[PATCH] D71799: [Attributor] AAUndefinedBehavior: Check for branches on undef value.

[Stefanos Baziotis via Phabricator via llvm-commits]

baziotis added a comment.

So, I tried to make a reduced test case that fails:

  define void @fails() {
  entry:
    br i1 undef, label %end, label %end
  
  end:
    %phi = phi i32* [ null, %entry ], [ null, %entry ]
    %a = load i32, i32* %phi, align 4
    ret void
  }

It's based on the test case `IPConstantProp/PR26044.ll`, which also fails. The interesting things are:

1. If we remove the load, it doesn't fail.
2. If we remove the phi it doesn't fail (that's also true for `PR26044.ll`).
3. As noted yesterday, if we remove the `changeToUnreachable` in `AAUndefinedBehavior::manifest()`, it doesn't fail (all the test cases basically fail because of this part).
4. Also, that `AAIsDead` seems to have a problem with it.

My guess is that the fact that we change a branch instruction to unreachable means that there's one less predecessor for the then and else blocks of the branch.
If one of these 2 hasn't been converted to unreachable and contains a `phi`, then we have problems as now the BB has one less predecessor than those listed in the phi.


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