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

[zhijian lin via llvm-commits]

================
@@ -9244,6 +9248,11 @@ SDValue SelectionDAG::getLoad(ISD::MemIndexedMode AM, ISD::LoadExtType ExtType,
 
   SDVTList VTs = Indexed ?
     getVTList(VT, Ptr.getValueType(), MVT::Other) : getVTList(VT, MVT::Other);
+
+  // Lower poison to undef.
+  if (Ptr.getNode()->isPoison())
----------------
diggerlin wrote:

The function `SelectionDAG::getLoad` is called before the the function  `SelectionDAGLegalize::LegalizeOp` which lower the ISD::POISON to ISD::UNDEF  is called .

and the function `SelectionDAG::getLoad` call  FindNodeOrInsertPos,

```
if (SDNode *E = FindNodeOrInsertPos(ID, dl, IP)) {
    cast<LoadSDNode>(E)->refineAlignment(MMO);
    return SDValue(E, 0);
  }
```
  
   if we do not lower the ISD::POISON to ISD::UNDEF, it will look loading from `poison` memory is different from loading from `undef` memory, it will have one more redundant instruction.



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