[PATCH] D123607: LowerSwitch: Avoid inserting NewDefault block

[Ruiling, Song via Phabricator via llvm-commits]

ruiling created this revision.
ruiling added reviewers: arsenm, foad, nhaehnle.
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The NewDefault was used to simplify the updating of PHI nodes, but it
causes some inefficiency for target that will run structurizer later. For
example, for a simple two-case switch, the extra NewDefault is causing
unstructured CFG like:

      O
     / \
    O   O
   / \ / \
  C1  ND C2
   \  |  /
    \ | /
      D

The change is to avoid the ND(NewDefault) block, that is we will get a
structured CFG for above example like:

       O
      / \
     /   \
    O     O
   / \   / \
  C1  \ /  C2
   \-> D <-/

The IR change introduced by this patch should be trivial to other targets,
so I am doing this unconditionally.

Fall-through among the cases will also cause unstructured CFG, but it need
more work and will be addressed in a separate change.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D123607

Files:
  llvm/lib/Transforms/Utils/LowerSwitch.cpp
  llvm/test/CodeGen/AMDGPU/multilevel-break.ll
  llvm/test/Transforms/FixIrreducible/switch.ll
  llvm/test/Transforms/LowerSwitch/2014-06-23-PHIlowering.ll
  llvm/test/Transforms/LowerSwitch/do-not-handle-impossible-values.ll
  llvm/test/Transforms/LowerSwitch/feature.ll
  llvm/test/Transforms/StructurizeCFG/interleaved-loop-order.ll
  llvm/test/Transforms/UnifyLoopExits/switch.ll
  llvm/test/Transforms/Util/lowerswitch.ll

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