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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>
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1 changed files with 2 additions and 6 deletions
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@ -236,8 +236,6 @@ ENDPROC(copy_user_generic_unrolled)
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ENTRY(copy_user_generic_string)
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CFI_STARTPROC
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ASM_STAC
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andl %edx,%edx
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jz 4f
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cmpl $8,%edx
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jb 2f /* less than 8 bytes, go to byte copy loop */
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ALIGN_DESTINATION
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@ -249,7 +247,7 @@ ENTRY(copy_user_generic_string)
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2: movl %edx,%ecx
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3: rep
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movsb
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4: xorl %eax,%eax
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xorl %eax,%eax
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ASM_CLAC
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ret
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@ -279,12 +277,10 @@ ENDPROC(copy_user_generic_string)
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ENTRY(copy_user_enhanced_fast_string)
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CFI_STARTPROC
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ASM_STAC
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andl %edx,%edx
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jz 2f
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movl %edx,%ecx
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1: rep
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movsb
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2: xorl %eax,%eax
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xorl %eax,%eax
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ASM_CLAC
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ret
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