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/** @file
*
* Wireshark - Network traffic analyzer
* By Gerald Combs <gerald@wireshark.org>
* Copyright 1998 Gerald Combs
*
* SPDX-License-Identifier: GPL-2.0-or-later
*/
#ifndef __WSUTIL_BITS_COUNT_ONES_H__
#define __WSUTIL_BITS_COUNT_ONES_H__
#include <inttypes.h>
/*
* The variable-precision SWAR algorithm is an interesting way to count
* the number of bits set in an integer:
*
* https://www.playingwithpointers.com/blog/swar.html
*
* See
*
* https://gcc.gnu.org/bugzilla/show_bug.cgi?id=36041
* https://danluu.com/assembly-intrinsics/
*
* for discussions of various forms of population-counting code on x86.
*
* See
*
* https://docs.microsoft.com/en-us/cpp/intrinsics/popcnt16-popcnt-popcnt64
*
* for MSVC's population count intrinsics.
*
* Note that not all x86 processors support the POPCOUNT instruction.
*
* Other CPUs may have population count instructions as well.
*/
static inline int
ws_count_ones(const uint64_t x)
{
uint64_t bits = x;
bits = bits - ((bits >> 1) & G_GUINT64_CONSTANT(0x5555555555555555));
bits = (bits & G_GUINT64_CONSTANT(0x3333333333333333)) + ((bits >> 2) & G_GUINT64_CONSTANT(0x3333333333333333));
bits = (bits + (bits >> 4)) & G_GUINT64_CONSTANT(0x0F0F0F0F0F0F0F0F);
return (int)((bits * G_GUINT64_CONSTANT(0x0101010101010101)) >> 56);
}
#endif /* __WSUTIL_BITS_COUNT_ONES_H__ */
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