string/memchr.c

   1 /* Copyright (C) 1991-2020 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3    Based on strlen implementation by Torbjorn Granlund (tege@sics.se),
   4    with help from Dan Sahlin (dan@sics.se) and
   5    commentary by Jim Blandy (jimb@ai.mit.edu);
   6    adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu),
   7    and implemented by Roland McGrath (roland@ai.mit.edu).
   8
   9    The GNU C Library is free software; you can redistribute it and/or
  10    modify it under the terms of the GNU Lesser General Public
  11    License as published by the Free Software Foundation; either
  12    version 2.1 of the License, or (at your option) any later version.
  13
  14    The GNU C Library is distributed in the hope that it will be useful,
  15    but WITHOUT ANY WARRANTY; without even the implied warranty of
  16    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  17    Lesser General Public License for more details.
  18
  19    You should have received a copy of the GNU Lesser General Public
  20    License along with the GNU C Library; if not, see
  21    <https://www.gnu.org/licenses/>.  */
  22
  23 #ifndef _LIBC
  24 # include <config.h>
  25 #endif
  26
  27 #include <string.h>
  28
  29 #include <stddef.h>
  30
  31 #include <limits.h>
  32
  33 #undef __memchr
  34 #ifdef _LIBC
  35 # undef memchr
  36 #endif
  37
  38 #ifndef weak_alias
  39 # define __memchr memchr
  40 #endif
  41
  42 #ifndef MEMCHR
  43 # define MEMCHR __memchr
  44 #endif
  45
  46 /* Search no more than N bytes of S for C.  */
  47 void *
  48 MEMCHR (void const *s, int c_in, size_t n)
  49 {
  50   /* On 32-bit hardware, choosing longword to be a 32-bit unsigned
  51      long instead of a 64-bit uintmax_t tends to give better
  52      performance.  On 64-bit hardware, unsigned long is generally 64
  53      bits already.  Change this typedef to experiment with
  54      performance.  */
  55   typedef unsigned long int longword;
  56
  57   const unsigned char *char_ptr;
  58   const longword *longword_ptr;
  59   longword repeated_one;
  60   longword repeated_c;
  61   unsigned char c;
  62
  63   c = (unsigned char) c_in;
  64
  65   /* Handle the first few bytes by reading one byte at a time.
  66      Do this until CHAR_PTR is aligned on a longword boundary.  */
  67   for (char_ptr = (const unsigned char *) s;
  68        n > 0 && (size_t) char_ptr % sizeof (longword) != 0;
  69        --n, ++char_ptr)
  70     if (*char_ptr == c)
  71       return (void *) char_ptr;
  72
  73   longword_ptr = (const longword *) char_ptr;
  74
  75   /* All these elucidatory comments refer to 4-byte longwords,
  76      but the theory applies equally well to any size longwords.  */
  77
  78   /* Compute auxiliary longword values:
  79      repeated_one is a value which has a 1 in every byte.
  80      repeated_c has c in every byte.  */
  81   repeated_one = 0x01010101;
  82   repeated_c = c | (c << 8);
  83   repeated_c |= repeated_c << 16;
  84   if (0xffffffffU < (longword) -1)
  85     {
  86       repeated_one |= repeated_one << 31 << 1;
  87       repeated_c |= repeated_c << 31 << 1;
  88       if (8 < sizeof (longword))
  89         {
  90           size_t i;
  91
  92           for (i = 64; i < sizeof (longword) * 8; i *= 2)
  93             {
  94               repeated_one |= repeated_one << i;
  95               repeated_c |= repeated_c << i;
  96             }
  97         }
  98     }
  99
 100   /* Instead of the traditional loop which tests each byte, we will test a
 101      longword at a time.  The tricky part is testing if *any of the four*
 102      bytes in the longword in question are equal to c.  We first use an xor
 103      with repeated_c.  This reduces the task to testing whether *any of the
 104      four* bytes in longword1 is zero.
 105
 106      We compute tmp =
 107        ((longword1 - repeated_one) & ~longword1) & (repeated_one << 7).
 108      That is, we perform the following operations:
 109        1. Subtract repeated_one.
 110        2. & ~longword1.
 111        3. & a mask consisting of 0x80 in every byte.
 112      Consider what happens in each byte:
 113        - If a byte of longword1 is zero, step 1 and 2 transform it into 0xff,
 114          and step 3 transforms it into 0x80.  A carry can also be propagated
 115          to more significant bytes.
 116        - If a byte of longword1 is nonzero, let its lowest 1 bit be at
 117          position k (0 <= k <= 7); so the lowest k bits are 0.  After step 1,
 118          the byte ends in a single bit of value 0 and k bits of value 1.
 119          After step 2, the result is just k bits of value 1: 2^k - 1.  After
 120          step 3, the result is 0.  And no carry is produced.
 121      So, if longword1 has only non-zero bytes, tmp is zero.
 122      Whereas if longword1 has a zero byte, call j the position of the least
 123      significant zero byte.  Then the result has a zero at positions 0, ...,
 124      j-1 and a 0x80 at position j.  We cannot predict the result at the more
 125      significant bytes (positions j+1..3), but it does not matter since we
 126      already have a non-zero bit at position 8*j+7.
 127
 128      So, the test whether any byte in longword1 is zero is equivalent to
 129      testing whether tmp is nonzero.  */
 130
 131   while (n >= sizeof (longword))
 132     {
 133       longword longword1 = *longword_ptr ^ repeated_c;
 134
 135       if ((((longword1 - repeated_one) & ~longword1)
 136            & (repeated_one << 7)) != 0)
 137         break;
 138       longword_ptr++;
 139       n -= sizeof (longword);
 140     }
 141
 142   char_ptr = (const unsigned char *) longword_ptr;
 143
 144   /* At this point, we know that either n < sizeof (longword), or one of the
 145      sizeof (longword) bytes starting at char_ptr is == c.  On little-endian
 146      machines, we could determine the first such byte without any further
 147      memory accesses, just by looking at the tmp result from the last loop
 148      iteration.  But this does not work on big-endian machines.  Choose code
 149      that works in both cases.  */
 150
 151   for (; n > 0; --n, ++char_ptr)
 152     {
 153       if (*char_ptr == c)
 154         return (void *) char_ptr;
 155     }
 156
 157   return NULL;
 158 }
 159 #ifdef weak_alias
 160 weak_alias (__memchr, memchr)
 161 #endif
 162 libc_hidden_builtin_def (memchr)