popcount32.c revision 1.4 
1/*	$NetBSD: popcount32.c,v 1.4 2011/08/21 21:25:04 dholland Exp $	*/
2/*-
3 * Copyright (c) 2009 The NetBSD Foundation, Inc.
4 * All rights reserved.
5 *
6 * This code is derived from software contributed to The NetBSD Foundation
7 * by Joerg Sonnenberger.
8 *
9 * Redistribution and use in source and binary forms, with or without
10 * modification, are permitted provided that the following conditions
11 * are met:
12 *
13 * 1. Redistributions of source code must retain the above copyright
14 *    notice, this list of conditions and the following disclaimer.
15 * 2. Redistributions in binary form must reproduce the above copyright
16 *    notice, this list of conditions and the following disclaimer in
17 *    the documentation and/or other materials provided with the
18 *    distribution.
19 *
20 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
21 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
22 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
23 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
24 * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
25 * INCIDENTAL, SPECIAL, EXEMPLARY OR CONSEQUENTIAL DAMAGES (INCLUDING,
26 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
27 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
28 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
29 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
30 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
31 * SUCH DAMAGE.
32 */
33
34#include <sys/cdefs.h>
35__RCSID("$NetBSD: popcount32.c,v 1.4 2011/08/21 21:25:04 dholland Exp $");
36
37#if !defined(_KERNEL) && !defined(_STANDALONE)
38#include <limits.h>
39#include <stdint.h>
40#include <strings.h>
41#else
42#include <lib/libkern/libkern.h>
43#include <machine/limits.h>
44#endif
45
46/*
47 * This a hybrid algorithm for bit counting between parallel counting and
48 * using multiplication.  The idea is to sum up the bits in each Byte, so
49 * that the final accumulation can be done with a single multiplication.
50 * If the platform has a slow multiplication instruction, it can be replaced
51 * by the commented out version below.
52 */
53
54unsigned int
55popcount32(uint32_t v)
56{
57	unsigned int c;
58
59	v = v - ((v >> 1) & 0x55555555U);
60	v = (v & 0x33333333U) + ((v >> 2) & 0x33333333U);
61	v = (v + (v >> 4)) & 0x0f0f0f0fU;
62	c = (v * 0x01010101U) >> 24;
63	/*
64	 * v = (v >> 16) + v;
65	 * v = (v >> 8) + v;
66	 * c = v & 255;
67	 */
68
69	return c;
70}
71
72#if UINT_MAX == 0xffffffffU
73__strong_alias(popcount, popcount32)
74#endif
75
76#if ULONG_MAX == 0xffffffffU
77__strong_alias(popcountl, popcount32)
78#endif
79