X86CodeEmitter.cpp   [plain text]

//===-- X86/X86CodeEmitter.cpp - Convert X86 code to machine code ---------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file contains the pass that transforms the X86 machine instructions into
// relocatable machine code.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "x86-emitter"
#include "X86InstrInfo.h"
#include "X86JITInfo.h"
#include "X86Subtarget.h"
#include "X86TargetMachine.h"
#include "X86Relocations.h"
#include "X86.h"
#include "llvm/LLVMContext.h"
#include "llvm/PassManager.h"
#include "llvm/CodeGen/JITCodeEmitter.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineModuleInfo.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/Function.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/MC/MCCodeEmitter.h"
#include "llvm/MC/MCExpr.h"
#include "llvm/MC/MCInst.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetOptions.h"
using namespace llvm;

STATISTIC(NumEmitted, "Number of machine instructions emitted");

namespace {
  template<class CodeEmitter>
  class Emitter : public MachineFunctionPass {
    const X86InstrInfo  *II;
    const TargetData    *TD;
    X86TargetMachine    &TM;
    CodeEmitter         &MCE;
    MachineModuleInfo   *MMI;
    intptr_t PICBaseOffset;
    bool Is64BitMode;
    bool IsPIC;
  public:
    static char ID;
    explicit Emitter(X86TargetMachine &tm, CodeEmitter &mce)
      : MachineFunctionPass(&ID), II(0), TD(0), TM(tm), 
      MCE(mce), PICBaseOffset(0), Is64BitMode(false),
      IsPIC(TM.getRelocationModel() == Reloc::PIC_) {}
    Emitter(X86TargetMachine &tm, CodeEmitter &mce,
            const X86InstrInfo &ii, const TargetData &td, bool is64)
      : MachineFunctionPass(&ID), II(&ii), TD(&td), TM(tm), 
      MCE(mce), PICBaseOffset(0), Is64BitMode(is64),
      IsPIC(TM.getRelocationModel() == Reloc::PIC_) {}

    bool runOnMachineFunction(MachineFunction &MF);

    virtual const char *getPassName() const {
      return "X86 Machine Code Emitter";
    }

    void emitInstruction(const MachineInstr &MI,
                         const TargetInstrDesc *Desc);
    
    void getAnalysisUsage(AnalysisUsage &AU) const {
      AU.setPreservesAll();
      AU.addRequired<MachineModuleInfo>();
      MachineFunctionPass::getAnalysisUsage(AU);
    }

  private:
    void emitPCRelativeBlockAddress(MachineBasicBlock *MBB);
    void emitGlobalAddress(GlobalValue *GV, unsigned Reloc,
                           intptr_t Disp = 0, intptr_t PCAdj = 0,
                           bool Indirect = false);
    void emitExternalSymbolAddress(const char *ES, unsigned Reloc);
    void emitConstPoolAddress(unsigned CPI, unsigned Reloc, intptr_t Disp = 0,
                              intptr_t PCAdj = 0);
    void emitJumpTableAddress(unsigned JTI, unsigned Reloc,
                              intptr_t PCAdj = 0);

    void emitDisplacementField(const MachineOperand *RelocOp, int DispVal,
                               intptr_t Adj = 0, bool IsPCRel = true);

    void emitRegModRMByte(unsigned ModRMReg, unsigned RegOpcodeField);
    void emitRegModRMByte(unsigned RegOpcodeField);
    void emitSIBByte(unsigned SS, unsigned Index, unsigned Base);
    void emitConstant(uint64_t Val, unsigned Size);

    void emitMemModRMByte(const MachineInstr &MI,
                          unsigned Op, unsigned RegOpcodeField,
                          intptr_t PCAdj = 0);

    unsigned getX86RegNum(unsigned RegNo) const;
  };

template<class CodeEmitter>
  char Emitter<CodeEmitter>::ID = 0;
} // end anonymous namespace.

