1  /* ffsl.h -- find the first set bit in a word.
       2     Copyright (C) 2011-2021 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  }