[llvm] [BasicBlockUtils] Fix SplitBlockPredecessors incorrect dominator insert (PR #107190)

[Nikita Popov via llvm-commits]

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@@ -1160,7 +1160,10 @@ static void UpdateAnalysisInformation(BasicBlock *OldBB, BasicBlock *NewBB,
       // Split block expects NewBB to have a non-empty set of predecessors.
       SmallVector<DominatorTree::UpdateType, 8> Updates;
       SmallPtrSet<BasicBlock *, 8> UniquePreds;
-      Updates.push_back({DominatorTree::Insert, NewBB, OldBB});
+      if (OldBB->getSinglePredecessor()) {
+        assert(OldBB->getSinglePredecessor() == NewBB);
+        Updates.push_back({DominatorTree::Insert, NewBB, OldBB});
+      }
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nikic wrote:

DTU updates are about edges in the CFG, not the DT. The point of DTU is to determine the correct DT updates from CFG updates, so this kind of reasoning in the caller should never be necessary.

https://github.com/llvm/llvm-project/pull/107190