined without a previous prototype declaration. This warning is issued even if the definition itself provides a prototype. The aim is to detect global functions that fail to be declared in header files. `-Wmissing-declarations' Warn if a global function is defined without a previous declaration. Do so even if the definition itself provides a prototype. Use this option to detect global functions that are not declared in header files. `-Wredundant-decls' Warn if anything is declared more than once in the same scope, even in cases where multiple declaration is valid and changes nothing. `-Wnested-externs' Warn if an `extern' declaration is encountered within an function. `-Winline' Warn if a function can not be inlined, and either it was declared as inline, or else the `-finline-functions' option was given. `-Wold-style-cast' Warn if an old-style (C-style) cast is used within a program. `-Woverloaded-virtual' Warn when a derived class function declaration may be an error in defining a virtual function (C++ only). In a derived class, the definitions of virtual functions must match the type signature of a virtual function declared in the base class. With this option, the compiler warns when you define a function with the same name as a virtual function, but with a type signature that does not match any declarations from the base class. `-Wsynth (C++ only)' Warn when g++'s synthesis behavior does not match that of cfront. For instance: struct A { operator int (); A& operator = (int); }; main () { A a,b; a = b; } In this example, g++ will synthesize a default `A& operator = (const A&);', while cfront will use the user-defined `operator ='. `-Werror' Make all warnings into errors.  File: gcc.info, Node: Debugging Options, Next: Optimize Options, Prev: Warning Options, Up: Invoking GCC Options for Debugging Your Program or GNU CC ============================================ GNU CC has various special options that are used for debugging either your program or GCC: `-g' Produce debugging information in the operating system's native format (stabs, 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+', or `-gdwarf-1' (see below). Unlike most other C compilers, GNU CC allows you to use `-g' with `-O'. The shortcuts taken by optimized code may occasionally produce surprising results: some variables you declared may not exist at all; flow of control may briefly move where you did not expect it; some statements may not be executed because they compute constant results or their values were already at hand; some statements may execute in different places because they were moved out of loops. Nevertheless it proves possible to debug optimized output. This makes it reasonable to use the optimizer for programs that might have bugs. The following options are useful when GNU CC is generated with the capability for more than one debugging format. `-ggdb' Produce debugging information 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. `-gLEVEL' `-ggdbLEVEL' `-gstabsLEVEL' `-gcoffLEVEL' `-gxcoffLEVEL' `-gdwarfLEVEL' `-gdwarf-2LEVEL' Request debugging information and also use LEVEL to specify how much information. The default level is 2. Level 1 produces minimal information, enough for making backtraces in parts of the program that you don't plan to debug. This includes descriptions of functions and external variables, but no information about local variables and no line numbers. Level 3 includes extra information, such as all the macro definitions present in the program. Some debuggers support macro expansion when you use `-g3'. `-p' Generate extra code to write profile information suitable for the analysis program `prof'. 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. `-a' Generate extra code to write profile information for basic blocks, which will record the number of times each basic block is executed, the basic block start address, and the function name containing the basic block. If `-g' is used, the line number and filename of the start of the basic block will also be recorded. If not overridden by the machine description, the default action is to append to the text file `bb.out'. This data could be analyzed by a program like `tcov'. Note, however, that the format of the data is not what `tcov' expects. Eventually GNU `gprof' should be extended to process this data. `-ax' Generate extra code to profile basic blocks. Your executable will produce output that is a superset of that produced when `-a' is used. Additional output is the source and target address of the basic blocks where a jump takes place, the number of times a jump is executed, and (optionally) the complete sequence of basic blocks being executed. The output is appended to file `bb.out'. You can examine different profiling aspects without recompilation. Your executable will read a list of function names from file `bb.in'. Profiling starts when a function on the list is entered and stops when that invocation is exited. To exclude a function from profiling, prefix its name with `-'. If a function name is not unique, you can disambiguate it by writing it in the form `/path/filename.d:functionname'. Your executable will write the available paths and filenames in file `bb.out'. Several function names have a special meaning: `__bb_jumps__' Write source, target and frequency of jumps to file `bb.out'. `__bb_hidecall__' Exclude function calls from frequency count. `__bb_showret__' Include function returns in frequency count. `__bb_trace__' Write the sequence of basic blocks executed to file `bbtrace.gz'. The file will be compressed using the program `gzip', which must exist in your `PATH'. On systems without the `popen' function, the file will be named `bbtrace' and will not be compressed. *Profiling for even a few seconds on these systems will produce a very large file.