[llvm-dev] [RFC] BasicAA considers address spaces?

Wed Aug 12 14:53:39 PDT 2015

[+Chandler]

----- Original Message -----
> From: "Jingyue Wu" <jingyue at google.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: llvm-dev at lists.llvm.org, "Justin Holewinski" <jholewinski at nvidia.com>
> Sent: Wednesday, August 12, 2015 3:39:52 PM
> Subject: Re: [llvm-dev] [RFC] BasicAA considers address spaces?
> 
> Then, we need to run this target-provided AA under -O3 to benefit the
> optimizations in O3.
> 
> I'm not sure what's the best practice of doing this. One way is pass
> 1) add a new interface TTI.getTargetAwareAliasAnalysis that returns
> the target-provided AA or nullptr
> 2) Embed TTI in PassManagerBuilder
> 3) When PassManagerBuilder::addInitialAliasAnalysisPass adds
> TTI.getTargetAwareAliasAnalysis to the pipeline.
> 
> Any better ideas?

I've cc'd Chandler so he can comment here.

Currently, TargetMachine can optionally return a TargetIRAnalysis object; this is then provided a function for which it can produce the actual TargetTransformInfo-derived object. TargetTransformInfoWrapperPass is the actual pass, and it just holds a pointer to the TargetIRAnalysis object.

In our case, we'd like to provide the target a way to add some AA pass to take advantage of target-specific knowledge regarding address spaces, intrinsics, and other IR constructs whose meaning is completely target specific. I'm not sure how well it fits into this model.

In addition, we have other use cases where targets want to add passes to handle target-specific things into the pipeline (http://reviews.llvm.org/D11782, for example, for doing target-specific loop-idiom recognition).

Plus, we already have a mechanism for allowing extension of the optimization pipeline, and I think the best way of approaching this (which is also useful for other use cases), is to provide the targets the ability to use this mechanism. I understand that this could be ripe for abuse (so we'd need to be vigilant in watching the targets), but nevertheless, how about this:

 1. Add a function to TargetMachine called, for example, registerPassManagerBuilderExtensions(PassManagerBuilder *PMB). This would need to be called, for example, by the code in EmitAssemblyHelper in Clang in tools/clang/lib/CodeGen/BackendUtil.cpp (just as it currently calls TM->getTargetIRAnalysis() and so that it can call createTargetTransformInfoWrapperPass) and also registers other extensions itself.

 2. registerPassManagerExtensions's job will be to call PassManagerBuilder::addExtension (or addGlobalExtension) as appropriate.

 3. Create a new extension point for AA passes, and call addExtensionsToPM inside PassManagerBuilder::addInitialAliasAnalysisPasses

This will automatically cover a wide variety of use cases, including this one. Thoughts?

