lib/memchr.c

/* Copyright (C) 1991, 1993, 1996-1997, 1999-2000, 2003-2004, 2006, 2008-2026
   Free Software Foundation, Inc.

   Based on strlen implementation by Torbjorn Granlund (tege@sics.se),
   with help from Dan Sahlin (dan@sics.se) and
   commentary by Jim Blandy (jimb@ai.mit.edu);
   adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu),
   and implemented by Roland McGrath (roland@ai.mit.edu).

   NOTE: The canonical source of this file is maintained with the GNU C Library.
   Bugs can be reported to bug-glibc@prep.ai.mit.edu.

   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/>.  */

#ifndef _LIBC
# include <config.h>
#endif

#include <string.h>

#include <stddef.h>

#if defined _LIBC
# include <memcopy.h>
#else
# define reg_char char
#endif

#include <limits.h>

#if HAVE_BP_SYM_H || defined _LIBC
# include <bp-sym.h>
#else
# define BP_SYM(sym) sym
#endif

#undef __memchr
#ifdef _LIBC
# undef memchr
#endif

#ifndef weak_alias
# define __memchr memchr
#endif

/* Search no more than N bytes of S for C.  */
void *
__memchr (void const *s, int c_in, size_t n)
{
  /* On 32-bit hardware, choosing longword to be a 32-bit unsigned
     long instead of a 64-bit uintmax_t tends to give better
     performance.  On 64-bit hardware, unsigned long is generally 64
     bits already.  Change this typedef to experiment with
     performance.  */
  typedef unsigned long int longword;

  unsigned reg_char c = (unsigned char) c_in;

  const longword *longword_ptr;

  /* Handle the first few bytes by reading one byte at a time.
     Do this until CHAR_PTR is aligned on a longword boundary.  */
  {
    const unsigned char *char_ptr;
    for (char_ptr = (const unsigned char *) s;
         n > 0 && (size_t) char_ptr % sizeof (longword) != 0;
         --n, ++char_ptr)
      if (*char_ptr == c)
        return (void *) char_ptr;

    longword_ptr = (const longword *) char_ptr;
  }

  /* All these elucidatory comments refer to 4-byte longwords,
     but the theory applies equally well to any size longwords.  */
  {
    /* Compute auxiliary longword values:
       repeated_one is a value which has a 1 in every byte.
       repeated_c has c in every byte.  */
    longword repeated_one = 0x01010101;
    longword repeated_c = c | (c << 8);
    repeated_c |= repeated_c << 16;
    if (0xffffffffU < (longword) -1)
      {
        repeated_one |= repeated_one << 31 << 1;
        repeated_c |= repeated_c << 31 << 1;
        if (8 < sizeof (longword))
          for (size_t i = 64; i < sizeof (longword) * 8; i *= 2)
            {
              repeated_one |= repeated_one << i;
              repeated_c |= repeated_c << i;
            }
      }

    /* Instead of the traditional loop which tests each byte, we will test a
       longword at a time.  The tricky part is testing if *any of the four*
       bytes in the longword in question are equal to c.  We first use an xor
       with repeated_c.  This reduces the task to testing whether *any of the
       four* bytes in longword1 is zero.

       We compute tmp =
         ((longword1 - repeated_one) & ~longword1) & (repeated_one << 7).
       That is, we perform the following operations:
         1. Subtract repeated_one.
         2. & ~longword1.
         3. & a mask consisting of 0x80 in every byte.
       Consider what happens in each byte:
         - If a byte of longword1 is zero, step 1 and 2 transform it into 0xff,
           and step 3 transforms it into 0x80.  A carry can also be propagated
           to more significant bytes.
         - If a byte of longword1 is nonzero, let its lowest 1 bit be at
           position k (0 <= k <= 7); so the lowest k bits are 0.  After step 1,
           the byte ends in a single bit of value 0 and k bits of value 1.
           After step 2, the result is just k bits of value 1: 2^k - 1.  After
           step 3, the result is 0.  And no carry is produced.
       So, if longword1 has only non-zero bytes, tmp is zero.
       Whereas if longword1 has a zero byte, call j the position of the least
       significant zero byte.  Then the result has a zero at positions 0, ...,
       j-1 and a 0x80 at position j.  We cannot predict the result at the more
       significant bytes (positions j+1..3), but it does not matter since we
       already have a non-zero bit at position 8*j+7.

       So, the test whether any byte in longword1 is zero is equivalent to
       testing whether tmp is nonzero.  */

    while (n >= sizeof (longword))
      {
        longword longword1 = *longword_ptr ^ repeated_c;

        if ((((longword1 - repeated_one) & ~longword1)
             & (repeated_one << 7)) != 0)
          break;
        longword_ptr++;
        n -= sizeof (longword);
      }
  }

  {
    const unsigned char *char_ptr = (const unsigned char *) longword_ptr;

    /* At this point, we know that either n < sizeof (longword), or one of the
       sizeof (longword) bytes starting at char_ptr is == c.  On little-endian
       machines, we could determine the first such byte without any further
       memory accesses, just by looking at the tmp result from the last loop
       iteration.  But this does not work on big-endian machines.  Choose code
       that works in both cases.  */

    for (; n > 0; --n, ++char_ptr)
      {
        if (*char_ptr == c)
          return (void *) char_ptr;
      }
  }

  return NULL;
}
#ifdef weak_alias
weak_alias (__memchr, BP_SYM (memchr))
#endif