[llvm-dev] enabling interleaved access loop vectorization

[Michael Kuperstein via llvm-dev]

On Fri, Aug 5, 2016 at 4:37 PM, Renato Golin <renato.golin at linaro.org>
wrote:

> On 6 August 2016 at 00:18, Michael Kuperstein <mkuper at google.com> wrote:
> > I agree that we can get *more* improvement with better cost modeling, but
> > I'd expect to be able to get *some* improvement the way things are right
> > now.
>
> Elena said she saw "some" improvements. :)
>
>
I didn't mean "some improvements, some regressions", I meant "some of the
improvement we'd expect from the full solution". :-)


>
> > That's why I'm curious about where we saw regressions - I'm wondering
> > whether there's really a significant cost modeling issue I'm missing, or
> > it's something that's easy to fix so that we can make forward progress,
> > while Ashutosh is working on the longer-term solution.
>
> Sounds like a task to try a few patterns and fiddle with the cost model.
>
> Arnold did a lot of those during the first months of the vectorizer,
> so it might be just a matter of finding the right heuristics, at least
> for the low hanging fruits.
>
> Of course, that'd also involve benchmarking everything else, to make
> sure the new heuristics doesn't introduce regressions on
> non-interleaved vectorisation.
>
>
I don't disagree with you.

All I'm saying is that before fiddling with the heuristics, it'd be good to
understand what exactly breaks if we simply flip the flag. If the answer
happens to be "nothing" - well, problem solved. Unfortunately, according to
Elena, that's not the answer.
I'm going to play with it with our internal benchmarks, but it's my
understanding that Elena/Ayal already have some idea of what the problems
are.
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