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    SubjectRe: [PATCH] optimize ia32 memmove
    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


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