lib/ffsl.h

/* ffsl.h -- find the first set bit in a word.
   Copyright (C) 2011-2026 Free Software Foundation, Inc.

   This file is free software: you can redistribute it and/or modify
   it under the terms of the GNU Lesser General Public License as
   published by the Free Software Foundation; either version 2.1 of the
   License, or (at your option) any later version.

   This file is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public License
   along with this program.  If not, see <https://www.gnu.org/licenses/>.  */

/* Written by Eric Blake.  */

/* This file is meant to be included by ffsl.c and ffsll.c, after
   they have defined FUNC and TYPE.  */

#include <config.h>

/* Specification.  */
#include <string.h>

#include <limits.h>
#include <strings.h>

#if defined _MSC_VER && !(__clang_major__ >= 4)
# include <intrin.h>
/* Copied from ffs.c.  */
static inline int
ffs (int i)
{
  /* _BitScanForward
     <https://docs.microsoft.com/en-us/cpp/intrinsics/bitscanforward-bitscanforward64> */
  unsigned long bit;
  if (_BitScanForward (&bit, i))
    return bit + 1;
  else
    return 0;
}
#endif

#if !defined FUNC || !defined TYPE
# error
#endif

int
FUNC (TYPE i)
{
#if __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4) \
    || (__clang_major__ >= 4)
  return GCC_BUILTIN (i);
#elif defined _MSC_VER && defined MSVC_BUILTIN
  /* _BitScanForward, _BitScanForward64
     <https://docs.microsoft.com/en-us/cpp/intrinsics/bitscanforward-bitscanforward64> */
  unsigned long bit;
  if (MSVC_BUILTIN (&bit, i))
    return bit + 1;
  else
    return 0;
#else
  unsigned TYPE j = i;
  /* Split j into chunks, and look at one chunk after the other.  */
  enum { chunk_bits = CHAR_BIT * sizeof (unsigned int) };
  /* The number of chunks is ceil (sizeof (TYPE) / sizeof (unsigned int))
     = (sizeof (TYPE) - 1) / sizeof (unsigned int) + 1. */
  enum { chunk_count = (sizeof (TYPE) - 1) / sizeof (unsigned int) + 1 };

  if (chunk_count > 1)
    {
      /* It is tempting to write  if (!j)  here, but if we do this,
         Solaris 10/x86 "cc -O" miscompiles the code.  */
      if (!i)
        return 0;
      /* Unroll the first loop round.  k = 0.  */
      if ((unsigned int) j)
        return ffs ((unsigned int) j);
      /* Generic loop.  */
      for (size_t k = 1; k < chunk_count - 1; k++)
        if ((unsigned int) (j >> (k * chunk_bits)) != 0)
          return k * chunk_bits + ffs ((unsigned int) (j >> (k * chunk_bits)));
    }
  /* Last loop round.  k = chunk_count - 1.  */
  return (chunk_count - 1) * chunk_bits
         + ffs ((unsigned int) (j >> ((chunk_count - 1) * chunk_bits)));
#endif
}