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[Bug libc/16394] New: i686/memmove.S always copies backwards when dst > src
- From: "maxim.kuvyrkov at gmail dot com" <sourceware-bugzilla at sourceware dot org>
- To: glibc-bugs at sourceware dot org
- Date: Sun, 05 Jan 2014 03:14:13 +0000
- Subject: [Bug libc/16394] New: i686/memmove.S always copies backwards when dst > src
- Auto-submitted: auto-generated
https://sourceware.org/bugzilla/show_bug.cgi?id=16394
Bug ID: 16394
Summary: i686/memmove.S always copies backwards when dst > src
Product: glibc
Version: unspecified
Status: NEW
Severity: enhancement
Priority: P2
Component: libc
Assignee: unassigned at sourceware dot org
Reporter: maxim.kuvyrkov at gmail dot com
CC: drepper.fsp at gmail dot com
Submitted by Yuriy Kaminskiy on libc-alpha@:
Compare:
=== cut string/memmove.c ===
rettype
MEMMOVE (a1, a2, len)
a1const void *a1;
a2const void *a2;
size_t len;
{
unsigned long int dstp = (long int) dest;
unsigned long int srcp = (long int) src;
/* This test makes the forward copying code be used whenever possible.
Reduces the working set. */
if (dstp - srcp >= len) /* *Unsigned* compare! */
{
/* Copy from the beginning to the end. */
=== cut sysdeps/i386/i686/memmove.S ===
....
movl LEN(%esp), %ecx
movl DEST(%esp), %edi
...
movl SRC(%esp), %esi
...
movl %edi, %eax
subl %esi, %eax
cmpl %eax, %edi
jae 3f
[...copy forward ...]
ret
3:
[...copy backward...]
=== cut ===
Obviously, the assembler code checks 'dstp - srcp >= dstp' (an awkward way to
check for dstp > srcp) instead of 'dstp - srcp > len', as was in the C code;
apparently this was /supposed/ to replicate same logic as in the C code, but
registers names was mixed up, and as "it works", nobody noticed. Fortunately,
it
seems only result in choosing suboptimal backward copy in non-overlapping case
when dst > src. Git blame says this mistaken check was already present when
this
code was first committed.
Patch attached:
--- glibc/sysdeps/i386/i686/memmove.S.orig
+++ glibc/sysdeps/i386/i686/memmove.S
@@ -63,8 +63,8 @@
movl %edi, %eax
subl %esi, %eax
- cmpl %eax, %edi
- jae 3f
+ cmpl %eax, %ecx
+ ja 3f
cld
shrl $1, %ecx
--
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