Re: [PATCH] optimize ia32 memmove

[Jeff Garzik]

Andrew Morton wrote:
> Jeff Garzik <jgarzik@xxxxxxxxx> wrote:
>
> > Linux Kernel Mailing List wrote:
> >
> > > ChangeSet 1.1496.22.32, 2003/12/29 21:45:30-08:00, akpm@xxxxxxxx
> > >
> > > 	[PATCH] optimize ia32 memmove
> > > 	
> > > 	From: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
> > > 	
> > > 	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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