Count leading zeros in a portable manner

clz.cpp

#include <iostream>
#include <limits>
#include <algorithm>
#include <cstdint>

int clz(uint32_t u) {
    uint32_t a = 0;
    uint32_t b = 32;
    uint32_t all = 0xFFFFFFFF;
    #pragma unroll
    for (int i = 0; i < 5; i++) {
        auto m = (a + b) / 2;
        auto mask = all << m;
        if (u & mask) a = m + 1;
        else          b = m;
    }
    return 32 - b;
}

bool test() {
    for (uint32_t i = 1;
         i != std::numeric_limits<uint32_t>::max();
         i++) {
        if (__builtin_clz(i) != clz(i)) {
            std::cout << i << " : " << __builtin_clz(i) << " " << clz(i) << std::endl;
            return false;
        }
    }
    return true;
}

int main(int argc, char** argv) {
    test();
    return 0;
}