lib/count-leading-zeros.h

   1 /* count-leading-zeros.h -- counts the number of leading 0 bits in a word.
   2    Copyright (C) 2012-2021 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 Eric Blake.  */
  18
  19 #ifndef COUNT_LEADING_ZEROS_H
  20 #define COUNT_LEADING_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_LEADING_ZEROS_INLINE
  30 # define COUNT_LEADING_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 leading 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_LEADING_ZEROS(BUILTIN, MSC_BUILTIN, TYPE)                \
  44   return x ? BUILTIN (x) : CHAR_BIT * sizeof x;
  45 #elif _MSC_VER
  46 # pragma intrinsic _BitScanReverse
  47 # pragma intrinsic _BitScanReverse64
  48 # define COUNT_LEADING_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_LEADING_ZEROS(BUILTIN, MSC_BUILTIN, TYPE)                \
  57     do                                                                  \
  58       {                                                                 \
  59         int count;                                                      \
  60         unsigned int leading_32;                                        \
  61         if (! x)                                                        \
  62           return CHAR_BIT * sizeof x;                                   \
  63         for (count = 0;                                                 \
  64              (leading_32 = ((x >> (sizeof (TYPE) * CHAR_BIT - 32))      \
  65                             & 0xffffffffU),                             \
  66               count < CHAR_BIT * sizeof x - 32 && !leading_32);         \
  67              count += 32)                                               \
  68           x = x << 31 << 1;                                             \
  69         return count + count_leading_zeros_32 (leading_32);             \
  70       }                                                                 \
  71     while (0)
  72
  73 /* Compute and return the number of leading zeros in X,
  74    where 0 < X < 2**32.  */
  75 COUNT_LEADING_ZEROS_INLINE int
  76 count_leading_zeros_32 (unsigned int x)
  77 {
  78   /* <https://github.com/gibsjose/BitHacks>
  79      <https://www.fit.vutbr.cz/~ibarina/pub/bithacks.pdf> */
  80   static const char de_Bruijn_lookup[32] = {
  81     31, 22, 30, 21, 18, 10, 29, 2, 20, 17, 15, 13, 9, 6, 28, 1,
  82     23, 19, 11, 3, 16, 14, 7, 24, 12, 4, 8, 25, 5, 26, 27, 0
  83   };
  84
  85   x |= x >> 1;
  86   x |= x >> 2;
  87   x |= x >> 4;
  88   x |= x >> 8;
  89   x |= x >> 16;
  90   return de_Bruijn_lookup[((x * 0x07c4acddU) & 0xffffffffU) >> 27];
  91 }
  92 #endif
  93
  94 /* Compute and return the number of leading zeros in X. */
  95 COUNT_LEADING_ZEROS_INLINE int
  96 count_leading_zeros (unsigned int x)
  97 {
  98   COUNT_LEADING_ZEROS (__builtin_clz, _BitScanReverse, unsigned int);
  99 }
 100
 101 /* Compute and return the number of leading zeros in X. */
 102 COUNT_LEADING_ZEROS_INLINE int
 103 count_leading_zeros_l (unsigned long int x)
 104 {
 105   COUNT_LEADING_ZEROS (__builtin_clzl, _BitScanReverse, unsigned long int);
 106 }
 107
 108 /* Compute and return the number of leading zeros in X. */
 109 COUNT_LEADING_ZEROS_INLINE int
 110 count_leading_zeros_ll (unsigned long long int x)
 111 {
 112   COUNT_LEADING_ZEROS (__builtin_clzll, _BitScanReverse64,
 113                        unsigned long long int);
 114 }
 115
 116 #ifdef __cplusplus
 117 }
 118 #endif
 119
 120 _GL_INLINE_HEADER_END
 121
 122 #endif /* COUNT_LEADING_ZEROS_H */