[PATCH] D48597: [LoopInstSimplify] Discard SCEV if simplification happened.

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri created this revision.
lebedev.ri added reviewers: uabelho, mkazantsev, chandlerc, sanjoy, evstupac.
Herald added subscribers: steven_wu, javed.absar, eraman, mehdi_amini.

Fixes PR37936 <https://bugs.llvm.org/show_bug.cgi?id=37936>.

The problem is as follows:

1. `indvars` marks `%dec` as `NUW`.
2. `loop-instsimplify` runs `instsimplify`, which constant-folds `%dec` to -1 (https://reviews.llvm.org/D47908)
3. `loop-reduce` tries to do some further modification, but crashes with an type assertion in cast, because `%dec` is no longer an `Instruction`,

If the runline is split into two, i.e. you first run `-indvars -loop-instsimplify`,
store that into a file, and then run `-loop-reduce`, there is no crash.

So it looks like the problem is due to `-loop-instsimplify` not discarding SCEV.
This does fix the crash (without the patch, that test file crashes),
although i do not know whether there is some other problem that should be solved.


Repository:
  rL LLVM

https://reviews.llvm.org/D48597

Files:
  lib/Transforms/Scalar/LoopInstSimplify.cpp
  test/Other/new-pm-defaults.ll
  test/Other/new-pm-thinlto-defaults.ll
  test/Transforms/LoopInstSimplify/forget-scev.ll

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