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

[Robinson, Paul via llvm-dev]

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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