llvm dev - Jan 2018 - RFC: [LV] any objections in moving isLegalMasked* check from Legal to CostModel? (Cleaning up LoopVectorizationLegality)

All,

I'm trying to refactor LoopVectorize such that it has better conformance to
VPlan vision going forward
(http://www.llvm.org/docs/Proposals/VectorizationPlan.html). All VP*Recipe class
definitions are now
moved to VPlan.h, and I have a patch under review to move
LoopVectorizationPlanner class out of
LoopVectorize.cpp (https://reviews.llvm.org/D41420).

Next thing I'm working on is LoopVectorizationLegality, and I noticed that
it has a component of
CostModel and optimization, which doesn't seem right from vectorizer's
architectural perspective.
It appears that we are currently abusing Legal as the attic to throw a lot of
things in order to avoid passing
many pointers around. From vectorizer's architectural point of view, we
should distinguish Legal from
"Vectorization Context Information" (I'd call it
LoopVectorizationAnalysisInfo), some of which (such as
induction, reduction, etc.) are populated during the Legal step. InterleaveInfo
shouldn't even be
a member of Legal. Nothing to do with Legality. It would be a good member of
LoopVectorizationAnalysisInfo.
Eventually, I'd like to see these under Analysis subtree (instead of
Transform), since they are indeed Analysis.

As a first step of this LoopVectorizationLegality cleanup, I propose to move the
following checks
(and member functions) to LoopVectorizationCostModel.
	isLegalMaskedStore
 	isLegalMaskedLoad
	isLegalMaskedScatter
 	isLegalMaskedGather
My assumption is that all SIMD architectures should support serialization of
those operations
at some cost  (e.g., lowering in CG prepare) and thus failing to vectorize due
to "false" return values
of those calls is incorrect behavior. I'll make sure to use a very high
initial cost such that this cleanup is
NFC for all practical purposes ---- and leave the tuning work as TODO.

The down side I can think of is that this will end up running more parts of
vectorizer for those kind
of loops ---- can expose pre-existing bugs and compile time would be a bit
longer since we are bailing
out later. Upside is that we can tune the cost model ---- if other parts of the
loop has enough
speedup, we don't have to give up entire vectorization simply because masked
load/store/gather/scatter
aren't supported on the target.

If anyone still thinks "early bailout" is valuable, splitting into a
separate HWLegal class would be
a cleaner approach than what we have today. We should be able to disable/enable
it under an option.

Let me know what you think.

Thanks,
Hideki Saito

> On 5 Jan 2018, at 21:01, Saito, Hideki via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> 
> All,
> 
> I'm trying to refactor LoopVectorize such that it has better
conformance to VPlan vision going forward
> (http://www.llvm.org/docs/Proposals/VectorizationPlan.html). All VP*Recipe
class definitions are now
> moved to VPlan.h, and I have a patch under review to move
LoopVectorizationPlanner class out of
> LoopVectorize.cpp (https://reviews.llvm.org/D41420).
> 
> Next thing I'm working on is LoopVectorizationLegality, and I noticed
that it has a component of
> CostModel and optimization, which doesn't seem right from
vectorizer's architectural perspective.
> It appears that we are currently abusing Legal as the attic to throw a lot
of things in order to avoid passing
> many pointers around. From vectorizer's architectural point of view, we
should distinguish Legal from
> "Vectorization Context Information" (I'd call it
LoopVectorizationAnalysisInfo), some of which (such as
> induction, reduction, etc.) are populated during the Legal step.
InterleaveInfo shouldn't even be
> a member of Legal. Nothing to do with Legality. It would be a good member
of LoopVectorizationAnalysisInfo.
> Eventually, I'd like to see these under Analysis subtree (instead of
Transform), since they are indeed Analysis.
> 
> As a first step of this LoopVectorizationLegality cleanup, I propose to
move the following checks
> (and member functions) to LoopVectorizationCostModel.
> 	isLegalMaskedStore
> 	isLegalMaskedLoad
> 	isLegalMaskedScatter
> 	isLegalMaskedGather
> My assumption is that all SIMD architectures should support serialization
of those operations
> at some cost  (e.g., lowering in CG prepare) and thus failing to vectorize
due to "false" return values
> of those calls is incorrect behavior. I'll make sure to use a very high
initial cost such that this cleanup is
> NFC for all practical purposes ---- and leave the tuning work as TODO.
> 
> The down side I can think of is that this will end up running more parts of
vectorizer for those kind
> of loops ---- can expose pre-existing bugs and compile time would be a bit
longer since we are bailing
> out later. Upside is that we can tune the cost model ---- if other parts of
the loop has enough
> speedup, we don't have to give up entire vectorization simply because
masked load/store/gather/scatter
> aren't supported on the target.
> 
> If anyone still thinks "early bailout" is valuable, splitting
into a separate HWLegal class would be
> a cleaner approach than what we have today. We should be able to
disable/enable it under an option.
> 
> Let me know what you think.
> 
> Thanks,
> Hideki Saito
> 
> 
I support this direction, and agree in principal that these are strictly cost
model queries, but are there actually any real world workloads where
gather/scatter scalarisation would be worth it, on any micro-architecture? If we
don’t have examples and the compile time cost is non-negligible then I think
we’d still like to keep the early bailouts in some form.’