* Note: `__bb_hidecall__' and `__bb_showret__' will not affect the sequence written to `bbtrace.gz'. Here's a short example using different profiling parameters in file `bb.in'. Assume function `foo' consists of basic blocks 1 and 2 and is called twice from block 3 of function `main'. After the calls, block 3 transfers control to block 4 of `main'. With `__bb_trace__' and `main' contained in file `bb.in', the following sequence of blocks is written to file `bbtrace.gz': 0 3 1 2 1 2 4. The return from block 2 to block 3 is not shown, because the return is to a point inside the block and not to the top. The block address 0 always indicates, that control is transferred to the trace from somewhere outside the observed functions. With `-foo' added to `bb.in', the blocks of function `foo' are removed from the trace, so only 0 3 4 remains. With `__bb_jumps__' and `main' contained in file `bb.in', jump frequencies will be written to file `bb.out'. The frequencies are obtained by constructing a trace of blocks and incrementing a counter for every neighbouring pair of blocks in the trace. The trace 0 3 1 2 1 2 4 displays the following frequencies: Jump from block 0x0 to block 0x3 executed 1 time(s) Jump from block 0x3 to block 0x1 executed 1 time(s) Jump from block 0x1 to block 0x2 executed 2 time(s) Jump from block 0x2 to block 0x1 executed 1 time(s) Jump from block 0x2 to block 0x4 executed 1 time(s) With `__bb_hidecall__', control transfer due to call instructions is removed from the trace, that is the trace is cut into three parts: 0 3 4, 0 1 2 and 0 1 2. With `__bb_showret__', control transfer due to return instructions is added to the trace. The trace becomes: 0 3 1 2 3 1 2 3 4. Note, that this trace is not the same, as the sequence written to `bbtrace.gz'. It is solely used for counting jump frequencies. `-fprofile-arcs' Instrument "arcs" during compilation. For each function of your program, GNU CC 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. Since not every arc in the program must be instrumented, programs compiled with this option run faster than programs compiled with `-a', which adds instrumentation code to every basic block in the program. The tradeoff: since `gcov' does not have execution counts for all branches, it must start with the execution counts for the instrumented branches, and then iterate over the program flow graph until the entire graph has been solved. Hence, `gcov' runs a little more slowly than a program which uses information from `-a'. `-fprofile-arcs' also makes it possible to estimate branch probabilities, and to calculate basic block execution counts. In general, basic block execution counts do not give enough information to estimate all branch probabilities. When the compiled program exits, it saves the arc execution counts to a file called `SOURCENAME.da'. Use the compiler option `-fbranch-probabilities' (*note Options that Control Optimization: Optimize Options.) when recompiling, to optimize using estimated branch probabilities. `-ftest-coverage' Create data files for the `gcov' code-coverage utility (*note `gcov': a GNU CC 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 (this last file is the output from `-fprofile-arcs'). `-Q' Makes the compiler print out each function name as it is compiled, and print some statistics about each pass when it finishes. `-dLETTERS' Says to make debugging dumps during compilation at times specified by LETTERS. This is used for debugging the compiler. The file names for most of the dumps are made by appending a word to the source file name (e.g. `foo.c.rtl' or `foo.c.jump'). Here are the possible letters for use in LETTERS, and their meanings: `M' Dump all macro definitions, at the end of preprocessing, and write no output. `N' Dump all macro names, at the end of preprocessing. `D' Dump all macro definitions, at the end of preprocessing, in addition to normal output. `y' Dump debugging information during parsing, to standard error. `r' Dump after RTL generation, to `FILE.rtl'. `x' Just generate RTL for a function instead of compiling it. Usually used with `r'. `j' Dump after first jump optimization, to `FILE.jump'. `s' Dump after CSE (including the jump optimization that sometimes follows CSE), to `FILE.cse'. `D' Dump after purging ADDRESSOF, to `FILE.addressof'. `L' Dump after loop optimization, to `FILE.loop'. `t' Dump after the second CSE pass (including the jump optimization that sometimes follows CSE), to `FILE.cse2'. `b' Dump after computing branch probabilities, to `FILE.bp'. `f' Dump after flow analysis, to `FILE.flow'. `c' Dump after instruction combination, to the file `FILE.combine'. `S' Dump after the first instruction scheduling pass, to `FILE.sched'. `l' Dump after local register allocation, to `FILE.lreg'. `g' Dump after global register allocation, to `FILE.greg'. `R' Dump after the second