Or better yet x |= MAC_computed[i] ^ MAC_received[i]; If they're not careful and the numeric type being subtracted is wider than CPU registers, depending

Or better yet x |= MAC_computed[i] ^ MAC_received[i]; If they're not careful and the numeric type being subtracted is wider than CPU registers, depending on architecture, the compiler-generated carry code to implement wider-than-register subtraction may introduce timing attacks. Wider-than-register xor is much much less likely to have such issues.