llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp

//===-- LegalizeDAG.cpp - Implement SelectionDAG::Legalize ----------------===//
// 
//                     The LLVM Compiler Infrastructure
//
// This file was developed by the LLVM research group and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
// 
//===----------------------------------------------------------------------===//
//
// This file implements the SelectionDAG::Legalize method.
//
//===----------------------------------------------------------------------===//

#include "llvm/CodeGen/SelectionDAG.h"
#include "llvm/CodeGen/MachineConstantPool.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/Target/TargetLowering.h"
#include "llvm/Constants.h"
#include <iostream>
using namespace llvm;

//===----------------------------------------------------------------------===//
/// SelectionDAGLegalize - This takes an arbitrary SelectionDAG as input and
/// hacks on it until the target machine can handle it.  This involves
/// eliminating value sizes the machine cannot handle (promoting small sizes to
/// large sizes or splitting up large values into small values) as well as
/// eliminating operations the machine cannot handle.
///
/// This code also does a small amount of optimization and recognition of idioms
/// as part of its processing.  For example, if a target does not support a
/// 'setcc' instruction efficiently, but does support 'brcc' instruction, this
/// will attempt merge setcc and brc instructions into brcc's.
///
namespace {
class SelectionDAGLegalize {
  TargetLowering &TLI;
  SelectionDAG &DAG;

  /// LegalizeAction - This enum indicates what action we should take for each
  /// value type the can occur in the program.
  enum LegalizeAction {
    Legal,            // The target natively supports this value type.
    Promote,          // This should be promoted to the next larger type.
    Expand,           // This integer type should be broken into smaller pieces.
  };

  /// TransformToType - For any value types we are promoting or expanding, this
  /// contains the value type that we are changing to.  For Expanded types, this
  /// contains one step of the expand (e.g. i64 -> i32), even if there are
  /// multiple steps required (e.g. i64 -> i16)
  MVT::ValueType TransformToType[MVT::LAST_VALUETYPE];

  /// ValueTypeActions - This is a bitvector that contains two bits for each
  /// value type, where the two bits correspond to the LegalizeAction enum.
  /// This can be queried with "getTypeAction(VT)".
  unsigned ValueTypeActions;

  /// NeedsAnotherIteration - This is set when we expand a large integer
  /// operation into smaller integer operations, but the smaller operations are
  /// not set.  This occurs only rarely in practice, for targets that don't have
  /// 32-bit or larger integer registers.
  bool NeedsAnotherIteration;

  /// LegalizedNodes - For nodes that are of legal width, and that have more
  /// than one use, this map indicates what regularized operand to use.  This
  /// allows us to avoid legalizing the same thing more than once.
  std::map<SDOperand, SDOperand> LegalizedNodes;

  /// ExpandedNodes - For nodes that need to be expanded, and which have more
  /// than one use, this map indicates which which operands are the expanded
  /// version of the input.  This allows us to avoid expanding the same node
  /// more than once.
  std::map<SDOperand, std::pair<SDOperand, SDOperand> > ExpandedNodes;

  /// setValueTypeAction - Set the action for a particular value type.  This
  /// assumes an action has not already been set for this value type.
  void setValueTypeAction(MVT::ValueType VT, LegalizeAction A) {
    ValueTypeActions |= A << (VT*2);
    if (A == Promote) {
      MVT::ValueType PromoteTo;
      if (VT == MVT::f32)
        PromoteTo = MVT::f64;
      else {
        unsigned LargerReg = VT+1;
        while (!TLI.hasNativeSupportFor((MVT::ValueType)LargerReg)) {
          ++LargerReg;
          assert(MVT::isInteger((MVT::ValueType)LargerReg) &&
                 "Nothing to promote to??");
        }
        PromoteTo = (MVT::ValueType)LargerReg;
      }

      assert(MVT::isInteger(VT) == MVT::isInteger(PromoteTo) &&
             MVT::isFloatingPoint(VT) == MVT::isFloatingPoint(PromoteTo) &&
             "Can only promote from int->int or fp->fp!");
      assert(VT < PromoteTo && "Must promote to a larger type!");
      TransformToType[VT] = PromoteTo;
    } else if (A == Expand) {
      assert(MVT::isInteger(VT) && VT > MVT::i8 &&
             "Cannot expand this type: target must support SOME integer reg!");
      // Expand to the next smaller integer type!
      TransformToType[VT] = (MVT::ValueType)(VT-1);
    }
  }

public:

