[llvm] [IR][JumpThreading] Fix infinite recursion on compare self-reference [updated] (PR #129501)

[Nikita Popov via llvm-commits]

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@@ -1519,12 +1526,24 @@ Constant *JumpThreadingPass::evaluateOnPredecessorEdge(BasicBlock *BB,
   }
 
   // If we have a CmpInst, try to fold it for each incoming edge into PredBB.
+  // Note that during the execution of the pass, phi nodes may become constant
+  // and may be removed, which can lead to self-referencing instructions in
+  // code that becomes unreachable. Consequently, we need to handle those
+  // instructions in unreachable code and check before going into recursion.
   if (CmpInst *CondCmp = dyn_cast<CmpInst>(V)) {
     if (CondCmp->getParent() == BB) {
-      Constant *Op0 =
-          evaluateOnPredecessorEdge(BB, PredPredBB, CondCmp->getOperand(0), DL);
-      Constant *Op1 =
-          evaluateOnPredecessorEdge(BB, PredPredBB, CondCmp->getOperand(1), DL);
+      Constant *Op0 = nullptr;
+      Constant *Op1 = nullptr;
+      if (Value *V0 = CondCmp->getOperand(0); !Visited.contains(V0)) {
+        Visited.insert(V0);
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nikic wrote:

I think it would be cleaner to do these checks in evaluateOnPredecessorEdge(), not at each call to evaluateOnPredecessorEdge. Doesn't really matter right now, but will make sure this keeps getting handled if this code handles more than icmp in the future.

Please also combine the contains and insert calls by using the return value of insert.

https://github.com/llvm/llvm-project/pull/129501