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    SubjectRe: Big git diff speedup by avoiding x86 "fast string" memcmp
    On 12/15/2010 06:06 AM, Nick Piggin wrote:
    > However instead of a normal memcmp, we could actually pad dentry
    > names out to sizeof(long) with zeros, and take advantage of that with
    > a memcmp that does not have to handle tails -- it would operate
    > entirely with longs.
    > That would avoid icache and branch regressions, and might speed up
    > the operation on some architectures. I just doubted whether it would
    > show an improvement to be worth doing at all. If it does, I'm all for it.

    I agree that the byte-compare or long-compare should give you very close
    results in modern pipeline CPUs. But surly 12 increments-and-test should
    show up against 3 (or even 2). I would say it must be a better plan.

    BTW the long approach if you assume that the beginning of the string
    is long aligned than it is only a matter of comparing the last byte
    with a mask, no branches. But I'm not saying, just make sure they are

    Just my $0.017

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      Last update: 2010-12-15 14:17    [W:0.019 / U:26.224 seconds]
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