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

[John McCall via llvm-dev]

> On Aug 17, 2017, at 1:56 PM, Nemanja Ivanovic via cfe-dev <cfe-dev at lists.llvm.org> wrote:
> 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. 

Right.  If someone can come up with a reasonable limiting principle here, that's one thing, but we do not want to end up with endless options to enable and disable individual optimizations from individual passes.

There are always ways to work around optimizations that seem to actually pessimize code.  In this case, you can probably just use volatile stores, but when there isn't such a convenient idiomatic solution, empty inline asm blocks will almost always do the trick.  The ability to subvert the optimizer like that is really crucial — for many reasons, really, but most importantly because it's usually a much better solution for the user.  In the short term, users want to get on with their lives without waiting for a new toolchain drop.  In the medium term, it's really nice for workarounds to be local (i.e. written explicitly in a specific piece of code) rather than global (passing a command-line option) because local changes only affect one piece of code and can be clearly commented (with intent, purpose, and a link to the compiler bug), whereas command-line options affect everything and have a way of continually accumulating until someone comes along ten years later wondering why you're building ten million lines of code with -fbifurcate-endosomes.

John.

> 
> On Thu, Aug 17, 2017 at 7:29 PM Robinson, Paul via llvm-dev <llvm-dev at lists.llvm.org <mailto: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 <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 <mailto: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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