  SelectionDAGLegalize(TargetLowering &TLI, SelectionDAG &DAG);

  /// Run - While there is still lowering to do, perform a pass over the DAG.
  /// Most regularization can be done in a single pass, but targets that require
  /// large values to be split into registers multiple times (e.g. i64 -> 4x
  /// i16) require iteration for these values (the first iteration will demote
  /// to i32, the second will demote to i16).
  void Run() {
    do {
      NeedsAnotherIteration = false;
      LegalizeDAG();
    } while (NeedsAnotherIteration);
  }

  /// getTypeAction - Return how we should legalize values of this type, either
  /// it is already legal or we need to expand it into multiple registers of
  /// smaller integer type, or we need to promote it to a larger type.
  LegalizeAction getTypeAction(MVT::ValueType VT) const {
    return (LegalizeAction)((ValueTypeActions >> (2*VT)) & 3);
  }

  /// isTypeLegal - Return true if this type is legal on this target.
  ///
  bool isTypeLegal(MVT::ValueType VT) const {
    return getTypeAction(VT) == Legal;
  }

private:
  void LegalizeDAG();

  SDOperand LegalizeOp(SDOperand O);
  void ExpandOp(SDOperand O, SDOperand &Lo, SDOperand &Hi);

  SDOperand getIntPtrConstant(uint64_t Val) {
    return DAG.getConstant(Val, TLI.getPointerTy());
  }
};
}


SelectionDAGLegalize::SelectionDAGLegalize(TargetLowering &tli,
                                           SelectionDAG &dag)
  : TLI(tli), DAG(dag), ValueTypeActions(0) {

  assert(MVT::LAST_VALUETYPE <= 16 &&
         "Too many value types for ValueTypeActions to hold!");
  
  // Inspect all of the ValueType's possible, deciding how to process them.
  for (unsigned IntReg = MVT::i1; IntReg <= MVT::i128; ++IntReg)
    // If TLI says we are expanding this type, expand it!
    if (TLI.getNumElements((MVT::ValueType)IntReg) != 1)
      setValueTypeAction((MVT::ValueType)IntReg, Expand);
    else if (!TLI.hasNativeSupportFor((MVT::ValueType)IntReg))
      // Otherwise, if we don't have native support, we must promote to a
      // larger type.
      setValueTypeAction((MVT::ValueType)IntReg, Promote);
  
  // If the target does not have native support for F32, promote it to F64.
  if (!TLI.hasNativeSupportFor(MVT::f32))
    setValueTypeAction(MVT::f32, Promote);
}

void SelectionDAGLegalize::LegalizeDAG() {
  SDOperand OldRoot = DAG.getRoot();
  SDOperand NewRoot = LegalizeOp(OldRoot);
  DAG.setRoot(NewRoot);

  ExpandedNodes.clear();
  LegalizedNodes.clear();

  // Remove dead nodes now.
  DAG.RemoveDeadNodes(OldRoot.Val);
}

SDOperand SelectionDAGLegalize::LegalizeOp(SDOperand Op) {
  // If this operation defines any values that cannot be represented in a
  // register on this target, make sure to expand it.
  if (Op.Val->getNumValues() == 1) {// Fast path == assertion only
    assert(getTypeAction(Op.Val->getValueType(0)) == Legal &&
           "For a single use value, caller should check for legality!");
  } else {
    for (unsigned i = 0, e = Op.Val->getNumValues(); i != e; ++i)
      switch (getTypeAction(Op.Val->getValueType(i))) {
      case Legal: break;  // Nothing to do.
      case Expand: {
        SDOperand T1, T2;
        ExpandOp(Op.getValue(i), T1, T2);
        assert(LegalizedNodes.count(Op) &&
               "Expansion didn't add legal operands!");
        return LegalizedNodes[Op];
      }
      case Promote:
        // FIXME: Implement promotion!
        assert(0 && "Promotion not implemented at all yet!");
      }
  }

