[LoopUnrollAndJam] Use Lazy strategy for DTU.

We can also apply the earlier updates to the lazy DTU, instead of
applying them directly.

Reviewers: kuhar, brzycki, asbirlea, SjoerdMeijer

Reviewed By: brzycki, asbirlea, SjoerdMeijer

Differential Revision: https://reviews.llvm.org/D66918

llvm-svn: 370391

llvm/lib/Transforms/Utils/LoopUnrollAndJam.cpp

@@ -517,6 +517,7 @@ LoopUnrollResult llvm::UnrollAndJamLoop(
     movePHIs(AftBlocksFirst[It], AftBlocksFirst[0]);
   }
 
+  DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
   // Dominator Tree. Remove the old links between Fore, Sub and Aft, adding the
   // new ones required.
   if (Count != 1) {
@@ -530,15 +531,14 @@ LoopUnrollResult llvm::UnrollAndJamLoop(
                            ForeBlocksLast.back(), SubLoopBlocksFirst[0]);
     DTUpdates.emplace_back(DominatorTree::UpdateKind::Insert,
                            SubLoopBlocksLast.back(), AftBlocksFirst[0]);
-    DT->applyUpdates(DTUpdates);
+    DTU.applyUpdatesPermissive(DTUpdates);
   }
 
   // Merge adjacent basic blocks, if possible.
   SmallPtrSet<BasicBlock *, 16> MergeBlocks;
   MergeBlocks.insert(ForeBlocksLast.begin(), ForeBlocksLast.end());
   MergeBlocks.insert(SubLoopBlocksLast.begin(), SubLoopBlocksLast.end());
   MergeBlocks.insert(AftBlocksLast.begin(), AftBlocksLast.end());
-  DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
   while (!MergeBlocks.empty()) {
     BasicBlock *BB = *MergeBlocks.begin();
     BranchInst *Term = dyn_cast<BranchInst>(BB->getTerminator());
@@ -555,6 +555,8 @@ LoopUnrollResult llvm::UnrollAndJamLoop(
     } else
       MergeBlocks.erase(BB);
   }
+  // Apply updates to the DomTree.
+  DT = &DTU.getDomTree();
 
   // At this point, the code is well formed.  We now do a quick sweep over the
   // inserted code, doing constant propagation and dead code elimination as we