[PATCH] D60834: [AMDGPU] Uniform values being used outside loop marked non-divergent

[Nicolai Hähnle via Phabricator via llvm-commits]

nhaehnle added a comment.

I think there are some misunderstandings here. None of the IR passes require LCSSA. The problem is in getting the divergence data into the SelectionDAG.

Specifically, you can have, in a weird mixture of IR and SelectionDAG:

  loop:
    ...
    %uni = ...                             ; Value is uniform here
    ...
    br i1 %div, label %loop, label %next   ; Divergent loop exit
  
  next:
    %0 = CopyFromReg N(corresponding to %uni)
    use %0

In this case, %0 must be labeled divergent. However, %0 does not exist at an IR level, and so the code in `isSDNodeSourceOfDivergence` can only query for the divergence of %uni. However, %uni itself is uniform.

One way to look at the problem is that `DivergenceAnalysis::isDivergent` is really "isDivergentAtDefinition", and what we need is a query "isDivergentAtUse". Implementing that query isn't entirely trivial, and LCSSA is effectively an alternative way of making the right query.


Repository:
  rL LLVM

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  https://reviews.llvm.org/D60834/new/

https://reviews.llvm.org/D60834