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file gcc.texi.

   This file documents the use and the internals of the GNU compiler.

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File: gcc.info,  Node: Dialect Options,  Next: Warning Options,  Prev: Overall Options,  Up: Invoking GCC

Options Controlling Dialect

   The following options control the dialect of C or C++ that the
compiler accepts:

     Support all ANSI standard C programs.

     This turns off certain features of GNU C that are incompatible
     with ANSI C, such as the `asm', `inline' and `typeof' keywords, and
     predefined macros such as `unix' and `vax' that identify the type
     of system you are using.  It also enables the undesirable and
     rarely used ANSI trigraph feature, and disallows `$' as part of

     The alternate keywords `__asm__', `__extension__', `__inline__'
     and `__typeof__' continue to work despite `-ansi'.  You would not
     want to use them in an ANSI C program, of course, but it useful to
     put them in header files that might be included in compilations
     done with `-ansi'.  Alternate predefined macros such as `__unix__'
     and `__vax__' are also available, with or without `-ansi'.

     The `-ansi' option does not cause non-ANSI programs to be rejected
     gratuitously.  For that, `-pedantic' is required in addition to
     `-ansi'.  *Note Warning Options::.

     The macro `__STRICT_ANSI__' is predefined when the `-ansi' option
     is used.  Some header files may notice this macro and refrain from
     declaring certain functions or defining certain macros that the
     ANSI standard doesn't call for; this is to avoid interfering with
     any programs that might use these names for other things.

     The functions `alloca', `abort', `exit', and `_exit' are not
     builtin functions when `-ansi' is used.

     Treat certain member functions as virtual, implicitly (C++ only). 
     This applies to all member functions declared in the same class
     with a "method-call" operator method (except for constructor
     functions and `new' or `delete' member operators).  In effect, all
     of these methods become "implicitly virtual."

     This does not mean that all calls to these methods will be made
     through the internal table of virtual functions.  There are some
     circumstances under which it is obvious that a call to a given
     virtual function can be made directly, and in these cases the
     calls still go direct.

     The effect of making all methods of a class with a declared
     `operator->()()' implicitly virtual using `-fall-virtual' extends
     also to all non-constructor methods of any class derived from such
     a class.

     Permit the use of `$' in identifiers (C++ only).  You can also use
     `-fno-dollars-in-identifiers' to explicitly prohibit use of `$'. 
     (GNU C++ allows `$' by default on some target systems but not

     Permit implicit conversion of `int' to enumeration types (C++
     only).  Normally GNU C++ allows conversion of `enum' to `int', but
     not the other way around.

     Do not recognize `asm', `inline' or `typeof' as a keyword.  These
     words may then be used as identifiers.  You can use `__asm__',
     `__inline__' and `__typeof__' instead. `-ansi' implies `-fno-asm'.

     Don't recognize built-in functions that do not begin with two
     leading underscores. Currently, the functions affected include
     `_exit', `abort', `abs', `alloca', `cos', `exit', `fabs', `labs',
     `memcmp', `memcpy', `sin', `sqrt', `strcmp', `strcpy', and

     The `-ansi' option prevents `alloca' and `_exit' from being
     builtin functions.

     Treat a function declaration with no arguments, such as `int foo
     ();', as C would treat it--as saying nothing about the number of
     arguments or their types (C++ only).  Normally, such a declaration
     in C++ means that the function `foo' takes no arguments.

     Permit assignment to `this' (C++ only).  The incorporation of
     user-defined free store management into C++ has made assignment to
     `this' an anachronism.  Therefore, by default it is invalid to
     assign to `this' within a class member function.  However, for
     backwards compatibility, you can make it valid with

     Support ANSI C trigraphs.  You don't want to know about this
     brain-damage.  The `-ansi' option implies `-trigraphs'.

     Attempt to support some aspects of traditional C compilers.

        * All `extern' declarations take effect globally even if they
          are written inside of a function definition.  This includes
          implicit declarations of functions.

        * The keywords `typeof', `inline', `signed', `const' and
          `volatile' are not recognized.  (You can still use the
          alternative keywords such as `__typeof__', `__inline__', and
          so on.)