/// createX86CodeEmitterPass - Return a pass that emits the collected X86 code
/// to the specified templated MachineCodeEmitter object.
FunctionPass *llvm::createX86JITCodeEmitterPass(X86TargetMachine &TM,
                                                JITCodeEmitter &JCE) {
  return new Emitter<JITCodeEmitter>(TM, JCE);
}

template<class CodeEmitter>
bool Emitter<CodeEmitter>::runOnMachineFunction(MachineFunction &MF) {
  MMI = &getAnalysis<MachineModuleInfo>();
  MCE.setModuleInfo(MMI);
  
  II = TM.getInstrInfo();
  TD = TM.getTargetData();
  Is64BitMode = TM.getSubtarget<X86Subtarget>().is64Bit();
  IsPIC = TM.getRelocationModel() == Reloc::PIC_;
  
  do {
    DEBUG(dbgs() << "JITTing function '" 
          << MF.getFunction()->getName() << "'\n");
    MCE.startFunction(MF);
    for (MachineFunction::iterator MBB = MF.begin(), E = MF.end(); 
         MBB != E; ++MBB) {
      MCE.StartMachineBasicBlock(MBB);
      for (MachineBasicBlock::const_iterator I = MBB->begin(), E = MBB->end();
           I != E; ++I) {
        const TargetInstrDesc &Desc = I->getDesc();
        emitInstruction(*I, &Desc);
        // MOVPC32r is basically a call plus a pop instruction.
        if (Desc.getOpcode() == X86::MOVPC32r)
          emitInstruction(*I, &II->get(X86::POP32r));
        NumEmitted++;  // Keep track of the # of mi's emitted
      }
    }
  } while (MCE.finishFunction(MF));

  return false;
}

/// emitPCRelativeBlockAddress - This method keeps track of the information
/// necessary to resolve the address of this block later and emits a dummy
/// value.
///
template<class CodeEmitter>
void Emitter<CodeEmitter>::emitPCRelativeBlockAddress(MachineBasicBlock *MBB) {
  // Remember where this reference was and where it is to so we can
  // deal with it later.
  MCE.addRelocation(MachineRelocation::getBB(MCE.getCurrentPCOffset(),
                                             X86::reloc_pcrel_word, MBB));
  MCE.emitWordLE(0);
}

/// emitGlobalAddress - Emit the specified address to the code stream assuming
/// this is part of a "take the address of a global" instruction.
///
template<class CodeEmitter>
void Emitter<CodeEmitter>::emitGlobalAddress(GlobalValue *GV, unsigned Reloc,
                                intptr_t Disp /* = 0 */,
                                intptr_t PCAdj /* = 0 */,
                                bool Indirect /* = false */) {
  intptr_t RelocCST = Disp;
  if (Reloc == X86::reloc_picrel_word)
    RelocCST = PICBaseOffset;
  else if (Reloc == X86::reloc_pcrel_word)
    RelocCST = PCAdj;
  MachineRelocation MR = Indirect
    ? MachineRelocation::getIndirectSymbol(MCE.getCurrentPCOffset(), Reloc,
                                           GV, RelocCST, false)
    : MachineRelocation::getGV(MCE.getCurrentPCOffset(), Reloc,
                               GV, RelocCST, false);
  MCE.addRelocation(MR);
  // The relocated value will be added to the displacement
  if (Reloc == X86::reloc_absolute_dword)
    MCE.emitDWordLE(Disp);
  else
    MCE.emitWordLE((int32_t)Disp);
}

/// emitExternalSymbolAddress - Arrange for the address of an external symbol to
/// be emitted to the current location in the function, and allow it to be PC
/// relative.
template<class CodeEmitter>
void Emitter<CodeEmitter>::emitExternalSymbolAddress(const char *ES,
                                                     unsigned Reloc) {
  intptr_t RelocCST = (Reloc == X86::reloc_picrel_word) ? PICBaseOffset : 0;

  // X86 never needs stubs because instruction selection will always pick
  // an instruction sequence that is large enough to hold any address
  // to a symbol.
  // (see X86ISelLowering.cpp, near 2039: X86TargetLowering::LowerCall)
  bool NeedStub = false;
  MCE.addRelocation(MachineRelocation::getExtSym(MCE.getCurrentPCOffset(),
                                                 Reloc, ES, RelocCST,
                                                 0, NeedStub));
  if (Reloc == X86::reloc_absolute_dword)
    MCE.emitDWordLE(0);
  else
    MCE.emitWordLE(0);
}

