Find the left-most bit

lmb.cpp

#include <cstdio>
#include <cstdint>
#include <cstdlib>

void
show_bit(uint32_t v) {
  for (int i = 0; i < 32; i++) {
    printf("%d", (v & (1ll << (31 - i)) ? 1 : 0));
  }
  printf("\n");
}

uint32_t
lmb(uint32_t v) {
  if (v == 0)return 0;
  else {
    return 1 << (31 ^ __builtin_clz(v));
  }
}

int
main(void) {
  for (uint32_t i = 0; i < 5; i++) {
    int v = rand();
    printf("\n");
    show_bit(v);
    show_bit(lmb(v));
  }
}

$ ./a.out
00000000000000000100000110100111
00000000000000000100000000000000

00010000110101100011101011110001
00010000000000000000000000000000

01100000101101111010110011011001
01000000000000000000000000000000

00111010101101010000110000101010
00100000000000000000000000000000

01000100001100011011011110000010
01000000000000000000000000000000