        * Comparisons between pointers and integers are always allowed.

        * Integer types `unsigned short' and `unsigned char' promote to
          `unsigned int'.

        * Out-of-range floating point literals are not an error.

        * String "constants" are not necessarily constant; they are
          stored in writable space, and identical looking constants are
          allocated separately.  (This is the same as the effect of

        * All automatic variables not declared `register' are preserved
          by `longjmp'.  Ordinarily, GNU C follows ANSI C: automatic
          variables not declared `volatile' may be clobbered.

        * In the preprocessor, comments convert to nothing at all,
          rather than to a space.  This allows traditional token

        * In the preprocessor, macro arguments are recognized within
          string constants in a macro definition (and their values are
          stringified, though without additional quote marks, when they
          appear in such a context).  The preprocessor always considers
          a string constant to end at a newline.

        * The predefined macro `__STDC__' is not defined when you use
          `-traditional', but `__GNUC__' is (since the GNU extensions
          which `__GNUC__' indicates are not affected by
          `-traditional').  If you need to write header files that work
          differently depending on whether `-traditional' is in use, by
          testing both of these predefined macros you can distinguish
          four situations: GNU C, traditional GNU C, other ANSI C
          compilers, and other old C compilers.

     You may wish to use `-fno-builtin' as well as `-traditional' if
     your program uses names that are normally GNU C builtin functions
     for other purposes of its own.

     Attempt to support some aspects of traditional C preprocessors.
     This includes the last three items in the table immediately above,
     but none of the other effects of `-traditional'.

     Allow conditional expressions with mismatched types in the second
     and third arguments.  The value of such an expression is void.

     Let the type `char' be unsigned, like `unsigned char'.

     Each kind of machine has a default for what `char' should be.  It
     is either like `unsigned char' by default or like `signed char' by

     Ideally, a portable program should always use `signed char' or
     `unsigned char' when it depends on the signedness of an object.
     But many programs have been written to use plain `char' and expect
     it to be signed, or expect it to be unsigned, depending on the
     machines they were written for.  This option, and its inverse, let
     you make such a program work with the opposite default.

     The type `char' is always a distinct type from each of `signed
     char' or `unsigned char', even though its behavior is always just
     like one of those two.

     Let the type `char' be signed, like `signed char'.

     Note that this is equivalent to `-fno-unsigned-char', which is the
     negative form of `-funsigned-char'.  Likewise, `-fno-signed-char'
     is equivalent to `-funsigned-char'.

     These options control whether a bitfield is signed or unsigned,
     when the declaration does not use either `signed' or `unsigned'. 
     By default, such a bitfield is signed, because this is consistent:
     the basic integer types such as `int' are signed types.

     However, when `-traditional' is used, bitfields are all unsigned
     no matter what.

     Store string constants in the writable data segment and don't
     uniquize them.  This is for compatibility with old programs which
     assume they can write into string constants.  `-traditional' also
     has this effect.

     Writing into string constants is a very bad idea; "constants"
     should be constant.

File: gcc.info,  Node: Warning Options,  Next: Debugging Options,  Prev: 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

   These options control the amount and kinds of warnings produced by

     Check the code for syntax errors, but don't emit any output.

     Inhibit all warning messages.

     Inhibit warning messages about the use of `#import'.

     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.

     Like `-pedantic', except that errors are produced rather than

     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 contains no side effects.

        * 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.

        * 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 };

     Warn about conversion between different enumeration types (C++

     Warn whenever a function or parameter is implicitly declared.

     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'.

     Warn whenever a local variable is unused aside from its
     declaration, whenever a function is declared static but never
     defined, and whenever a statement computes a result that is
     explicitly not used.

     If you want to prevent a warning for a particular variable, you
     can use this macro:

          #define USE(var) \
            static void * use_##var = (&use_##var, (void *) &var)
          USE (string);

     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.

     Warn whenever a comment-start sequence `/*' appears in a comment.

     Warn if any trigraphs are encountered (assuming they are enabled).

     Check calls to `printf' and `scanf', etc., to make sure that the
     arguments supplied have types appropriate to the format string

     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.

