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 <[email protected]> Signed-off-by: Fenghua Yu <[email protected]> Link: http://lkml.kernel.org/r/[email protected] Signed-off-by: H. Peter Anvin <[email protected]>
vbatts · Nov 17, 2013 · f4cb1cc · f4cb1cc
1 parent 1213959
commit f4cb1cc
Showing 1 changed file with 2 additions and 6 deletions.
diff --git 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