[SelectionDAG] Introduce ISD::PTRADD (#140017)

This opcode represents the addition of a pointer value (first operand)
and an integer offset (second operand). PTRADD nodes are only generated
if the TargetMachine opts in by overriding
TargetMachine::shouldPreservePtrArith().

The PTRADD node and respective visitPTRADD() function were adapted by
@rgwott from the CHERI/Morello LLVM tree.
Original authors: @davidchisnall, @jrtc27, @arichardson.

The changes in this PR were extracted from PR #105669.

---------

Co-authored-by: David Chisnall <github@theravensnest.org>
Co-authored-by: Jessica Clarke <jrtc27@jrtc27.com>
Co-authored-by: Alexander Richardson <alexrichardson@google.com>
Co-authored-by: Rodolfo Wottrich <rodolfo.wottrich@arm.com>

Author: 5 people

llvm/include/llvm/CodeGen/ISDOpcodes.h

@@ -1511,6 +1511,11 @@ enum NodeType {
   // Outputs: [rv], output chain, glue
   PATCHPOINT,
 
+  // PTRADD represents pointer arithmetic semantics, for targets that opt in
+  // using shouldPreservePtrArith().
+  // ptr = PTRADD ptr, offset
+  PTRADD,
+
 // Vector Predication
 #define BEGIN_REGISTER_VP_SDNODE(VPSDID, ...) VPSDID,
 #include "llvm/IR/VPIntrinsics.def"

llvm/include/llvm/CodeGen/SelectionDAGNodes.h

@@ -213,6 +213,7 @@ class SDValue {
   inline bool isTargetOpcode() const;
   inline bool isMachineOpcode() const;
   inline bool isUndef() const;
+  inline bool isAnyAdd() const;
   inline unsigned getMachineOpcode() const;
   inline const DebugLoc &getDebugLoc() const;
   inline void dump() const;
@@ -697,6 +698,11 @@ END_TWO_BYTE_PACK()
     return NodeType == ISD::UNDEF || NodeType == ISD::POISON;
   }
 
+  /// Returns true if the node type is ADD or PTRADD.
+  bool isAnyAdd() const {
+    return NodeType == ISD::ADD || NodeType == ISD::PTRADD;
+  }
+
   /// Test if this node is a memory intrinsic (with valid pointer information).
   bool isMemIntrinsic() const { return SDNodeBits.IsMemIntrinsic; }
 
@@ -1270,6 +1276,8 @@ inline bool SDValue::isUndef() const {
   return Node->isUndef();
 }
 
+inline bool SDValue::isAnyAdd() const { return Node->isAnyAdd(); }
+
 inline bool SDValue::use_empty() const {
   return !Node->hasAnyUseOfValue(ResNo);
 }

llvm/include/llvm/CodeGen/TargetLowering.h

@@ -3487,6 +3487,15 @@ class TargetLoweringBase {
   /// doing arithmetic on boolean types
   virtual bool shouldExpandCmpUsingSelects(EVT VT) const { return false; }
 
+  /// True if target has some particular form of dealing with pointer arithmetic
+  /// semantics for pointers with the given value type. False if pointer
+  /// arithmetic should not be preserved for passes such as instruction
+  /// selection, and can fallback to regular arithmetic.
+  /// This should be removed when PTRADD nodes are widely supported by backends.
+  virtual bool shouldPreservePtrArith(const Function &F, EVT PtrVT) const {
+    return false;
+  }
+
   /// Does this target support complex deinterleaving
   virtual bool isComplexDeinterleavingSupported() const { return false; }
 

llvm/include/llvm/Target/TargetSelectionDAG.td

@@ -407,7 +407,7 @@ def tblockaddress: SDNode<"ISD::TargetBlockAddress",  SDTPtrLeaf, [],
 
 def add        : SDNode<"ISD::ADD"       , SDTIntBinOp   ,
                         [SDNPCommutative, SDNPAssociative]>;
-def ptradd     : SDNode<"ISD::ADD"       , SDTPtrAddOp, []>;
+def ptradd     : SDNode<"ISD::PTRADD"    , SDTPtrAddOp, []>;
 def sub        : SDNode<"ISD::SUB"       , SDTIntBinOp>;
 def mul        : SDNode<"ISD::MUL"       , SDTIntBinOp,
                         [SDNPCommutative, SDNPAssociative]>;