  // If there is more than one use of this, see if we already legalized it.
  // There is no use remembering values that only have a single use, as the map
  // entries will never be reused.
  if (!Op.Val->hasOneUse()) {
    std::map<SDOperand, SDOperand>::iterator I = LegalizedNodes.find(Op);
    if (I != LegalizedNodes.end()) return I->second;
  }

  SDOperand Tmp1, Tmp2;

  SDOperand Result = Op;
  SDNode *Node = Op.Val;
  LegalizeAction Action;

  switch (Node->getOpcode()) {
  default:
    std::cerr << "NODE: "; Node->dump(); std::cerr << "\n";
    assert(0 && "Do not know how to legalize this operator!");
    abort();
  case ISD::EntryToken:
  case ISD::FrameIndex:
  case ISD::GlobalAddress:
  case ISD::ExternalSymbol:
  case ISD::ConstantPool:
  case ISD::CopyFromReg:            // Nothing to do.
    assert(getTypeAction(Node->getValueType(0)) == Legal &&
           "This must be legal!");
    break;
  case ISD::Constant:
    // We know we don't need to expand constants here, constants only have one
    // value and we check that it is fine above.

    // FIXME: Maybe we should handle things like targets that don't support full
    // 32-bit immediates?
    break;
  case ISD::ConstantFP: {
    // Spill FP immediates to the constant pool if the target cannot directly
    // codegen them.  Targets often have some immediate values that can be
    // efficiently generated into an FP register without a load.  We explicitly
    // leave these constants as ConstantFP nodes for the target to deal with.

    ConstantFPSDNode *CFP = cast<ConstantFPSDNode>(Node);

    // Check to see if this FP immediate is already legal.
    bool isLegal = false;
    for (TargetLowering::legal_fpimm_iterator I = TLI.legal_fpimm_begin(),
           E = TLI.legal_fpimm_end(); I != E; ++I)
      if (CFP->isExactlyValue(*I)) {
        isLegal = true;
        break;
      }

    if (!isLegal) {
      // Otherwise we need to spill the constant to memory.
      MachineConstantPool *CP = DAG.getMachineFunction().getConstantPool();

      bool Extend = false;

      // If a FP immediate is precise when represented as a float, we put it
      // into the constant pool as a float, even if it's is statically typed
      // as a double.
      MVT::ValueType VT = CFP->getValueType(0);
      bool isDouble = VT == MVT::f64;
      ConstantFP *LLVMC = ConstantFP::get(isDouble ? Type::DoubleTy :
                                             Type::FloatTy, CFP->getValue());
      if (isDouble && CFP->isExactlyValue((float)CFP->getValue())) {
        LLVMC = cast<ConstantFP>(ConstantExpr::getCast(LLVMC, Type::FloatTy));
        VT = MVT::f32;
        Extend = true;
      }
      
      SDOperand CPIdx = DAG.getConstantPool(CP->getConstantPoolIndex(LLVMC),
                                            TLI.getPointerTy());
      Result = DAG.getLoad(VT, DAG.getEntryNode(), CPIdx);
      
