Re: [PATCH] optimize ia32 memmove

[Andrew Morton]

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.

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