The given case does "carry += *x++; carry += *y++;". One workaround is to rewrite that as "carry = (carry + *x++) + *y++;". The parens force the first addition to be 64-bit, which in turn forces the second to be 64-bit, while preventing the reassociation that allowed the erroneous 32-bit addition.

The compiler is allowed to combine and reassociate additions and other associative operations. However, some combinations of mixed-width operations, where the desired type is wider than int, may be reassociated incorrectly in a way that allows the operation in a narrow type than intended. In this case, it's expressed as "c += X; c += Y", where X and Y are int32_t and c is int64_t. The compiler is turning that into "c += X + Y", which adds X and Y as int32_t and thus doesn't keep the 33rd bit.