gistfile1.txt

/tmp
➜ ./a.out
0
1
2
1
8
1
32

numbits.c

#include <stdio.h>

int p2(unsigned int n) {
  int r;
  __asm__( "popcnt %1, %0; "
      : "=r" (r)
      : "r" (n)
      : "%rdi" );
  return r;

}

int main() {

  printf("%d\n",p2(0));
  printf("%d\n",p2(2));
  printf("%d\n",p2(3));
  printf("%d\n",p2(4));
  printf("%d\n",p2(255));
  printf("%d\n",p2(256));
  printf("%d\n",p2(-1));

  return 0;

}