[LLVMdev] AliasAnalysis update interface - a tale of sorrow and woe

[Chandler Carruth]

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 report_fatal_error 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
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