lib/memchr.c

   1 /* Copyright (C) 1991,93,96,97,99,2000 Free Software Foundation, Inc.
   2    Based on strlen implementation by Torbjorn Granlund (tege@sics.se),
   3    with help from Dan Sahlin (dan@sics.se) and
   4    commentary by Jim Blandy (jimb@ai.mit.edu);
   5    adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu),
   6    and implemented by Roland McGrath (roland@ai.mit.edu).
   7
   8 NOTE: The canonical source of this file is maintained with the GNU C Library.
   9 Bugs can be reported to bug-glibc@prep.ai.mit.edu.
  10
  11 This program is free software; you can redistribute it and/or modify it
  12 under the terms of the GNU General Public License as published by the
  13 Free Software Foundation; either version 2, or (at your option) any
  14 later version.
  15
  16 This program is distributed in the hope that it will be useful,
  17 but WITHOUT ANY WARRANTY; without even the implied warranty of
  18 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  19 GNU General Public License for more details.
  20
  21 You should have received a copy of the GNU General Public License
  22 along with this program; if not, write to the Free Software
  23 Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307,
  24 USA.  */
  25
  26 #ifdef HAVE_CONFIG_H
  27 # include <config.h>
  28 #endif
  29
  30 #undef __ptr_t
  31 #if defined (__cplusplus) || (defined (__STDC__) && __STDC__)
  32 # define __ptr_t void *
  33 #else /* Not C++ or ANSI C.  */
  34 # define __ptr_t char *
  35 #endif /* C++ or ANSI C.  */
  36
  37 #if defined _LIBC
  38 # include <string.h>
  39 # include <memcopy.h>
  40 #else
  41 # define reg_char char
  42 #endif
  43
  44 #if HAVE_STDLIB_H || defined _LIBC
  45 # include <stdlib.h>
  46 #endif
  47
  48 #if HAVE_LIMITS_H || defined _LIBC
  49 # include <limits.h>
  50 #endif
  51
  52 #define LONG_MAX_32_BITS 2147483647
  53
  54 #ifndef LONG_MAX
  55 # define LONG_MAX LONG_MAX_32_BITS
  56 #endif
  57
  58 #include <sys/types.h>
  59 #if HAVE_BP_SYM_H || defined _LIBC
  60 # include <bp-sym.h>
  61 #else
  62 # define BP_SYM(sym) sym
  63 #endif
  64
  65 #undef memchr
  66 #undef __memchr
  67
  68 /* Search no more than N bytes of S for C.  */
  69 __ptr_t
  70 __memchr (s, c_in, n)
  71      const __ptr_t s;
  72      int c_in;
  73      size_t n;
  74 {
  75   const unsigned char *char_ptr;
  76   const unsigned long int *longword_ptr;
  77   unsigned long int longword, magic_bits, charmask;
  78   unsigned reg_char c;
  79
  80   c = (unsigned char) c_in;
  81
  82   /* Handle the first few characters by reading one character at a time.
  83      Do this until CHAR_PTR is aligned on a longword boundary.  */
  84   for (char_ptr = (const unsigned char *) s;
  85        n > 0 && ((unsigned long int) char_ptr
  86                  & (sizeof (longword) - 1)) != 0;
  87        --n, ++char_ptr)
  88     if (*char_ptr == c)
  89       return (__ptr_t) char_ptr;
  90
  91   /* All these elucidatory comments refer to 4-byte longwords,
  92      but the theory applies equally well to 8-byte longwords.  */
  93
  94   longword_ptr = (unsigned long int *) char_ptr;
  95
  96   /* Bits 31, 24, 16, and 8 of this number are zero.  Call these bits
  97      the "holes."  Note that there is a hole just to the left of
  98      each byte, with an extra at the end:
