forked from osmocom/wireshark
52 lines
1.3 KiB
C
52 lines
1.3 KiB
C
/** @file
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*
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* Wireshark - Network traffic analyzer
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* By Gerald Combs <gerald@wireshark.org>
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* Copyright 1998 Gerald Combs
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*
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* SPDX-License-Identifier: GPL-2.0-or-later
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*/
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#ifndef __WSUTIL_BITS_COUNT_ONES_H__
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#define __WSUTIL_BITS_COUNT_ONES_H__
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#include <glib.h>
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/*
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* The variable-precision SWAR algorithm is an interesting way to count
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* the number of bits set in an integer:
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*
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* https://www.playingwithpointers.com/blog/swar.html
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*
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* See
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*
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* https://gcc.gnu.org/bugzilla/show_bug.cgi?id=36041
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* https://danluu.com/assembly-intrinsics/
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*
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* for discussions of various forms of population-counting code on x86.
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*
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* See
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*
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* https://docs.microsoft.com/en-us/cpp/intrinsics/popcnt16-popcnt-popcnt64
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*
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* for MSVC's population count intrinsics.
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*
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* Note that not all x86 processors support the POPCOUNT instruction.
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*
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* Other CPUs may have population count instructions as well.
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*/
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static inline int
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ws_count_ones(const guint64 x)
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{
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guint64 bits = x;
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bits = bits - ((bits >> 1) & G_GUINT64_CONSTANT(0x5555555555555555));
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bits = (bits & G_GUINT64_CONSTANT(0x3333333333333333)) + ((bits >> 2) & G_GUINT64_CONSTANT(0x3333333333333333));
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bits = (bits + (bits >> 4)) & G_GUINT64_CONSTANT(0x0F0F0F0F0F0F0F0F);
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return (int)((bits * G_GUINT64_CONSTANT(0x0101010101010101)) >> 56);
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}
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#endif /* __WSUTIL_BITS_COUNT_ONES_H__ */
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