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(gcc-295.info)Other Builtins

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Other built-in functions provided by GNU CC
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   GNU CC provides a large number of built-in functions other than the
ones mentioned above.  Some of these are for internal use in the
processing of exceptions or variable-length argument lists and will not
be documented here because they may change from time to time; we do not
recommend general use of these functions.

   The remaining functions are provided for optimization purposes.

   GNU CC includes builtin versions of many of the functions in the
standard C library.  These will always be treated as having the same
meaning as the C library function even if you specify the
`-fno-builtin' (Note: C Dialect Options.) option.  These functions
correspond to the C library functions `alloca', `ffs', `abs', `fabsf',
`fabs', `fabsl', `labs', `memcpy', `memcmp', `strcmp', `strcpy',
`strlen', `sqrtf', `sqrt', `sqrtl', `sinf', `sin', `sinl', `cosf',
`cos', and `cosl'.

   You can use the builtin function `__builtin_constant_p' to determine
if a value is known to be constant at compile-time and hence that GNU
CC can perform constant-folding on expressions involving that value.
The argument of the function is the value to test.  The function
returns the integer 1 if the argument is known to be a compile-time
constant and 0 if it is not known to be a compile-time constant.  A
return of 0 does not indicate that the value is *not* a constant, but
merely that GNU CC cannot prove it is a constant with the specified
value of the `-O' option.

   You would typically use this function in an embedded application
where memory was a critical resource.  If you have some complex
calculation, you may want it to be folded if it involves constants, but
need to call a function if it does not.  For example:

     #define Scale_Value(X)  \
       (__builtin_constant_p (X) ? ((X) * SCALE + OFFSET) : Scale (X))

   You may use this builtin function in either a macro or an inline
function.  However, if you use it in an inlined function and pass an
argument of the function as the argument to the builtin, GNU CC will
never return 1 when you call the inline function with a string constant
or constructor expression (Note: Constructors.) and will not return 1
when you pass a constant numeric value to the inline function unless you
specify the `-O' option.