I'm not sure I understand what you're suggesting. x ^ y will obviously never be 0 when x != y, but there is no quick way to locate where they differ to turn it

I'm not sure I understand what you're suggesting. x ^ y will obviously never be 0 when x != y, but there is no quick way to locate where they differ to turn it into a mask. When x != y, either x - y or y - x will be negative and therefore have the MSB set.