[Dominators][AMDGPU] Don't use virtual exit node in findNearestCommonDominator. Cleanup MachinePostDominators.

Summary:
This patch fixes a bug that originated from passing a virtual exit block (nullptr) to `MachinePostDominatorTee::findNearestCommonDominator` and resulted in assertion failures inside its callee. It also applies a small cleanup to the class.

The patch introduces a new function in PDT that given a list of `MachineBasicBlock`s finds their NCD. The new overload of `findNearestCommonDominator` handles virtual root correctly.

Note that similar handling of virtual root nodes is not necessary in (forward) `DominatorTree`s, as right now they don't use virtual roots.

Reviewers: tstellar, tpr, nhaehnle, arsenm, NutshellySima, grosser, hliao

Reviewed By: hliao

Subscribers: hliao, kzhuravl, jvesely, wdng, yaxunl, dstuttard, t-tye, hiraditya, llvm-commits

Tags: #amdgpu, #llvm

Differential Revision: https://reviews.llvm.org/D67974

llvm-svn: 372874

1 files changed, 10 insertions, 10 deletions

diff --git a/llvm/lib/Target/AMDGPU/SILowerI1Copies.cpp b/llvm/lib/Target/AMDGPU/SILowerI1Copies.cpp
index 86242f1fb64..b4541253635 100644
--- a/llvm/lib/Target/AMDGPU/SILowerI1Copies.cpp
+++ b/llvm/lib/Target/AMDGPU/SILowerI1Copies.cpp
@@ -589,12 +589,12 @@ void SILowerI1Copies::lowerPhis() {
 
       // Phis in a loop that are observed outside the loop receive a simple but
       // conservatively correct treatment.
-      MachineBasicBlock *PostDomBound = &MBB;
-      for (MachineInstr &Use : MRI->use_instructions(DstReg)) {
-        PostDomBound =
-            PDT->findNearestCommonDominator(PostDomBound, Use.getParent());
-      }
+      std::vector<MachineBasicBlock *> DomBlocks = {&MBB};
+      for (MachineInstr &Use : MRI->use_instructions(DstReg))
+        DomBlocks.push_back(Use.getParent());
 
+      MachineBasicBlock *PostDomBound =
+          PDT->findNearestCommonDominator(DomBlocks);
       unsigned FoundLoopLevel = LF.findLoop(PostDomBound);
 
       SSAUpdater.Initialize(DstReg);
@@ -711,12 +711,12 @@ void SILowerI1Copies::lowerCopiesToI1() {
 
       // Defs in a loop that are observed outside the loop must be transformed
       // into appropriate bit manipulation.
-      MachineBasicBlock *PostDomBound = &MBB;
-      for (MachineInstr &Use : MRI->use_instructions(DstReg)) {
-        PostDomBound =
-            PDT->findNearestCommonDominator(PostDomBound, Use.getParent());
-      }
+      std::vector<MachineBasicBlock *> DomBlocks = {&MBB};
+      for (MachineInstr &Use : MRI->use_instructions(DstReg))
+        DomBlocks.push_back(Use.getParent());
 
+      MachineBasicBlock *PostDomBound =
+          PDT->findNearestCommonDominator(DomBlocks);
       unsigned FoundLoopLevel = LF.findLoop(PostDomBound);
       if (FoundLoopLevel) {
         SSAUpdater.Initialize(DstReg);