[PATCH] D132450: StructurizeCFG: Set Undef for non-predecessors in setPhiValues()

[Ruiling, Song via Phabricator via llvm-commits]

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During structurization process, we may place non-predecessor blocks
between the predecessors of a block in the structurized CFG. Take
the typical while-break case as an example:

  |

/---A(v=...)

| / \ |
|

^ B   C

| \ / |
|

\---L |

  \ /
   E (r=phi (v:C)...)

After structurization, the CFG would be look like:

  |

/---A

|   | \ |
|   | C |
|   | / |
| F1 <https://reviews.llvm.org/F1> |
|

^   |\

|   | B |
|   | / |
| F2 <https://reviews.llvm.org/F2> |
|   | \ |
|   | L |
|

\   |/

  \--F3
     |
     E

We can see that block B is placed between the predecessors(C/L) of E.
During phi reconstruction, to achieve the same sematics as before, we
are reconstructing the PHIs as:

  F1: v1 = phi (v:C), (undef:A)
  F3: r = phi (v1:F2), ...

But this is also saying that `v1` would be live through B, which is not
quite necessary. The idea in the change is to say the incoming value
from B is Undef for the PHI in E. With this change, the reconstructed
PHI would be:

  F1: v1 = phi (v:C), (undef:A)
  F2: v2 = phi (v1:F1), (undef:B)
  F3: r  = phi (v2:F2), ...


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D132450

Files:
  llvm/lib/Transforms/Scalar/StructurizeCFG.cpp
  llvm/test/CodeGen/AMDGPU/multilevel-break.ll
  llvm/test/CodeGen/AMDGPU/while-break.ll
  llvm/test/Transforms/StructurizeCFG/workarounds/needs-fr-ule.ll

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