Answer by Piotr Siupa for What is the fastest/most efficient way to find the highest set bit (msb) in an integer in C?

Here is a fast solution for C that works in GCC and Clang; ready to be copied and pasted.

#include <limits.h>unsigned int fls(const unsigned int value){    return (unsigned int)1 << ((sizeof(unsigned int) * CHAR_BIT) - __builtin_clz(value) - 1);}unsigned long flsl(const unsigned long value){    return (unsigned long)1 << ((sizeof(unsigned long) * CHAR_BIT) - __builtin_clzl(value) - 1);}unsigned long long flsll(const unsigned long long value){    return (unsigned long long)1 << ((sizeof(unsigned long long) * CHAR_BIT) - __builtin_clzll(value) - 1);}

And a little improved version for C++.

#include <climits>constexpr unsigned int fls(const unsigned int value){    return (unsigned int)1 << ((sizeof(unsigned int) * CHAR_BIT) - __builtin_clz(value) - 1);}constexpr unsigned long fls(const unsigned long value){    return (unsigned long)1 << ((sizeof(unsigned long) * CHAR_BIT) - __builtin_clzl(value) - 1);}constexpr unsigned long long fls(const unsigned long long value){    return (unsigned long long)1 << ((sizeof(unsigned long long) * CHAR_BIT) - __builtin_clzll(value) - 1);}

The code assumes that value won't be 0. If you want to allow 0, you need to modify it.