[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:

I don't really understand this fix. The updates here are just the CFG changes, and as far as I can tell we do indeed always insert a CFG edge from NewBB to OldBB, so it should be reflected in the updates.

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