lib/integer_length.c

   1 /* integer_length - find most significant bit in an 'unsigned int'.
   2    Copyright (C) 2011-2018 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 Bruno Haible <bruno@clisp.org>, 2011.  */
  18
  19 #include <config.h>
  20
  21 /* Specification.  */
  22 #include "integer_length.h"
  23
  24 #include <limits.h>
  25
  26 #include "float+.h"
  27
  28 /* MSVC with option -fp:strict refuses to compile constant initializers that
  29    contain floating-point operations.  Pacify this compiler.  */
  30 #ifdef _MSC_VER
  31 # pragma fenv_access (off)
  32 #endif
  33
  34 #define NBITS (sizeof (unsigned int) * CHAR_BIT)
  35
  36 int
  37 integer_length (unsigned int x)
  38 {
  39 #if __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4)
  40   if (x == 0)
  41     return 0;
  42   else
  43     return NBITS - __builtin_clz (x);
  44 #else
  45 # if defined DBL_EXPBIT0_WORD && defined DBL_EXPBIT0_BIT
  46   if (NBITS <= DBL_MANT_BIT)
  47     {
  48       /* Use 'double' operations.
  49          Assumes an IEEE 754 'double' implementation.  */
  50 #  define DBL_EXP_MASK ((DBL_MAX_EXP - DBL_MIN_EXP) | 7)
  51 #  define DBL_EXP_BIAS (DBL_EXP_MASK / 2 - 1)
  52 #  define NWORDS \
  53     ((sizeof (double) + sizeof (unsigned int) - 1) / sizeof (unsigned int))
  54       typedef union { double value; unsigned int word[NWORDS]; }
  55               memory_double;
  56
  57       if (x == 0)
  58         return 0;
  59       else
  60         {
  61           memory_double m;
  62           unsigned int exponent;
  63
  64           if (1)
  65             {
  66               /* Use a single integer to floating-point conversion.  */
  67               m.value = x;
  68             }
  69           else
  70             {
  71               /* Use a single floating-point subtraction.  */
  72               /* 2^(DBL_MANT_DIG-1).  */
  73               static const double TWO_DBL_MANT_DIG =
  74                 /* Assume DBL_MANT_DIG <= 5 * 31.
  75                    Use the identity
  76                    n = floor(n/5) + floor((n+1)/5) + ... + floor((n+4)/5).  */
  77                 (double) (1U << ((DBL_MANT_DIG - 1) / 5))
  78                 * (double) (1U << ((DBL_MANT_DIG - 1 + 1) / 5))
  79                 * (double) (1U << ((DBL_MANT_DIG - 1 + 2) / 5))
  80                 * (double) (1U << ((DBL_MANT_DIG - 1 + 3) / 5))
  81                 * (double) (1U << ((DBL_MANT_DIG - 1 + 4) / 5));
  82
  83               /* Construct 2^(DBL_MANT_DIG-1) + x by hand.  */
  84               m.word[DBL_EXPBIT0_WORD] =
  85                 (DBL_MANT_DIG + DBL_EXP_BIAS) << DBL_EXPBIT0_BIT;
  86               m.word[1 - DBL_EXPBIT0_WORD] = x;
  87
  88               /* Subtract 2^(DBL_MANT_DIG-1).  */
  89               m.value = m.value - TWO_DBL_MANT_DIG;
  90             }
  91
  92           exponent =
  93             (m.word[DBL_EXPBIT0_WORD] >> DBL_EXPBIT0_BIT) & DBL_EXP_MASK;
  94           return exponent - DBL_EXP_BIAS;
  95         }
  96     }
  97   else
  98 # endif
  99     if (NBITS == 32)
 100       {
 101         /* 6 comparisons.  */
 102         if (x != 0)
 103           {
 104             int result = 1;
 105             if (x >= 0x10000)
 106               {
 107                 x = x >> 16;
 108                 result += 16;
 109               }
 110             if (x >= 0x100)
 111               {
 112                 x = x >> 8;
 113                 result += 8;
 114               }
 115             if (x >= 0x10)
 116               {
 117                 x = x >> 4;
 118                 result += 4;
 119               }
 120             if (x >= 0x4)
 121               {
 122                 x = x >> 2;
 123                 result += 2;
 124               }
 125             if (x >= 0x2)
 126               {
 127                 x = x >> 1;
 128                 result += 1;
 129               }
 130             return result;
 131           }
 132         else
 133           return 0;
 134       }
 135     else
 136       {
 137         /* Naive loop.
 138            Works for any value of NBITS.  */
 139         int j;
 140
 141         for (j = NBITS - 1; j >= 0; j--)
 142           if (x & (1U << j))
 143             return j + 1;
 144         return 0;
 145       }
 146 #endif
 147 }