Besides just timing it (excellent!) did you take a look at what it compiles to? https://godbolt.org/z/C0ZeE5 https://godbolt.org/z/C0ZeE5 It's faster because

Besides just timing it (excellent!) did you take a look at what it compiles to? https://godbolt.org/z/C0ZeE5 https://godbolt.org/z/C0ZeE5 It's faster because both GCC and Clang now optimize loops with n&(n-1) to use AVX2 SIMD! I haven't looked closely to confirm, but I think they may in fact even do Harley-Seal for "naive popcount" loops. C is no longer portable assembly. If you want to test whether a particular algorithm is faster than another, you probably need to write assembly --- or at least confirm that the compiler did what you thought it did.