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

Wed Aug 12 15:02:31 PDT 2015

----- Original Message -----
> From: "Daniel Berlin" <dberlin at dberlin.org>
> 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 4:36:54 PM
> Subject: Re: [llvm-dev] [RFC] BasicAA considers address spaces?
> 
> So, the one downside of doing it this way is that the information
> never will get to the other parts of the world.
> We definitely don't want to go crazy and have tons of target-specific
> alias info encoded into the AA interface.
> 
> I agree also, in general, this belongs in an AA pass and not the
> interface (IE i don't think basicaa needs it).
> 
> However, for things like "memory spaces being disjoint", some other
> AA
> passes may want to be able to know that.
> IE If CFL-AA can tell that memory spaces are disjoint, it could avoid
> unioning in stuff that can't really alias.  Without such a hook, it
> would be relegated to calling something like alias() on every pair :P
> 
> So i'm torn on how best to make that happen, without people going off
> and completely making things like basicaa super target-specific using
> hooks (such that it ends up providing pretty crappy answers unless
> you
> implement 60 different target hooks)

We might be able to consider address spaces somewhat special in this regard -- they're already somewhat special. Like target-specific intrinsics, they are an IR-level feature with target-defined semantics. As a result, I feel it would be justified to add some additional IR-level support to provide some more information to the generic IR analysis. We could, for example, use module-level metadata to encode address-space disjointness information, and that could be leveraged by BasicAA, etc.

This fits with the role of metadata, in that if the metadata were dropped, the IR analysis would simply fallback to making the same kinds of conservative decisions it makes now.

 -Hal

> 
> On Wed, Aug 12, 2015 at 12:24 PM, Hal Finkel <hfinkel at anl.gov> 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
> >> > >
> >> > >
> >> > >
> >> > > _______________________________________________
> >> > > 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
> 

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