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SubjectRe: [PATCH] optimize ia32 memmove
Andrew Morton wrote:
> Jeff Garzik <jgarzik@pobox.com> wrote:
>
>>Linux Kernel Mailing List wrote:
>>
>>>ChangeSet 1.1496.22.32, 2003/12/29 21:45:30-08:00, akpm@osdl.org
>>>
>>> [PATCH] optimize ia32 memmove
>>>
>>> From: Manfred Spraul <manfred@colorfullife.com>
>>>
>>> The memmove implementation of i386 is not optimized: it uses movsb, which is
>>> far slower than movsd. The optimization is trivial: if dest is less than
>>> source, then call memcpy(). markw tried it on a 4xXeon with dbt2, it saved
>>> around 300 million cpu ticks in cache_flusharray():
>>
>>[...]
>>
>>>diff -Nru a/include/asm-i386/string.h b/include/asm-i386/string.h
>>>--- a/include/asm-i386/string.h Mon Dec 29 23:13:20 2003
>>>+++ b/include/asm-i386/string.h Mon Dec 29 23:13:20 2003
>>>@@ -299,14 +299,9 @@
>>> static inline void * memmove(void * dest,const void * src, size_t n)
>>> {
>>> int d0, d1, d2;
>>>-if (dest<src)
>>>-__asm__ __volatile__(
>>>- "rep\n\t"
>>>- "movsb"
>>>- : "=&c" (d0), "=&S" (d1), "=&D" (d2)
>>>- :"0" (n),"1" (src),"2" (dest)
>>>- : "memory");
>>>-else
>>>+if (dest<src) {
>>>+ memcpy(dest,src,n);
>>>+} else
>>> __asm__ __volatile__(
>>> "std\n\t"
>>> "rep\n\t"
>>
>>Dumb question, though... what about the overlap case, when dest<src ?
>> It seems to me this change is ignoring that.
>>
>
>
> "if dest is less that source, then call memcpy". If the move is to a
> higher address we do it the old way.


I'm confused... that doesn't say anything to me about overlap.

They can still overlap: Consider if dest is 1 byte less than src, and
n==128...

Jeff



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