What is different about C++ math.h abs() compared to my abs()

Question

I am currently writing some glsl like vector math classes in C++, and I just implemented an abs() function like this:

template
static         


        

Answer 1

There are well-known tricks for computing the absolute value of a two's complement signed number. If the number is negative, flip all the bits and add 1, that is, xor with -1 and subtract -1. If it is positive, do nothing, that is, xor with 0 and subtract 0.

int my_abs(int x)
{
    int s = x >> 31;
    return (x ^ s) - s;
}