[llvm-dev] BasicAA depending on PhiValues analysis

[Alexey Zhikhartsev via llvm-dev]

Hi all,

I encountered a problem where an optimization that uses BasicAA is very
fragile due to BasicAA's "weak" reliance on the PhiValues analysis.

The whole story is this: our team is in the process of upgrading to the
newest version of LLVM, and I discovered that on the newest LLVM, Global
Value Numbering doesn't apply an important optimization anymore.

A little bit of background: GVN depends on Basic Alias Analysis, and
BasicAA depends on PhiValues. However, the dependence between GVN and AA is
"strong":

   getAnalysis<AAResultsWrapperPass>().getAAResults()

Whereas between BasicAA and PhiValues it is "weak" (note "IfAvailable"):

   auto *PVWP = getAnalysisIfAvailable<PhiValuesWrapperPass>();

So, in the old LLVM, BasicAA that runs before GVN, happens to find cached
PhiValues analysis, gives good results to GVN, and GVN applies the
optimization. But on the new LLVM, BasicAA doesn't have PhiValues, gives
poor results to GVN, and GVN gives up. It looks to me that whether BasicAA
gets its hands on PhiValues is almost entirely up to chance, which makes
the clients of BasicAA very fragile, meaning that it's hard to be sure if
an optimization you need will get applied.

I see the discussion in https://reviews.llvm.org/D44564 but I'm pretty much
out of my depth there and I'm not sure if a solution to the problem above
has been mentioned.

Maybe the new pass manager has this problem solved?

Anyhow, I would appreciate any input, thank you in advance.

Best,
Alex
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