lib/ffsl.h

   1 /* ffsl.h -- find the first set bit in a word.
   2    Copyright (C) 2011-2019 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 /* 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 FUNC || !defined TYPE
  31 # error
  32 #endif
  33
  34 int
  35 FUNC (TYPE i)
  36 {
  37 #if (__GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4)) && defined GCC_BUILTIN
  38   return GCC_BUILTIN (i);
  39 #else
  40   unsigned TYPE j = i;
  41   /* Split j into chunks, and look at one chunk after the other.  */
  42   enum { chunk_bits = CHAR_BIT * sizeof (unsigned int) };
  43   /* The number of chunks is ceil (sizeof (TYPE) / sizeof (unsigned int))
  44      = (sizeof (TYPE) - 1) / sizeof (unsigned int) + 1. */
  45   enum { chunk_count = (sizeof (TYPE) - 1) / sizeof (unsigned int) + 1 };
  46
  47   if (chunk_count > 1)
  48     {
  49       size_t k;
  50
  51       /* It is tempting to write  if (!j)  here, but if we do this,
  52          Solaris 10/x86 "cc -O" miscompiles the code.  */
  53       if (!i)
  54         return 0;
  55       /* Unroll the first loop round.  k = 0.  */
  56       if ((unsigned int) j)
  57         return ffs ((unsigned int) j);
  58       /* Generic loop.  */
  59       for (k = 1; k < chunk_count - 1; k++)
  60         if ((unsigned int) (j >> (k * chunk_bits)) != 0)
  61           return k * chunk_bits + ffs ((unsigned int) (j >> (k * chunk_bits)));
  62     }
  63   /* Last loop round.  k = chunk_count - 1.  */
  64   return (chunk_count - 1) * chunk_bits
  65          + ffs ((unsigned int) (j >> ((chunk_count - 1) * chunk_bits)));
  66 #endif
  67 }