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[BasicAA] Handle recursive queries more efficiently

Authored by nikic on Nov 10 2020, 11:43 AM.

Description

[BasicAA] Handle recursive queries more efficiently

An alias query currently works out roughly like this:

  • Look up location pair in cache.
  • Perform BasicAA logic (including cache lookup and insertion...)
  • Perform a recursive query using BestAAResults.
    • Look up location pair in cache (and thus do not recurse into BasicAA)
    • Query all the other AA providers.
  • Query all the other AA providers.

This is a lot of unnecessary work, all ultimately caused by the
BestAAResults query at the end of aliasCheck(). The reason we perform
it, is that aliasCheck() is getting called recursively, and we of
course want those recursive queries to also make use of other AA
providers, not just BasicAA. We can solve this by making the recursive
queries directly use BestAAResults (which will check both BasicAA
and other providers), rather than recursing into aliasCheck().

There are some tradeoffs:

  • We can no longer pass through the precomputed underlying object to aliasCheck(). This is not a major concern, because nowadays getUnderlyingObject() is quite cheap.
  • Results from other AA providers are no longer cached inside BasicAA. The way this worked was already a bit iffy, in that a result could be cached, but if it was MayAlias, we'd still end up re-querying other providers anyway. If we want to cache non-BasicAA results, we should do that in a more principled manner.

In any case, despite those tradeoffs, this works out to be a decent
compile-time improvment. I think it also simplifies the mental model
of how BasicAA works. It took me quite a while to fully understand
how these things interact.

Differential Revision: https://reviews.llvm.org/D90094