lib/count-leading-zeros.h

   1 /* count-leading-zeros.h -- counts the number of leading 0 bits in a word.
   2    Copyright (C) 2012-2024 Free Software Foundation, Inc.
   3
   4    This file is free software: you can redistribute it and/or modify
   5    it under the terms of the GNU Lesser General Public License as
   6    published by the Free Software Foundation; either version 2.1 of the
   7    License, or (at your option) any later version.
   8
   9    This file 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 Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser 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 /* This file uses _GL_INLINE_HEADER_BEGIN, _GL_INLINE.  */
  23 #if !_GL_CONFIG_H_INCLUDED
  24  #error "Please include config.h first."
  25 #endif
  26
  27 #include <limits.h>
  28 #include <stdlib.h>
  29
  30 _GL_INLINE_HEADER_BEGIN
  31 #ifndef COUNT_LEADING_ZEROS_INLINE
  32 # define COUNT_LEADING_ZEROS_INLINE _GL_INLINE
  33 #endif
  34
  35 #ifdef __cplusplus
  36 extern "C" {
  37 #endif
  38
  39 /* Assuming the GCC builtin is BUILTIN and the MSC builtin is MSC_BUILTIN,
  40    expand to code that computes the number of leading zeros of the local
  41    variable 'x' of type TYPE (an unsigned integer type) and return it
  42    from the current function.  */
  43 #if __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4) \
  44     || (__clang_major__ >= 4)
  45 # define COUNT_LEADING_ZEROS(BUILTIN, MSC_BUILTIN, TYPE)                \
  46   return x ? BUILTIN (x) : CHAR_BIT * sizeof x;
  47 #elif _MSC_VER
  48 extern unsigned char _BitScanReverse (unsigned long *, unsigned long);
  49 # pragma intrinsic (_BitScanReverse)
  50 # if defined _M_X64
  51 extern unsigned char _BitScanReverse64 (unsigned long *, unsigned long long);
  52 #  pragma intrinsic (_BitScanReverse64)
  53 # endif
  54 # define COUNT_LEADING_ZEROS(BUILTIN, MSC_BUILTIN, TYPE)                \
  55     do                                                                  \
  56       {                                                                 \
  57         unsigned long result;                                           \
  58         if (MSC_BUILTIN (&result, x))                                   \
  59           return CHAR_BIT * sizeof x - 1 - result;                      \
  60         return CHAR_BIT * sizeof x;                                     \
  61       }                                                                 \
  62     while (0)
  63 #else
  64 # define COUNT_LEADING_ZEROS(BUILTIN, MSC_BUILTIN, TYPE)                \
  65     do                                                                  \
  66       {                                                                 \
  67         int count;                                                      \
  68         unsigned int leading_32;                                        \
  69         if (! x)                                                        \
  70           return CHAR_BIT * sizeof x;                                   \
  71         for (count = 0;                                                 \
  72              (leading_32 = ((x >> (sizeof (TYPE) * CHAR_BIT - 32))      \
  73                             & 0xffffffffU),                             \
  74               count < CHAR_BIT * sizeof x - 32 && !leading_32);         \
  75              count += 32)                                               \
  76           x = x << 31 << 1;                                             \
  77         return count + count_leading_zeros_32 (leading_32);             \
  78       }                                                                 \
  79     while (0)
  80
  81 /* Compute and return the number of leading zeros in X,
  82    where 0 < X < 2**32.  */
  83 COUNT_LEADING_ZEROS_INLINE int
  84 count_leading_zeros_32 (unsigned int x)
  85 {
  86   /* <https://github.com/gibsjose/BitHacks>
  87      <https://www.fit.vutbr.cz/~ibarina/pub/bithacks.pdf> */
  88   static const char de_Bruijn_lookup[32] = {
  89     31, 22, 30, 21, 18, 10, 29, 2, 20, 17, 15, 13, 9, 6, 28, 1,
  90     23, 19, 11, 3, 16, 14, 7, 24, 12, 4, 8, 25, 5, 26, 27, 0
  91   };
  92
  93   x |= x >> 1;
  94   x |= x >> 2;
  95   x |= x >> 4;
  96   x |= x >> 8;
  97   x |= x >> 16;
  98   return de_Bruijn_lookup[((x * 0x07c4acddU) & 0xffffffffU) >> 27];
  99 }
 100 #endif
 101
 102 /* Compute and return the number of leading zeros in X. */
 103 COUNT_LEADING_ZEROS_INLINE int
 104 count_leading_zeros (unsigned int x)
 105 {
 106   COUNT_LEADING_ZEROS (__builtin_clz, _BitScanReverse, unsigned int);
 107 }
 108
 109 /* Compute and return the number of leading zeros in X. */
 110 COUNT_LEADING_ZEROS_INLINE int
 111 count_leading_zeros_l (unsigned long int x)
 112 {
 113   COUNT_LEADING_ZEROS (__builtin_clzl, _BitScanReverse, unsigned long int);
 114 }
 115
 116 /* Compute and return the number of leading zeros in X. */
 117 COUNT_LEADING_ZEROS_INLINE int
 118 count_leading_zeros_ll (unsigned long long int x)
 119 {
 120 #if (defined _MSC_VER && !defined __clang__) && !defined _M_X64
 121   /* 32-bit MSVC does not have _BitScanReverse64, only _BitScanReverse.  */
 122   unsigned long result;
 123   if (_BitScanReverse (&result, (unsigned long) (x >> 32)))
 124     return CHAR_BIT * sizeof x - 1 - 32 - result;
 125   if (_BitScanReverse (&result, (unsigned long) x))
 126     return CHAR_BIT * sizeof x - 1 - result;
 127   return CHAR_BIT * sizeof x;
 128 #else
 129   COUNT_LEADING_ZEROS (__builtin_clzll, _BitScanReverse64,
 130                        unsigned long long int);
 131 #endif
 132 }
 133
 134 #ifdef __cplusplus
 135 }
 136 #endif
 137
 138 _GL_INLINE_HEADER_END
 139
 140 #endif /* COUNT_LEADING_ZEROS_H */