llvm dev - Jun 2019 - RFC: Memcpy inlining in IR

Hi all,

For GlobalISel, we’re exploring options for implementing inlining optimizations
for memcpy and friends. However, looking around the existing implementation, I
don’t see anything that would particularly be problematic for us to do it at the
IR level.

The existing TLI hooks to specify how certain memcpy calls should be lowered
doesn’t have anything too SelectionDAG specific, and an IR lowering pass could
be shared in future between SDAG and GISel. Does anyone see issues with this?

Thanks,
Amara

On Thu, Jun 20, 2019 at 6:42 AM Amara Emerson via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> Hi all,
>
> For GlobalISel, we’re exploring options for implementing inlining
optimizations for memcpy and friends. However, looking around the existing
implementation, I don’t see anything that would particularly be problematic for
us to do it at the IR level.
>
> The existing TLI hooks to specify how certain memcpy calls should be
lowered doesn’t have anything too SelectionDAG specific, and an IR lowering pass
could be shared in future between SDAG and GISel. Does anyone see issues with
this?
Sounds similar to https://reviews.llvm.org/D60318
It should be done *really* late in the middle-end pipeline though.
> Thanks,
> Amara
Roman.
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

Looks like there are a lot of opinions where memcpy expansion/inlining needs to
happen: (late) IR, or if it is a backend problem, see also for example
https://reviews.llvm.org/D35035. Complicating factor here is that efficient
memcpy lowering is crucial for performance and code-size (and they occur a lot).

Either way, I agree that the TLI hooks are not SelectionDAG specific, they can
be used in an IR lowering pass.

Cheers,
Sjoerd.

________________________________
From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Roman
Lebedev via llvm-dev <llvm-dev at lists.llvm.org>
Sent: 20 June 2019 08:04
To: Amara Emerson
Cc: llvm-dev
Subject: Re: [llvm-dev] RFC: Memcpy inlining in IR

On Thu, Jun 20, 2019 at 6:42 AM Amara Emerson via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> Hi all,
>
> For GlobalISel, we’re exploring options for implementing inlining
optimizations for memcpy and friends. However, looking around the existing
implementation, I don’t see anything that would particularly be problematic for
us to do it at the IR level.
>
> The existing TLI hooks to specify how certain memcpy calls should be
lowered doesn’t have anything too SelectionDAG specific, and an IR lowering pass
could be shared in future between SDAG and GISel. Does anyone see issues with
this?
Sounds similar to https://reviews.llvm.org/D60318
It should be done *really* late in the middle-end pipeline though.
> Thanks,
> Amara
Roman.
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
_______________________________________________
LLVM Developers mailing list
llvm-dev at lists.llvm.org
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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> On Jun 19, 2019, at 11:41 PM, Amara Emerson via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> Hi all,
> 
> For GlobalISel, we’re exploring options for implementing inlining
optimizations for memcpy and friends. However, looking around the existing
implementation, I don’t see anything that would particularly be problematic for
us to do it at the IR level.
> 
> The existing TLI hooks to specify how certain memcpy calls should be
lowered doesn’t have anything too SelectionDAG specific, and an IR lowering pass
could be shared in future between SDAG and GISel. Does anyone see issues with
this?
> 
> Thanks,
> Amara
We already have lib/Transforms/Utils/LowerMemIntrinsics.cpp, there just isn’t a
general pass that expands these for targets. AMDGPU already always use this for
memcpy handling.


-Matt

> On Jun 20, 2019, at 3:54 PM, Matt Arsenault <arsenm2 at gmail.com>
wrote:
> 
> 
> 
>> On Jun 19, 2019, at 11:41 PM, Amara Emerson via llvm-dev <llvm-dev
at lists.llvm.org> wrote:
>> 
>> Hi all,
>> 
>> For GlobalISel, we’re exploring options for implementing inlining
optimizations for memcpy and friends. However, looking around the existing
implementation, I don’t see anything that would particularly be problematic for
us to do it at the IR level.
>> 
>> The existing TLI hooks to specify how certain memcpy calls should be
lowered doesn’t have anything too SelectionDAG specific, and an IR lowering pass
could be shared in future between SDAG and GISel. Does anyone see issues with
this?
>> 
>> Thanks,
>> Amara
> 
> We already have lib/Transforms/Utils/LowerMemIntrinsics.cpp, there just
isn’t a general pass that expands these for targets. AMDGPU already always use
this for memcpy handling.
> 
> 
> -Matt
> 
Sure, that might end up sharing some code but the key thing is to use the TLI
hooks to implement the same optimizations that SelectionDAG currently does.

Amara