[LoopUnroll] Use Lazy strategy for DTU used for MergeBlockIntoPredecessor.

We do not access the DT in the loop, so we do not have to apply updates
eagerly. We can apply them lazyly and flush them after we are done
merging blocks.

As follow-up work, we might be able to use the DTU above as well,
instead of manually updating the DT.

This brings the example from PR43134 from ~100s to ~4s for a relase +
assertions build on my machine.

Reviewers: efriedma, kuhar, asbirlea, brzycki

Reviewed By: kuhar, brzycki

Differential Revision: https://reviews.llvm.org/D66911

llvm-svn: 370292

1 files changed, 3 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Utils/LoopUnroll.cpp b/llvm/lib/Transforms/Utils/LoopUnroll.cpp
index 4a1edb3700c..a7590fc3254 100644
--- a/llvm/lib/Transforms/Utils/LoopUnroll.cpp
+++ b/llvm/lib/Transforms/Utils/LoopUnroll.cpp
@@ -870,7 +870,7 @@ LoopUnrollResult llvm::UnrollLoop(Loop *L, UnrollLoopOptions ULO, LoopInfo *LI,
   assert(!DT || !UnrollVerifyDomtree ||
          DT->verify(DominatorTree::VerificationLevel::Fast));
 
-  DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
+  DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
   // Merge adjacent basic blocks, if possible.
   for (BasicBlock *Latch : Latches) {
     BranchInst *Term = dyn_cast<BranchInst>(Latch->getTerminator());
@@ -890,6 +890,8 @@ LoopUnrollResult llvm::UnrollLoop(Loop *L, UnrollLoopOptions ULO, LoopInfo *LI,
       }
     }
   }
+  // Apply updates to the DomTree.
+  DT = &DTU.getDomTree();
 
   // At this point, the code is well formed.  We now simplify the unrolled loop,
   // doing constant propagation and dead code elimination as we go.