lib/count-leading-zeros.h

   1 /* count-leading-zeros.h -- counts the number of leading 0 bits in a word.
   2    Copyright (C) 2012-2018 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 /* Assuming the GCC builtin is BUILTIN and the MSC builtin is MSC_BUILTIN,
  34    expand to code that computes the number of leading zeros of the local
  35    variable 'x' of type TYPE (an unsigned integer type) and return it
  36    from the current function.  */
  37 #if __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4)
  38 # define COUNT_LEADING_ZEROS(BUILTIN, MSC_BUILTIN, TYPE)                \
  39   return x ? BUILTIN (x) : CHAR_BIT * sizeof x;
  40 #elif _MSC_VER
  41 # pragma intrinsic _BitScanReverse
  42 # pragma intrinsic _BitScanReverse64
  43 # define COUNT_LEADING_ZEROS(BUILTIN, MSC_BUILTIN, TYPE)                \
  44     do                                                                  \
  45       {                                                                 \
  46         unsigned long result;                                           \
  47         return MSC_BUILTIN (&result, x) ? result : CHAR_BIT * sizeof x; \
  48       }                                                                 \
  49     while (0)
  50 #else
  51 # define COUNT_LEADING_ZEROS(BUILTIN, MSC_BUILTIN, TYPE)                \
  52     do                                                                  \
  53       {                                                                 \
  54         int count;                                                      \
  55         unsigned int leading_32;                                        \
  56         if (! x)                                                        \
  57           return CHAR_BIT * sizeof x;                                   \
  58         for (count = 0;                                                 \
  59              (leading_32 = ((x >> (sizeof (TYPE) * CHAR_BIT - 32))      \
  60                             & 0xffffffffU),                             \
  61               count < CHAR_BIT * sizeof x - 32 && !leading_32);         \
  62              count += 32)                                               \
  63           x = x << 31 << 1;                                             \
  64         return count + count_leading_zeros_32 (leading_32);             \
  65       }                                                                 \
  66     while (0)
  67
  68 /* Compute and return the number of leading zeros in X,
  69    where 0 < X < 2**32.  */
  70 COUNT_LEADING_ZEROS_INLINE int
  71 count_leading_zeros_32 (unsigned int x)
  72 {
  73   /* <https://github.com/gibsjose/BitHacks>
  74      <http://www.fit.vutbr.cz/~ibarina/pub/bithacks.pdf> */
  75   static const char de_Bruijn_lookup[32] = {
  76     31, 22, 30, 21, 18, 10, 29, 2, 20, 17, 15, 13, 9, 6, 28, 1,
  77     23, 19, 11, 3, 16, 14, 7, 24, 12, 4, 8, 25, 5, 26, 27, 0
  78   };
  79
  80   x |= x >> 1;
  81   x |= x >> 2;
  82   x |= x >> 4;
  83   x |= x >> 8;
  84   x |= x >> 16;
  85   return de_Bruijn_lookup[((x * 0x07c4acddU) & 0xffffffffU) >> 27];
  86 }
  87 #endif
  88
  89 /* Compute and return the number of leading zeros in X. */
  90 COUNT_LEADING_ZEROS_INLINE int
  91 count_leading_zeros (unsigned int x)
  92 {
  93   COUNT_LEADING_ZEROS (__builtin_clz, _BitScanReverse, unsigned int);
  94 }
  95
  96 /* Compute and return the number of leading zeros in X. */
  97 COUNT_LEADING_ZEROS_INLINE int
  98 count_leading_zeros_l (unsigned long int x)
  99 {
 100   COUNT_LEADING_ZEROS (__builtin_clzl, _BitScanReverse, unsigned long int);
 101 }
 102
 103 #if HAVE_UNSIGNED_LONG_LONG_INT
 104 /* Compute and return the number of leading zeros in X. */
 105 COUNT_LEADING_ZEROS_INLINE int
 106 count_leading_zeros_ll (unsigned long long int x)
 107 {
 108   COUNT_LEADING_ZEROS (__builtin_clzll, _BitScanReverse64,
 109                        unsigned long long int);
 110 }
 111 #endif
 112
 113 _GL_INLINE_HEADER_END
 114
 115 #endif /* COUNT_LEADING_ZEROS_H */