[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, 28 insertions, 16 deletions

diff --git a/llvm/lib/CodeGen/MachinePostDominators.cpp b/llvm/lib/CodeGen/MachinePostDominators.cpp
index 7f220ed1fd8..f2fc9f814f8 100644
--- a/llvm/lib/CodeGen/MachinePostDominators.cpp
+++ b/llvm/lib/CodeGen/MachinePostDominators.cpp
@@ -13,6 +13,8 @@
 
 #include "llvm/CodeGen/MachinePostDominators.h"
 
+#include "llvm/ADT/STLExtras.h"
+
 using namespace llvm;
 
 namespace llvm {
@@ -25,33 +27,43 @@ char MachinePostDominatorTree::ID = 0;
 INITIALIZE_PASS(MachinePostDominatorTree, "machinepostdomtree",
                 "MachinePostDominator Tree Construction", true, true)
 
-MachinePostDominatorTree::MachinePostDominatorTree() : MachineFunctionPass(ID) {
+MachinePostDominatorTree::MachinePostDominatorTree()
+    : MachineFunctionPass(ID), PDT(nullptr) {
   initializeMachinePostDominatorTreePass(*PassRegistry::getPassRegistry());
-  DT = new PostDomTreeBase<MachineBasicBlock>();
 }
 
-FunctionPass *
-MachinePostDominatorTree::createMachinePostDominatorTreePass() {
+FunctionPass *MachinePostDominatorTree::createMachinePostDominatorTreePass() {
   return new MachinePostDominatorTree();
 }
 
-bool
-MachinePostDominatorTree::runOnMachineFunction(MachineFunction &F) {
-  DT->recalculate(F);
+bool MachinePostDominatorTree::runOnMachineFunction(MachineFunction &F) {
+  PDT = std::make_unique<PostDomTreeT>();
+  PDT->recalculate(F);
   return false;
 }
 
-MachinePostDominatorTree::~MachinePostDominatorTree() {
-  delete DT;
-}
-
-void
-MachinePostDominatorTree::getAnalysisUsage(AnalysisUsage &AU) const {
+void MachinePostDominatorTree::getAnalysisUsage(AnalysisUsage &AU) const {
   AU.setPreservesAll();
   MachineFunctionPass::getAnalysisUsage(AU);
 }
 
-void
-MachinePostDominatorTree::print(llvm::raw_ostream &OS, const Module *M) const {
-  DT->print(OS);
+MachineBasicBlock *MachinePostDominatorTree::findNearestCommonDominator(
+    ArrayRef<MachineBasicBlock *> Blocks) const {
+  assert(!Blocks.empty());
+
+  MachineBasicBlock *NCD = Blocks.front();
+  for (MachineBasicBlock *BB : Blocks.drop_front()) {
+    NCD = PDT->findNearestCommonDominator(NCD, BB);
+
+    // Stop when the root is reached.
+    if (PDT->isVirtualRoot(PDT->getNode(NCD)))
+      return nullptr;
+  }
+
+  return NCD;
+}
+
+void MachinePostDominatorTree::print(llvm::raw_ostream &OS,
+                                     const Module *M) const {
+  PDT->print(OS);
 }