 -Hal

> 
> 
> On Wed, Aug 12, 2015 at 12:24 PM, Hal Finkel via llvm-dev <
> llvm-dev at lists.llvm.org > wrote:
> 
> 
> ----- Original Message -----
> > From: "Hal Finkel via llvm-dev" < llvm-dev at lists.llvm.org >
> > To: "Daniel Berlin" < dberlin at dberlin.org >
> > Cc: llvm-dev at lists.llvm.org , "Justin Holewinski" <
> > jholewinski at nvidia.com >
> > Sent: Wednesday, August 12, 2015 2:06:41 PM
> > Subject: Re: [llvm-dev] [RFC] BasicAA considers address spaces?
> > 
> > ----- Original Message -----
> > > From: "Daniel Berlin" < dberlin at dberlin.org >
> > > To: "Jingyue Wu" < jingyue at google.com >
> > > Cc: "Hal Finkel" < hfinkel at anl.gov >, llvm-dev at lists.llvm.org ,
> > > "Justin Holewinski" < jholewinski at nvidia.com >
> > > Sent: Wednesday, August 12, 2015 2:03:34 PM
> > > Subject: Re: [llvm-dev] [RFC] BasicAA considers address spaces?
> > > 
> > > SGTM
> > 
> > +1
> 
> Actually, upon further reflection, I don't we should create a new AA
> interface for this unless that's really necessary. AA passes can
> currently provide alias, pointsToConstantMemory, getArgModRefInfo,
> etc. and the target can quite reasonably want to enhance many of
> these.
> 
> Unless there's some reason why we can't, I think we should provide
> the target the ability to provide an AA analysis pass to be inserted
> in the chain, or maybe TTI should just *be* an AA pass that gets
> inserted into the chain?
> 
> -Hal
> 
> 
> 
> > 
> > -Hal
> > 
> > > 
> > > On Wed, Aug 12, 2015 at 12:00 PM, Jingyue Wu via llvm-dev
> > > < llvm-dev at lists.llvm.org > wrote:
> > > > I was lost from the thread at some point.
> > > > 
> > > > Making the interface more general sounds good to me. This helps
> > > > to
> > > > solve
> > > > Escha's concern that targets can know more about aliasing than
> > > > just
> > > > comparing address spaces.
> > > > 
> > > > If there are no objections, I'll
> > > > 1) add a new interface to TTI such as isTriviallyDisjoint. It
> > > > returns false
> > > > by default.
> > > > 2) create a new AA that checks this interface, and add it to
> > > > the
> > > > AA
> > > > chain.
> > > > It could be named TargetAwareAliasAnalysis.
> > > > 
> > > > Jingyue
> > > > 
> > > > On Sun, Aug 9, 2015 at 2:34 PM, Hal Finkel via llvm-dev
> > > > < llvm-dev at lists.llvm.org > wrote:
> > > >> 
> > > >> ----- Original Message -----
> > > >> > From: "escha" < escha at apple.com >
> > > >> > To: "Hal Finkel" < hfinkel at anl.gov >
> > > >> > Cc: "Matt Arsenault" < Matthew.Arsenault at amd.com >,
> > > >> > llvm-dev at lists.llvm.org , "Justin Holewinski"
> > > >> > < jholewinski at nvidia.com >
> > > >> > Sent: Sunday, August 9, 2015 7:46:26 AM
> > > >> > Subject: Re: [llvm-dev] [RFC] BasicAA considers address
> > > >> > spaces?
> > > >> > 
> > > >> > Personally I feel the most intuitive approach would be to
> > > >> > have
> > > >> > an
> > > >> > equivalent of isTriviallyDisjoint for IR; we already have a
> > > >> > model
> > > >> > for how it would work, and it could be a TTI call. I’ve kind
> > > >> > of
> > > >> > wanted this for a while because there’s a lot of
> > > >> > address-space-esque
> > > >> > aliasing relationships that can’t be easily modeled on the
> > > >> > IR
> > > >> > level.
> > > >> > 
> > > >> > For example (in our model), we have some constraints like
> > > >> > this:
> > > >> > 
> > > >> > Global memory can’t alias local memory.
> > > >> > Global writeable memory can’t alias global readonly memory
> > > >> > (different
> > > >> > address spaces).
> > > >> > Stack memory can’t alias global memory (different address
> > > >> > spaces).
> > > >> > Texture reads cannot alias texture writes, because you can’t
> > > >> > bind a
> > > >> > texture as readable and writeable at the same time. Texture
> > > >> > writes,
> > > >> > however, can alias each other.
> > > >> > Vertex shader outputs can’t really alias each other, even
> > > >> > though
> > > >> > they
> > > >> > are technically “stores”.
> > > >> > (there’s more where that came from)
> > > >> > 
> > > >> > These are all very trivial to express in code (the trivially
> > > >> > disjoint
> > > >> > function in our backend is like 50 lines of code to cover
> > > >> > all
> > > >> > the
> > > >> > cases), but a few of them are slightly more complex than
> > > >> > “address
> > > >> > space A can’t alias B”, so having a generic callback might
> > > >> > be
> > > >> > nicer
> > > >> > and more powerful than a “does address space A alias address
> > > >> > space
> > > >> > B” callback, I think.
> > > >> 
> > > >> Could you provide a specific example of a case where the
> > > >> address
> > > >> space is
> > > >> not enough? [maybe you did above, but if so, I don't know
> > > >> which
> > > >> one].
> > > >> 
> > > >> Perhaps we should just do the most generic thing: Provide an
> > > >> AA/TTI shim
> > > >> which allows any target provide an implementation of AA (as
> > > >> part
> > > >> of the
> > > >> chain of AA passes). Thoughts?
> > > >> 
> > > >> -Hal
> > > >> 
> > > >> > 
> > > >> > —escha
> > > >> 
> > > >> --
> > > >> Hal Finkel
> > > >> Assistant Computational Scientist
> > > >> Leadership Computing Facility
> > > >> Argonne National Laboratory
> > > >> _______________________________________________
> > > >> LLVM Developers mailing list
> > > >> llvm-dev at lists.llvm.org http://llvm.cs.uiuc.edu
> > > >> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> > > > 
> > > > 
> > > > 
> > > > _______________________________________________
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> > > > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> > > > 
> > > 
> > 
> > --
> > Hal Finkel
> > Assistant Computational Scientist
> > Leadership Computing Facility
> > Argonne National Laboratory
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org http://llvm.cs.uiuc.edu
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> > 
> 
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org http://llvm.cs.uiuc.edu
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> 
> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory