[PATCH] D70593: [LCSSA] Don't use VH callbacks to invalidate SCEV when creating LCSSA phis

[Daniil Suchkov via Phabricator via llvm-commits]

DaniilSuchkov created this revision.
DaniilSuchkov added reviewers: fhahn, efriedma, reames, sanjoy.google.
Herald added subscribers: llvm-commits, javed.absar, hiraditya.
Herald added a project: LLVM.
DaniilSuchkov retitled this revision from "[LCSSA] Don't use VH callbacks to invalidate SCEV" to "[LCSSA] Don't use VH callbacks to invalidate SCEV when creating LCSSA phis".
DaniilSuchkov added a parent revision: D70423: [SCEV] Introduce ScalarEvolutionVerifierPass.

In general ValueHandleBase::ValueIsRAUWd shouldn't be called when not all uses of the value were actually replaced, though, currently formLCSSAForInstructions calls it when it inserts LCSSA-phis.

Calls of ValueHandleBase::ValueIsRAUWd were added to LCSSA specifically to update/invalidate SCEV. In the best case these calls duplicate some of the work already done by SE->forgetValue, though in case when SCEV of the value is SCEVUnknown, SCEV replaces the underlying value of SCEVUnknown with the new value (i.e. acts like LCSSA-phi actually fully replaces the value it is created for), which leads to SCEV being corrupted because LCSSA-phi rarely dominates all uses of its inputs.

This fixes bug https://bugs.llvm.org/show_bug.cgi?id=44058.


https://reviews.llvm.org/D70593

Files:
  llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
  llvm/lib/Transforms/Utils/LCSSA.cpp
  llvm/test/Transforms/LCSSA/pr44058.ll

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