Cross platform implementation of ffsll. Find the index of the first set bit of a 64 bit unsigned int. It uses the intrinsics available via clang, gcc and mvcc.

bit_index.cpp

#include <stdint.h>
#include <iostream>
#include <boost/lexical_cast.hpp>

#ifndef ffsll
#ifdef _WIN32
inline char
ffsll(uint64_t x)
{
    char index = 0x00;
    _BitScanForward64(&index, x);
    return index;
}
#endif
#define ffsll(x) __builtin_ffsll(x)
#endif

int
main()
{
    std::cout << boost::lexical_cast<std::string>(((0 & -0) * 0x07EDD5E59A4E28C2ULL) >> 58) << std::endl;
    std::cout << boost::lexical_cast<std::string>(ffsll(0)) << std::endl;
}