#include <assert.h>
#include <limits.h>

/*
 * Generate mask indicating leftmost 1 in x.  Assume w=32.
 * For example, 0xFF00 -> 0x8000, and 0x6600 --> 0x4000.
 * If x = 0, then return 0.
 */
int leftmost_one(unsigned x) {
  unsigned t = x;
  t >>= 1;

  t |= t >> 1;
  t |= t >> 2;
  t |= t >> 4;
  t |= t >> 8;
  t |= t >> 16;

  return (t + 1) & x;
}

int main() {
  for (unsigned x = 0; ; x++) {
    assert((x == 0 && leftmost_one(x) == 0) || (x != 0 && leftmost_one(x) == (1 << (31 - __builtin_clz(x)))));

    if (x == UINT_MAX) {
      break;
    }
  }
}