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authorUlrich Drepper <drepper@redhat.com>2009-03-15 09:03:38 +0000
committerUlrich Drepper <drepper@redhat.com>2009-03-15 09:03:38 +0000
commit71a5bd3e177e7748cf8993b0577d65d8986b44bc (patch)
tree71bcdbf9bd0c2ef2805ec41888065cffcfa40a74 /string/strlen.c
parentfad070abc4292bd82ef0ea5977162fdc710a2e64 (diff)
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[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.
Diffstat (limited to 'string/strlen.c')
-rw-r--r--string/strlen.c52
1 files changed, 3 insertions, 49 deletions
diff --git a/string/strlen.c b/string/strlen.c
index 9bc9db68f7..5f22ce9509 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. */