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -417,7 +417,8 @@ namespace {
     SDValue visitMERGE_VALUES(SDNode *N);
     SDValue visitADD(SDNode *N);
     SDValue visitADDLike(SDNode *N);
-    SDValue visitADDLikeCommutative(SDValue N0, SDValue N1, SDNode *LocReference);
+    SDValue visitADDLikeCommutative(SDValue N0, SDValue N1,
+                                    SDNode *LocReference);
     SDValue visitSUB(SDNode *N);
     SDValue visitADDSAT(SDNode *N);
     SDValue visitSUBSAT(SDNode *N);
@@ -1100,7 +1101,7 @@ bool DAGCombiner::reassociationCanBreakAddressingModePattern(unsigned Opc,
   // (load/store (add, (add, x, y), offset2)) ->
   // (load/store (add, (add, x, offset2), y)).
 
-  if (N0.getOpcode() != ISD::ADD)
+  if (!N0.isAnyAdd())
     return false;
 
   // Check for vscale addressing modes.
@@ -2393,7 +2394,7 @@ static bool canFoldInAddressingMode(SDNode *N, SDNode *Use, SelectionDAG &DAG,
   }
 
   TargetLowering::AddrMode AM;
-  if (N->getOpcode() == ISD::ADD) {
+  if (N->isAnyAdd()) {
     AM.HasBaseReg = true;
     ConstantSDNode *Offset = dyn_cast<ConstantSDNode>(N->getOperand(1));
     if (Offset)

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -5642,7 +5642,7 @@ bool SelectionDAG::isADDLike(SDValue Op, bool NoWrap) const {
 
 bool SelectionDAG::isBaseWithConstantOffset(SDValue Op) const {
   return Op.getNumOperands() == 2 && isa<ConstantSDNode>(Op.getOperand(1)) &&
-         (Op.getOpcode() == ISD::ADD || isADDLike(Op));
+         (Op.isAnyAdd() || isADDLike(Op));
 }
 
 bool SelectionDAG::isKnownNeverNaN(SDValue Op, bool SNaN,
@@ -7351,10 +7351,18 @@ SDValue SelectionDAG::getNode(unsigned Opcode, const SDLoc &DL, EVT VT,
   case ISD::OR:
   case ISD::XOR:
   case ISD::ADD:
+  case ISD::PTRADD:
   case ISD::SUB:
     assert(VT.isInteger() && "This operator does not apply to FP types!");
     assert(N1.getValueType() == N2.getValueType() &&
            N1.getValueType() == VT && "Binary operator types must match!");
+    // The equal operand types requirement is unnecessarily strong for PTRADD.
+    // However, the SelectionDAGBuilder does not generate PTRADDs with different
+    // operand types, and we'd need to re-implement GEP's non-standard wrapping
+    // logic everywhere where PTRADDs may be folded or combined to properly
+    // support them. If/when we introduce pointer types to the SDAG, we will
+    // need to relax this constraint.
+
     // (X ^|+- 0) -> X.  This commonly occurs when legalizing i64 values, so
     // it's worth handling here.
     if (N2CV && N2CV->isZero())
@@ -7717,6 +7725,7 @@ SDValue SelectionDAG::getNode(unsigned Opcode, const SDLoc &DL, EVT VT,
       std::swap(N1, N2);
     } else {
       switch (Opcode) {
+      case ISD::PTRADD:
       case ISD::SUB:
         // fold op(undef, arg2) -> undef, fold op(poison, arg2) ->poison.
         return N1.getOpcode() == ISD::POISON ? getPOISON(VT) : getUNDEF(VT);
@@ -7744,6 +7753,7 @@ SDValue SelectionDAG::getNode(unsigned Opcode, const SDLoc &DL, EVT VT,
         return getConstant(0, DL, VT);
       [[fallthrough]];
     case ISD::ADD:
+    case ISD::PTRADD:
     case ISD::SUB:
     case ISD::UDIV:
     case ISD::SDIV:
@@ -8165,6 +8175,9 @@ SDValue SelectionDAG::getMemBasePlusOffset(SDValue Ptr, SDValue Offset,
                                            const SDNodeFlags Flags) {
   assert(Offset.getValueType().isInteger());
   EVT BasePtrVT = Ptr.getValueType();
+  if (TLI->shouldPreservePtrArith(this->getMachineFunction().getFunction(),
+                                  BasePtrVT))
+    return getNode(ISD::PTRADD, DL, BasePtrVT, Ptr, Offset, Flags);
   return getNode(ISD::ADD, DL, BasePtrVT, Ptr, Offset, Flags);
 }
 