      if (Extend) Result = DAG.getNode(ISD::FP_EXTEND, MVT::f64, Result);
    }
    break;
  }
  case ISD::ADJCALLSTACKDOWN:
  case ISD::ADJCALLSTACKUP:
    Tmp1 = LegalizeOp(Node->getOperand(0));  // Legalize the chain.
    // There is no need to legalize the size argument (Operand #1)
    if (Tmp1 != Node->getOperand(0))
      Result = DAG.getNode(Node->getOpcode(), MVT::Other, Tmp1,
                           Node->getOperand(1));
    break;
  case ISD::CALL:
    Tmp1 = LegalizeOp(Node->getOperand(0));  // Legalize the chain.
    Tmp2 = LegalizeOp(Node->getOperand(1));  // Legalize the callee.
    if (Tmp1 != Node->getOperand(0) || Tmp2 != Node->getOperand(1)) {
      std::vector<MVT::ValueType> RetTyVTs;
      RetTyVTs.reserve(Node->getNumValues());
      for (unsigned i = 0, e = Node->getNumValues(); i != e; ++i)
        RetTyVTs.push_back(Node->getValueType(i));
      Result = SDOperand(DAG.getCall(RetTyVTs, Tmp1, Tmp2), Op.ResNo);
    }
    break;

  case ISD::BR:
    Tmp1 = LegalizeOp(Node->getOperand(0));  // Legalize the chain.
    if (Tmp1 != Node->getOperand(0))
      Result = DAG.getNode(ISD::BR, MVT::Other, Tmp1, Node->getOperand(1));
    break;

  case ISD::BRCOND:
    Tmp1 = LegalizeOp(Node->getOperand(0));  // Legalize the chain.
    // FIXME: booleans might not be legal!
    Tmp2 = LegalizeOp(Node->getOperand(1));  // Legalize the condition.
    // Basic block destination (Op#2) is always legal.
    if (Tmp1 != Node->getOperand(0) || Tmp2 != Node->getOperand(1))
      Result = DAG.getNode(ISD::BRCOND, MVT::Other, Tmp1, Tmp2,
                           Node->getOperand(2));
    break;

  case ISD::LOAD:
    Tmp1 = LegalizeOp(Node->getOperand(0));  // Legalize the chain.
    Tmp2 = LegalizeOp(Node->getOperand(1));  // Legalize the pointer.
    if (Tmp1 != Node->getOperand(0) ||
        Tmp2 != Node->getOperand(1))
      Result = DAG.getLoad(Node->getValueType(0), Tmp1, Tmp2);
    break;

  case ISD::EXTRACT_ELEMENT:
    // Get both the low and high parts.
    ExpandOp(Node->getOperand(0), Tmp1, Tmp2);
    if (cast<ConstantSDNode>(Node->getOperand(1))->getValue())
      Result = Tmp2;  // 1 -> Hi
    else
      Result = Tmp1;  // 0 -> Lo
    break;

  case ISD::CopyToReg:
    Tmp1 = LegalizeOp(Node->getOperand(0));  // Legalize the chain.
    
    switch (getTypeAction(Node->getOperand(1).getValueType())) {
    case Legal:
      // Legalize the incoming value (must be legal).
      Tmp2 = LegalizeOp(Node->getOperand(1));
      if (Tmp1 != Node->getOperand(0) || Tmp2 != Node->getOperand(1))
        Result = DAG.getCopyToReg(Tmp1, Tmp2,
                                  cast<CopyRegSDNode>(Node)->getReg());
      break;
    case Expand: {
      SDOperand Lo, Hi;
      ExpandOp(Node->getOperand(1), Lo, Hi);      
      unsigned Reg = cast<CopyRegSDNode>(Node)->getReg();
      Result = DAG.getCopyToReg(Tmp1, Lo, Reg);
      Result = DAG.getCopyToReg(Result, Hi, Reg+1);
      assert(isTypeLegal(Result.getValueType()) &&
             "Cannot expand multiple times yet (i64 -> i16)");
      break;
    }
    case Promote:
      assert(0 && "Don't know what it means to promote this!");
      abort();
    }
    break;

