[llvm-dev] enabling interleaved access loop vectorization

[Michael Kuperstein via llvm-dev]

Thanks Ayal!

I'll take a look at DENBench.

As another data point - I tried enabling this on our internal benchmarks.
I'm seeing one regression, and it seems to be a regression of the "good"
kind - without interleaving we don't vectorize the innermost loop, and with
interleaving we do. The vectorized loop is actually significantly faster
when benchmarked in isolation, but in this specific instance, the static
loop count is unknown, and the dynamic loop count happens to almost always
be 1 - and this lives inside a hot outer loop.
That's something we ought to be handling through PGO (or, conceivably,
outer loop vectorization :-) ).

Michael

On Mon, Aug 8, 2016 at 3:21 PM, Zaks, Ayal <ayal.zaks at intel.com> wrote:

> > We also need to understand what to do with edge elements in the vector
> if their loading is not required. We, probably, should issue a masked load
> in this case.
>
>
>
> The existing code solves such edge cases where the last element of an
> InterleaveGroup is absent by making sure the last iteration (and up to last
> VF iterations) are peeled and executed scalarly; see requiresScalarEpilogue.
>
>
>
>
>
> > All regressions that we see are in 32-bit mode.
>
>
>
> One place to find them, using the default BaseT::getInterleavedMemoryOpCost(),
> is DENBench’s RGB conversions.
>
>
>
> Ayal.
>
>
>
> *From:* Demikhovsky, Elena
> *Sent:* Monday, August 08, 2016 00:09
> *To:* Michael Kuperstein <mkuper at google.com>; Renato Golin <
> renato.golin at linaro.org>
> *Cc:* Matthew Simpson <mssimpso at codeaurora.org>; Nema, Ashutosh <
> Ashutosh.Nema at amd.com>; Sanjay Patel <spatel at rotateright.com>; llvm-dev <
> llvm-dev at lists.llvm.org>; Zaks, Ayal <ayal.zaks at intel.com>
> *Subject:* RE: [llvm-dev] enabling interleaved access loop vectorization
>
>
>
> We checked the gathered data again. All regressions that we see are in
> 32-bit mode. The 64-bit mode looks good overall.
>
>
>
> -          * Elena*
>
>
>
> *From:* Michael Kuperstein [mailto:mkuper at google.com <mkuper at google.com>]
> *Sent:* Saturday, August 06, 2016 02:56
> *To:* Renato Golin <renato.golin at linaro.org>
> *Cc:* Demikhovsky, Elena <elena.demikhovsky at intel.com>; Matthew Simpson <
> mssimpso at codeaurora.org>; Nema, Ashutosh <Ashutosh.Nema at amd.com>; Sanjay
> Patel <spatel at rotateright.com>; llvm-dev <llvm-dev at lists.llvm.org>; Zaks,
> Ayal <ayal.zaks at intel.com>
> *Subject:* Re: [llvm-dev] enabling interleaved access loop vectorization
>
>
>
>
>
>
>
> 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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