#include <stdio.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 = t >> 1;

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

  return (t + 1) & x;
}

int main() {
  printf("%.8X %.8X\n", 0x06050403, leftmost_one(0x06050403));
  printf("%.8X %.8X\n", 0xF0F0F0F0, leftmost_one(0xF0F0F0F0));
  printf("%.8X %.8X\n", 0x00000000, leftmost_one(0x00000000));
  printf("%.8X %.8X\n", 0x00000001, leftmost_one(0x00000001));
}