/// emitConstPoolAddress - Arrange for the address of an constant pool
/// to be emitted to the current location in the function, and allow it to be PC
/// relative.
template<class CodeEmitter>
void Emitter<CodeEmitter>::emitConstPoolAddress(unsigned CPI, unsigned Reloc,
                                   intptr_t Disp /* = 0 */,
                                   intptr_t PCAdj /* = 0 */) {
  intptr_t RelocCST = 0;
  if (Reloc == X86::reloc_picrel_word)
    RelocCST = PICBaseOffset;
  else if (Reloc == X86::reloc_pcrel_word)
    RelocCST = PCAdj;
  MCE.addRelocation(MachineRelocation::getConstPool(MCE.getCurrentPCOffset(),
                                                    Reloc, CPI, RelocCST));
  // The relocated value will be added to the displacement
  if (Reloc == X86::reloc_absolute_dword)
    MCE.emitDWordLE(Disp);
  else
    MCE.emitWordLE((int32_t)Disp);
}

/// emitJumpTableAddress - Arrange for the address of a jump table to
/// be emitted to the current location in the function, and allow it to be PC
/// relative.
template<class CodeEmitter>
void Emitter<CodeEmitter>::emitJumpTableAddress(unsigned JTI, unsigned Reloc,
                                   intptr_t PCAdj /* = 0 */) {
  intptr_t RelocCST = 0;
  if (Reloc == X86::reloc_picrel_word)
    RelocCST = PICBaseOffset;
  else if (Reloc == X86::reloc_pcrel_word)
    RelocCST = PCAdj;
  MCE.addRelocation(MachineRelocation::getJumpTable(MCE.getCurrentPCOffset(),
                                                    Reloc, JTI, RelocCST));
  // The relocated value will be added to the displacement
  if (Reloc == X86::reloc_absolute_dword)
    MCE.emitDWordLE(0);
  else
    MCE.emitWordLE(0);
}

template<class CodeEmitter>
unsigned Emitter<CodeEmitter>::getX86RegNum(unsigned RegNo) const {
  return X86RegisterInfo::getX86RegNum(RegNo);
}

inline static unsigned char ModRMByte(unsigned Mod, unsigned RegOpcode,
                                      unsigned RM) {
  assert(Mod < 4 && RegOpcode < 8 && RM < 8 && "ModRM Fields out of range!");
  return RM | (RegOpcode << 3) | (Mod << 6);
}

template<class CodeEmitter>
void Emitter<CodeEmitter>::emitRegModRMByte(unsigned ModRMReg,
                                            unsigned RegOpcodeFld){
  MCE.emitByte(ModRMByte(3, RegOpcodeFld, getX86RegNum(ModRMReg)));
}

template<class CodeEmitter>
void Emitter<CodeEmitter>::emitRegModRMByte(unsigned RegOpcodeFld) {
  MCE.emitByte(ModRMByte(3, RegOpcodeFld, 0));
}

template<class CodeEmitter>
void Emitter<CodeEmitter>::emitSIBByte(unsigned SS, 
                                       unsigned Index,
                                       unsigned Base) {
  // SIB byte is in the same format as the ModRMByte...
  MCE.emitByte(ModRMByte(SS, Index, Base));
}

template<class CodeEmitter>
void Emitter<CodeEmitter>::emitConstant(uint64_t Val, unsigned Size) {
  // Output the constant in little endian byte order...
  for (unsigned i = 0; i != Size; ++i) {
    MCE.emitByte(Val & 255);
    Val >>= 8;
  }
}

/// isDisp8 - Return true if this signed displacement fits in a 8-bit 
/// sign-extended field. 
static bool isDisp8(int Value) {
  return Value == (signed char)Value;
}

static bool gvNeedsNonLazyPtr(const MachineOperand &GVOp,
                              const TargetMachine &TM) {
  // For Darwin-64, simulate the linktime GOT by using the same non-lazy-pointer
  // mechanism as 32-bit mode.
  if (TM.getSubtarget<X86Subtarget>().is64Bit() && 
      !TM.getSubtarget<X86Subtarget>().isTargetDarwin())
    return false;
  
  // Return true if this is a reference to a stub containing the address of the
  // global, not the global itself.
  return isGlobalStubReference(GVOp.getTargetFlags());
}

template<class CodeEmitter>
void Emitter<CodeEmitter>::emitDisplacementField(const MachineOperand *RelocOp,
                                                 int DispVal,
                                                 intptr_t Adj /* = 0 */,
                                                 bool IsPCRel /* = true */) {
  // If this is a simple integer displacement that doesn't require a relocation,
  // emit it now.
  if (!RelocOp) {
    emitConstant(DispVal, 4);
    return;
  }

