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x86-64: Handle byte-wise tail copying in memcpy() without a loop 

While hard to measure, reducing the number of possibly/likely
mis-predicted branches can generally be expected to be slightly
better.

Other than apparent at the first glance, this also doesn't grow
the function size (the alignment gap to the next function just
gets smaller).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/4F218584020000780006F422@nat28.tlf.novell.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Jan Beulich 2012-01-26 15:55:32 +00:00 committed by Ingo Molnar
parent 2ab560911a
commit 9d8e22777e
1 changed files with 10 additions and 9 deletions
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19
arch/x86/lib/memcpy_64.S
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@ -164,18 +164,19 @@ ENTRY(memcpy)
retq
.p2align 4
.Lless_3bytes:
cmpl $0, %edx
je .Lend
subl $1, %edx
jb .Lend
/*
* Move data from 1 bytes to 3 bytes.
*/
.Lloop_1:
movb (%rsi), %r8b
movb %r8b, (%rdi)
incq %rdi
incq %rsi
decl %edx
jnz .Lloop_1
movzbl (%rsi), %ecx
jz .Lstore_1byte
movzbq 1(%rsi), %r8
movzbq (%rsi, %rdx), %r9
movb %r8b, 1(%rdi)
movb %r9b, (%rdi, %rdx)
.Lstore_1byte:
movb %cl, (%rdi)
.Lend:
retq