[PATCH] D24651: [SCEV] Try harder to find UB for NSW/NUW instr

[Christof Douma via llvm-commits]

christof created this revision.
christof added reviewers: broune, sanjoy, sbaranga.
christof added a subscriber: llvm-commits.
christof set the repository for this revision to rL LLVM.
Herald added subscribers: mzolotukhin, mehdi_amini, sanjoy.

Extended UB analysis of SCEV with the case where an instruction marked as non-wrapping (I) is later used in a latch control expression. In this case, if I holds a poison value, the loop can be made to iterate forever which is undefined behavior.

For wrapping of I to be UB, the latch control expression must execute if I is executed. To proof this, the PostDominatorTree is needed by SCEV. If a post-domination path is found, it is checked that progress from I to the latch control expression is guaranteed. That is, trapping instruction and infinite loops must be absent on that execution path.

This patch is 1/2 to fix PR28429. Known issues:

- Contains a work around for the LoopPassManager that has trouble with the newly added dependency of the PostDominator pass. This can be approved upon to make the PostDominatorTree pass run less often
- LoopUnroll uses SCEV and breaks PostDominatorTree, giving an segfault if a function contains multiple loops.

I hope to get a review to see if this work is OK to commit once the known issues listed above are fixed.

Repository:
  rL LLVM

https://reviews.llvm.org/D24651

Files:
  include/llvm/Analysis/ScalarEvolution.h
  include/llvm/Analysis/ValueTracking.h
  lib/Analysis/LoopPassManager.cpp
  lib/Analysis/ScalarEvolution.cpp
  lib/Analysis/ValueTracking.cpp
  lib/Transforms/Utils/LoopUtils.cpp
  test/Analysis/ScalarEvolution/flags-from-poison.ll
  test/Analysis/ScalarEvolution/trip-count12.ll
  test/Analysis/ScalarEvolution/trip-count9.ll
  test/Other/pass-pipelines.ll
  test/Transforms/SLPVectorizer/X86/consecutive-access.ll
  unittests/Analysis/ScalarEvolutionTest.cpp

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