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Re: [PATCH] sysdeps/arm/armv7/multiarch/memcpy_impl.S: Improve performance.

On Tue, Sep 3, 2013 at 11:18 AM, Carlos O'Donell <> wrote:
> We have one, it's the glibc microbenchmark, and we want to expand it,
> otherwise when ACME comes with their patch for ARM and breaks performance
> for targets that Linaro cares about I have no way to reject the patch
> objectively :-)

Can you be objective in analyzing performance when two different
people have differing opinions on what performance preconditions
should be coded against?

There are some cases that are obvious.. we know that from pipeline
analysis that certain instruction sequences can hinder performance.
That is objective and can be measured by a benchmark, but saying that
a particular change penalizes X sized copies but helps Y sized copies
when there are no published performance preconditions isn't.  It's a
difference in opinion of what's important.

PowerPC has had the luxury of not having their performance
pre-conditions contested.  PowerPC string performance is optimized
based upon customer data-set analysis.  So PowerPC's preconditions are
pretty concrete...  Optimize for aligned data in excess of 128-bytes
(I believe).

> You need to statistically analyze the numbers, assign weights to ranges,
> and come up with some kind of number that evaluates the results based
> on *some* formula. That is the only way we are going to keep moving
> performance forward (against some kind of criteria).

This sounds like establishing preconditions (what types of data will
be optimized for).

Unless technology evolves that you can statistically analyze data in
real time and adjust the implementation based on what you find (an
implementation with a different set of preconditions) to account for
this you're going to end up with a lot of in-fighting over

I've run into situations where I recommended that a customer code
their own string function implementation because they continually
encountered unaligned-data when copying-by-value in C++ functions and
PowerPC's string function implementations penalized unaligned copies
in preference for aligned copies.


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