/* Population count from Hacker's Delight. */
always_inline uword
count_set_bits (uword x)
{
#ifdef __POPCNT__
#if uword_bits == 64
  return __builtin_popcountll (x);
#else
  return __builtin_popcount (x);
#endif
#else
#if uword_bits == 64
  const uword c1 = 0x5555555555555555;
  const uword c2 = 0x3333333333333333;
  const uword c3 = 0x0f0f0f0f0f0f0f0f;
#else
  const uword c1 = 0x55555555;
  const uword c2 = 0x33333333;
  const uword c3 = 0x0f0f0f0f;
#endif

  /* Sum 1 bit at a time. */
  x = x - ((x >> (uword) 1) & c1);

  /* 2 bits at a time. */
  x = (x & c2) + ((x >> (uword) 2) & c2);

  /* 4 bits at a time. */
  x = (x + (x >> (uword) 4)) & c3;

  /* 8, 16, 32 bits at a time. */
  x = x + (x >> (uword) 8);
  x = x + (x >> (uword) 16);
#if uword_bits == 64
  x = x + (x >> (uword) 32);
#endif

  return x & (2 * BITS (uword) - 1);
#endif
}
 

計算1個字長記憶體裡置1的bit數目。 在編譯器沒有內建支援的時候，使用了Hacker's Delight演算法，這個演算法以後詳述。