  // Otherwise, this is something that requires a relocation.  Emit it as such
  // now.
  unsigned RelocType = Is64BitMode ?
    (IsPCRel ? X86::reloc_pcrel_word : X86::reloc_absolute_word_sext)
    : (IsPIC ? X86::reloc_picrel_word : X86::reloc_absolute_word);
  if (RelocOp->isGlobal()) {
    // In 64-bit static small code model, we could potentially emit absolute.
    // But it's probably not beneficial. If the MCE supports using RIP directly
    // do it, otherwise fallback to absolute (this is determined by IsPCRel). 
    //  89 05 00 00 00 00     mov    %eax,0(%rip)  # PC-relative
    //  89 04 25 00 00 00 00  mov    %eax,0x0      # Absolute
    bool Indirect = gvNeedsNonLazyPtr(*RelocOp, TM);
    emitGlobalAddress(RelocOp->getGlobal(), RelocType, RelocOp->getOffset(),
                      Adj, Indirect);
  } else if (RelocOp->isSymbol()) {
    emitExternalSymbolAddress(RelocOp->getSymbolName(), RelocType);
  } else if (RelocOp->isCPI()) {
    emitConstPoolAddress(RelocOp->getIndex(), RelocType,
                         RelocOp->getOffset(), Adj);
  } else {
    assert(RelocOp->isJTI() && "Unexpected machine operand!");
    emitJumpTableAddress(RelocOp->getIndex(), RelocType, Adj);
  }
}

template<class CodeEmitter>
void Emitter<CodeEmitter>::emitMemModRMByte(const MachineInstr &MI,
                                            unsigned Op,unsigned RegOpcodeField,
                                            intptr_t PCAdj) {
  const MachineOperand &Op3 = MI.getOperand(Op+3);
  int DispVal = 0;
  const MachineOperand *DispForReloc = 0;
  
  // Figure out what sort of displacement we have to handle here.
  if (Op3.isGlobal()) {
    DispForReloc = &Op3;
  } else if (Op3.isSymbol()) {
    DispForReloc = &Op3;
  } else if (Op3.isCPI()) {
    if (!MCE.earlyResolveAddresses() || Is64BitMode || IsPIC) {
      DispForReloc = &Op3;
    } else {
      DispVal += MCE.getConstantPoolEntryAddress(Op3.getIndex());
      DispVal += Op3.getOffset();
    }
  } else if (Op3.isJTI()) {
    if (!MCE.earlyResolveAddresses() || Is64BitMode || IsPIC) {
      DispForReloc = &Op3;
    } else {
      DispVal += MCE.getJumpTableEntryAddress(Op3.getIndex());
    }
  } else {
    DispVal = Op3.getImm();
  }

  const MachineOperand &Base     = MI.getOperand(Op);
  const MachineOperand &Scale    = MI.getOperand(Op+1);
  const MachineOperand &IndexReg = MI.getOperand(Op+2);

  unsigned BaseReg = Base.getReg();
  
  // Handle %rip relative addressing.
  if (BaseReg == X86::RIP ||
      (Is64BitMode && DispForReloc)) { // [disp32+RIP] in X86-64 mode
    assert(IndexReg.getReg() == 0 && Is64BitMode &&
           "Invalid rip-relative address");
    MCE.emitByte(ModRMByte(0, RegOpcodeField, 5));
    emitDisplacementField(DispForReloc, DispVal, PCAdj, true);
    return;
  }

  // Indicate that the displacement will use an pcrel or absolute reference
  // by default. MCEs able to resolve addresses on-the-fly use pcrel by default
  // while others, unless explicit asked to use RIP, use absolute references.
  bool IsPCRel = MCE.earlyResolveAddresses() ? true : false;

