wsutil/bits_count_ones.h

   1 /** @file
   2  *
   3  * Wireshark - Network traffic analyzer
   4  * By Gerald Combs <gerald@wireshark.org>
   5  * Copyright 1998 Gerald Combs
   6  *
   7  * SPDX-License-Identifier: GPL-2.0-or-later
   8  */
   9
  10 #ifndef __WSUTIL_BITS_COUNT_ONES_H__
  11 #define __WSUTIL_BITS_COUNT_ONES_H__
  12
  13 #include <inttypes.h>
  14
  15 /*
  16  * The variable-precision SWAR algorithm is an interesting way to count
  17  * the number of bits set in an integer:
  18  *
  19  *     https://www.playingwithpointers.com/blog/swar.html
  20  *
  21  * See
  22  *
  23  *     https://gcc.gnu.org/bugzilla/show_bug.cgi?id=36041
  24  *     https://danluu.com/assembly-intrinsics/
  25  *
  26  * for discussions of various forms of population-counting code on x86.
  27  *
  28  * See
  29  *
  30  *     https://docs.microsoft.com/en-us/cpp/intrinsics/popcnt16-popcnt-popcnt64
  31  *
  32  * for MSVC's population count intrinsics.
  33  *
  34  * Note that not all x86 processors support the POPCOUNT instruction.
  35  *
  36  * Other CPUs may have population count instructions as well.
  37  */
  38
  39 static inline int
  40 ws_count_ones(const uint64_t x)
  41 {
  42         uint64_t bits = x;
  43
  44         bits = bits - ((bits >> 1) & UINT64_C(0x5555555555555555));
  45         bits = (bits & UINT64_C(0x3333333333333333)) + ((bits >> 2) & UINT64_C(0x3333333333333333));
  46         bits = (bits + (bits >> 4)) & UINT64_C(0x0F0F0F0F0F0F0F0F);
  47
  48         return (int)((bits * UINT64_C(0x0101010101010101)) >> 56);
  49 }
  50
  51 #endif /* __WSUTIL_BITS_COUNT_ONES_H__ */