[clang] [LifetimeSafety] Introduce a liveness-based lifetime policy (PR #159991)

[Yitzhak Mandelbaum via cfe-commits]

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@@ -867,29 +868,30 @@ class DataflowAnalysis {
         std::conditional_t<Dir == Direction::Forward, ForwardDataflowWorklist,
                            BackwardDataflowWorklist>;
     Worklist W(Cfg, AC);
+    llvm::SmallBitVector Seen(Cfg.getNumBlockIDs() + 1);
 
     const CFGBlock *Start = isForward() ? &Cfg.getEntry() : &Cfg.getExit();
     InStates[Start] = D.getInitialState();
+    Seen.set(Start->getBlockID());
     W.enqueueBlock(Start);
 
-    llvm::SmallBitVector Visited(Cfg.getNumBlockIDs() + 1);
-
     while (const CFGBlock *B = W.dequeue()) {
       Lattice StateIn = getInState(B);
       Lattice StateOut = transferBlock(B, StateIn);
       OutStates[B] = StateOut;
-      Visited.set(B->getBlockID());
       for (const CFGBlock *AdjacentB : isForward() ? B->succs() : B->preds()) {
         if (!AdjacentB)
           continue;
         Lattice OldInState = getInState(AdjacentB);
-        Lattice NewInState = D.join(OldInState, StateOut);
+        bool SeeingFirstTime = !Seen.test(AdjacentB->getBlockID());
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ymand wrote:

Why not use optionals? like `std::optional<Lattice> OldInState`?

https://github.com/llvm/llvm-project/pull/159991