2.12.4 __builtin_constant_p()

__builtin_constant_p is a compiler builtin function. It takes a single numeric argument and returns 1 if the argument is known to be a compile-time constant. A return value of 0 means that the compiler can not determine that the argument is a compile-time constant or not. A typical use of this builtin function is in manual compile-time optimizations in macros.