lib/ffsl.h

   1 /* ffsl.h -- find the first set bit in a word.
   2    Copyright (C) 2011-2024 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 /* This file is meant to be included by ffsl.c and ffsll.c, after
  20    they have defined FUNC and TYPE.  */
  21
  22 #include <config.h>
  23
  24 /* Specification.  */
  25 #include <string.h>
  26
  27 #include <limits.h>
  28 #include <strings.h>
  29
  30 #if defined _MSC_VER && !(__clang_major__ >= 4)
  31 # include <intrin.h>
  32 /* Copied from ffs.c.  */
  33 static inline int
  34 ffs (int i)
  35 {
  36   /* _BitScanForward
  37      <https://docs.microsoft.com/en-us/cpp/intrinsics/bitscanforward-bitscanforward64> */
  38   unsigned long bit;
  39   if (_BitScanForward (&bit, i))
  40     return bit + 1;
  41   else
  42     return 0;
  43 }
  44 #endif
  45
  46 #if !defined FUNC || !defined TYPE
  47 # error
  48 #endif
  49
  50 int
  51 FUNC (TYPE i)
  52 {
  53 #if __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4) \
  54     || (__clang_major__ >= 4)
  55   return GCC_BUILTIN (i);
  56 #elif defined _MSC_VER && defined MSVC_BUILTIN
  57   /* _BitScanForward, _BitScanForward64
  58      <https://docs.microsoft.com/en-us/cpp/intrinsics/bitscanforward-bitscanforward64> */
  59   unsigned long bit;
  60   if (MSVC_BUILTIN (&bit, i))
  61     return bit + 1;
  62   else
  63     return 0;
  64 #else
  65   unsigned TYPE j = i;
  66   /* Split j into chunks, and look at one chunk after the other.  */
  67   enum { chunk_bits = CHAR_BIT * sizeof (unsigned int) };
  68   /* The number of chunks is ceil (sizeof (TYPE) / sizeof (unsigned int))
  69      = (sizeof (TYPE) - 1) / sizeof (unsigned int) + 1. */
  70   enum { chunk_count = (sizeof (TYPE) - 1) / sizeof (unsigned int) + 1 };
  71
  72   if (chunk_count > 1)
  73     {
  74       size_t k;
  75
  76       /* It is tempting to write  if (!j)  here, but if we do this,
  77          Solaris 10/x86 "cc -O" miscompiles the code.  */
  78       if (!i)
  79         return 0;
  80       /* Unroll the first loop round.  k = 0.  */
  81       if ((unsigned int) j)
  82         return ffs ((unsigned int) j);
  83       /* Generic loop.  */
  84       for (k = 1; k < chunk_count - 1; k++)
  85         if ((unsigned int) (j >> (k * chunk_bits)) != 0)
  86           return k * chunk_bits + ffs ((unsigned int) (j >> (k * chunk_bits)));
  87     }
  88   /* Last loop round.  k = chunk_count - 1.  */
  89   return (chunk_count - 1) * chunk_bits
  90          + ffs ((unsigned int) (j >> ((chunk_count - 1) * chunk_bits)));
  91 #endif
  92 }