instruction scheduling pass, to `FILE.sched2'. `J' Dump after last jump optimization, to `FILE.jump2'. `d' Dump after delayed branch scheduling, to `FILE.dbr'. `k' Dump after conversion from registers to stack, to `FILE.stack'. `a' Produce all the dumps listed above. `m' Print statistics on memory usage, at the end of the run, to standard error. `p' Annotate the assembler output with a comment indicating which pattern and alternative was used. `A' Annotate the assembler output with miscellaneous debugging information. `-fpretend-float' When running a cross-compiler, pretend that the target machine uses the same floating point format as the host machine. This causes incorrect output of the actual floating constants, but the actual instruction sequence will probably be the same as GNU CC would make when running on the target machine. `-save-temps' Store the usual "temporary" intermediate files permanently; place them in the current directory and name them based on the source file. Thus, compiling `foo.c' with `-c -save-temps' would produce files `foo.i' and `foo.s', as well as `foo.o'. `-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, GNU CC does not compile or link anything; it just prints the file name. `-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 cpp: No such file or directory'. To resolve this you either need to put `cpp' 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::.  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. Without `-O', the compiler only allocates variables declared `register' in registers. The resulting compiled code is a little worse than produced by PCC without `-O'. With `-O', the compiler tries to reduce code size and execution time. When you specify `-O', the compiler turns on `-fthread-jumps' and `-fdefer-pop' on all machines. The compiler turns on `-fdelayed-branch' on machines that have delay slots, and `-fomit-frame-pointer' on machines that can support debugging even without a frame pointer. On some machines the compiler also turns on other flags. `-O2' Optimize even more. GNU CC 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 and function inlining. 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. `-O3' Optimize yet more. `-O3' turns on all optimizations specified by `-O2' and also turns on the `inline-functions' option. `-O0' Do not optimize. 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. `-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 Registers::. `-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. `-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. GNU CC 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. `-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' This option allows GCC to violate some ANSI or IEEE rules and/or specifications in the interest of optimizing code for speed. For example, it allows the compiler to assume arguments to the `sqrt' function are non-negative numbers and that no floating-point values are NaNs. 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 ANSI rules/specifications for math functions. 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. `-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. `-fexpensive-optimizations' Perform a number of minor optimizations that are relatively expensive. `-fdelayed-branch' If supported for the target machine, attempt to reorder instructions to exploit instruction slots available after delayed branch instructions. `-fschedule-insns' If supported for the target machine, attempt to reorder instructions to eliminate execution stalls due to required data being unavailable. This helps machines that have slow floating point or memory load instructions by allowing other instructions to be issued until the result of the load or floating point instruction is required. `-fschedule-insns2' Similar to `-fschedule-insns', but requests an additional pass of instruction scheduling after register allocation has been done. This is especially useful on machines with a relatively small number of registers and where memory load instructions take more than one cycle. `-ffunction-sections' Place each function into its own section in the output file if the target supports arbitrary sections. The function's name determines the section's name in the output file. Use this option 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 this option when there are significant benefits from doing so. When you specify this option, 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 enabled by default on certain machines, usually those which have no call-preserved registers to use instead. `-funroll-loops' Perform the optimization of loop unrolling. This is only done for loops whose number of iterations can be determined at compile time or run time. `-funroll-loop' implies both `-fstrength-reduce' and `-frerun-cse-after-loop'. `-funroll-all-loops' Perform the optimization of loop unrolling. This is done for all loops and usually makes programs run more slowly. `-funroll-all-loops' implies `-fstrength-reduce' as well as `-frerun-cse-after-loop'. `-fno-peephole' Disable any machine-specific peephole optimizations. `-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 guessing the path a branch might take. With `-fbranch-probabilities', GNU CC 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.