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      1 /* ffs.c -- find the first set bit in a word.
      2    Copyright (C) 2011-2022 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 #include <config.h>
     20 
     21 /* Specification.  */
     22 #include <strings.h>
     23 
     24 #include <limits.h>
     25 
     26 #if defined _MSC_VER && !(__clang_major__ >= 4)
     27 # include <intrin.h>
     28 #endif
     29 
     30 int
     31 ffs (int i)
     32 {
     33 #if __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4) || (__clang_major__ >= 4)
     34   return __builtin_ffs (i);
     35 #elif defined _MSC_VER
     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 #else
     44   /* <https://github.com/gibsjose/BitHacks>
     45      gives this deBruijn constant for a branch-less computation, although
     46      that table counted trailing zeros rather than bit position.  This
     47      requires 32-bit int, we fall back to a naive algorithm on the rare
     48      platforms where that assumption is not true.  */
     49   if (CHAR_BIT * sizeof i == 32)
     50     {
     51       static unsigned int table[] = {
     52         1, 2, 29, 3, 30, 15, 25, 4, 31, 23, 21, 16, 26, 18, 5, 9,
     53         32, 28, 14, 24, 22, 20, 17, 8, 27, 13, 19, 7, 12, 6, 11, 10
     54       };
     55       unsigned int u = i;
     56       unsigned int bit = u & -u;
     57       return table[(bit * 0x077cb531U) >> 27] - !i;
     58     }
     59   else
     60     {
     61       unsigned int j;
     62       for (j = 0; j < CHAR_BIT * sizeof i; j++)
     63         if (i & (1U << j))
     64           return j + 1;
     65       return 0;
     66     }
     67 #endif
     68 }
     69