  case ISD::RET:
    Tmp1 = LegalizeOp(Node->getOperand(0));  // Legalize the chain.
    switch (Node->getNumOperands()) {
    case 2:  // ret val
      switch (getTypeAction(Node->getOperand(1).getValueType())) {
      case Legal:
        Tmp2 = LegalizeOp(Node->getOperand(1));
        if (Tmp2 != Node->getOperand(1))
          Result = DAG.getNode(ISD::RET, MVT::Other, Tmp1, Tmp2);
        break;
      case Expand: {
        SDOperand Lo, Hi;
        ExpandOp(Node->getOperand(1), Lo, Hi);
        Result = DAG.getNode(ISD::RET, MVT::Other, Tmp1, Lo, Hi);
        break;                             
      }
      case Promote:
        assert(0 && "Can't promote return value!");
      }
      break;
    case 1:  // ret void
      if (Tmp1 != Node->getOperand(0))
        Result = DAG.getNode(ISD::RET, MVT::Other, Tmp1);
      break;
    default: { // ret <values>
      std::vector<SDOperand> NewValues;
      NewValues.push_back(Tmp1);
      for (unsigned i = 1, e = Node->getNumOperands(); i != e; ++i)
        switch (getTypeAction(Node->getOperand(i).getValueType())) {
        case Legal:
          NewValues.push_back(LegalizeOp(Node->getOperand(1)));
          break;
        case Expand: {
          SDOperand Lo, Hi;
          ExpandOp(Node->getOperand(i), Lo, Hi);
          NewValues.push_back(Lo);
          NewValues.push_back(Hi);
          break;                             
        }
        case Promote:
          assert(0 && "Can't promote return value!");
        }
      Result = DAG.getNode(ISD::RET, MVT::Other, NewValues);
      break;
    }
    }
    break;
  case ISD::STORE:
    Tmp1 = LegalizeOp(Node->getOperand(0));  // Legalize the chain.
    Tmp2 = LegalizeOp(Node->getOperand(2));  // Legalize the pointer.

    switch (getTypeAction(Node->getOperand(1).getValueType())) {
    case Legal: {
      SDOperand Val = LegalizeOp(Node->getOperand(1));
      if (Val != Node->getOperand(1) || Tmp1 != Node->getOperand(0) ||
          Tmp2 != Node->getOperand(2))
        Result = DAG.getNode(ISD::STORE, MVT::Other, Tmp1, Val, Tmp2);
      break;
    }
    case Promote:
      assert(0 && "FIXME: promote for stores not implemented!");
    case Expand:
      SDOperand Lo, Hi;
      ExpandOp(Node->getOperand(1), Lo, Hi);

      if (!TLI.isLittleEndian())
        std::swap(Lo, Hi);

      // FIXME: These two stores are independent of each other!
      Result = DAG.getNode(ISD::STORE, MVT::Other, Tmp1, Lo, Tmp2);

      unsigned IncrementSize;
      switch (Lo.getValueType()) {
      default: assert(0 && "Unknown ValueType to expand to!");
      case MVT::i32: IncrementSize = 4; break;
      case MVT::i16: IncrementSize = 2; break;
      case MVT::i8:  IncrementSize = 1; break;
      }
      Tmp2 = DAG.getNode(ISD::ADD, Tmp2.getValueType(), Tmp2,
                         getIntPtrConstant(IncrementSize));
      assert(isTypeLegal(Tmp2.getValueType()) &&
             "Pointers must be legal!");
      Result = DAG.getNode(ISD::STORE, MVT::Other, Result, Hi, Tmp2);
    }
    break;
  case ISD::SELECT: {
    // FIXME: BOOLS MAY REQUIRE PROMOTION!
    Tmp1 = LegalizeOp(Node->getOperand(0));   // Cond
    Tmp2 = LegalizeOp(Node->getOperand(1));   // TrueVal
    SDOperand Tmp3 = LegalizeOp(Node->getOperand(2));   // FalseVal