     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;
              case 2: x = 4;
              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

            int save_y;
            if (change_y) save_y = y, y = new_y;
            if (change_y) y = save_y;

     This has no bug because `save_y' is used only if it is set.

     Some spurious warnings can be avoided if you declare as `volatile'
     all the functions you use that never return. *Note Function

     Warn if parentheses are omitted in certain contexts.

     All of the above `-W' options combined.  These are all the options
     which pertain to usage that we recommend avoiding and that we
     believe is easy to avoid, even in conjunction with macros.

   The remaining `-W...' options are not implied by `-Wall' because
they warn about constructions that we consider reasonable to use, on
occasion, in clean programs.

     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'.

     Warn whenever a local variable shadows another local variable.

     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.

     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

     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 *'.

     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.

     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.

     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

     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.)

     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.)

     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.

     Warn if anything is declared more than once in the same scope,
     even in cases where multiple declaration is valid and changes

     Warn if an `extern' declaration is encountered within an function.

     Warn if a function can not be inlined, and either it was declared
     as inline, or else the `-finline-functions' option was given.

     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:

     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+', or
     `-gdwarf' (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.

     Produce debugging information in the native format (if that is
     supported), including GDB extensions if at all possible.

     Produce debugging information in stabs format (if that is
     supported), without GDB extensions.  This is the format used by
     DBX on most BSD systems.

     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.

     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.

     Produce debugging information in XCOFF format (if that is
     supported). This is the format used by the DBX debugger on IBM
     RS/6000 systems.

     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.

     Produce debugging information in DWARF format (if that is
     supported). This is the format used by SDB on most System V
     Release 4 systems.

     Produce debugging information in DWARF 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.

     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'.

     Generate extra code to write profile information suitable for the
     analysis program `prof'.

     Generate extra code to write profile information suitable for the
     analysis program `gprof'.

     Generate extra code to write profile information for basic blocks,
     which will record the number of times each basic block is executed.
     This data could be analyzed by a program like `tcov'.  Note,
     however, that the format of the data is not what `tcov' expects.
     Eventually GNU `gprof' should be extended to process this data.

     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:

          Dump all macro definitions, at the end of preprocessing, and
          write no output.

          Dump all macro names, at the end of preprocessing.

          Dump all macro definitions, at the end of preprocessing, in
          addition to normal output.

          Dump debugging information during parsing, to standard error.

          Dump after RTL generation, to `FILE.rtl'.

          Just generate RTL for a function instead of compiling it. 
          Usually used with `r'.

          Dump after first jump optimization, to `FILE.jump'.

          Dump after CSE (including the jump optimization that sometimes
          follows CSE), to `FILE.cse'.

          Dump after loop optimization, to `FILE.loop'.

          Dump after the second CSE pass (including the jump
          optimization that sometimes follows CSE), to `FILE.cse2'.

          Dump after flow analysis, to `FILE.flow'.

          Dump after instruction combination, to `FILE.combine'.

          Dump after the first instruction scheduling pass, to

          Dump after local register allocation, to

          Dump after global register allocation, to

          Dump after the second instruction scheduling pass, to

          Dump after last jump optimization, to `FILE.jump2'.

          Dump after delayed branch scheduling, to `FILE.dbr'.

          Dump after conversion from registers to stack, to

          Produce all the dumps listed above.

          Print statistics on memory usage, at the end of the run, to
          standard error.

          Annotate the assembler output with a comment indicating which
          pattern and alternative was used.

     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.

     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'.

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:

     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', only variables declared `register' are allocated in
     registers.  The resulting compiled code is a little worse than
     produced by PCC without `-O'.

     With `-O', the compiler tries to reduce code size and execution

     When `-O' is specified, `-fthread-jumps' and `-fdelayed-branch'
     are turned on.  On some machines other flags may also be turned on.

     Optimize even more.  Nearly all supported optimizations that do not
     involve a space-speed tradeoff are performed.  As compared to `-O',
     this option increases both compilation time and the performance of
     the generated code.

     `-O2' turns on all `-fFLAG' options that enable more optimization,
     except for `-funroll-loops', `-funroll-all-loops' and

     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.

     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.  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.

