wsutil/bits_ctz.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_CTZ_H__
  11 #define __WSUTIL_BITS_CTZ_H__
  12
  13 #include <inttypes.h>
  14
  15 /* ws_ctz == trailing zeros == position of lowest set bit [0..63] */
  16 /* ws_ilog2 == position of highest set bit == 63 - leading zeros [0..63] */
  17
  18 /* The return value of both ws_ctz and ws_ilog2 is undefined for x == 0 */
  19
  20 #if defined(__GNUC__) && ((__GNUC__ > 3) || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4))
  21
  22 static inline int
  23 ws_ctz(uint64_t x)
  24 {
  25         return __builtin_ctzll(x);
  26 }
  27
  28 static inline int
  29 ws_ilog2(uint64_t x)
  30 {
  31         return 63 - __builtin_clzll(x);
  32 }
  33
  34 #else
  35
  36 static inline int
  37 __ws_ctz32(uint32_t x)
  38 {
  39         /* From http://graphics.stanford.edu/~seander/bithacks.html#ZerosOnRightMultLookup */
  40         static const uint8_t table[32] = {
  41                 0,   1, 28,  2, 29, 14, 24, 3, 30, 22, 20, 15, 25, 17,  4, 8,
  42                 31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18,  6, 11,  5, 10, 9
  43         };
  44
  45         return table[((uint32_t)((x & -(int32_t)x) * 0x077CB531U)) >> 27];
  46 }
  47
  48 static inline int
  49 ws_ctz(uint64_t x)
  50 {
  51         uint32_t hi = x >> 32;
  52         uint32_t lo = (uint32_t) x;
  53
  54         if (lo == 0)
  55                 return 32 + __ws_ctz32(hi);
  56         else
  57                 return __ws_ctz32(lo);
  58 }
  59
  60 static inline int
  61 __ws_ilog2_32(uint32_t x)
  62 {
  63         /* From http://graphics.stanford.edu/~seander/bithacks.html#IntegerLogDeBruijn */
  64         static const uint8_t table[32] =        {
  65                 0,  9,  1, 10, 13, 21,  2, 29, 11, 14, 16, 18, 22, 25,  3, 30,
  66                 8, 12, 20, 28, 15, 17, 24,  7, 19, 27, 23,  6, 26,  5,  4, 31
  67         };
  68
  69         x |= x >> 1;
  70         x |= x >> 2;
  71         x |= x >> 4;
  72         x |= x >> 8;
  73         x |= x >> 16;
  74
  75         return table[((uint32_t)(x * 0x07C4ACDDU)) >> 27];
  76 }
  77
  78 static inline int
  79 ws_ilog2(uint64_t x)
  80 {
  81         uint32_t hi = x >> 32;
  82         uint32_t lo = (uint32_t) x;
  83
  84         if (hi == 0)
  85                 return __ws_ilog2_32(lo);
  86         else
  87                 return 32 + __ws_ilog2_32(hi);
  88 }
  89
  90 #endif
  91
  92 #endif /* __WSUTIL_BITS_CTZ_H__ */