wsutil/bits_count_ones.h

   1 /*
   2  * bits_count_ones.h
   3  *
   4  * $Id$
   5  *
   6  * Wireshark - Network traffic analyzer
   7  * By Gerald Combs <gerald@wireshark.org>
   8  * Copyright 1998 Gerald Combs
   9  *
  10  * This program is free software; you can redistribute it and/or
  11  * modify it under the terms of the GNU General Public License
  12  * as published by the Free Software Foundation; either version 2
  13  * of the License, or (at your option) any later version.
  14  *
  15  * This program is distributed in the hope that it will be useful,
  16  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  17  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  18  * GNU General Public License for more details.
  19  *
  20  * You should have received a copy of the GNU General Public License
  21  * along with this program; if not, write to the Free Software
  22  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  23  *
  24  */
  25
  26 #ifndef __WSUTIL_BITS_COUNT_ONES_H__
  27 #define __WSUTIL_BITS_COUNT_ONES_H__
  28
  29 #include "config.h"
  30
  31 #include <glib.h>
  32
  33 /*
  34  * The variable-precision SWAR algorithm is an interesting way to count
  35  * the number of bits set in an integer. While its performance is very
  36  * good (two times faster than gcc's __builtin_popcount [1] and
  37  * 16 instructions when compiled with gcc -O3)
  38  * http://playingwithpointers.com/swar.html
  39  */
  40
  41 static inline int
  42 ws_count_ones(const guint32 x)
  43 {
  44         int bits = x;
  45
  46         bits = bits - ((bits >> 1) & 0x55555555);
  47         bits = (bits & 0x33333333) + ((bits >> 2) & 0x33333333);
  48         bits = (bits + (bits >> 4)) & 0x0F0F0F0F;
  49
  50         return (bits * 0x01010101) >> 24;
  51 }
  52
  53 #endif /* __WSUTIL_BITS_COUNT_ONES_H__ */