Module Name: src
Committed By: dsl
Date: Sat Jul 11 08:48:52 UTC 2009
Modified Files:
src/common/lib/libc/arch/x86_64/string: strlen.S
Log Message:
Change comments about algorithms, 99.6% for random data isn't 'rare' in my book!
(The 'rare' case is any byte values 0x80-0xff.)
To generate a diff of this commit:
cvs rdiff -u -r1.1 -r1.2 src/common/lib/libc/arch/x86_64/string/strlen.S
Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.
Modified files:
Index: src/common/lib/libc/arch/x86_64/string/strlen.S
diff -u src/common/lib/libc/arch/x86_64/string/strlen.S:1.1 src/common/lib/libc/arch/x86_64/string/strlen.S:1.2
--- src/common/lib/libc/arch/x86_64/string/strlen.S:1.1 Tue Dec 20 19:28:51 2005
+++ src/common/lib/libc/arch/x86_64/string/strlen.S Sat Jul 11 08:48:51 2009
@@ -6,7 +6,7 @@
#include <machine/asm.h>
#if defined(LIBC_SCCS)
- RCSID("$NetBSD: strlen.S,v 1.1 2005/12/20 19:28:51 christos Exp $")
+ RCSID("$NetBSD: strlen.S,v 1.2 2009/07/11 08:48:51 dsl Exp $")
#endif
ENTRY(strlen)
@@ -74,9 +74,9 @@
* (3) ((x - 0x01....01) & 0x80....80)
*
* always evaluates to a non-zero value if any of the bytes in
- * the original word is zero. However, in rare cases, it also
- * evaluates to a non-zero value when none of the bytes in the
- * original word is zero.
+ * the original word is zero or has the top bit set.
+ * For strings that are likely to only contain 7-bit ascii these
+ * false positives will be rare.
*
* To account for possible false positives, each byte of the
* original word must be checked when the expression evaluates to
@@ -110,7 +110,7 @@
je .Lloop
/*
- * In rare cases, the above loop may exit prematurely. We must
+ * For bytes 0x80-0xff the above loop will exit prematurely. We must
* return to the loop if none of the bytes in the word equal 0.
*/
cmpb $0,-8(%rax) /* 1st byte == 0? */
