NetBSD 6.1.5 - man page for __builtin_constant_p (netbsd section 3)
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__BUILTIN_CONSTANT_P(3) BSD Library Functions Manual __BUILTIN_CONSTANT_P(3)
__builtin_constant_p -- GNU extension to determine compile time constants
The __builtin_constant_p() is a GNU extension for determining whether a value is known to be
constant at compile time. The function is closely related to the concept of ``constant
folding'' used by modern optimizing compilers.
If the value is known to be a compile-time constant, a value 1 is returned. If
__builtin_constant_p() returns 0, the value is not a compile-time constant in the sense that
gcc(1) was unable to determine whether the value is constant or not.
A typical example of the use of __builtin_constant_p() involves a situation where it may be
desirable to fold the computation if it involves a constant, but a function call is needed
otherwise. For instance, bswap16(3) is defined in NetBSD as:
#define bswap16(x) \
(__builtin_constant_p((x)) ? \
__byte_swap_u16_constant(x) : __BYTE_SWAP_U16_VARIABLE(x))
gcc(1), __builtin_object_size(3), __builtin_return_address(3)
This is a non-standard, compiler-specific extension.
BSD December 19, 2010 BSD
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