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

[Jingyue Wu via llvm-dev]

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
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