ou can use the built-in function __builtin_constant_p to determine if a value is known to be constant at compile
time and hence that GCC 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 GCC cannot prove it is a constant with the specified value of the -O option.