     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.

     Force memory operands to be copied into registers before doing
     arithmetic on them.  This may produce better code by making all
     memory references potential common subexpressions.  When they are
     not common subexpressions, instruction combination should
     eliminate the separate register-load.  I am interested in hearing
     about the difference this makes.

     Force memory address constants to be copied into registers before
     doing arithmetic on them.  This may produce better code just as
     `-fforce-mem' may.  I am interested in hearing about the
     difference this makes.

     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

     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::.

     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

     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.

     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.

     Don't make member functions inline by default merely because they
     are defined inside the class scope (C++ only).

     Do not put function addresses in registers; make each instruction
     that calls a constant function contain the function's address

     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.

     This option allows GCC to violate some ANSI or IEEE
     rules/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.

     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

     Elide constructors when this seems plausible (C++ only).  With this
     option, GNU C++ initializes `y' directly from the call to `foo'
     without going through a temporary in the following code:

          A foo ();
          A y = foo ();

     Without this option, GNU C++ first initializes `y' by calling the
     appropriate constructor for type `A'; then assigns the result of
     `foo' to a temporary; and, finally, replaces the initial value of
     `y' with the temporary.

     The default behavior (`-fno-elide-constructors') is specified by
     the draft ANSI C++ standard.  If your program's constructors have
     side effects, `-felide-constructors' can change your program's
     behavior, since some constructor calls may be omitted.

     Use heuristics to compile faster (C++ only).  These heuristics are
     not enabled by default, since they are only effective for certain
     input files.  Other input files compile more slowly.

     The first time the compiler must build a call to a member function
     (or reference to a data member), it must (1) determine whether the
     class implements member functions of that name; (2) resolve which
     member function to call (which involves figuring out what sorts of
     type conversions need to be made); and (3) check the visibility of
     the member function to the caller.  All of this adds up to slower
     compilation. Normally, the second time a call is made to that
     member function (or reference to that data member), it must go
     through the same lengthy process again.  This means that code like

          cout << "This " << p << " has " << n << " legs.\n";

     makes six passes through all three steps.  By using a software
     cache, a "hit" significantly reduces this cost.  Unfortunately,
     using the cache introduces another layer of mechanisms which must
     be implemented, and so incurs its own overhead. 
     `-fmemoize-lookups' enables the software cache.

     Because access privileges (visibility) to members and member
     functions may differ from one function context to the next, G++
     may need to flush the cache.  With the `-fmemoize-lookups' flag,
     the cache is flushed after every function that is compiled.  The
     `-fsave-memoized' flag enables the same software cache, but when
     the compiler determines that the context of the last function
     compiled would yield the same access privileges of the next
     function to compile, it preserves the cache. This is most helpful
     when defining many member functions for the same class: with the
     exception of member functions which are friends of other classes,
     each member function has exactly the same access privileges as
     every other, and the cache need not be flushed.

   The following options control specific optimizations.  The `-O2'
option turns on all of these optimizations except `-funroll-loops' and
`-funroll-all-loops'.  The `-O' option usually turns on the
`-fthread-jumps' and `-fdelayed-branch' options, but specific machines
may change the default optimizations.

   You can use the following flags in the rare cases when "fine-tuning"
of optimizations to be performed is desired.

     Perform the optimizations of loop strength reduction and
     elimination of iteration variables.

     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.

     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

     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'.

     Re-run common subexpression elimination after loop optimizations
     has been performed.

     Perform a number of minor optimizations that are relatively

     If supported for the target machine, attempt to reorder
     instructions to exploit instruction slots available after delayed
     branch instructions.

     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.

     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.

     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.

     Perform the optimization of loop unrolling.  This is only done for
     loops whose number of iterations can be determined at compile time
     or run time. `-funroll-loop' implies `-fstrength-reduce' and

     Perform the optimization of loop unrolling.  This is done for all
     loops and usually makes programs run more slowly. 
     `-funroll-all-loops' implies `-fstrength-reduce' and

     Disable any machine-specific peephole optimizations.

File: gcc.info,  Node: Preprocessor Options,  Next: Assembler Options,  Prev: Optimize Options,  Up: Invoking GCC

Options Controlling the Preprocessor

   These options control the C preprocessor, which is run on each C
source file before actual compilation.

