[PATCH] D104569: [SimplifyCFG] Fix SimplifyBranchOnICmpChain to be undef/poison safe.

[Hyeongyu Kim via Phabricator via llvm-commits]

hyeongyukim created this revision.
Herald added a subscriber: hiraditya.
hyeongyukim requested review of this revision.
Herald added a project: LLVM.
Herald added a subscriber: llvm-commits.

This patch fixes the problem of SimplifyBranchOnICmpChain that occurs
when extra values are Undef or poison.

Suppose the %mode is 51 and the %Cond is poison, and let's look at the
case below.

  	%A = icmp ne i32 %mode, 0
  	%B = icmp ne i32 %mode, 51
  	%C = select i1 %A, i1 %B, i1 false
  	%D = select i1 %C, i1 %Cond, i1 false
  	br i1 %D, label %T, label %F
  =>
  	br i1 %Cond, label %switch.early.test, label %F
  switch.early.test:
  	switch i32 %mode, label %T [
  		i32 51, label %F
  		i32 0, label %F
  	]

incorrectness: https://alive2.llvm.org/ce/z/BWScX

Code before transformation will not raise UB because %C and %D is false,
and it will not use %Cond. But after transformation, %Cond is being used
immediately, and it will raise UB.

This problem can be solved by adding freeze instruction.

correctness: https://alive2.llvm.org/ce/z/x9x4oY


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D104569

Files:
  llvm/lib/Transforms/Utils/SimplifyCFG.cpp
  llvm/test/Transforms/SimplifyCFG/switch_create-custom-dl.ll
  llvm/test/Transforms/SimplifyCFG/switch_create.ll
  llvm/test/Transforms/SimplifyCFG/switch_msan.ll

-------------- next part --------------
A non-text attachment was scrubbed...
Name: D104569.353099.patch
Type: text/x-patch
Size: 9953 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20210618/67e71667/attachment.bin>