[all-commits] [llvm/llvm-project] a3904c: [BasicAA] Handle recursive queries more efficiently

[Nikita Popov via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: a3904cc77f181cff7355357688edfc392a236f5d
      https://github.com/llvm/llvm-project/commit/a3904cc77f181cff7355357688edfc392a236f5d
  Author: Nikita Popov <nikita.ppv at gmail.com>
  Date:   2021-01-14 (Thu, 14 Jan 2021)

  Changed paths:
    M llvm/include/llvm/Analysis/BasicAliasAnalysis.h
    M llvm/lib/Analysis/BasicAliasAnalysis.cpp
    M llvm/lib/Analysis/GlobalsModRef.cpp

  Log Message:
  -----------
  [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