[llvm] [ControlFlowHub] Fix duplicate DomTree updates when branch successors are identical (PR #176620)

[Shilei Tian via llvm-commits]

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@@ -248,6 +250,11 @@ static void reconnectPhis(BasicBlock *Out, BasicBlock *GuardBlock,
       Value *V = PoisonValue::get(Phi->getType());
       if  (Phi->getBasicBlockIndex(BB) != -1) {
         V = Phi->removeIncomingValue(BB, false);
+        // When both successors are the same (Succ0 == Succ1), there are two
+        // edges from BB to Out, so we need to remove the second PHI entry too.
+        if (Succ0 && Succ1 && Succ0 == Succ1 &&
+            Phi->getBasicBlockIndex(BB) != -1)
+          Phi->removeIncomingValue(BB, false);
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shiltian wrote:

Yup, the `Phi` here can have two identical incoming BB and value, since it is a list not a set. I was thinking about using a set for phi node in the past for a separate issue, but that change is too intrusive, and I might miss some design choice and history here.

https://github.com/llvm/llvm-project/pull/176620