src/compiler-rt/lib/builtins/popcountsi2.c

   1 /* ===-- popcountsi2.c - Implement __popcountsi2 ---------------------------===
   2  *
   3  *                     The LLVM Compiler Infrastructure
   4  *
   5  * This file is dual licensed under the MIT and the University of Illinois Open
   6  * Source Licenses. See LICENSE.TXT for details.
   7  *
   8  * ===----------------------------------------------------------------------===
   9  *
  10  * This file implements __popcountsi2 for the compiler_rt library.
  11  *
  12  * ===----------------------------------------------------------------------===
  13  */
  14
  15 #include "int_lib.h"
  16
  17 /* Returns: count of 1 bits */
  18
  19 COMPILER_RT_ABI si_int
  20 __popcountsi2(si_int a)
  21 {
  22     su_int x = (su_int)a;
  23     x = x - ((x >> 1) & 0x55555555);
  24     /* Every 2 bits holds the sum of every pair of bits */
  25     x = ((x >> 2) & 0x33333333) + (x & 0x33333333);
  26     /* Every 4 bits holds the sum of every 4-set of bits (3 significant bits) */
  27     x = (x + (x >> 4)) & 0x0F0F0F0F;
  28     /* Every 8 bits holds the sum of every 8-set of bits (4 significant bits) */
  29     x = (x + (x >> 16));
  30     /* The lower 16 bits hold two 8 bit sums (5 significant bits).*/
  31     /*    Upper 16 bits are garbage */
  32     return (x + (x >> 8)) & 0x0000003F;  /* (6 significant bits) */
  33 }