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

Fri Aug 7 17:33:46 PDT 2015

On Aug 7, 2015, at 8:28 PM, Hal Finkel <hfinkel at anl.gov<mailto:hfinkel at anl.gov>> wrote:

________________________________
From: "Jingyue Wu" <jingyue at google.com<mailto:jingyue at google.com>>
To: "Matt Arsenault" <Matthew.Arsenault at amd.com<mailto:Matthew.Arsenault at amd.com>>
Cc: llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>, "Hal Finkel" <hfinkel at anl.gov<mailto:hfinkel at anl.gov>>, "Justin Holewinski" <jholewinski at nvidia.com<mailto:jholewinski at nvidia.com>>, "Eli Bendersky" <eliben at google.com<mailto:eliben at google.com>>, "Xuetian Weng" <xweng at google.com<mailto:xweng at google.com>>
Sent: Friday, August 7, 2015 3:44:29 PM
Subject: Re: [RFC] BasicAA considers address spaces?

On Fri, Aug 7, 2015 at 12:01 PM, Matt Arsenault <Matthew.Arsenault at amd.com<mailto:Matthew.Arsenault at amd.com>> wrote:
On 08/07/2015 11:35 AM, Jingyue Wu wrote:
+ the new llvm-dev

On Fri, Aug 7, 2015 at 11:30 AM, Jingyue Wu <jingyue at google.com<mailto:jingyue at google.com>> wrote:
Hi folks,

Unsurprisingly, leveraging the fact that certain address spaces don't alias can significantly improve alias analysis precision and enhance (observably 2x performance gain) load/store optimizations such as LICM and DSE.

This sounds to me an overdue feature. I saw several discussion threads on that direction, but none of them really happened.

(1) http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20111010/129615.html. Justin Holewinski proposed to add an address-space alias analysis that treats pointers in different address spaces not aliasing. This patch got shot down because, in some targets, address spaces may alias. For example, in CUDA+NVPTX, addrspace(0) aliases everyone.

(2) http://lists.cs.uiuc.edu/pipermail/llvmdev/2013-August/064620.html. Michele Scandale proposed address space extensions in IR metadata which TBAA could then leverage to prove non-aliasing. This approach didn't fly either because address spaces are target-specific. The front end doesn't know enough to decide aliasing.

So, can we make BasicAA to consider address spaces? Specifically, I am proposing:
a) adding a new interface like TTI::addressSpacesAlias(unsigned, unsigned), and
b) adding a little piece of logic in BasicAA that reports "no alias" if address spaces don't alias.

This approach addresses the issue brought up in (2) because TTI can see the entire codegen. It also resolves the issue that shut down (1) because it allows address spaces to alias in a target-defined way. Actually, John Criswell did mention in that thread the idea of embedding alias info in TargetData. Now that we have TTI, it seems a better place to hold target-specific alias info than DataLayout.

Any comments?

Jingyue

We definitely need something to handle this.

I think a TTI::addressSpaceAlias is a good idea, although I don't think this fully solves the language vs. target address space distinction a metadata based approach was supposed to handle although it would be easier to implement. For our GPU purposes this would mostly be enough. For example, the fact that in OpenCL local vs. global pointers won't alias is a useful distinction, even though for a CPU target those will all be mapped to

Thanks for pointing this out, Matt. In that case, I'd suggest LLVM have both TTI- and metadata-based approaches, the former for targets being more knowledgeable, and the latter for front-ends being more knowledgeable. They are quite orthogonal.

I don't understand this. If the frontend has more target knowledge than the target, something seems wrong. Could you please provide an example of when this could be a useful setup, and such knowledge should not be moved into the target?

Part of the problem is the loss of information going from source to a target-dependent representation.  If you compile an OpenCL kernel to LLVM IR for x86, both global and local memory may map to address space 0.  So the target would not be able to state that a pointer to global memory does not alias a pointer to local memory.  But in the source language, you do have that information.

 -Hal

address space 0 so in some cases the frontend knows more about aliasing address spaces than the target.

-Matt

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

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