[llvm] Introducing a new ISD::POISON SDNode to represent the poison value in the IR. (PR #125883)

[Matt Arsenault via llvm-commits]

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@@ -977,6 +977,11 @@ void SelectionDAGLegalize::LegalizeOp(SDNode *Node) {
   TargetLowering::LegalizeAction Action = TargetLowering::Legal;
   bool SimpleFinishLegalizing = true;
   switch (Node->getOpcode()) {
+  case ISD::POISON: {
+    SDValue UndefNode = DAG.getUNDEF(Node->getValueType(0));
+    ReplaceNode(Node, UndefNode.getNode());
+    break;
+  }
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arsenm wrote:

Yes, it's fine to leave it as it's copying what undef is already doing. But really this should have mandatory legality for legal types 

https://github.com/llvm/llvm-project/pull/125883