1. — Built-in Function: int __builtin_constant_p (exp)
You 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.
2. __ARMEB__:
if defined big endian, it's defined by the compiler when it is invoked with -EB (for big-endian).
