Test ffsll

ffsll.cpp

#include <assert.h>
#include <inttypes.h>
#include <stddef.h>
#include <stdlib.h>
#include <stdio.h>

typedef uint64_t slot_t;
static constexpr size_t bits_per_slot = sizeof(slot_t) * 8;

static inline ssize_t count_0s_gcc(slot_t slot_val, size_t start_pos)
{

  size_t pos = __builtin_ffsll(slot_val >> start_pos);
  if (pos == 0)
    return sizeof(slot_t)*8 - start_pos;
  return pos - 1;
}

static inline ssize_t count_0s_other(slot_t slot_val, size_t start_pos)
{
  size_t pos = start_pos;
  slot_t mask = slot_t(1) << pos;
  while (pos < bits_per_slot && (slot_val & mask) == 0) {
    mask <<= 1;
    pos++;
  }
  return pos - start_pos;
}


int main(int argc, char** argv)
{
  slot_t v = 0;
  for (int i=0;i<100000000;i++)
    {
      size_t len = rand()%16 + 1;
      slot_t val = slot_t(0 - rand()%2) >> (bits_per_slot - len);
      v = (v << len) | val;
      //printf("%d %llx\n", len, val);
      //printf("%16.16llx\n", v);
      size_t start_pos = rand()%bits_per_slot;
      assert(count_0s_gcc(v, start_pos) ==
	     count_0s_other(v, start_pos));
    }
  return 0;
}