llvm dev - May 2020 - Codegen pass configs dependent on function attributes?

I’ve put up a patch here: https://reviews.llvm.org/D79769
<https://reviews.llvm.org/D79769> that adds a unified pipeline that
targets can opt-into. It has some similarities with forcing fallbacks, but uses
a different mechanism to do so to preserve the abort behavior. It therefore
requires that every GISel pass needs to explicitly check whether the GISel
selector is being requested rather than just using the FailedISel property.

Cheers,
Amara
> On May 5, 2020, at 6:56 PM, Quentin Colombet <qcolombet at apple.com>
wrote:
> 
> Hi Amara,
> 
> I’ve done that internally for testing purposes and basically the existing
GISel pipeline with fallbacks just did what I wanted.
> In other words, I always used the GISel pipeline but I added an attribute
that triggered a fallback to SDISel for the functions I wanted to compile with
SDISel. I had to disable the global-isel-abort thing though. That said, that
should be easily fixable.
> 
> For the record, I was doing the attribute check in lowerFormalArgument
IIRC. I.e., something that is called for every single function at the beginning
of the GISel pipeline.
> 
> Cheers,
> -Quentin
> 
>> On May 5, 2020, at 2:48 PM, Amara Emerson <aemerson at apple.com>
wrote:
>> 
>> Hi all.
>> 
>> I’m trying to get GlobalISel to work better with LTO. At the moment if
you enable it via -fglobal-isel, it only adds the -mllvm -global-isel and
related options to the cc1 invocation. With LTO, that doesn’t work as we need to
encode codegen options into the bitcode, usually via function attributes.
>> 
>> Does anyone have any ideas on how to achieve this? The only way I can
see it working is if we have a unified codegen pipeline for both GISel and
SelectionDAG. Then use a function attribute to tell the GISel passes to skip and
leave it to FastISel/SelectionDAG which runs afterwards. Likewise,
FastISel/SelectionDAG would need to skip the function if it was marked for GISel
compilation and we didn’t trigger a fallback.
>> 
>> Amara
> 
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This looks like per-function codegen pipeline. If there are existing cases of
doing this, we could refer to these to implement it in an intuitive way.
Overall, I think it is a bit overreaching and brings more downsides than
benefits. If we do this per-TU during linking stage, could we read Module
metadata and pass the flag to TargetPassConfig through TargetMachine::Options?

________________________________________
From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Amara
Emerson via llvm-dev <llvm-dev at lists.llvm.org>
Sent: Tuesday, May 12, 2020 12:17 AM
To: Quentin Colombet
Cc: LLVM Developers' List; Matt Arsenault
Subject: Re: [llvm-dev] Codegen pass configs dependent on function     
attributes?

I’ve put up a patch here: https://reviews.llvm.org/D79769 that adds a unified
pipeline that targets can opt-into. It has some similarities with forcing
fallbacks, but uses a different mechanism to do so to preserve the abort
behavior. It therefore requires that every GISel pass needs to explicitly check
whether the GISel selector is being requested rather than just using the
FailedISel property.

Cheers,
Amara

On May 5, 2020, at 6:56 PM, Quentin Colombet <qcolombet at
apple.com<mailto:qcolombet at apple.com>> wrote:

Hi Amara,

I’ve done that internally for testing purposes and basically the existing GISel
pipeline with fallbacks just did what I wanted.
In other words, I always used the GISel pipeline but I added an attribute that
triggered a fallback to SDISel for the functions I wanted to compile with
SDISel. I had to disable the global-isel-abort thing though. That said, that
should be easily fixable.

For the record, I was doing the attribute check in lowerFormalArgument IIRC.
I.e., something that is called for every single function at the beginning of the
GISel pipeline.

Cheers,
-Quentin

On May 5, 2020, at 2:48 PM, Amara Emerson <aemerson at
apple.com<mailto:aemerson at apple.com>> wrote:

Hi all.

I’m trying to get GlobalISel to work better with LTO. At the moment if you
enable it via -fglobal-isel, it only adds the -mllvm -global-isel and related
options to the cc1 invocation. With LTO, that doesn’t work as we need to encode
codegen options into the bitcode, usually via function attributes.

Does anyone have any ideas on how to achieve this? The only way I can see it
working is if we have a unified codegen pipeline for both GISel and
SelectionDAG. Then use a function attribute to tell the GISel passes to skip and
leave it to FastISel/SelectionDAG which runs afterwards. Likewise,
FastISel/SelectionDAG would need to skip the function if it was marked for GISel
compilation and we didn’t trigger a fallback.

Amara

Yes, per TU selection sounds like a reasonable compromise. I’ll look into that.

