[llvm-dev] CFLAA

[David Callahan via llvm-dev]

(Adding "LLVM Dev")

My variant is up as https://reviews.llvm.org/D23876
-david


From: George Burgess IV <george.burgess.iv at gmail.com<mailto:george.burgess.iv at gmail.com>>
Date: Wednesday, August 24, 2016 at 3:17 PM
To: David Callahan <dcallahan at fb.com<mailto:dcallahan at fb.com>>
Subject: Re: CFLAA

Hi!

> I see there is on going work with alias analysis and it appears the prior CFLAA has been abandoned.

There was quite a bit of refactoring done, yeah. The original CFLAA is now called CFLSteens, and graph construction was moved to its own bit. We also have CFLAnders, which is based more heavily on the paper by Zheng and Rugina (e.g. no stratifiedsets magic).

> I have a variant of it where I reworked how compression was done to be less conservative, reworked the interprocedural to do simulated but bounded inlining, and added code to do on-demand testing of CFL paths on both compressed and full graphs.

Awesome!

> Happy to share the patch with you if you are interested as well as some data collected

Yes, please. Would you mind if I CC'ed llvm-dev on this thread (and a few people specifically, who also might find this interesting)?

> However I was not able to see any performance improvements in the code. In fact on a various benchmarks there were noticeable regressions in measured performance of the generated code. Have you noticed any similar problems?

I know that a number of people people in the community expressed concerns about how other passes will perform with better AA results (e.g. If LICM becomes more aggressive, register pressure may increase, which may cause us to spill when we haven't before, etc). So, such a problem isn't unthinkable. :)

On Wed, Aug 24, 2016 at 2:56 PM, David Callahan <dcallahan at fb.com<mailto:dcallahan at fb.com>> wrote:
Hi Greg,

I see there is on going work with alias analysis and it appears the prior CFLAA has been abandoned.

I have a variant of it where I reworked how compression was done to be less conservative, reworked the interprocedural to do simulated but bounded inlining, and added code to do on-demand testing of CFL paths on both compressed and full graphs.

I reached a point where the ahead-of-time compression was linear but still very accurate compared to on-demand path search and there were noticeable improvements in the alias analysis results and impacted transformations.  Happy to share the patch with you if you are interested as well as some data collected.

However I was not able to see any performance improvements in the code. In fact on a various benchmarks there were noticeable regressions in measured performance of the generated code. Have you noticed any similar problems?

--david

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