string/strlen.c

   1 /* Copyright (C) 1991, 1993, 1997, 2000, 2003 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3    Written by Torbjorn Granlund (tege@sics.se),
   4    with help from Dan Sahlin (dan@sics.se);
   5    commentary by Jim Blandy (jimb@ai.mit.edu).
   6
   7    The GNU C Library is free software; you can redistribute it and/or
   8    modify it under the terms of the GNU Lesser General Public
   9    License as published by the Free Software Foundation; either
  10    version 2.1 of the License, or (at your option) any later version.
  11
  12    The GNU C Library is distributed in the hope that it will be useful,
  13    but WITHOUT ANY WARRANTY; without even the implied warranty of
  14    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  15    Lesser General Public License for more details.
  16
  17    You should have received a copy of the GNU Lesser General Public
  18    License along with the GNU C Library; if not, write to the Free
  19    Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
  20    02111-1307 USA.  */
  21
  22 #include <string.h>
  23 #include <stdlib.h>
  24
  25 #undef strlen
  26
  27 /* Return the length of the null-terminated string STR.  Scan for
  28    the null terminator quickly by testing four bytes at a time.  */
  29 size_t
  30 strlen (str)
  31      const char *str;
  32 {
  33   const char *char_ptr;
  34   const unsigned long int *longword_ptr;
  35   unsigned long int longword, magic_bits, himagic, lomagic;
  36
  37   /* Handle the first few characters by reading one character at a time.
  38      Do this until CHAR_PTR is aligned on a longword boundary.  */
  39   for (char_ptr = str; ((unsigned long int) char_ptr
  40                         & (sizeof (longword) - 1)) != 0;
  41        ++char_ptr)
  42     if (*char_ptr == '\0')
  43       return char_ptr - str;
  44
  45   /* All these elucidatory comments refer to 4-byte longwords,
  46      but the theory applies equally well to 8-byte longwords.  */
  47
  48   longword_ptr = (unsigned long int *) char_ptr;
  49
  50   /* Bits 31, 24, 16, and 8 of this number are zero.  Call these bits
  51      the "holes."  Note that there is a hole just to the left of
  52      each byte, with an extra at the end:
  53
  54      bits:  01111110 11111110 11111110 11111111
  55      bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
  56
  57      The 1-bits make sure that carries propagate to the next 0-bit.
  58      The 0-bits provide holes for carries to fall into.  */
  59   magic_bits = 0x7efefeffL;
  60   himagic = 0x80808080L;
  61   lomagic = 0x01010101L;
  62   if (sizeof (longword) > 4)
  63     {
  64       /* 64-bit version of the magic.  */
  65       /* Do the shift in two steps to avoid a warning if long has 32 bits.  */
  66       magic_bits = ((0x7efefefeL << 16) << 16) | 0xfefefeffL;
  67       himagic = ((himagic << 16) << 16) | himagic;
  68       lomagic = ((lomagic << 16) << 16) | lomagic;
  69     }
  70   if (sizeof (longword) > 8)
  71     abort ();
  72
  73   /* Instead of the traditional loop which tests each character,
  74      we will test a longword at a time.  The tricky part is testing
  75      if *any of the four* bytes in the longword in question are zero.  */
  76   for (;;)
  77     {
  78       /* We tentatively exit the loop if adding MAGIC_BITS to
  79          LONGWORD fails to change any of the hole bits of LONGWORD.
  80
  81          1) Is this safe?  Will it catch all the zero bytes?
  82          Suppose there is a byte with all zeros.  Any carry bits
  83          propagating from its left will fall into the hole at its
  84          least significant bit and stop.  Since there will be no
  85          carry from its most significant bit, the LSB of the
  86          byte to the left will be unchanged, and the zero will be
  87          detected.
  88
  89          2) Is this worthwhile?  Will it ignore everything except
  90          zero bytes?  Suppose every byte of LONGWORD has a bit set
  91          somewhere.  There will be a carry into bit 8.  If bit 8
  92          is set, this will carry into bit 16.  If bit 8 is clear,
  93          one of bits 9-15 must be set, so there will be a carry
  94          into bit 16.  Similarly, there will be a carry into bit
  95          24.  If one of bits 24-30 is set, there will be a carry
  96          into bit 31, so all of the hole bits will be changed.
  97
  98          The one misfire occurs when bits 24-30 are clear and bit
  99          31 is set; in this case, the hole at bit 31 is not
 100          changed.  If we had access to the processor carry flag,
 101          we could close this loophole by putting the fourth hole
 102          at bit 32!
 103
 104          So it ignores everything except 128's, when they're aligned
 105          properly.  */
 106
 107       longword = *longword_ptr++;
 108
 109       if (
 110 #if 0
 111           /* Add MAGIC_BITS to LONGWORD.  */
 112           (((longword + magic_bits)
 113
 114             /* Set those bits that were unchanged by the addition.  */
 115             ^ ~longword)
 116
 117            /* Look at only the hole bits.  If any of the hole bits
 118               are unchanged, most likely one of the bytes was a
 119               zero.  */
 120            & ~magic_bits)
 121 #else
 122           ((longword - lomagic) & himagic)
 123 #endif
 124           != 0)
 125         {
 126           /* Which of the bytes was the zero?  If none of them were, it was
 127              a misfire; continue the search.  */
 128
 129           const char *cp = (const char *) (longword_ptr - 1);
 130
 131           if (cp[0] == 0)
 132             return cp - str;
 133           if (cp[1] == 0)
 134             return cp - str + 1;
 135           if (cp[2] == 0)
 136             return cp - str + 2;
 137           if (cp[3] == 0)
 138             return cp - str + 3;
 139           if (sizeof (longword) > 4)
 140             {
 141               if (cp[4] == 0)
 142                 return cp - str + 4;
 143               if (cp[5] == 0)
 144                 return cp - str + 5;
 145               if (cp[6] == 0)
 146                 return cp - str + 6;
 147               if (cp[7] == 0)
 148                 return cp - str + 7;
 149             }
 150         }
 151     }
 152 }
 153 libc_hidden_builtin_def (strlen)