Cheers,
Amara
> On May 12, 2020, at 6:21 PM, Chen, Yuanfang <Yuanfang.Chen at
sony.com> wrote:
> 
> This looks like per-function codegen pipeline. If there are existing cases
of doing this, we could refer to these to implement it in an intuitive way.
Overall, I think it is a bit overreaching and brings more downsides than
benefits. If we do this per-TU during linking stage, could we read Module
metadata and pass the flag to TargetPassConfig through TargetMachine::Options?
> 
> ________________________________________
> From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of
Amara Emerson via llvm-dev <llvm-dev at lists.llvm.org>
> Sent: Tuesday, May 12, 2020 12:17 AM
> To: Quentin Colombet
> Cc: LLVM Developers' List; Matt Arsenault
> Subject: Re: [llvm-dev] Codegen pass configs dependent on function     
attributes?
> 
> I’ve put up a patch here: https://reviews.llvm.org/D79769 that adds a
unified pipeline that targets can opt-into. It has some similarities with
forcing fallbacks, but uses a different mechanism to do so to preserve the abort
behavior. It therefore requires that every GISel pass needs to explicitly check
whether the GISel selector is being requested rather than just using the
FailedISel property.
> 
> Cheers,
> Amara
> 
> On May 5, 2020, at 6:56 PM, Quentin Colombet <qcolombet at
apple.com<mailto:qcolombet at apple.com>> wrote:
> 
> Hi Amara,
> 
> I’ve done that internally for testing purposes and basically the existing
GISel pipeline with fallbacks just did what I wanted.
> In other words, I always used the GISel pipeline but I added an attribute
that triggered a fallback to SDISel for the functions I wanted to compile with
SDISel. I had to disable the global-isel-abort thing though. That said, that
should be easily fixable.
> 
> For the record, I was doing the attribute check in lowerFormalArgument
IIRC. I.e., something that is called for every single function at the beginning
of the GISel pipeline.
> 
> Cheers,
> -Quentin
> 
> On May 5, 2020, at 2:48 PM, Amara Emerson <aemerson at
apple.com<mailto:aemerson at apple.com>> wrote:
> 
> Hi all.
> 
> I’m trying to get GlobalISel to work better with LTO. At the moment if you
enable it via -fglobal-isel, it only adds the -mllvm -global-isel and related
options to the cc1 invocation. With LTO, that doesn’t work as we need to encode
codegen options into the bitcode, usually via function attributes.
> 
> Does anyone have any ideas on how to achieve this? The only way I can see
it working is if we have a unified codegen pipeline for both GISel and
SelectionDAG. Then use a function attribute to tell the GISel passes to skip and
leave it to FastISel/SelectionDAG which runs afterwards. Likewise,
FastISel/SelectionDAG would need to skip the function if it was marked for GISel
compilation and we didn’t trigger a fallback.
> 
> Amara
> 
>

On Tue, May 12, 2020 at 6:22 PM Chen, Yuanfang via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> This looks like per-function codegen pipeline. If there are existing cases
> of doing this, we could refer to these to implement it in an intuitive way.
> Overall, I think it is a bit overreaching and brings more downsides than
> benefits. If we do this per-TU during linking stage, could we read Module
> metadata and pass the flag to TargetPassConfig through
> TargetMachine::Options?
>
Do you mean per-original-source-TU? That amounts to per-function under LTO
(because LTO will merge functions from different TUs into one Module - so
then parts of a Module will do one thing, and parts will do the other).

If you want to do it on a Module-level, Module-level metadata can be used.
It supports merging/override, so if two front-end compilations asked for
different things (one asked for GlobalISel, one asked to not have it) you
could have a rule that either fails IR linking due to the conflict, or that
picks one way or the other (GlobalISel overrides if any input has picked
GlobalISel).

> ________________________________________
> From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of
Amara
> Emerson via llvm-dev <llvm-dev at lists.llvm.org>
> Sent: Tuesday, May 12, 2020 12:17 AM
> To: Quentin Colombet
> Cc: LLVM Developers' List; Matt Arsenault
> Subject: Re: [llvm-dev] Codegen pass configs dependent on function
> attributes?
>
> I’ve put up a patch here: https://reviews.llvm.org/D79769 that adds a
> unified pipeline that targets can opt-into. It has some similarities with
> forcing fallbacks, but uses a different mechanism to do so to preserve the
> abort behavior. It therefore requires that every GISel pass needs to
> explicitly check whether the GISel selector is being requested rather than
> just using the FailedISel property.
>
> Cheers,
> Amara
>
> On May 5, 2020, at 6:56 PM, Quentin Colombet <qcolombet at
apple.com<mailto:
> qcolombet at apple.com>> wrote:
>
> Hi Amara,
>
> I’ve done that internally for testing purposes and basically the existing
> GISel pipeline with fallbacks just did what I wanted.
> In other words, I always used the GISel pipeline but I added an attribute
> that triggered a fallback to SDISel for the functions I wanted to compile
> with SDISel. I had to disable the global-isel-abort thing though. That
> said, that should be easily fixable.
>
> For the record, I was doing the attribute check in lowerFormalArgument
> IIRC. I.e., something that is called for every single function at the
> beginning of the GISel pipeline.
>
> Cheers,
> -Quentin
>
> On May 5, 2020, at 2:48 PM, Amara Emerson <aemerson at
apple.com<mailto:
> aemerson at apple.com>> wrote:
>
> Hi all.
>
> I’m trying to get GlobalISel to work better with LTO. At the moment if you
> enable it via -fglobal-isel, it only adds the -mllvm -global-isel and
> related options to the cc1 invocation. With LTO, that doesn’t work as we
> need to encode codegen options into the bitcode, usually via function
> attributes.
>
> Does anyone have any ideas on how to achieve this? The only way I can see
> it working is if we have a unified codegen pipeline for both GISel and
> SelectionDAG. Then use a function attribute to tell the GISel passes to
> skip and leave it to FastISel/SelectionDAG which runs afterwards. Likewise,
> FastISel/SelectionDAG would need to skip the function if it was marked for
> GISel compilation and we didn’t trigger a fallback.
>
> Amara
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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