title: Why does glibc's strlen need to be so complicated to run quickly?
body: <p>I was looking through the <code>strlen</code> code <a href="http://web.archive.org/web/20190826112007/http://www.stdlib.net/%7Ecolmmacc/strlen.c.html" rel="nofollow noreferrer">here</a> and I was wondering if the optimizations used in the code are really needed? For example, why wouldn't something like the following work equally good or better?</p>
  <pre><code>unsigned long strlen(char s[]) {
      unsigned long i;
      for (i = 0; s[i] != '\0'; i++)
          continue;
      return i;
  }
  </code></pre>
  <p>Isn't simpler code better and/or easier for the compiler to optimize?</p>
  <p>The code of <code>strlen</code> on the page behind the link looks like this:</p>
  <blockquote>
  <pre><code>/* Copyright (C) 1991, 1993, 1997, 2000, 2003 Free Software Foundation, Inc.
     This file is part of the GNU C Library.
     Written by Torbjorn Granlund (tege@sics.se),
     with help from Dan Sahlin (dan@sics.se);
     commentary by Jim Blandy (jimb@ai.mit.edu).
  
     The GNU C Library is free software; you can redistribute it and/or
     modify it under the terms of the GNU Lesser General Public
     License as published by the Free Software Foundation; either
     version 2.1 of the License, or (at your option) any later version.
  
     The GNU C Library is distributed in the hope that it will be useful,
     but WITHOUT ANY WARRANTY; without even the implied warranty of
     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
     Lesser General Public License for more details.
  
     You should have received a copy of the GNU Lesser General Public
     License along with the GNU C Library; if not, write to the Free
     Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
     02111-1307 USA.  */
  
  #include &lt;string.h&gt;
  #include &lt;stdlib.h&gt;
  
  #undef strlen
  
  /* Return the length of the null-terminated string STR.  Scan for
     the null terminator quickly by testing four bytes at a time.  */
  size_t
  strlen (str)
       const char *str;
  {
    const char *char_ptr;
    const unsigned long int *longword_ptr;
    unsigned long int longword, magic_bits, himagic, lomagic;
  
    /* Handle the first few characters by reading one character at a time.
       Do this until CHAR_PTR is aligned on a longword boundary.  */
    for (char_ptr = str; ((unsigned long int) char_ptr
              &amp; (sizeof (longword) - 1)) != 0;
         ++char_ptr)
      if (*char_ptr == '\0')
        return char_ptr - str;
  
    /* All these elucidatory comments refer to 4-byte longwords,
       but the theory applies equally well to 8-byte longwords.  */
  
    longword_ptr = (unsigned long int *) char_ptr;
  
    /* Bits 31, 24, 16, and 8 of this number are zero.  Call these bits
       the &quot;holes.&quot;  Note that there is a hole just to the left of
       each byte, with an extra at the end:
  
       bits:  01111110 11111110 11111110 11111111
       bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
  
       The 1-bits make sure that carries propagate to the next 0-bit.
       The 0-bits provide holes for carries to fall into.  */
    magic_bits = 0x7efefeffL;
    himagic = 0x80808080L;
    lomagic = 0x01010101L;
    if (sizeof (longword) &gt; 4)
      {
        /* 64-bit version of the magic.  */
        /* Do the shift in two steps to avoid a warning if long has 32 bits.  */
        magic_bits = ((0x7efefefeL &lt;&lt; 16) &lt;&lt; 16) | 0xfefefeffL;
        himagic = ((himagic &lt;&lt; 16) &lt;&lt; 16) | himagic;
        lomagic = ((lomagic &lt;&lt; 16) &lt;&lt; 16) | lomagic;
      }
    if (sizeof (longword) &gt; 8)
      abort ();
  
    /* Instead of the traditional loop which tests each character,
       we will test a longword at a time.  The tricky part is testing
       if *any of the four* bytes in the longword in question are zero.  */
    for (;;)
      {
        /* We tentatively exit the loop if adding MAGIC_BITS to
       LONGWORD fails to change any of the hole bits of LONGWORD.
  
       1) Is this safe?  Will it catch all the zero bytes?
       Suppose there is a byte with all zeros.  Any carry bits
       propagating from its left will fall into the hole at its
       least significant bit and stop.  Since there will be no
       carry from its most significant bit, the LSB of the
       byte to the left will be unchanged, and the zero will be
       detected.
  
       2) Is this worthwhile?  Will it ignore everything except
       zero bytes?  Suppose every byte of LONGWORD has a bit set
       somewhere.  There will be a carry into bit 8.  If bit 8
       is set, this will carry into bit 16.  If bit 8 is clear,
       one of bits 9-15 must be set, so there will be a carry
       into bit 16.  Similarly, there will be a carry into bit
       24.  If one of bits 24-30 is set, there will be a carry
       into bit 31, so all of the hole bits will be changed.
  
       The one misfire occurs when bits 24-30 are clear and bit
       31 is set; in this case, the hole at bit 31 is not
       changed.  If we had access to the processor carry flag,
       we could close this loophole by putting the fourth hole
       at bit 32!
  
       So it ignores everything except 128's, when they're aligned
       properly.  */
  
        longword = *longword_ptr++;
  
        if (
  #if 0
        /* Add MAGIC_BITS to LONGWORD.  */
        (((longword + magic_bits)
  
          /* Set those bits that were unchanged by the addition.  */
          ^ ~longword)
  
         /* Look at only the hole bits.  If any of the hole bits
            are unchanged, most likely one of the bytes was a
            zero.  */
         &amp; ~magic_bits)
  #else
        ((longword - lomagic) &amp; himagic)
  #endif
        != 0)
      {
        /* Which of the bytes was the zero?  If none of them were, it was
           a misfire; continue the search.  */
  
        const char *cp = (const char *) (longword_ptr - 1);
  
        if (cp[0] == 0)
          return cp - str;
        if (cp[1] == 0)
          return cp - str + 1;
        if (cp[2] == 0)
          return cp - str + 2;
        if (cp[3] == 0)
          return cp - str + 3;
        if (sizeof (longword) &gt; 4)
          {
            if (cp[4] == 0)
          return cp - str + 4;
            if (cp[5] == 0)
          return cp - str + 5;
            if (cp[6] == 0)
          return cp - str + 6;
            if (cp[7] == 0)
          return cp - str + 7;
          }
      }
      }
  }
  libc_hidden_builtin_def (strlen)
  </code></pre>
  </blockquote>
  <p>Why does this version run quickly?</p>
  <p>Isn't it doing a lot of unnecessary work?</p>