[LLVMdev] AliasAnalysis update interface - a tale of sorrow and woe (original) (raw)

Hal Finkel hfinkel at anl.gov
Wed Jul 1 09:37:46 PDT 2015

----- Original Message -----

From: "Daniel Berlin" <dberlin at dberlin.org> To: "Chandler Carruth" <chandlerc at gmail.com> Cc: "LLVM Developers Mailing List" <llvmdev at cs.uiuc.edu>, "Hal Finkel" <hfinkel at anl.gov>, "Daniel Berlin" <dannyb at google.com> Sent: Wednesday, July 1, 2015 11:31:49 AM Subject: Re: [LLVMdev] AliasAnalysis update interface - a tale of sorrow and woe

My opinion: The interface is essentially broken and unused. We know it doesn't work. We should remove it. Given that we need to sit down and think about stateful alias analysis due to the new pass manager, it seems like a good time to do that.

+1

-Hal

On Wed, Jul 1, 2015 at 1:18 AM, Chandler Carruth <chandlerc at gmail.com> wrote: > Greetings folks, > > As I'm working on refactoring the AA interfaces in LLVM to prepare > for the > new pass manager, I keep hitting issues. Some of the complexity > that is > hitting this stems from the update API, which consists of three > virtual > functions: > > deleteValue(Value *V) > copyValue(Value *From, Value *To) > addEscapingUse(Use &U) > > These interfaces are very rarely called. Here are the only passes > that > ever bother to use these: > - MemoryDependenceAnalysis.cpp > - BasicBlockUtils.cpp > - LoopSimplify.cpp > - MergedLoadStoreMotion.cpp > - GVN.cpp > > That's it. This is not a complete set of code that can delete a > load or > store, nor a complete list of things which can fold something into > a Phi > node or otherwise trigger an "escaping" use. Maybe these interfaces > used to > be much more widely used prior to the introduction of > AliasSetTracker? Now > that utility is used instead. Either way, basic things like CSE, > store->load > forwarding in instcombine, etc., all seem like they should be > updating AA, > but they aren't.... > > So, if these update APIs are really important, I'm pretty sure that > LLVM is > completely broken already... > > But in fact, almost nothing overrides these APIs. The only pass in > tree that > uses them at all is GlobalsModRef, which might explain why the > world hasn't > melted due to us not calling these update routines reliably. > > So I looked into GlobalsModRef to understand why it is overriding > these. It > doesn't override copyValue at all. That API point appears to be > completely > dead. So my first question is: can I remove copyValue? Is there > some out of > tree user that desperately needs this? If so, I'd like to > understand why. > > The APIs it does override are deleteValue to nix things in its > cache, and > addEscapingUse, which it implements by just deleting the used value > from its > cache. This last one seems the most interesting. Only GVN calls it, > but GVN > does call it. So I commented out the code in addEscapingUse so that > it > became a no-op. No test failed. =/ So I added an assert to its > addEscapingUse method. No test failed. So I added a > reportfatalerror to > the addEscapingUse method and did an LTO run over clang's bitcode > which > finally did reach this code path. > > addEscapingUse was added in 2011 by r122787 without any test case. > There is > no mention of this fixing a bug. It looks like it may have been > intended to > support something thata was never added to the tree. > > So I'd like to remove addEscapingUse since we used to not have it, > and we've > never bothered to test it and I can't get anything to fail without > it, and > GlobalsModRef is the only user and that is only reached during LTO. > Thoughts? > > My final question is deleteValue. Again, only GlobalsModRef > implements this > extension point. This was added in 2006 in r30684, also without any > changes > to any test. It claims to implement a test file, but that test > isn't changed > in the commit. Indeed, commenting out the body of deleteValue in > GlobalsModRef causes no test to fail. =/ Fortunately, putting an > assert here > does trip in the regression test suite, so the code is reached, > just not > exercised. > > Either way, the use case for the deleteValue at least makes perfect > sense. > But there is (IMO) a much better way to accomplish the same task: > use a > ValueHandle to trigger the update on deletion. This will prevent > widespread > failure to use the deleteValue API to update alias analysis. So I > would like > to replace deleteValue with a ValueHandle approach. But I'm more > than a > little nervous implementing this, *as no test actually uses the > behavior*! > At least the code is reached... > > I think generally, the update API in the AliasAnalysis interface > isn't > working well. It is poorly and incompletely used. It isn't even > clear that > it is necessary in the face of tools like value handles. Thoughts > about just > removing the entire thing and falling back to value handles > everywhere? > > -Chandler > _> ________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev >

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

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