It's UB for positive integers, so the compiler can assume it never happens. It's correct for negative integers, so the compiler just optimizes it for negative i

It's UB for positive integers, so the compiler can assume it never happens. It's correct for negative integers, so the compiler just optimizes it for negative integers and ignores the positive ones - in this case with the end result that it works for positive integers as well. If the argument is changed to unsigned int then it seems to be correct and portable because the C standard[1] requires UINT_MAX to be of the form 2^n - 1 (which makes it wrap from odd to even when overflowing). So the one with signed argument is UB for positive integers but happens to work by accident (but the nasal demons may still lurk just around the corner, e.g. the compiler may also assume that it's never called with a positive argument which may affect the compilation of callers). The one with unsigned argument is correct because unsigned overflow is well-defined to wrap around to zero, combined with the restrictions on max unsigned int. [1]: From C17 6.2.6.2: For unsigned integer types other than unsigned char, the bits of the object representation shall be divided into two groups: value bits and padding bits (there need not be any of the latter). If there are N value bits, each bit shall represent a different power of 2 between 1 and 2*(N-1), so that objects of that type shall be capable of representing values from 0 to 2*(N-1) using a pure binary representation; this shall be known as the value representation.