Re: [PATCH] optimize ia32 memmove

[Jeff Garzik <jgarzik@pobox.com>]

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