lib/memchr.c

   1 /* Copyright (C) 1991, 1993, 1996, 1997, 1999, 2000, 2003, 2004, 2006 Free
   2    Software Foundation, Inc.
   3
   4    Based on strlen implementation by Torbjorn Granlund (tege@sics.se),
   5    with help from Dan Sahlin (dan@sics.se) and
   6    commentary by Jim Blandy (jimb@ai.mit.edu);
   7    adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu),
   8    and implemented by Roland McGrath (roland@ai.mit.edu).
   9
  10 NOTE: The canonical source of this file is maintained with the GNU C Library.
  11 Bugs can be reported to bug-glibc@prep.ai.mit.edu.
  12
  13 This program is free software; you can redistribute it and/or modify it
  14 under the terms of the GNU General Public License as published by the
  15 Free Software Foundation; either version 2, or (at your option) any
  16 later version.
  17
  18 This program is distributed in the hope that it will be useful,
  19 but WITHOUT ANY WARRANTY; without even the implied warranty of
  20 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  21 GNU General Public License for more details.
  22
  23 You should have received a copy of the GNU General Public License
  24 along with this program; if not, write to the Free Software Foundation,
  25 Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.  */
  26
  27 #ifndef _LIBC
  28 # include <config.h>
  29 #endif
  30
  31 #include <string.h>
  32
  33 #include <stddef.h>
  34
  35 #if defined _LIBC
  36 # include <memcopy.h>
  37 #else
  38 # define reg_char char
  39 #endif
  40
  41 #include <limits.h>
  42
  43 #if HAVE_BP_SYM_H || defined _LIBC
  44 # include <bp-sym.h>
  45 #else
  46 # define BP_SYM(sym) sym
  47 #endif
  48
  49 #undef memchr
  50 #undef __memchr
  51
  52 /* Search no more than N bytes of S for C.  */
  53 void *
  54 __memchr (void const *s, int c_in, size_t n)
  55 {
  56   const unsigned char *char_ptr;
  57   const unsigned long int *longword_ptr;
  58   unsigned long int longword, magic_bits, charmask;
  59   unsigned reg_char c;
  60   int i;
  61
  62   c = (unsigned char) c_in;
  63
  64   /* Handle the first few characters by reading one character at a time.
  65      Do this until CHAR_PTR is aligned on a longword boundary.  */
  66   for (char_ptr = (const unsigned char *) s;
  67        n > 0 && (size_t) char_ptr % sizeof longword != 0;
  68        --n, ++char_ptr)
  69     if (*char_ptr == c)
  70       return (void *) char_ptr;
  71
  72   /* All these elucidatory comments refer to 4-byte longwords,
  73      but the theory applies equally well to any size longwords.  */
  74
  75   longword_ptr = (const unsigned long int *) char_ptr;
  76
  77   /* Bits 31, 24, 16, and 8 of this number are zero.  Call these bits
  78      the "holes."  Note that there is a hole just to the left of
  79      each byte, with an extra at the end:
  80
  81      bits:  01111110 11111110 11111110 11111111
  82      bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
  83
  84      The 1-bits make sure that carries propagate to the next 0-bit.
  85      The 0-bits provide holes for carries to fall into.  */
  86
  87   /* Set MAGIC_BITS to be this pattern of 1 and 0 bits.
  88      Set CHARMASK to be a longword, each of whose bytes is C.  */
  89
  90   magic_bits = 0xfefefefe;
  91   charmask = c | (c << 8);
  92   charmask |= charmask << 16;
  93 #if 0xffffffffU < ULONG_MAX
  94   magic_bits |= magic_bits << 32;
  95   charmask |= charmask << 32;
  96   if (8 < sizeof longword)
  97     for (i = 64; i < sizeof longword * 8; i *= 2)
  98       {
  99         magic_bits |= magic_bits << i;
 100         charmask |= charmask << i;
 101       }
 102 #endif
 103   magic_bits = (ULONG_MAX >> 1) & (magic_bits | 1);
 104
 105   /* Instead of the traditional loop which tests each character,
 106      we will test a longword at a time.  The tricky part is testing
 107      if *any of the four* bytes in the longword in question are zero.  */
 108   while (n >= sizeof longword)
 109     {
 110       /* We tentatively exit the loop if adding MAGIC_BITS to
 111          LONGWORD fails to change any of the hole bits of LONGWORD.
 112
 113          1) Is this safe?  Will it catch all the zero bytes?
