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 a30ca15be21c8a182b3bb937ac612d1863a046ee..ffe4eb9f09eba6a9dac5804df07857b0672da366 100644 (file)
--- a/arch/x86/lib/copy_user_64.S
+++ b/arch/x86/lib/copy_user_64.S
@@ -236,8 +236,6 @@ ENDPROC(copy_user_generic_unrolled)
 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 @@ ENTRY(copy_user_generic_string)
 2:     movl %edx,%ecx
 3:     rep
        movsb
-4:     xorl %eax,%eax
+       xorl %eax,%eax
        ASM_CLAC
        ret
 
@@ -279,12 +277,10 @@ ENDPROC(copy_user_generic_string)
 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