    if (Tmp1 != Node->getOperand(0) ||
        Tmp2 != Node->getOperand(1) ||
        Tmp3 != Node->getOperand(2))
      Result = DAG.getNode(ISD::SELECT, Node->getValueType(0), Tmp1, Tmp2,Tmp3);
    break;
  }
  case ISD::SETCC:
    switch (getTypeAction(Node->getOperand(0).getValueType())) {
    case Legal:
      Tmp1 = LegalizeOp(Node->getOperand(0));   // LHS
      Tmp2 = LegalizeOp(Node->getOperand(1));   // RHS
      if (Tmp1 != Node->getOperand(0) || Tmp2 != Node->getOperand(1))
        Result = DAG.getSetCC(cast<SetCCSDNode>(Node)->getCondition(),
                              Tmp1, Tmp2);
      break;
    case Promote:
      assert(0 && "Can't promote setcc operands yet!");
      break;
    case Expand: 
      SDOperand LHSLo, LHSHi, RHSLo, RHSHi;
      ExpandOp(Node->getOperand(0), LHSLo, LHSHi);
      ExpandOp(Node->getOperand(1), RHSLo, RHSHi);
      switch (cast<SetCCSDNode>(Node)->getCondition()) {
      case ISD::SETEQ:
      case ISD::SETNE:
        Tmp1 = DAG.getNode(ISD::XOR, LHSLo.getValueType(), LHSLo, RHSLo);
        Tmp2 = DAG.getNode(ISD::XOR, LHSLo.getValueType(), LHSHi, RHSHi);
        Tmp1 = DAG.getNode(ISD::OR, Tmp1.getValueType(), Tmp1, Tmp2);
        Result = DAG.getSetCC(cast<SetCCSDNode>(Node)->getCondition(), Tmp1,
                              DAG.getConstant(0, Tmp1.getValueType()));
        break;
      default:
        // FIXME: This generated code sucks.
        ISD::CondCode LowCC;
        switch (cast<SetCCSDNode>(Node)->getCondition()) {
        default: assert(0 && "Unknown integer setcc!");
        case ISD::SETLT:
        case ISD::SETULT: LowCC = ISD::SETULT; break;
        case ISD::SETGT:
        case ISD::SETUGT: LowCC = ISD::SETUGT; break;
        case ISD::SETLE:
        case ISD::SETULE: LowCC = ISD::SETULE; break;
        case ISD::SETGE:
        case ISD::SETUGE: LowCC = ISD::SETUGE; break;
        }
        
        // Tmp1 = lo(op1) < lo(op2)   // Always unsigned comparison
        // Tmp2 = hi(op1) < hi(op2)   // Signedness depends on operands
        // dest = hi(op1) == hi(op2) ? Tmp1 : Tmp2;

        // NOTE: on targets without efficient SELECT of bools, we can always use
        // this identity: (B1 ? B2 : B3) --> (B1 & B2)|(!B1&B3)
        Tmp1 = DAG.getSetCC(LowCC, LHSLo, RHSLo);
        Tmp2 = DAG.getSetCC(cast<SetCCSDNode>(Node)->getCondition(),
                            LHSHi, RHSHi);
        Result = DAG.getSetCC(ISD::SETEQ, LHSHi, RHSHi);
        Result = DAG.getNode(ISD::SELECT, MVT::i1, Result, Tmp1, Tmp2);
        break;
      }
    }
    break;

  case ISD::ADD:
  case ISD::SUB:
  case ISD::MUL:
  case ISD::UDIV:
  case ISD::SDIV:
  case ISD::UREM:
  case ISD::SREM:
  case ISD::AND:
  case ISD::OR:
  case ISD::XOR:
  case ISD::SHL:
  case ISD::SRL:
  case ISD::SRA:
    Tmp1 = LegalizeOp(Node->getOperand(0));   // LHS
    Tmp2 = LegalizeOp(Node->getOperand(1));   // RHS
    if (Tmp1 != Node->getOperand(0) ||
        Tmp2 != Node->getOperand(1))
      Result = DAG.getNode(Node->getOpcode(), Node->getValueType(0), Tmp1,Tmp2);
    break;
  case ISD::ZERO_EXTEND:
  case ISD::SIGN_EXTEND:
  case ISD::TRUNCATE:
  case ISD::FP_EXTEND:
  case ISD::FP_ROUND:
    switch (getTypeAction(Node->getOperand(0).getValueType())) {
    case Legal:
      Tmp1 = LegalizeOp(Node->getOperand(0));
      if (Tmp1 != Node->getOperand(0))
        Result = DAG.getNode(Node->getOpcode(), Node->getValueType(0), Tmp1);
      break;
    default:
      assert(0 && "Do not know how to expand or promote this yet!");
    }
    break;
  }