 114          Suppose there is a byte with all zeros.  Any carry bits
 115          propagating from its left will fall into the hole at its
 116          least significant bit and stop.  Since there will be no
 117          carry from its most significant bit, the LSB of the
 118          byte to the left will be unchanged, and the zero will be
 119          detected.
 120
 121          2) Is this worthwhile?  Will it ignore everything except
 122          zero bytes?  Suppose every byte of LONGWORD has a bit set
 123          somewhere.  There will be a carry into bit 8.  If bit 8
 124          is set, this will carry into bit 16.  If bit 8 is clear,
 125          one of bits 9-15 must be set, so there will be a carry
 126          into bit 16.  Similarly, there will be a carry into bit
 127          24.  If one of bits 24-30 is set, there will be a carry
 128          into bit 31, so all of the hole bits will be changed.
 129
 130          The one misfire occurs when bits 24-30 are clear and bit
 131          31 is set; in this case, the hole at bit 31 is not
 132          changed.  If we had access to the processor carry flag,
 133          we could close this loophole by putting the fourth hole
 134          at bit 32!
 135
 136          So it ignores everything except 128's, when they're aligned
 137          properly.
 138
 139          3) But wait!  Aren't we looking for C, not zero?
 140          Good point.  So what we do is XOR LONGWORD with a longword,
 141          each of whose bytes is C.  This turns each byte that is C
 142          into a zero.  */
 143
 144       longword = *longword_ptr++ ^ charmask;
 145
 146       /* Add MAGIC_BITS to LONGWORD.  */
 147       if ((((longword + magic_bits)
 148
 149             /* Set those bits that were unchanged by the addition.  */
 150             ^ ~longword)
 151
 152            /* Look at only the hole bits.  If any of the hole bits
 153               are unchanged, most likely one of the bytes was a
 154               zero.  */
 155            & ~magic_bits) != 0)
 156         {
 157           /* Which of the bytes was C?  If none of them were, it was
 158              a misfire; continue the search.  */
 159
 160           const unsigned char *cp = (const unsigned char *) (longword_ptr - 1);
 161
 162           if (cp[0] == c)
 163             return (void *) cp;
 164           if (cp[1] == c)
 165             return (void *) &cp[1];
 166           if (cp[2] == c)
 167             return (void *) &cp[2];
 168           if (cp[3] == c)
 169             return (void *) &cp[3];
 170           if (4 < sizeof longword && cp[4] == c)
 171             return (void *) &cp[4];
 172           if (5 < sizeof longword && cp[5] == c)
 173             return (void *) &cp[5];
 174           if (6 < sizeof longword && cp[6] == c)
 175             return (void *) &cp[6];
 176           if (7 < sizeof longword && cp[7] == c)
 177             return (void *) &cp[7];
 178           if (8 < sizeof longword)
 179             for (i = 8; i < sizeof longword; i++)
 180               if (cp[i] == c)
 181                 return (void *) &cp[i];
 182         }
 183
 184       n -= sizeof longword;
 185     }
 186
 187   char_ptr = (const unsigned char *) longword_ptr;
 188
 189   while (n-- > 0)
 190     {
 191       if (*char_ptr == c)
 192         return (void *) char_ptr;
 193       else
 194         ++char_ptr;
 195     }
 196
 197   return 0;
 198 }
 199 #ifdef weak_alias
 200 weak_alias (__memchr, BP_SYM (memchr))
 201 #endif