Simplest way to check if two integers have same sign?

Question

Which is the simplest way to check if two integers have same sign? Is there any short bitwise trick to do this?

Answer 1

Assuming twos complement arithmetic (http://en.wikipedia.org/wiki/Two_complement):

inline bool same_sign(int x, int y) {
    return (x^y) >= 0;
}

This can take as little as two instructions and less than 1ns on a modern processor with optimization.

Not assuming twos complement arithmetic:

inline bool same_sign(int x, int y) {
    return (x<0) == (y<0);
}

This may require one or two extra instructions and take a little longer.

Using multiplication is a bad idea because it is vulnerable to overflow.