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

[Craig Topper via llvm-commits]

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@@ -690,8 +690,17 @@ END_TWO_BYTE_PACK()
   /// \<target\>ISD namespace).
   bool isTargetOpcode() const { return NodeType >= ISD::BUILTIN_OP_END; }
 
-  /// Return true if the type of the node type undefined.
-  bool isUndef() const { return NodeType == ISD::UNDEF; }
+  /// Returns true if the node type is UNDEF or, when DoNotIncludeExplicitPoison
+  /// is false, POISON.
+  /// @param DoNotIncludeExplicitPoison Determines whether to check only for
+  /// UNDEF.
+  bool isUndef(bool DoNotIncludeExplicitPoison = false) const {
+    return NodeType == ISD::UNDEF ||
+           (!DoNotIncludeExplicitPoison && NodeType == ISD::POISON);
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topperc wrote:

I think Matt is saying that isUndef() should always check ISD::UNDEF and ISD::POISON. If some optimization wants to check only undef, they can use `getOpcode() == ISD::UNDEF` or `isUndef() && !isPoison()`.

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