  if (!Op.Val->hasOneUse()) {
    bool isNew = LegalizedNodes.insert(std::make_pair(Op, Result)).second;
    assert(isNew && "Got into the map somehow?");
  }

  return Result;
}


/// ExpandOp - Expand the specified SDOperand into its two component pieces
/// Lo&Hi.  Note that the Op MUST be an expanded type.  As a result of this, the
/// LegalizeNodes map is filled in for any results that are not expanded, the
/// ExpandedNodes map is filled in for any results that are expanded, and the
/// Lo/Hi values are returned.
void SelectionDAGLegalize::ExpandOp(SDOperand Op, SDOperand &Lo, SDOperand &Hi){
  MVT::ValueType VT = Op.getValueType();
  MVT::ValueType NVT = TransformToType[VT];
  SDNode *Node = Op.Val;
  assert(getTypeAction(VT) == Expand && "Not an expanded type!");
  assert(MVT::isInteger(VT) && "Cannot expand FP values!");
  assert(MVT::isInteger(NVT) && NVT < VT &&
         "Cannot expand to FP value or to larger int value!");

  // If there is more than one use of this, see if we already expanded it.
  // There is no use remembering values that only have a single use, as the map
  // entries will never be reused.
  if (!Node->hasOneUse()) {
    std::map<SDOperand, std::pair<SDOperand, SDOperand> >::iterator I
      = ExpandedNodes.find(Op);
    if (I != ExpandedNodes.end()) {
      Lo = I->second.first;
      Hi = I->second.second;
      return;
    }
  }

  // If we are lowering to a type that the target doesn't support, we will have
  // to iterate lowering.
  if (!isTypeLegal(NVT))
    NeedsAnotherIteration = true;

  LegalizeAction Action;
  switch (Node->getOpcode()) {
  default:
    std::cerr << "NODE: "; Node->dump(); std::cerr << "\n";
    assert(0 && "Do not know how to expand this operator!");
    abort();
  case ISD::Constant: {
    uint64_t Cst = cast<ConstantSDNode>(Node)->getValue();
    Lo = DAG.getConstant(Cst, NVT);
    Hi = DAG.getConstant(Cst >> MVT::getSizeInBits(NVT), NVT);
    break;
  }

  case ISD::CopyFromReg: {
    unsigned Reg = cast<CopyRegSDNode>(Node)->getReg();
    // Aggregate register values are always in consequtive pairs.
    Lo = DAG.getCopyFromReg(Reg, NVT);
    Hi = DAG.getCopyFromReg(Reg+1, NVT);
    assert(isTypeLegal(NVT) && "Cannot expand this multiple times yet!");
    break;
  }

  case ISD::LOAD: {
    SDOperand Ch = LegalizeOp(Node->getOperand(0));   // Legalize the chain.
    SDOperand Ptr = LegalizeOp(Node->getOperand(1));  // Legalize the pointer.
    Lo = DAG.getLoad(NVT, Ch, Ptr);

    // Increment the pointer to the other half.
    unsigned IncrementSize;
    switch (Lo.getValueType()) {
    default: assert(0 && "Unknown ValueType to expand to!");
    case MVT::i32: IncrementSize = 4; break;
    case MVT::i16: IncrementSize = 2; break;
    case MVT::i8:  IncrementSize = 1; break;
    }
    Ptr = DAG.getNode(ISD::ADD, Ptr.getValueType(), Ptr,
                      getIntPtrConstant(IncrementSize));
    // FIXME: This load is independent of the first one.
    Hi = DAG.getLoad(NVT, Lo.getValue(1), Ptr);
    
