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

A new ISD::POISON SDNode is introduced to represent the `poison value`
in the IR, replacing the previous use of ISD::UNDEF.

Author: diggerlin

llvm/include/llvm/CodeGen/ISDOpcodes.h

@@ -217,6 +217,9 @@ enum NodeType {
   /// UNDEF - An undefined node.
   UNDEF,
 
+  /// POISON - A poison node.
+  POISON,
+
   /// FREEZE - FREEZE(VAL) returns an arbitrary value if VAL is UNDEF (or
   /// is evaluated to UNDEF), or returns VAL otherwise. Note that each
   /// read of UNDEF can yield different value, but FREEZE(UNDEF) cannot.

llvm/include/llvm/CodeGen/SelectionDAG.h

@@ -1130,6 +1130,9 @@ class SelectionDAG {
     return getNode(ISD::UNDEF, SDLoc(), VT);
   }
 
+  /// Return a POISON node. POISON does not have a useful SDLoc.
+  SDValue getPOISON(EVT VT) { return getNode(ISD::POISON, SDLoc(), VT); }
+
   /// Return a node that represents the runtime scaling 'MulImm * RuntimeVL'.
   SDValue getVScale(const SDLoc &DL, EVT VT, APInt MulImm,
                     bool ConstantFold = true);

llvm/include/llvm/CodeGen/SelectionDAGNodes.h

@@ -692,8 +692,10 @@ 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 POISON.
+  bool isUndef() const {
+    return NodeType == ISD::UNDEF || NodeType == ISD::POISON;
+  }
 
   /// Test if this node is a memory intrinsic (with valid pointer information).
   bool isMemIntrinsic() const { return SDNodeBits.IsMemIntrinsic; }

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -16287,7 +16287,8 @@ SDValue DAGCombiner::visitFREEZE(SDNode *N) {
   // Finally, recreate the node, it's operands were updated to use
   // frozen operands, so we just need to use it's "original" operands.
   SmallVector<SDValue> Ops(N0->ops());
-  // Special-handle ISD::UNDEF, each single one of them can be it's own thing.
+  // TODO: ISD::UNDEF and ISD::POISON should get separate handling, but best
+  // leave for a future patch.
   for (SDValue &Op : Ops) {
     if (Op.isUndef())
       Op = DAG.getFreeze(Op);

llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp

@@ -986,6 +986,19 @@ void SelectionDAGLegalize::LegalizeOp(SDNode *Node) {
   TargetLowering::LegalizeAction Action = TargetLowering::Legal;
   bool SimpleFinishLegalizing = true;
   switch (Node->getOpcode()) {
+  // TODO: Currently, POISON is being lowered to UNDEF here. However, there is
+  // an open concern that this transformation may not be ideal, as targets
+  // should ideally handle POISON directly. Changing this behavior would require
+  // adding support for POISON in TableGen, which is a large change.
+  // Additionally, many existing test cases rely on the current behavior (e.g.,
+  // llvm/test/CodeGen/PowerPC/vec_shuffle.ll). A broader discussion and
+  // incremental changes might be needed to properly
+  // support POISON without breaking existing targets and tests.
+  case ISD::POISON: {
+    SDValue UndefNode = DAG.getUNDEF(Node->getValueType(0));
+    ReplaceNode(Node, UndefNode.getNode());
+    break;
+  }
   case ISD::INTRINSIC_W_CHAIN:
   case ISD::INTRINSIC_WO_CHAIN:
   case ISD::INTRINSIC_VOID:
@@ -3169,6 +3182,7 @@ bool SelectionDAGLegalize::ExpandNode(SDNode *Node) {
     for (unsigned i = 0; i < Node->getNumValues(); i++)
       Results.push_back(Node->getOperand(i));
     break;
+  case ISD::POISON:
   case ISD::UNDEF: {
     EVT VT = Node->getValueType(0);
     if (VT.isInteger())

llvm/lib/CodeGen/SelectionDAG/LegalizeFloatTypes.cpp

@@ -2845,6 +2845,7 @@ void DAGTypeLegalizer::PromoteFloatResult(SDNode *N, unsigned ResNo) {
 
     case ISD::SINT_TO_FP:
     case ISD::UINT_TO_FP: R = PromoteFloatRes_XINT_TO_FP(N); break;
+    case ISD::POISON:
     case ISD::UNDEF:      R = PromoteFloatRes_UNDEF(N); break;
     case ISD::ATOMIC_SWAP: R = BitcastToInt_ATOMIC_SWAP(N); break;
     case ISD::VECREDUCE_FADD:

