Recommit "[LoopUnroll] Directly update DT instead of DTU."

This reverts commit c5ea42bcf48c8f3d3e35a6bff620b06d2a499108.

Recommit the patch with a fix for loops where the exiting terminator is
not a branch instruction. In that case, ExitInfos may be empty. In
addition to checking if there's a single exiting block also check if
there's a single ExitInfo.

A test case has been added in f92b35392ed8e4631.

1 files changed, 19 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
index 9b35faf1..58a226f 100644
--- a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
+++ b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
@@ -179,7 +179,8 @@ bool llvm::DeleteDeadPHIs(BasicBlock *BB, const TargetLibraryInfo *TLI,
 bool llvm::MergeBlockIntoPredecessor(BasicBlock *BB, DomTreeUpdater *DTU,
                                      LoopInfo *LI, MemorySSAUpdater *MSSAU,
                                      MemoryDependenceResults *MemDep,
-                                     bool PredecessorWithTwoSuccessors) {
+                                     bool PredecessorWithTwoSuccessors,
+                                     DominatorTree *DT) {
   if (BB->hasAddressTaken())
     return false;
 
@@ -232,10 +233,21 @@ bool llvm::MergeBlockIntoPredecessor(BasicBlock *BB, DomTreeUpdater *DTU,
     FoldSingleEntryPHINodes(BB, MemDep);
   }
 
+  if (DT) {
+    assert(!DTU && "cannot use both DT and DTU for updates");
+    DomTreeNode *PredNode = DT->getNode(PredBB);
+    DomTreeNode *BBNode = DT->getNode(BB);
+    if (PredNode) {
+      assert(BBNode && "PredNode unreachable but BBNode reachable?");
+      for (DomTreeNode *C : to_vector(BBNode->children()))
+        C->setIDom(PredNode);
+    }
+  }
   // DTU update: Collect all the edges that exit BB.
   // These dominator edges will be redirected from Pred.
   std::vector<DominatorTree::UpdateType> Updates;
   if (DTU) {
+    assert(!DT && "cannot use both DT and DTU for updates");
     // To avoid processing the same predecessor more than once.
     SmallPtrSet<BasicBlock *, 8> SeenSuccs;
     SmallPtrSet<BasicBlock *, 2> SuccsOfPredBB(succ_begin(PredBB),
@@ -311,6 +323,12 @@ bool llvm::MergeBlockIntoPredecessor(BasicBlock *BB, DomTreeUpdater *DTU,
   if (DTU)
     DTU->applyUpdates(Updates);
 
+  if (DT) {
+    assert(succ_empty(BB) &&
+           "successors should have been transferred to PredBB");
+    DT->eraseNode(BB);
+  }
+
   // Finally, erase the old block and update dominator info.
   DeleteDeadBlock(BB, DTU);