[AMDGPU] lower-switch in preISel as a workaround for legacy DA

Summary:
The default target of the switch instruction may sometimes be an
"unreachable" block, when it is guaranteed that one of the cases is
always taken. The dominator tree concludes that such a switch
instruction does not have an immediate post dominator. This confuses
divergence analysis, which is unable to propagate sync dependence to
the targets of the switch instruction.

As a workaround, the AMDGPU target now invokes lower-switch as a
preISel pass. LowerSwitch is designed to handle the unreachable
default target correctly, allowing the divergence analysis to locate
the correct immediate dominator of the now-lowered switch.

Reviewers: arsenm, nhaehnle

Reviewed By: nhaehnle

Subscribers: kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, llvm-commits, simoll

Differential Revision: https://reviews.llvm.org/D52221

llvm-svn: 342722

1 files changed, 1 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Scalar/StructurizeCFG.cpp b/llvm/lib/Transforms/Scalar/StructurizeCFG.cpp
index f58f79f8b14..41edcb56aab 100644
--- a/llvm/lib/Transforms/Scalar/StructurizeCFG.cpp
+++ b/llvm/lib/Transforms/Scalar/StructurizeCFG.cpp
@@ -275,6 +275,7 @@ public:
     AU.addRequired<LoopInfoWrapperPass>();
 
     AU.addPreserved<DominatorTreeWrapperPass>();
+    AU.addPreservedID(LowerSwitchID);
     RegionPass::getAnalysisUsage(AU);
   }
 };