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

[Hyeongyu Kim via Phabricator via llvm-commits]

hyeongyukim added a comment.

  entry:
    %cmp = icmp ult i8 %c, 33                           ; ExtraCase
    %cmp4 = icmp eq i8 %c, 46
    %or.cond = or i1 %cmp, %cmp4
    %cmp9 = icmp eq i8 %c, 44
    %or.cond1 = or i1 %or.cond, %cmp9
    %cmp14 = icmp eq i8 %c, 58
    %or.cond2 = or i1 %or.cond1, %cmp14
    %cmp19 = icmp eq i8 %c, 59
    %or.cond3 = or i1 %or.cond2, %cmp19
    %cmp24 = icmp eq i8 %c, 60
    %or.cond4 = or i1 %or.cond3, %cmp24
    %cmp29 = icmp eq i8 %c, 62
    %or.cond5 = or i1 %or.cond4, %cmp29
    %cmp34 = icmp eq i8 %c, 34
    %or.cond6 = or i1 %or.cond5, %cmp34
    %cmp39 = icmp eq i8 %c, 92
    %or.cond7 = or i1 %or.cond6, %cmp39
    br i1 %or.cond7, label %lor.end, label %lor.rhs    ; BI
  ====
      // Currently isGuaranteedNotToBePoison returns false.
      if (!isGuaranteedNotToBePoison(ExtraCase, AC, BI, DT))
        ExtraCase = Builder.CreateFreeze(ExtraCase);

`%or.cond7` cannot be poison because `%or.cond7` is a branching condition.
Also, `%cmp` cannot be poison because `%or.cond7` is not poison.
But `isGuaranteedNotToBePosion` is returning false, and an unnecessary freeze is generated.

I think this situation does not appear to be considered in the current implementation of `isGuaranteedNotToBePosion`.
Therefore, it seems that `isGuaranteedNotToBePosion` should be modified so that this case can be dealt with. (I would like to add a code that handles the case when 'CtxI' is a branching instruction.)

Of course, I may have misused `isGuaranteedNotToBePosition`, and if there is anything wrong with me, please let me know.


Repository:
  rG LLVM Github Monorepo

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https://reviews.llvm.org/D104569