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -4284,8 +4284,8 @@ void SelectionDAGBuilder::visitGetElementPtr(const User &I) {
             (int64_t(Offset) >= 0 && NW.hasNoUnsignedSignedWrap()))
           Flags |= SDNodeFlags::NoUnsignedWrap;
 
-        N = DAG.getNode(ISD::ADD, dl, N.getValueType(), N,
-                        DAG.getConstant(Offset, dl, N.getValueType()), Flags);
+        N = DAG.getMemBasePlusOffset(
+            N, DAG.getConstant(Offset, dl, N.getValueType()), dl, Flags);
       }
     } else {
       // IdxSize is the width of the arithmetic according to IR semantics.
@@ -4329,7 +4329,7 @@ void SelectionDAGBuilder::visitGetElementPtr(const User &I) {
 
         OffsVal = DAG.getSExtOrTrunc(OffsVal, dl, N.getValueType());
 
-        N = DAG.getNode(ISD::ADD, dl, N.getValueType(), N, OffsVal, Flags);
+        N = DAG.getMemBasePlusOffset(N, OffsVal, dl, Flags);
         continue;
       }
 
@@ -4389,7 +4389,7 @@ void SelectionDAGBuilder::visitGetElementPtr(const User &I) {
       SDNodeFlags AddFlags;
       AddFlags.setNoUnsignedWrap(NW.hasNoUnsignedWrap());
 
-      N = DAG.getNode(ISD::ADD, dl, N.getValueType(), N, IdxN, AddFlags);
+      N = DAG.getMemBasePlusOffset(N, IdxN, dl, AddFlags);
     }
   }
 
@@ -9173,8 +9173,7 @@ bool SelectionDAGBuilder::visitMemPCpyCall(const CallInst &I) {
   Size = DAG.getSExtOrTrunc(Size, sdl, Dst.getValueType());
 
   // Adjust return pointer to point just past the last dst byte.
-  SDValue DstPlusSize = DAG.getNode(ISD::ADD, sdl, Dst.getValueType(),
-                                    Dst, Size);
+  SDValue DstPlusSize = DAG.getMemBasePlusOffset(Dst, Size, sdl);
   setValue(&I, DstPlusSize);
   return true;
 }
@@ -11265,10 +11264,9 @@ TargetLowering::LowerCallTo(TargetLowering::CallLoweringInfo &CLI) const {
     MachineFunction &MF = CLI.DAG.getMachineFunction();
     Align HiddenSRetAlign = MF.getFrameInfo().getObjectAlign(DemoteStackIdx);
     for (unsigned i = 0; i < NumValues; ++i) {
-      SDValue Add =
-          CLI.DAG.getNode(ISD::ADD, CLI.DL, PtrVT, DemoteStackSlot,
-                          CLI.DAG.getConstant(Offsets[i], CLI.DL, PtrVT),
-                          SDNodeFlags::NoUnsignedWrap);
+      SDValue Add = CLI.DAG.getMemBasePlusOffset(
+          DemoteStackSlot, CLI.DAG.getConstant(Offsets[i], CLI.DL, PtrVT),
+          CLI.DL, SDNodeFlags::NoUnsignedWrap);
       SDValue L = CLI.DAG.getLoad(
           RetTys[i], CLI.DL, CLI.Chain, Add,
           MachinePointerInfo::getFixedStack(CLI.DAG.getMachineFunction(),

llvm/lib/CodeGen/SelectionDAG/SelectionDAGDumper.cpp

@@ -270,6 +270,7 @@ std::string SDNode::getOperationName(const SelectionDAG *G) const {
 
   // Binary operators
   case ISD::ADD:                        return "add";
+  case ISD::PTRADD:                     return "ptradd";
   case ISD::SUB:                        return "sub";
   case ISD::MUL:                        return "mul";
   case ISD::MULHU:                      return "mulhu";