[BZ #5807]

2009-03-15  Ulrich Drepper  <drepper@redhat.com>
	[BZ #5807]
	* string/strlen.c (strlen): Fix omission in the expression to test
	for NUL bytes.

1 files changed, 3 insertions, 49 deletions

diff --git a/string/strlen.c b/string/strlen.c
index 9bc9db6..5f22ce9 100644
--- a/string/strlen.c
+++ b/string/strlen.c
@@ -1,4 +1,4 @@
-/* Copyright (C) 1991, 1993, 1997, 2000, 2003 Free Software Foundation, Inc.
+/* Copyright (C) 1991,1993,1997,2000,2003,2009 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);
@@ -32,7 +32,7 @@ strlen (str)
 {
   const char *char_ptr;
   const unsigned long int *longword_ptr;
-  unsigned long int longword, magic_bits, himagic, lomagic;
+  unsigned long int longword, 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.  */
@@ -56,14 +56,12 @@ strlen (str)
 
      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) > 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 << 16) << 16) | 0xfefefeffL;
       himagic = ((himagic << 16) << 16) | himagic;
       lomagic = ((lomagic << 16) << 16) | lomagic;
     }
@@ -75,53 +73,9 @@ strlen (str)
      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.  */
-	   & ~magic_bits)
-#else
-	  ((longword - lomagic) & himagic)
-#endif
-	  != 0)
+      if (((longword - lomagic) & ~longword & himagic) != 0)
 	{
 	  /* Which of the bytes was the zero?  If none of them were, it was
 	     a misfire; continue the search.  */