   If you use the `-E' option, nothing is done except preprocessing.
Some of these options make sense only together with `-E' because they
cause the preprocessor output to be unsuitable for actual compilation.

`-include FILE'
     Process FILE as input before processing the regular input file. In
     effect, the contents of FILE are compiled first.  Any `-D' and
     `-U' options on the command line are always processed before
     `-include FILE', regardless of the order in which they are
     written.  All the `-include' and `-imacros' options are processed
     in the order in which they are written.

`-imacros FILE'
     Process FILE as input, discarding the resulting output, before
     processing the regular input file.  Because the output generated
     from FILE is discarded, the only effect of `-imacros FILE' is to
     make the macros defined in FILE available for use in the main

     Any `-D' and `-U' options on the command line are always processed
     before `-imacros FILE', regardless of the order in which they are
     written.  All the `-include' and `-imacros' options are processed
     in the order in which they are written.

     Do not search the standard system directories for header files. 
     Only the directories you have specified with `-I' options (and the
     current directory, if appropriate) are searched.  *Note Directory
     Options::, for information on `-I'.

     By using both `-nostdinc' and `-I-', you can limit the include-file
     search path to only those directories you specify explicitly.

     Do not search for header files in the C++-specific standard
     directories, but do still search the other standard directories.
     (This option is used when building `libg++'.)

     Do not predefine any nonstandard macros.  (Including architecture

     Run only the C preprocessor.  Preprocess all the C source files
     specified and output the results to standard output or to the
     specified output file.

     Tell the preprocessor not to discard comments.  Used with the `-E'

     Tell the preprocessor not to generate `#line' commands. Used with
     the `-E' option.

     Tell the preprocessor to output a rule suitable for `make'
     describing the dependencies of each object file.  For each source
     file, the preprocessor outputs one `make'-rule whose target is the
     object file name for that source file and whose dependencies are
     all the files `#include'd in it.  This rule may be a single line
     or may be continued with `\'-newline if it is long.  The list of
     rules is printed on standard output instead of the preprocessed C

     `-M' implies `-E'.

     Another way to specify output of a `make' rule is by setting the
     environment variable `DEPENDENCIES_OUTPUT' (*note Environment

     Like `-M' but the output mentions only the user header files
     included with `#include "FILE"'.  System header files included
     with `#include <FILE>' are omitted.

     Like `-M' but the dependency information is written to files with
     names made by replacing `.o' with `.d' at the end of the output
     file names.  This is in addition to compiling the input files as
     specified--`-MD' does not inhibit ordinary compilation the way
     `-M' does.

     The Mach utility `md' can be used to merge the `.d' files into a
     single dependency file suitable for using with the `make' command.

     Like `-MD' except mention only user header files, not system
     header files.

     Print the name of each header file used, in addition to other
     normal activities.

     Assert the answer ANSWER for QUESTION, in case it is tested with a
     preprocessor conditional such as `#if #QUESTION(ANSWER)'.  `-A-'
     disables the standard assertions that normally describe the target

     Define macro MACRO with the string `1' as its definition.

     Define macro MACRO as DEFN.  All instances of `-D' on the command
     line are processed before any `-U' options.

     Undefine macro MACRO.  `-U' options are evaluated after all `-D'
     options, but before any `-include' and `-imacros' options.

     Tell the preprocessor to output only a list of the macro
     definitions that are in effect at the end of preprocessing.  Used
     with the `-E' option.

     Tell the preprocessing to pass all macro definitions into the
     output, in their proper sequence in the rest of the output.

     Like `-dD' except that the macro arguments and contents are
     omitted. Only `#define NAME' is included in the output.

     Support ANSI C trigraphs.  You don't want to know about this
     brain-damage.  The `-ansi' option also has this effect.

File: gcc.info,  Node: Assembler Options,  Next: Link Options,  Prev: Preprocessor Options,  Up: Invoking GCC

Passing Options to the Assembler

     Pass OPTION as an option to the assembler.  If OPTION contains
     commas, it is split into multiple options at the commas.