[PATCH] D44715: [MemorySSA] Fix exponential compile-time updating MemorySSA.

[Eli Friedman via Phabricator via llvm-commits]

efriedma created this revision.
efriedma added a reviewer: dberlin.
Herald added a subscriber: Prazek.

MemorySSAUpdater::getPreviousDefRecursive is a recursive algorithm, for each block, it computes the previous definition for each predecessor, then takes those definitions and combines them.  But currently it doesn't remember results which it already computed; this means it can visit the same block multiple times, which adds up to exponential time overall.

To fix this, this patch adds a cache.  If we computed the result for a block already, we don't need to visit it again because we'll come up with the same result. Well, unless we RAUW a MemoryPHI; in that case, the TrackingVH will be updated automatically.

I'm not sure this is the best fix, but it seems to work.

The testcase isn't really a test for the bug, but it adds coverage for the case where tryRemoveTrivialPhi erases an existing PHI node.  (It's hard to write a good regression test for a performance issue.)


Repository:
  rL LLVM

https://reviews.llvm.org/D44715

Files:
  include/llvm/Analysis/MemorySSAUpdater.h
  lib/Analysis/MemorySSAUpdater.cpp
  test/Transforms/GVNHoist/hoist-simplify-phi.ll

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