string/rawmemchr.c

   1 /* Copyright (C) 1991-2015 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    <http://www.gnu.org/licenses/>.  */
  22
  23 #ifdef HAVE_CONFIG_H
  24 #include <config.h>
  25 #endif
  26
  27 #undef __ptr_t
  28 #define __ptr_t void *
  29
  30 #if defined (_LIBC)
  31 # include <string.h>
  32 # include <memcopy.h>
  33 # include <stdlib.h>
  34 #endif
  35
  36 #if defined (HAVE_LIMITS_H) || defined (_LIBC)
  37 # include <limits.h>
  38 #endif
  39
  40 #define LONG_MAX_32_BITS 2147483647
  41
  42 #ifndef LONG_MAX
  43 #define LONG_MAX LONG_MAX_32_BITS
  44 #endif
  45
  46 #include <sys/types.h>
  47
  48 #undef memchr
  49
  50 #ifndef RAWMEMCHR
  51 # define RAWMEMCHR __rawmemchr
  52 #endif
  53
  54 /* Find the first occurrence of C in S.  */
  55 __ptr_t
  56 RAWMEMCHR (const __ptr_t s, int c_in)
  57 {
  58   const unsigned char *char_ptr;
  59   const unsigned long int *longword_ptr;
  60   unsigned long int longword, magic_bits, charmask;
  61   unsigned char c;
  62
  63   c = (unsigned char) c_in;
  64
  65   /* Handle the first few characters by reading one character at a time.
  66      Do this until CHAR_PTR is aligned on a longword boundary.  */
  67   for (char_ptr = (const unsigned char *) s;
  68        ((unsigned long int) char_ptr & (sizeof (longword) - 1)) != 0;
  69        ++char_ptr)
  70     if (*char_ptr == c)
  71       return (__ptr_t) char_ptr;
  72
  73   /* All these elucidatory comments refer to 4-byte longwords,
  74      but the theory applies equally well to 8-byte longwords.  */
  75
  76   longword_ptr = (unsigned long int *) char_ptr;
  77
  78   /* Bits 31, 24, 16, and 8 of this number are zero.  Call these bits
  79      the "holes."  Note that there is a hole just to the left of
  80      each byte, with an extra at the end:
  81
  82      bits:  01111110 11111110 11111110 11111111
  83      bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
  84
  85      The 1-bits make sure that carries propagate to the next 0-bit.
  86      The 0-bits provide holes for carries to fall into.  */
  87   magic_bits = -1;
  88   magic_bits = magic_bits / 0xff * 0xfe << 1 >> 1 | 1;
  89
  90   /* Set up a longword, each of whose bytes is C.  */
  91   charmask = c | (c << 8);
  92   charmask |= charmask << 16;
  93 #if LONG_MAX > LONG_MAX_32_BITS
  94   charmask |= charmask << 32;
  95 #endif
  96
  97   /* Instead of the traditional loop which tests each character,
  98      we will test a longword at a time.  The tricky part is testing
  99      if *any of the four* bytes in the longword in question are zero.  */
 100   while (1)
 101     {
 102       /* We tentatively exit the loop if adding MAGIC_BITS to
 103          LONGWORD fails to change any of the hole bits of LONGWORD.
 104
 105          1) Is this safe?  Will it catch all the zero bytes?
 106          Suppose there is a byte with all zeros.  Any carry bits
 107          propagating from its left will fall into the hole at its
 108          least significant bit and stop.  Since there will be no
 109          carry from its most significant bit, the LSB of the
 110          byte to the left will be unchanged, and the zero will be
 111          detected.
 112
 113          2) Is this worthwhile?  Will it ignore everything except
 114          zero bytes?  Suppose every byte of LONGWORD has a bit set
 115          somewhere.  There will be a carry into bit 8.  If bit 8
 116          is set, this will carry into bit 16.  If bit 8 is clear,
 117          one of bits 9-15 must be set, so there will be a carry
 118          into bit 16.  Similarly, there will be a carry into bit
 119          24.  If one of bits 24-30 is set, there will be a carry
 120          into bit 31, so all of the hole bits will be changed.
 121
 122          The one misfire occurs when bits 24-30 are clear and bit
 123          31 is set; in this case, the hole at bit 31 is not
 124          changed.  If we had access to the processor carry flag,
 125          we could close this loophole by putting the fourth hole
 126          at bit 32!
 127
 128          So it ignores everything except 128's, when they're aligned
 129          properly.
 130
 131          3) But wait!  Aren't we looking for C, not zero?
 132          Good point.  So what we do is XOR LONGWORD with a longword,
 133          each of whose bytes is C.  This turns each byte that is C
 134          into a zero.  */
 135
 136       longword = *longword_ptr++ ^ charmask;
 137
 138       /* Add MAGIC_BITS to LONGWORD.  */
 139       if ((((longword + magic_bits)
 140
 141             /* Set those bits that were unchanged by the addition.  */
 142             ^ ~longword)
 143
 144            /* Look at only the hole bits.  If any of the hole bits
 145               are unchanged, most likely one of the bytes was a
 146               zero.  */
 147            & ~magic_bits) != 0)
 148         {
 149           /* Which of the bytes was C?  If none of them were, it was
 150              a misfire; continue the search.  */
 151
 152           const unsigned char *cp = (const unsigned char *) (longword_ptr - 1);
 153
 154           if (cp[0] == c)
 155             return (__ptr_t) cp;
 156           if (cp[1] == c)
 157             return (__ptr_t) &cp[1];
 158           if (cp[2] == c)
 159             return (__ptr_t) &cp[2];
 160           if (cp[3] == c)
 161             return (__ptr_t) &cp[3];
 162 #if LONG_MAX > 2147483647
 163           if (cp[4] == c)
 164             return (__ptr_t) &cp[4];
 165           if (cp[5] == c)
 166             return (__ptr_t) &cp[5];
 167           if (cp[6] == c)
 168             return (__ptr_t) &cp[6];
 169           if (cp[7] == c)
 170             return (__ptr_t) &cp[7];
 171 #endif
 172         }
 173     }
 174 }
 175 libc_hidden_def (__rawmemchr)
 176 weak_alias (__rawmemchr, rawmemchr)