[llvm-dev] enabling interleaved access loop vectorization

[Zaks, Ayal via llvm-dev]

> 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]
Sent: Saturday, August 06, 2016 02:56
To: Renato Golin <renato.golin at linaro.org<mailto:renato.golin at linaro.org>>
Cc: Demikhovsky, Elena <elena.demikhovsky at intel.com<mailto:elena.demikhovsky at intel.com>>; Matthew Simpson <mssimpso at codeaurora.org<mailto:mssimpso at codeaurora.org>>; Nema, Ashutosh <Ashutosh.Nema at amd.com<mailto:Ashutosh.Nema at amd.com>>; Sanjay Patel <spatel at rotateright.com<mailto:spatel at rotateright.com>>; llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>; Zaks, Ayal <ayal.zaks at intel.com<mailto: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<mailto:renato.golin at linaro.org>> wrote:
On 6 August 2016 at 00:18, Michael Kuperstein <mkuper at google.com<mailto: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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