From f4cb1cc18f364d761d5614eb6293cccc6647f259 Mon Sep 17 00:00:00 2001 From: Fenghua Yu Date: Sat, 16 Nov 2013 12:37:01 -0800 Subject: [PATCH] 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 Signed-off-by: Fenghua Yu Link: http://lkml.kernel.org/r/1384634221-6006-1-git-send-email-fenghua.yu@intel.com Signed-off-by: H. Peter Anvin --- arch/x86/lib/copy_user_64.S | 8 ++------ 1 file changed, 2 insertions(+), 6 deletions(-) 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 @@ 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 -- 2.7.4