> The proper way for a compiler to handle this, would be to add a single overflow check before the loop, which branches to a scalar translation of the loop. Most realistic code will need a scalar version anyway, to deal with the prolog/epilog of the unrolled loop for iteration counts that aren't multiples of the unrolling factor.

That's true, I agree that that would be a clever way to handle this particular case. It would still happily invoke undefined behavior if the indices don't match the array's length, of course. Many assumptions about the programmer knowing what they are doing goes into the optimization of C code.

> Surely you agree that treating unsigned overflow differently from signed does not make any sense semantically?

Yes. Silently wrapping unsigned overflow is also very often semantically meaningless.