__builtin_constant_p
—
GNU extension to determine compile time constants
int
__builtin_constant_p
(value);
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))
This is a non-standard, compiler-specific extension.