llvm/lib/CodeGen/SelectionDAG/LegalizeIntegerTypes.cpp

@@ -118,6 +118,7 @@ void DAGTypeLegalizer::PromoteIntegerResult(SDNode *N, unsigned ResNo) {
   case ISD::VP_SRL:      Res = PromoteIntRes_SRL(N); break;
   case ISD::VP_TRUNCATE:
   case ISD::TRUNCATE:    Res = PromoteIntRes_TRUNCATE(N); break;
+  case ISD::POISON:
   case ISD::UNDEF:       Res = PromoteIntRes_UNDEF(N); break;
   case ISD::VAARG:       Res = PromoteIntRes_VAARG(N); break;
   case ISD::VSCALE:      Res = PromoteIntRes_VSCALE(N); break;
@@ -2932,6 +2933,7 @@ void DAGTypeLegalizer::ExpandIntegerResult(SDNode *N, unsigned ResNo) {
   case ISD::MERGE_VALUES: SplitRes_MERGE_VALUES(N, ResNo, Lo, Hi); break;
   case ISD::SELECT:       SplitRes_Select(N, Lo, Hi); break;
   case ISD::SELECT_CC:    SplitRes_SELECT_CC(N, Lo, Hi); break;
+  case ISD::POISON:
   case ISD::UNDEF:        SplitRes_UNDEF(N, Lo, Hi); break;
   case ISD::FREEZE:       SplitRes_FREEZE(N, Lo, Hi); break;
   case ISD::SETCC:        ExpandIntRes_SETCC(N, Lo, Hi); break;

llvm/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp

@@ -71,6 +71,7 @@ void DAGTypeLegalizer::ScalarizeVectorResult(SDNode *N, unsigned ResNo) {
   case ISD::SELECT:            R = ScalarizeVecRes_SELECT(N); break;
   case ISD::SELECT_CC:         R = ScalarizeVecRes_SELECT_CC(N); break;
   case ISD::SETCC:             R = ScalarizeVecRes_SETCC(N); break;
+  case ISD::POISON:
   case ISD::UNDEF:             R = ScalarizeVecRes_UNDEF(N); break;
   case ISD::VECTOR_SHUFFLE:    R = ScalarizeVecRes_VECTOR_SHUFFLE(N); break;
   case ISD::IS_FPCLASS:        R = ScalarizeVecRes_IS_FPCLASS(N); break;
@@ -1137,6 +1138,7 @@ void DAGTypeLegalizer::SplitVectorResult(SDNode *N, unsigned ResNo) {
   case ISD::VP_MERGE:
   case ISD::VP_SELECT:    SplitRes_Select(N, Lo, Hi); break;
   case ISD::SELECT_CC:    SplitRes_SELECT_CC(N, Lo, Hi); break;
+  case ISD::POISON:
   case ISD::UNDEF:        SplitRes_UNDEF(N, Lo, Hi); break;
   case ISD::BITCAST:           SplitVecRes_BITCAST(N, Lo, Hi); break;
   case ISD::BUILD_VECTOR:      SplitVecRes_BUILD_VECTOR(N, Lo, Hi); break;
@@ -4592,6 +4594,7 @@ void DAGTypeLegalizer::WidenVectorResult(SDNode *N, unsigned ResNo) {
   case ISD::SELECT_CC:         Res = WidenVecRes_SELECT_CC(N); break;
   case ISD::VP_SETCC:
   case ISD::SETCC:             Res = WidenVecRes_SETCC(N); break;
+  case ISD::POISON:
   case ISD::UNDEF:             Res = WidenVecRes_UNDEF(N); break;
   case ISD::VECTOR_SHUFFLE:
     Res = WidenVecRes_VECTOR_SHUFFLE(cast<ShuffleVectorSDNode>(N));

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -5387,6 +5387,9 @@ bool SelectionDAG::isGuaranteedNotToBeUndefOrPoison(SDValue Op,
   case ISD::CopyFromReg:
     return true;
 
+  case ISD::POISON:
+    return false;
+
   case ISD::UNDEF:
     return PoisonOnly;
 

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -1817,7 +1817,7 @@ SDValue SelectionDAGBuilder::getValueImpl(const Value *V) {
       return DAG.getConstantFP(*CFP, getCurSDLoc(), VT);
 
     if (isa<UndefValue>(C) && !V->getType()->isAggregateType())
-      return DAG.getUNDEF(VT);
+      return isa<PoisonValue>(C) ? DAG.getPOISON(VT) : DAG.getUNDEF(VT);
 
     if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(C)) {
       visit(CE->getOpcode(), *CE);