  // Is a SIB byte needed?
  // If no BaseReg, issue a RIP relative instruction only if the MCE can 
  // resolve addresses on-the-fly, otherwise use SIB (Intel Manual 2A, table
  // 2-7) and absolute references.
  unsigned BaseRegNo = -1U;
  if (BaseReg != 0 && BaseReg != X86::RIP)
    BaseRegNo = getX86RegNum(BaseReg);

  if (// The SIB byte must be used if there is an index register.
      IndexReg.getReg() == 0 && 
      // The SIB byte must be used if the base is ESP/RSP/R12, all of which
      // encode to an R/M value of 4, which indicates that a SIB byte is
      // present.
      BaseRegNo != N86::ESP &&
      // If there is no base register and we're in 64-bit mode, we need a SIB
      // byte to emit an addr that is just 'disp32' (the non-RIP relative form).
      (!Is64BitMode || BaseReg != 0)) {
    if (BaseReg == 0 ||          // [disp32]     in X86-32 mode
        BaseReg == X86::RIP) {   // [disp32+RIP] in X86-64 mode
      MCE.emitByte(ModRMByte(0, RegOpcodeField, 5));
      emitDisplacementField(DispForReloc, DispVal, PCAdj, true);
      return;
    }
    
    // If the base is not EBP/ESP and there is no displacement, use simple
    // indirect register encoding, this handles addresses like [EAX].  The
    // encoding for [EBP] with no displacement means [disp32] so we handle it
    // by emitting a displacement of 0 below.
    if (!DispForReloc && DispVal == 0 && BaseRegNo != N86::EBP) {
      MCE.emitByte(ModRMByte(0, RegOpcodeField, BaseRegNo));
      return;
    }
    
    // Otherwise, if the displacement fits in a byte, encode as [REG+disp8].
    if (!DispForReloc && isDisp8(DispVal)) {
      MCE.emitByte(ModRMByte(1, RegOpcodeField, BaseRegNo));
      emitConstant(DispVal, 1);
      return;
    }
    
    // Otherwise, emit the most general non-SIB encoding: [REG+disp32]
    MCE.emitByte(ModRMByte(2, RegOpcodeField, BaseRegNo));
    emitDisplacementField(DispForReloc, DispVal, PCAdj, IsPCRel);
    return;
  }
  
  // Otherwise we need a SIB byte, so start by outputting the ModR/M byte first.
  assert(IndexReg.getReg() != X86::ESP &&
         IndexReg.getReg() != X86::RSP && "Cannot use ESP as index reg!");

  bool ForceDisp32 = false;
  bool ForceDisp8  = false;
  if (BaseReg == 0) {
    // If there is no base register, we emit the special case SIB byte with
    // MOD=0, BASE=4, to JUST get the index, scale, and displacement.
    MCE.emitByte(ModRMByte(0, RegOpcodeField, 4));
    ForceDisp32 = true;
  } else if (DispForReloc) {
    // Emit the normal disp32 encoding.
    MCE.emitByte(ModRMByte(2, RegOpcodeField, 4));
    ForceDisp32 = true;
  } else if (DispVal == 0 && BaseRegNo != N86::EBP) {
    // Emit no displacement ModR/M byte
    MCE.emitByte(ModRMByte(0, RegOpcodeField, 4));
  } else if (isDisp8(DispVal)) {
    // Emit the disp8 encoding...
    MCE.emitByte(ModRMByte(1, RegOpcodeField, 4));
    ForceDisp8 = true;           // Make sure to force 8 bit disp if Base=EBP
  } else {
    // Emit the normal disp32 encoding...
    MCE.emitByte(ModRMByte(2, RegOpcodeField, 4));
  }

  // Calculate what the SS field value should be...
  static const unsigned SSTable[] = { ~0, 0, 1, ~0, 2, ~0, ~0, ~0, 3 };
  unsigned SS = SSTable[Scale.getImm()];

  if (BaseReg == 0) {
    // Handle the SIB byte for the case where there is no base, see Intel 
    // Manual 2A, table 2-7. The displacement has already been output.
    unsigned IndexRegNo;
    if (IndexReg.getReg())
      IndexRegNo = getX86RegNum(IndexReg.getReg());
    else // Examples: [ESP+1*<noreg>+4] or [scaled idx]+disp32 (MOD=0,BASE=5)
      IndexRegNo = 4;
    emitSIBByte(SS, IndexRegNo, 5);
  } else {
    unsigned BaseRegNo = getX86RegNum(BaseReg);
    unsigned IndexRegNo;
    if (IndexReg.getReg())
      IndexRegNo = getX86RegNum(IndexReg.getReg());
    else
      IndexRegNo = 4;   // For example [ESP+1*<noreg>+4]
    emitSIBByte(SS, IndexRegNo, BaseRegNo);
  }

