Messages in this thread | | | Date | Tue, 30 Dec 2003 02:56:55 -0500 | From | Jeff Garzik <> | Subject | Re: [PATCH] optimize ia32 memmove |
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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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