Make MergeBlockIntoPredecessor conformant to the precondition of calling DTU.applyUpdates

Summary:
It is mentioned in the document of DTU that "It is illegal to submit any update that has already been submitted, i.e., you are supposed not to insert an existent edge or delete a nonexistent edge." It is dangerous to violet this rule because DomTree and PostDomTree occasionally crash on this scenario.

This patch fixes `MergeBlockIntoPredecessor`, making it conformant to this precondition.

Reviewers: kuhar, brzycki, chandlerc

Reviewed By: brzycki

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58444

llvm-svn: 355105

1 files changed, 3 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
index 0557b7d7442..81031b1a61e 100644
--- a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
+++ b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
@@ -186,7 +186,9 @@ bool llvm::MergeBlockIntoPredecessor(BasicBlock *BB, DomTreeUpdater *DTU,
     Updates.push_back({DominatorTree::Delete, PredBB, BB});
     for (auto I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
       Updates.push_back({DominatorTree::Delete, BB, *I});
-      Updates.push_back({DominatorTree::Insert, PredBB, *I});
+      // This successor of BB may already have PredBB as a predecessor.
+      if (llvm::find(successors(PredBB), *I) == succ_end(PredBB))
+        Updates.push_back({DominatorTree::Insert, PredBB, *I});
     }
   }