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

[zhijian lin via llvm-commits]

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@@ -977,6 +977,22 @@ void SelectionDAGLegalize::LegalizeOp(SDNode *Node) {
   TargetLowering::LegalizeAction Action = TargetLowering::Legal;
   bool SimpleFinishLegalizing = true;
   switch (Node->getOpcode()) {
+  // FIXME: If the node represents a poison value, replace it with an undef
+  // value.
+  //  A poison value results from an erroneous operation but does not cause
+  //  immediate undefined behavior, allowing speculative execution.
+  //  Since most operations propagate poison, it is valid to replace poison
+  //  with an undef value, which can take any legal value of the same type.
+  //  This ensures that downstream computations do not rely on poison semantics.
+  //  Poison is more restrictive than undef. Since we replace poison with undef
+  //  here, the poison information will be lost after the code is executed. In
+  //  the futher, If we need to retain the poison information after the code is
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diggerlin wrote:

> I suspect most of the added type legalization is creating UNDEF instead of POISON.

Yes, I agree, but it only most of  type legalization is creating UNDEF instead of POISON. so there are still some POISON are not converted o UNDEF in the type legalization, So checking and convert  ISD::POISON  to UNDEF  in the function SelectionDAGLegalize::LegalizeOp(SDNode *Node) . if all the ISD::POISON  to UNDEF  are converted in the  type legalization in the future.  We can delete the code.

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