gdb/gnulib/memchr.c

   1 /* Copyright (C) 1991, 1993, 1996, 1997, 1999, 2000, 2003, 2004, 2006, 2009
   2 Free 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 3 of the License, or 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, see <http://www.gnu.org/licenses/>.  */
  25
  26 #ifndef _LIBC
  27 # include <config.h>
  28 #endif
  29
  30 #include <string.h>
  31
  32 #include <stddef.h>
  33
  34 #if defined _LIBC
  35 # include <memcopy.h>
  36 #else
  37 # define reg_char char
  38 #endif
  39
  40 #include <limits.h>
  41
  42 #if HAVE_BP_SYM_H || defined _LIBC
  43 # include <bp-sym.h>
  44 #else
  45 # define BP_SYM(sym) sym
  46 #endif
  47
  48 #undef memchr
  49 #undef __memchr
  50
  51 /* Search no more than N bytes of S for C.  */
  52 void *
  53 __memchr (void const *s, int c_in, size_t n)
  54 {
  55   const unsigned char *char_ptr;
  56   const unsigned long int *longword_ptr;
  57   unsigned long int longword, magic_bits, charmask;
  58   unsigned reg_char c;
  59   int i;
  60
  61   c = (unsigned char) c_in;
  62
  63   /* Handle the first few characters by reading one character at a time.
  64      Do this until CHAR_PTR is aligned on a longword boundary.  */
  65   for (char_ptr = (const unsigned char *) s;
  66        n > 0 && (size_t) char_ptr % sizeof longword != 0;
  67        --n, ++char_ptr)
  68     if (*char_ptr == c)
  69       return (void *) char_ptr;
  70
  71   /* All these elucidatory comments refer to 4-byte longwords,
  72      but the theory applies equally well to any size longwords.  */
  73
  74   longword_ptr = (const unsigned long int *) char_ptr;
  75
  76   /* Bits 31, 24, 16, and 8 of this number are zero.  Call these bits
  77      the "holes."  Note that there is a hole just to the left of
  78      each byte, with an extra at the end:
  79
  80      bits:  01111110 11111110 11111110 11111111
  81      bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
  82
  83      The 1-bits make sure that carries propagate to the next 0-bit.
  84      The 0-bits provide holes for carries to fall into.  */
  85
  86   /* Set MAGIC_BITS to be this pattern of 1 and 0 bits.
  87      Set CHARMASK to be a longword, each of whose bytes is C.  */
  88
  89   magic_bits = 0xfefefefe;
  90   charmask = c | (c << 8);
  91   charmask |= charmask << 16;
  92 #if 0xffffffffU < ULONG_MAX
  93   magic_bits |= magic_bits << 32;
  94   charmask |= charmask << 32;
  95   if (8 < sizeof longword)
  96     for (i = 64; i < sizeof longword * 8; i *= 2)
  97       {
  98         magic_bits |= magic_bits << i;
  99         charmask |= charmask << i;
 100       }
 101 #endif
 102   magic_bits = (ULONG_MAX >> 1) & (magic_bits | 1);
 103
 104   /* Instead of the traditional loop which tests each character,
 105      we will test a longword at a time.  The tricky part is testing
 106      if *any of the four* bytes in the longword in question are zero.  */
 107   while (n >= sizeof longword)
 108     {
 109       /* We tentatively exit the loop if adding MAGIC_BITS to
 110          LONGWORD fails to change any of the hole bits of LONGWORD.
 111
 112          1) Is this safe?  Will it catch all the zero bytes?
 113          Suppose there is a byte with all zeros.  Any carry bits
 114          propagating from its left will fall into the hole at its
 115          least significant bit and stop.  Since there will be no
 116          carry from its most significant bit, the LSB of the
 117          byte to the left will be unchanged, and the zero will be
 118          detected.
 119
 120          2) Is this worthwhile?  Will it ignore everything except
 121          zero bytes?  Suppose every byte of LONGWORD has a bit set
 122          somewhere.  There will be a carry into bit 8.  If bit 8
 123          is set, this will carry into bit 16.  If bit 8 is clear,
 124          one of bits 9-15 must be set, so there will be a carry
 125          into bit 16.  Similarly, there will be a carry into bit
 126          24.  If one of bits 24-30 is set, there will be a carry
 127          into bit 31, so all of the hole bits will be changed.
 128
 129          The one misfire occurs when bits 24-30 are clear and bit
 130          31 is set; in this case, the hole at bit 31 is not
 131          changed.  If we had access to the processor carry flag,
 132          we could close this loophole by putting the fourth hole
 133          at bit 32!
 134
 135          So it ignores everything except 128's, when they're aligned
 136          properly.
 137
 138          3) But wait!  Aren't we looking for C, not zero?
 139          Good point.  So what we do is XOR LONGWORD with a longword,
 140          each of whose bytes is C.  This turns each byte that is C
 141          into a zero.  */
 142
 143       longword = *longword_ptr++ ^ charmask;
 144
 145       /* Add MAGIC_BITS to LONGWORD.  */
 146       if ((((longword + magic_bits)
 147
 148             /* Set those bits that were unchanged by the addition.  */
 149             ^ ~longword)
 150
 151            /* Look at only the hole bits.  If any of the hole bits
 152               are unchanged, most likely one of the bytes was a
 153               zero.  */
 154            & ~magic_bits) != 0)
 155         {
 156           /* Which of the bytes was C?  If none of them were, it was
 157              a misfire; continue the search.  */
 158
 159           const unsigned char *cp = (const unsigned char *) (longword_ptr - 1);
 160
 161           if (cp[0] == c)
 162             return (void *) cp;
 163           if (cp[1] == c)
 164             return (void *) &cp[1];
 165           if (cp[2] == c)
 166             return (void *) &cp[2];
 167           if (cp[3] == c)
 168             return (void *) &cp[3];
 169           if (4 < sizeof longword && cp[4] == c)
 170             return (void *) &cp[4];
 171           if (5 < sizeof longword && cp[5] == c)
 172             return (void *) &cp[5];
 173           if (6 < sizeof longword && cp[6] == c)
 174             return (void *) &cp[6];
 175           if (7 < sizeof longword && cp[7] == c)
 176             return (void *) &cp[7];
 177           if (8 < sizeof longword)
 178             for (i = 8; i < sizeof longword; i++)
 179               if (cp[i] == c)
 180                 return (void *) &cp[i];
 181         }
 182
 183       n -= sizeof longword;
 184     }
 185
 186   char_ptr = (const unsigned char *) longword_ptr;
 187
 188   while (n-- > 0)
 189     {
 190       if (*char_ptr == c)
 191         return (void *) char_ptr;
 192       else
 193         ++char_ptr;
 194     }
 195
 196   return 0;
 197 }
 198 #ifdef weak_alias
 199 weak_alias (__memchr, BP_SYM (memchr))
 200 #endif