Fast and efficient log2 function with gcc builtin

log2.c

#include <stdio.h>
#include <stdint.h>

// log 2 - with ceil rounding
int32_t log2_uint32 (uint32_t u)
{
  if (u == 0) { return INT32_MIN; }

  return ((int32_t)31 - (int32_t)__builtin_clz(u));
 }

int main(int argc, char *argv[])
{
  printf("\n%d", log2_uint32(0x0));
  printf("\n%d", log2_uint32(0x1));
  printf("\n%d", log2_uint32(0x2));
  printf("\n%d", log2_uint32(0x3));
  printf("\n%d", log2_uint32(0x4));
  printf("\n%d", log2_uint32(0x10));
  printf("\n%d", log2_uint32(0x20));
  printf("\n%d", log2_uint32(0x40));
  printf("\n%d", log2_uint32(0x80));
  printf("\n%d", log2_uint32(7000));
  return 0;
}