lib/count-trailing-zeros.h

   1 /* count-trailing-zeros.h -- counts the number of trailing 0 bits in a word.
   2    Copyright 2013-2020 Free Software Foundation, Inc.
   3
   4    This program is free software: you can redistribute it and/or modify
   5    it under the terms of the GNU General Public License as published by
   6    the Free Software Foundation; either version 3 of the License, or
   7    (at your option) any later version.
   8
   9    This program is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12    GNU General Public License for more details.
  13
  14    You should have received a copy of the GNU General Public License
  15    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
  16
  17 /* Written by Paul Eggert.  */
  18
  19 #ifndef COUNT_TRAILING_ZEROS_H
  20 #define COUNT_TRAILING_ZEROS_H 1
  21
  22 #include <limits.h>
  23 #include <stdlib.h>
  24
  25 #ifndef _GL_INLINE_HEADER_BEGIN
  26  #error "Please include config.h first."
  27 #endif
  28 _GL_INLINE_HEADER_BEGIN
  29 #ifndef COUNT_TRAILING_ZEROS_INLINE
  30 # define COUNT_TRAILING_ZEROS_INLINE _GL_INLINE
  31 #endif
  32
  33 #ifdef __cplusplus
  34 extern "C" {
  35 #endif
  36
  37 /* Assuming the GCC builtin is BUILTIN and the MSC builtin is MSC_BUILTIN,
  38    expand to code that computes the number of trailing zeros of the local
  39    variable 'x' of type TYPE (an unsigned integer type) and return it
  40    from the current function.  */
  41 #if __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4) \
  42     || (__clang_major__ >= 4)
  43 # define COUNT_TRAILING_ZEROS(BUILTIN, MSC_BUILTIN, TYPE)               \
  44   return x ? BUILTIN (x) : CHAR_BIT * sizeof x;
  45 #elif _MSC_VER
  46 # pragma intrinsic _BitScanForward
  47 # pragma intrinsic _BitScanForward64
  48 # define COUNT_TRAILING_ZEROS(BUILTIN, MSC_BUILTIN, TYPE)               \
  49     do                                                                  \
  50       {                                                                 \
  51         unsigned long result;                                           \
  52         return MSC_BUILTIN (&result, x) ? result : CHAR_BIT * sizeof x; \
  53       }                                                                 \
  54     while (0)
  55 #else
  56 # define COUNT_TRAILING_ZEROS(BUILTIN, MSC_BUILTIN, TYPE)               \
  57     do                                                                  \
  58       {                                                                 \
  59         int count = 0;                                                  \
  60         if (! x)                                                        \
  61           return CHAR_BIT * sizeof x;                                   \
  62         for (count = 0;                                                 \
  63              (count < CHAR_BIT * sizeof x - 32                          \
  64               && ! (x & 0xffffffffU));                                  \
  65              count += 32)                                               \
  66           x = x >> 31 >> 1;                                             \
  67         return count + count_trailing_zeros_32 (x);                     \
  68       }                                                                 \
  69     while (0)
  70
  71 /* Compute and return the number of trailing zeros in the least
  72    significant 32 bits of X.  One of these bits must be nonzero.  */
  73 COUNT_TRAILING_ZEROS_INLINE int
  74 count_trailing_zeros_32 (unsigned int x)
  75 {
  76   /* <https://github.com/gibsjose/BitHacks>
  77      <https://www.fit.vutbr.cz/~ibarina/pub/bithacks.pdf> */
  78   static const char de_Bruijn_lookup[32] = {
  79     0, 1, 28, 2, 29, 14, 24, 3, 30, 22, 20, 15, 25, 17, 4, 8,
  80     31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18, 6, 11, 5, 10, 9
  81   };
  82   return de_Bruijn_lookup[(((x & -x) * 0x077cb531U) & 0xffffffffU) >> 27];
  83 }
  84 #endif
  85
  86 /* Compute and return the number of trailing zeros in X. */
  87 COUNT_TRAILING_ZEROS_INLINE int
  88 count_trailing_zeros (unsigned int x)
  89 {
  90   COUNT_TRAILING_ZEROS (__builtin_ctz, _BitScanForward, unsigned int);
  91 }
  92
  93 /* Compute and return the number of trailing zeros in X. */
  94 COUNT_TRAILING_ZEROS_INLINE int
  95 count_trailing_zeros_l (unsigned long int x)
  96 {
  97   COUNT_TRAILING_ZEROS (__builtin_ctzl, _BitScanForward, unsigned long int);
  98 }
  99
 100 /* Compute and return the number of trailing zeros in X. */
 101 COUNT_TRAILING_ZEROS_INLINE int
 102 count_trailing_zeros_ll (unsigned long long int x)
 103 {
 104   COUNT_TRAILING_ZEROS (__builtin_ctzll, _BitScanForward64,
 105                         unsigned long long int);
 106 }
 107
 108 #ifdef __cplusplus
 109 }
 110 #endif
 111
 112 _GL_INLINE_HEADER_END
 113
 114 #endif