[Pixman] [PATCH 06/12] vmx: implement fast path vmx_composite_over_n_8888_8888_ca

[Oded Gabbay]

On Wed, Jul 15, 2015 at 3:32 PM, Pekka Paalanen <ppaalanen at gmail.com> wrote:
> Oh. That's totally not what I expected. I was going to ask what
> versions these benchmark results are comparing, because you didn't
> mention, but I never expected *that*.
>
> This is not what the users will be seeing, as they won't be using just
> one of these patches, but the cumulative effect.
Correct, but I think that when we benchmark different fast-paths, we
would like to see the added value of each new fast path separately. At
least that was my thinking but I guess your thinking it least valid as
mine.

>
> If not documented, I'd expect the benchmark results to be before vs.
> after this patch, but since this patch is a part of a series, all the
> earlier patches in the series already applied.
>
> I'd really prefer an explicit mention of what was benchmarked.
Fair enough, I will add a note to each patch.

>
>> > Or if we don't care about that, why?
>> I think that the speedups in this specific patch are more substantial
>> than the slowdowns. If it was the other way around, than I would have
>> removed this patch, like I did with another patch, which Siarhei
>> rejected because of it.
>
> But in theory, you should not get any slowdowns, right? Or did you
> actually expect that some things will slow down?
I'm not so sure I won't get any slowdowns. I guess it depends on the
size of the image and the amount of alignment that needs to be done
for that image

e.g. if we have many small images, and for each image we need to do
unaligned operations first to make sure we are 16-bytes aligned, then
the unaligned operations may take more cycles then the cycles that are
saved from doing the vmx operations. There could be extreme cases,
where there is one vmx operation on aligned data and all the rest of
the operations are unaligned. Now, in the C implementation, you don't
care about unalignment, so you always work in 4 byte quanitites.

Does that make sense ?

    Oded
>
>
> Thanks,
> pq