xref: /src/common/lib/libc/string/popcount32.c

/*	$NetBSD: popcount32.c,v 1.1 2009/07/21 14:55:32 joerg Exp $	*/
/*-
 * Copyright (c) 2009 The NetBSD Foundation, Inc.
 * All rights reserved.
 *
 * This code is derived from software contributed to The NetBSD Foundation
 * by Joerg Sonnenberger.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
 * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY OR CONSEQUENTIAL DAMAGES (INCLUDING,
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

#include <sys/cdefs.h>
__RCSID("$NetBSD: popcount32.c,v 1.1 2009/07/21 14:55:32 joerg Exp $");

#if !defined(_KERNEL) && !defined(_STANDALONE)
#include <strings.h>
#else
#include <lib/libkern/libkern.h>
#endif

/*
 * This a hybrid algorithm for bit counting between parallel counting and
 * using multiplication.  The idea is to sum up the bits in each Byte, so
 * that the final accumulation can be done with a single multiplication.
 * If the platform has a slow multiplication instruction, it can be replaced
 * by the commented out version below.
 */

unsigned int
popcount32(uint32_t v)
{
	unsigned int c;

	v = v - ((v >> 1) & 0x55555555U);
	v = (v & 0x33333333U) + ((v >> 2) & 0x33333333U);
	v = (v + (v >> 4)) & 0x0f0f0f0fU;
	c = (v * 0x01010101U) >> 24;
	/*
	 * v = (v >> 16) + v;
	 * v = (v >> 8) + v;
	 * c = v & 255;
	 */

	return c;
}

#if UINT_MAX == 0xffffffffUL
__strong_alias(popcount, popcount32);
#endif

#if ULONG_MAX == 0xffffffffU
__strong_alias(popcountl, popcount32);
#endif