x86-64, copy_user: Remove zero byte check before copy user buffer.
Operation of rep movsb instruction handles zero byte copy. As pointed out by
Linus, there is no need to check zero size in kernel. Removing this redundant
check saves a few cycles in copy user functions.
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Link: http://lkml.kernel.org/r/1384634221-6006-1-git-send-email-fenghua.yu@intel.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
diff --git a/arch/x86/lib/copy_user_64.S b/arch/x86/lib/copy_user_64.S
index a30ca15..ffe4eb9 100644
--- a/arch/x86/lib/copy_user_64.S
+++ b/arch/x86/lib/copy_user_64.S
@@ -236,8 +236,6 @@
ENTRY(copy_user_generic_string)
CFI_STARTPROC
ASM_STAC
- andl %edx,%edx
- jz 4f
cmpl $8,%edx
jb 2f /* less than 8 bytes, go to byte copy loop */
ALIGN_DESTINATION
@@ -249,7 +247,7 @@
2: movl %edx,%ecx
3: rep
movsb
-4: xorl %eax,%eax
+ xorl %eax,%eax
ASM_CLAC
ret
@@ -279,12 +277,10 @@
ENTRY(copy_user_enhanced_fast_string)
CFI_STARTPROC
ASM_STAC
- andl %edx,%edx
- jz 2f
movl %edx,%ecx
1: rep
movsb
-2: xorl %eax,%eax
+ xorl %eax,%eax
ASM_CLAC
ret