Re: [PATCH] optimize ia32 memmove

[Manfred Spraul <manfred@colorfullife.com>]

Jeremy Fitzhardinge wrote:

 >On Tue, 2003-12-30 at 01:58, Linus Torvalds wrote:
 >
 >>But then anything that does the loads in ascending order is still ok, so
 >>it shouldn't matter - by the time "dest" has been overwritten, the source
 >>data has already been read. And all the "memcpy()"  implementations had
 >>better do that anyway, in order to get nice memory access patterns. "rep
 >>movsl" certainly does.

AMD recommends to perform bulk copies backwards: That defeats the hw 
prefecher, and results in even better access patterns. Doesn't matter in 
this case, memmove is never used for bulk copies.

 >A PPC memcpy may end up clearing the destination before reading the
 >source (using the cache-line zeroing instruction, to prevent the
 >destination from being spuriously read to populate the cache line).
 >
The change is i386 only, no effect on other archs.
I found the unoptimized memmove in oprofiles of dbt2 testruns: slab 
contains a few memmoves to keep it's recently used arrays in strict LIFO 
order. Typically perhaps 100 bytes.

--
    Manfred