llvm dev - Aug 2017 - [cfe-dev] Disable memset synthesis

On Tue, Aug 15, 2017 at 9:37 PM, Tim Northover via cfe-dev <
cfe-dev at lists.llvm.org> wrote:
> On 15 August 2017 at 19:38, bharathi seshadri via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > I find that GCC has an option -fno-tree-loop-distribute-patterns that
> > can be used to disable memcpy/memset synthesis. I wonder if there is
> > something similar in llvm/clang.
>
> I have no idea what that means, but we almost certainly don't have any
> option with similar semantics. Clang does not provide options to
> control specific optimization passes like that.
>
I think Sony exposes an option to disable idiom recognition in PS4
compiler. This seems like one of those areas where users keep asking for
something and we keep insisting that what they think they want isn't
actually what they want, i.e. disabling idiom recognition blocks mid-level
canonicalization and that leads to missing optimizations and bad
performance, etc. However, the user feedback has been persistent, and in
the interests of not having to hear about it again, we might want to
consider giving users the rope they need to hang themselves. It would let
them work around real performance problems today rather than waiting for
the next version of the compiler that will lower memset/memcpy/memcmp
better.
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Right, it's a balance between those snooty compiler developers who think
they always know best versus those pesky real-world code authors who think they
can hand-tune their code to do better than what the compiler comes up with.
Offhand I don't know how often our licensees use the option in production,
but it surely gives them a tool that lets them do their own measurements, and
only come back to us when there is something worthwhile to report.
--paulr

From: cfe-dev [mailto:cfe-dev-bounces at lists.llvm.org] On Behalf Of Reid
Kleckner via cfe-dev
Sent: Wednesday, August 16, 2017 1:39 PM
To: Tim Northover
Cc: LLVM Developers Mailing List; cfe-dev at lists.llvm.org Developers; bharathi
seshadri
Subject: Re: [cfe-dev] [llvm-dev] Disable memset synthesis

On Tue, Aug 15, 2017 at 9:37 PM, Tim Northover via cfe-dev <cfe-dev at
lists.llvm.org<mailto:cfe-dev at lists.llvm.org>> wrote:
On 15 August 2017 at 19:38, bharathi seshadri via llvm-dev
<llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
wrote:
> I find that GCC has an option -fno-tree-loop-distribute-patterns that
> can be used to disable memcpy/memset synthesis. I wonder if there is
> something similar in llvm/clang.
I have no idea what that means, but we almost certainly don't have any
option with similar semantics. Clang does not provide options to
control specific optimization passes like that.

I think Sony exposes an option to disable idiom recognition in PS4 compiler.
This seems like one of those areas where users keep asking for something and we
keep insisting that what they think they want isn't actually what they want,
i.e. disabling idiom recognition blocks mid-level canonicalization and that
leads to missing optimizations and bad performance, etc. However, the user
feedback has been persistent, and in the interests of not having to hear about
it again, we might want to consider giving users the rope they need to hang
themselves. It would let them work around real performance problems today rather
than waiting for the next version of the compiler that will lower
memset/memcpy/memcmp better.
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My concern wasn't a phylosophical one but a pragmatic one. Learning about
poor choices when lowering memset is probably quite useful. Having a flag
that just turns off idiom recognition for it may just work around the
problem. But the problem may still exist.
In any case, I'm not fundamentally against such a flag but it just seems
like something that could
1. Hide a problem
2. Get a bit unwieldy - today it's memset, maybe tomorrow memcpy, etc. And
then does a single flag turn off all idiom recognition? A separate flag for
each? Maybe groups (i.e. Memory functions). And so on.

On Thu, Aug 17, 2017 at 7:29 PM Robinson, Paul via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Right, it's a balance between those snooty compiler developers who
think
> they always know best versus those pesky real-world code authors who think
> they can hand-tune their code to do better than what the compiler comes up
> with.
>
> Offhand I don't know how often our licensees use the option in
production,
> but it surely gives them a tool that lets them do their own measurements,
> and only come back to us when there is something worthwhile to report.
>
> --paulr
>
>
>
> *From:* cfe-dev [mailto:cfe-dev-bounces at lists.llvm.org] *On Behalf Of
*Reid
> Kleckner via cfe-dev
> *Sent:* Wednesday, August 16, 2017 1:39 PM
> *To:* Tim Northover
> *Cc:* LLVM Developers Mailing List; cfe-dev at lists.llvm.org Developers;
> bharathi seshadri
> *Subject:* Re: [cfe-dev] [llvm-dev] Disable memset synthesis
>
>
>
> On Tue, Aug 15, 2017 at 9:37 PM, Tim Northover via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
> On 15 August 2017 at 19:38, bharathi seshadri via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > I find that GCC has an option -fno-tree-loop-distribute-patterns that
> > can be used to disable memcpy/memset synthesis. I wonder if there is
> > something similar in llvm/clang.
>
> I have no idea what that means, but we almost certainly don't have any
> option with similar semantics. Clang does not provide options to
> control specific optimization passes like that.
>
>
>
> I think Sony exposes an option to disable idiom recognition in PS4
> compiler. This seems like one of those areas where users keep asking for
> something and we keep insisting that what they think they want isn't
> actually what they want, i.e. disabling idiom recognition blocks mid-level
> canonicalization and that leads to missing optimizations and bad
> performance, etc. However, the user feedback has been persistent, and in
> the interests of not having to hear about it again, we might want to
> consider giving users the rope they need to hang themselves. It would let
> them work around real performance problems today rather than waiting for
> the next version of the compiler that will lower memset/memcpy/memcmp
> better.
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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