But yes stdc_count_ones is indeed the intrinsic you'd want here, and only a few years after I stopped writing C, so thanks for mentioning that.
std::popcount is C++ but it's also kinda miserable that it took until C++ 20 and yet they still only landed the unsigned integer types, even though C++ 20 also insists the signed integers have two's complement representation, so the signed integers do have these desirable properties in fact but you can't use that.
