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1 office 1 /*
2 * bits_count_ones.h
3 *
4 * Wireshark - Network traffic analyzer
5 * By Gerald Combs <gerald@wireshark.org>
6 * Copyright 1998 Gerald Combs
7 *
8 * This program is free software; you can redistribute it and/or
9 * modify it under the terms of the GNU General Public License
10 * as published by the Free Software Foundation; either version 2
11 * of the License, or (at your option) any later version.
12 *
13 * This program is distributed in the hope that it will be useful,
14 * but WITHOUT ANY WARRANTY; without even the implied warranty of
15 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
16 * GNU General Public License for more details.
17 *
18 * You should have received a copy of the GNU General Public License
19 * along with this program; if not, write to the Free Software
20 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
21 *
22 */
23  
24 #ifndef __WSUTIL_BITS_COUNT_ONES_H__
25 #define __WSUTIL_BITS_COUNT_ONES_H__
26  
27 #include "config.h"
28  
29 #include <glib.h>
30  
31 /*
32 * The variable-precision SWAR algorithm is an interesting way to count
33 * the number of bits set in an integer. While its performance is very
34 * good (two times faster than gcc's __builtin_popcount [1] and
35 * 16 instructions when compiled with gcc -O3)
36 * http://playingwithpointers.com/swar.html
37 */
38  
39 static inline int
40 ws_count_ones(const guint64 x)
41 {
42 guint64 bits = x;
43  
44 bits = bits - ((bits >> 1) & G_GUINT64_CONSTANT(0x5555555555555555));
45 bits = (bits & G_GUINT64_CONSTANT(0x3333333333333333)) + ((bits >> 2) & G_GUINT64_CONSTANT(0x3333333333333333));
46 bits = (bits + (bits >> 4)) & G_GUINT64_CONSTANT(0x0F0F0F0F0F0F0F0F);
47  
48 return (int)((bits * G_GUINT64_CONSTANT(0x0101010101010101)) >> 56);
49 }
50  
51 #endif /* __WSUTIL_BITS_COUNT_ONES_H__ */