  99
 100      bits:  01111110 11111110 11111110 11111111
 101      bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
 102
 103      The 1-bits make sure that carries propagate to the next 0-bit.
 104      The 0-bits provide holes for carries to fall into.  */
 105
 106   if (sizeof (longword) != 4 && sizeof (longword) != 8)
 107     abort ();
 108
 109 #if LONG_MAX <= LONG_MAX_32_BITS
 110   magic_bits = 0x7efefeff;
 111 #else
 112   magic_bits = ((unsigned long int) 0x7efefefe << 32) | 0xfefefeff;
 113 #endif
 114
 115   /* Set up a longword, each of whose bytes is C.  */
 116   charmask = c | (c << 8);
 117   charmask |= charmask << 16;
 118 #if LONG_MAX > LONG_MAX_32_BITS
 119   charmask |= charmask << 32;
 120 #endif
 121
 122   /* Instead of the traditional loop which tests each character,
 123      we will test a longword at a time.  The tricky part is testing
 124      if *any of the four* bytes in the longword in question are zero.  */
 125   while (n >= sizeof (longword))
 126     {
 127       /* We tentatively exit the loop if adding MAGIC_BITS to
 128          LONGWORD fails to change any of the hole bits of LONGWORD.
 129
 130          1) Is this safe?  Will it catch all the zero bytes?
 131          Suppose there is a byte with all zeros.  Any carry bits
 132          propagating from its left will fall into the hole at its
 133          least significant bit and stop.  Since there will be no
 134          carry from its most significant bit, the LSB of the
 135          byte to the left will be unchanged, and the zero will be
 136          detected.
 137
 138          2) Is this worthwhile?  Will it ignore everything except
 139          zero bytes?  Suppose every byte of LONGWORD has a bit set
 140          somewhere.  There will be a carry into bit 8.  If bit 8
 141          is set, this will carry into bit 16.  If bit 8 is clear,
 142          one of bits 9-15 must be set, so there will be a carry
 143          into bit 16.  Similarly, there will be a carry into bit
 144          24.  If one of bits 24-30 is set, there will be a carry
 145          into bit 31, so all of the hole bits will be changed.
 146
 147          The one misfire occurs when bits 24-30 are clear and bit
 148          31 is set; in this case, the hole at bit 31 is not
 149          changed.  If we had access to the processor carry flag,
 150          we could close this loophole by putting the fourth hole
 151          at bit 32!
 152
 153          So it ignores everything except 128's, when they're aligned
 154          properly.
 155
 156          3) But wait!  Aren't we looking for C, not zero?
 157          Good point.  So what we do is XOR LONGWORD with a longword,
 158          each of whose bytes is C.  This turns each byte that is C
 159          into a zero.  */
 160
 161       longword = *longword_ptr++ ^ charmask;
 162
 163       /* Add MAGIC_BITS to LONGWORD.  */
 164       if ((((longword + magic_bits)
 165
 166             /* Set those bits that were unchanged by the addition.  */
 167             ^ ~longword)
 168
 169            /* Look at only the hole bits.  If any of the hole bits
 170               are unchanged, most likely one of the bytes was a
 171               zero.  */
 172            & ~magic_bits) != 0)
 173         {
 174           /* Which of the bytes was C?  If none of them were, it was
 175              a misfire; continue the search.  */
 176
 177           const unsigned char *cp = (const unsigned char *) (longword_ptr - 1);
 178
 179           if (cp[0] == c)
 180             return (__ptr_t) cp;
 181           if (cp[1] == c)
 182             return (__ptr_t) &cp[1];
 183           if (cp[2] == c)
 184             return (__ptr_t) &cp[2];
 185           if (cp[3] == c)
 186             return (__ptr_t) &cp[3];
 187 #if LONG_MAX > 2147483647
 188           if (cp[4] == c)
 189             return (__ptr_t) &cp[4];
 190           if (cp[5] == c)
 191             return (__ptr_t) &cp[5];
 192           if (cp[6] == c)
 193             return (__ptr_t) &cp[6];
 194           if (cp[7] == c)
 195             return (__ptr_t) &cp[7];
 196 #endif
 197         }
 198
 199       n -= sizeof (longword);
 200     }
 201
 202   char_ptr = (const unsigned char *) longword_ptr;
 203
 204   while (n-- > 0)
 205     {
 206       if (*char_ptr == c)
 207         return (__ptr_t) char_ptr;
 208       else
 209         ++char_ptr;
 210     }
 211
 212   return 0;
 213 }
 214 #ifdef weak_alias
 215 weak_alias (__memchr, BP_SYM (memchr))
 216 #endif