    // Remember that we legalized the chain.
    bool isNew = LegalizedNodes.insert(std::make_pair(Op.getValue(1),
                                                      Hi.getValue(1))).second;
    assert(isNew && "This node was already legalized!");
    if (!TLI.isLittleEndian())
      std::swap(Lo, Hi);
    break;
  }
  case ISD::CALL: {
    SDOperand Chain  = LegalizeOp(Node->getOperand(0));  // Legalize the chain.
    SDOperand Callee = LegalizeOp(Node->getOperand(1));  // Legalize the callee.

    assert(Node->getNumValues() == 2 && Op.ResNo == 0 &&
           "Can only expand a call once so far, not i64 -> i16!");

    std::vector<MVT::ValueType> RetTyVTs;
    RetTyVTs.reserve(3);
    RetTyVTs.push_back(NVT);
    RetTyVTs.push_back(NVT);
    RetTyVTs.push_back(MVT::Other);
    SDNode *NC = DAG.getCall(RetTyVTs, Chain, Callee);
    Lo = SDOperand(NC, 0);
    Hi = SDOperand(NC, 1);

    // Insert the new chain mapping.
    bool isNew = LegalizedNodes.insert(std::make_pair(Op.getValue(1),
                                                      Hi.getValue(2))).second;
    assert(isNew && "This node was already legalized!");
    break;
  }
  case ISD::AND:
  case ISD::OR:
  case ISD::XOR: {   // Simple logical operators -> two trivial pieces.
    SDOperand LL, LH, RL, RH;
    ExpandOp(Node->getOperand(0), LL, LH);
    ExpandOp(Node->getOperand(1), RL, RH);
    Lo = DAG.getNode(Node->getOpcode(), NVT, LL, RL);
    Hi = DAG.getNode(Node->getOpcode(), NVT, LH, RH);
    break;
  }
  case ISD::SELECT: {
    SDOperand C, LL, LH, RL, RH;
    // FIXME: BOOLS MAY REQUIRE PROMOTION!
    C = LegalizeOp(Node->getOperand(0));
    ExpandOp(Node->getOperand(1), LL, LH);
    ExpandOp(Node->getOperand(2), RL, RH);
    Lo = DAG.getNode(ISD::SELECT, NVT, C, LL, RL);
    Hi = DAG.getNode(ISD::SELECT, NVT, C, LH, RH);
    break;
  }
  case ISD::SIGN_EXTEND: {
    // The low part is just a sign extension of the input (which degenerates to
    // a copy).
    Lo = DAG.getNode(ISD::SIGN_EXTEND, NVT, LegalizeOp(Node->getOperand(0)));
    
    // The high part is obtained by SRA'ing all but one of the bits of the lo
    // part.
    unsigned SrcSize = MVT::getSizeInBits(Node->getOperand(0).getValueType());
    Hi = DAG.getNode(ISD::SRA, NVT, Lo, DAG.getConstant(SrcSize-1, MVT::i8));
    break;
  }
  case ISD::ZERO_EXTEND:
    // The low part is just a zero extension of the input (which degenerates to
    // a copy).
    Lo = DAG.getNode(ISD::ZERO_EXTEND, NVT, LegalizeOp(Node->getOperand(0)));
    
    // The high part is just a zero.
    Hi = DAG.getConstant(0, NVT);
    break;
  }

  // Remember in a map if the values will be reused later.
  if (!Node->hasOneUse()) {
    bool isNew = ExpandedNodes.insert(std::make_pair(Op,
                                            std::make_pair(Lo, Hi))).second;
    assert(isNew && "Value already expanded?!?");
  }
}


// SelectionDAG::Legalize - This is the entry point for the file.
//
void SelectionDAG::Legalize(TargetLowering &TLI) {
  /// run - This is the main entry point to this class.
  ///
  SelectionDAGLegalize(TLI, *this).Run();
}