[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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@@ -7601,6 +7604,20 @@ SDValue SelectionDAG::getNode(unsigned Opcode, const SDLoc &DL, EVT VT,
     }
   }
 
+  // TODO: According to https://llvm.org/docs/UndefinedBehavior.html#poison-values,
+  // most instructions return poison if any of their inputs are poison.
+  // A notable exception is the select instruction, which is poison if and only if
+  // the condition is poison or the selected value is poison.
+
+  // Currently, we only handle the case where the opcode is ISD::ADD and the second input is poison.
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topperc wrote:

I didn't ask for this change. I said we can do it in a follow up.

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