Messages in this thread | | | Date | Mon, 29 Dec 2003 23:51:58 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH] optimize ia32 memmove |
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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.
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