Fast sign of integer in C

Question

There is a sign function in C:

int sign(int x)
{
    if(x > 0) return 1;
    if(x < 0) return -1;
    return 0;
}

Unfortunately, comp

Answer 1

If s(x) is a function that returns the sign-bit of x (you implemented it by ((unsigned int)x)>>31), you can combine s(x) and s(-x) in some way. Here is a "truth table":

x > 0: s(x) = 0; s(-x) = 1; your function must return 1

x < 0: s(x) = 1; s(-x) = 0; your function must return -1

x = 0: s(x) = 0; s(-x) = 0; your function must return 0

So you can combine them in the following way:

s(-x) - s(x)