[PATCH] D25810: [SCEV] Memoize visitMulExpr results in SCEVRewriteVisitor. Fix PR18606

[Li Huang via llvm-commits]

lihuang created this revision.
lihuang added reviewers: sanjoy, mehdi_amini, mzolotukhin.
lihuang added a subscriber: llvm-commits.
lihuang added a dependency: D25794: [SCEV] Add a threshold to restrict number of mul operands to be inlined into SCEV.

[SCEV] Memoize visitMulExpr results in SCEVRewriteVisitor. Fix PR18606

When SCEVRewriteVisitor traverses the SCEV DAG, it may visit the same SCEV multiple times if this SCEV is referenced by multiple other SCEVs. This has exponential time complexity in the worst case. Memoizing the results will avoid re-visiting the same SCEV.

It's legal to memoize the rewrite results because SCEVRewriteVisitor is state-less, i.e., it should return the same rewrite result each time for the same input SCEV. Currently only memoize results of visitMulExpr because other functions are much cheaper in the worst case.

This patch depends on https://reviews.llvm.org/D25794, and they together fix PR18606.


https://reviews.llvm.org/D25810

Files:
  include/llvm/Analysis/ScalarEvolutionExpressions.h
  test/Analysis/ScalarEvolution/pr18606.ll

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