C log2 behavior test

/*
log2 behavior tester
copyright 2021 Damian Yerrick
insert GNU All-Permissive License here

build and run:
gcc -Wall -Wextra -Og -fsanitize=undefined log2test.c -lm -o log2test && log2test | less

Notice how the inaccuracy creeps in at 25 bits for cast to float
or 49 bits for cast to double.

*/

#include <stdio.h>
#include <inttypes.h>  // same as stdint.h plus
#include <math.h>

/**
 * calculates floor(log[2](i)), or -1 if i < 1
 * but can a compiler recognize it and convert it to clz?
 */
signed log2ul (uint_fast32_t x) {
  signed result = 0;
  if (x < 3) return x - 1;  // 0->-1, 1->0, 2->1
  while (x > 1 << 16) { result += 16; x >>= 16; }
  while (x > 1 <<  4) { result +=  4; x >>=  4; }
  while (x > 1      ) { result +=  1; x >>=  1; }
  return result;
}

/**
 * calculates floor(log[2](i)), or -1 if i < 1
 * but can a compiler recognize it and convert it to clz?
 */
signed log2ull (unsigned long long x) {
  signed result = 0;
  if (x < 3) return x - 1;  // 0->-1, 1->0, 2->1
  while (x > 1 << 16) { result += 16; x >>= 16; }
  while (x > 1 <<  4) { result +=  4; x >>=  4; }
  while (x > 1      ) { result +=  1; x >>=  1; }
  return result;
}

void u32_trial(unsigned shiftamt) {
  uint32_t arg = (uint32_t)1 << (shiftamt - 1);  // e.g. 8->128
  arg = (arg - 1) * 2;  // e.g. 8->254
  printf("trial for log2(1 << %u) using uint32_t\n",
         shiftamt);
  for (unsigned b = 5; b > 0 && arg > 0; --b, ++arg) {
    signed loop_result = log2ul(arg);
    double float_result = log2((float)arg);
    signed floor_float = floor(float_result);
    double double_result = log2((double)arg);
    signed floor_double = floor(double_result);
    printf("log2(%10"PRIu32") = %2d int, %7.4f=>%2d float, %7.4f=>%2d double\n",
           arg, loop_result, float_result, floor_float,
           double_result, floor_double);
  }
}

void u64_trial(unsigned shiftamt) {
  uint64_t arg = (uint64_t)1 << (shiftamt - 1);  // e.g. 8->128
  arg = (arg - 1) * 2;  // e.g. 8->254
  printf("trial for log2(1 << %u) using uint64_t\n",
         shiftamt);
  for (unsigned b = 5; b > 0 && arg > 0; --b, ++arg) {
    signed loop_result = log2ul(arg);
    double float_result = log2((float)arg);
    signed floor_float = floor(float_result);
    double double_result = log2((double)arg);
    signed floor_double = floor(double_result);
    printf("log2(%10"PRIu64") = %2d int, %7.4f=>%2d float, %7.4f=>%2d double\n",
           arg, loop_result, float_result, floor_float,
           double_result, floor_double);
  }
}

int main(void) {
  for (unsigned i = 4; i <= 32; ++i) u32_trial(i);
  for (unsigned i = 4; i <= 64; ++i) u64_trial(i);
  return 0;
}