Amara

Amara,
>I support this direction
Thanks for the support.
>but are there actually any real world workloads where gather/scatter
scalarisation would be worth it, on any micro-architecture? If we don’t have
examples and the compile time cost is non-negligible then I think we’d still
like to keep the early >bailouts in some form.’
It's not like I have specific application code in mind. This all depends on
what your code has and how often the emulated code has to
kick-in. I'm more than happy to implement HWLegal class, and have early
bailout available under the switch. I'm also fine having early
bail out as the default if that helps sweeten the deal. :)

My intention is to make LV components more modular, reusable, and easier to
maintain. Moving things where they actually belong
is part of that picture. That makes it a lot easier for us to place those things
within VPlan infrastructure and explain why that's
the best place.

Thanks,
Hideki

-----Original Message-----
From: aemerson at apple.com [mailto:aemerson at apple.com] 
Sent: Friday, January 05, 2018 3:38 PM
To: Saito, Hideki <hideki.saito at intel.com>
Cc: llvm-dev at lists.llvm.org; Hal Finkel <hfinkel at anl.gov>;
Demikhovsky, Elena <elena.demikhovsky at intel.com>; Amara Emerson
<amara.emerson at arm.com>; Stotzer, Eric <estotzer at ti.com>;
Nemanja Ivanovic <nemanja.i.ibm at gmail.com>; Kreitzer, David L
<david.l.kreitzer at intel.com>; Nuzman, Dorit <dorit.nuzman at
intel.com>; Adam Nemet <anemet at apple.com>; James Molloy
<James.Molloy at arm.com>; Sander De Smalen <Sander.DeSmalen at
arm.com>; Zaks, Ayal <ayal.zaks at intel.com>; Graham Hunter
<Graham.Hunter at arm.com>; Michael Kuperstein <mkuper at
google.com>; Caballero, Diego <diego.caballero at intel.com>; Sanjay
Patel <spatel at rotateright.com>; Simon Pilgrim <llvm-dev at
redking.me.uk>; Tian, Xinmin <xinmin.tian at intel.com>; Nema, Ashutosh
<Ashutosh.Nema at amd.com>
Subject: Re: [llvm-dev] RFC: [LV] any objections in moving isLegalMasked* check
from Legal to CostModel? (Cleaning up LoopVectorizationLegality)
> On 5 Jan 2018, at 21:01, Saito, Hideki via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> 
> All,
> 
> I'm trying to refactor LoopVectorize such that it has better 
> conformance to VPlan vision going forward 
> (http://www.llvm.org/docs/Proposals/VectorizationPlan.html). All 
> VP*Recipe class definitions are now moved to VPlan.h, and I have a patch
under review to move LoopVectorizationPlanner class out of LoopVectorize.cpp
(https://reviews.llvm.org/D41420).
> 
> Next thing I'm working on is LoopVectorizationLegality, and I noticed 
> that it has a component of CostModel and optimization, which doesn't
seem right from vectorizer's architectural perspective.
> It appears that we are currently abusing Legal as the attic to throw a 
> lot of things in order to avoid passing many pointers around. From 
> vectorizer's architectural point of view, we should distinguish Legal 
> from "Vectorization Context Information" (I'd call it 
> LoopVectorizationAnalysisInfo), some of which (such as induction,
reduction, etc.) are populated during the Legal step. InterleaveInfo
shouldn't even be a member of Legal. Nothing to do with Legality. It would
be a good member of LoopVectorizationAnalysisInfo.
> Eventually, I'd like to see these under Analysis subtree (instead of
Transform), since they are indeed Analysis.
> 
> As a first step of this LoopVectorizationLegality cleanup, I propose 
> to move the following checks (and member functions) to
LoopVectorizationCostModel.
> 	isLegalMaskedStore
> 	isLegalMaskedLoad
> 	isLegalMaskedScatter
> 	isLegalMaskedGather
> My assumption is that all SIMD architectures should support 
> serialization of those operations at some cost  (e.g., lowering in CG 
> prepare) and thus failing to vectorize due to "false" return
values of
> those calls is incorrect behavior. I'll make sure to use a very high
initial cost such that this cleanup is NFC for all practical purposes ---- and
leave the tuning work as TODO.
> 
> The down side I can think of is that this will end up running more 
> parts of vectorizer for those kind of loops ---- can expose 
> pre-existing bugs and compile time would be a bit longer since we are 
> bailing out later. Upside is that we can tune the cost model ---- if 
> other parts of the loop has enough speedup, we don't have to give up
entire vectorization simply because masked load/store/gather/scatter aren't
supported on the target.
> 
> If anyone still thinks "early bailout" is valuable, splitting
into a
> separate HWLegal class would be a cleaner approach than what we have today.
We should be able to disable/enable it under an option.
> 
> Let me know what you think.
> 
> Thanks,
> Hideki Saito
> 
> 
I support this direction, and agree in principal that these are strictly cost
model queries, but are there actually any real world workloads where
gather/scatter scalarisation would be worth it, on any micro-architecture? If we
don’t have examples and the compile time cost is non-negligible then I think
we’d still like to keep the early bailouts in some form.’

Amara