  // Do we need to output a displacement?
  if (ForceDisp8) {
    emitConstant(DispVal, 1);
  } else if (DispVal != 0 || ForceDisp32) {
    emitDisplacementField(DispForReloc, DispVal, PCAdj, IsPCRel);
  }
}

template<class CodeEmitter>
void Emitter<CodeEmitter>::emitInstruction(const MachineInstr &MI,
                                           const TargetInstrDesc *Desc) {
  DEBUG(dbgs() << MI);

  MCE.processDebugLoc(MI.getDebugLoc(), true);

  unsigned Opcode = Desc->Opcode;

  // Emit the lock opcode prefix as needed.
  if (Desc->TSFlags & X86II::LOCK)
    MCE.emitByte(0xF0);

  // Emit segment override opcode prefix as needed.
  switch (Desc->TSFlags & X86II::SegOvrMask) {
  case X86II::FS:
    MCE.emitByte(0x64);
    break;
  case X86II::GS:
    MCE.emitByte(0x65);
    break;
  default: llvm_unreachable("Invalid segment!");
  case 0: break;  // No segment override!
  }

  // Emit the repeat opcode prefix as needed.
  if ((Desc->TSFlags & X86II::Op0Mask) == X86II::REP)
    MCE.emitByte(0xF3);

  // Emit the operand size opcode prefix as needed.
  if (Desc->TSFlags & X86II::OpSize)
    MCE.emitByte(0x66);

  // Emit the address size opcode prefix as needed.
  if (Desc->TSFlags & X86II::AdSize)
    MCE.emitByte(0x67);

  bool Need0FPrefix = false;
  switch (Desc->TSFlags & X86II::Op0Mask) {
  case X86II::TB:  // Two-byte opcode prefix
  case X86II::T8:  // 0F 38
  case X86II::TA:  // 0F 3A
    Need0FPrefix = true;
    break;
  case X86II::TF: // F2 0F 38
    MCE.emitByte(0xF2);
    Need0FPrefix = true;
    break;
  case X86II::REP: break; // already handled.
  case X86II::XS:   // F3 0F
    MCE.emitByte(0xF3);
    Need0FPrefix = true;
    break;
  case X86II::XD:   // F2 0F
    MCE.emitByte(0xF2);
    Need0FPrefix = true;
    break;
  case X86II::D8: case X86II::D9: case X86II::DA: case X86II::DB:
  case X86II::DC: case X86II::DD: case X86II::DE: case X86II::DF:
    MCE.emitByte(0xD8+
                 (((Desc->TSFlags & X86II::Op0Mask)-X86II::D8)
                                   >> X86II::Op0Shift));
    break; // Two-byte opcode prefix
  default: llvm_unreachable("Invalid prefix!");
  case 0: break;  // No prefix!
  }

  // Handle REX prefix.
  if (Is64BitMode) {
    if (unsigned REX = X86InstrInfo::determineREX(MI))
      MCE.emitByte(0x40 | REX);
  }

  // 0x0F escape code must be emitted just before the opcode.
  if (Need0FPrefix)
    MCE.emitByte(0x0F);

  switch (Desc->TSFlags & X86II::Op0Mask) {
  case X86II::TF:    // F2 0F 38
  case X86II::T8:    // 0F 38
    MCE.emitByte(0x38);
    break;
  case X86II::TA:    // 0F 3A
    MCE.emitByte(0x3A);
    break;
  }

  // If this is a two-address instruction, skip one of the register operands.
  unsigned NumOps = Desc->getNumOperands();
  unsigned CurOp = 0;
  if (NumOps > 1 && Desc->getOperandConstraint(1, TOI::TIED_TO) != -1)
    ++CurOp;
  else if (NumOps > 2 && Desc->getOperandConstraint(NumOps-1, TOI::TIED_TO)== 0)
    // Skip the last source operand that is tied_to the dest reg. e.g. LXADD32
    --NumOps;

  unsigned char BaseOpcode = X86II::getBaseOpcodeFor(Desc->TSFlags);
  switch (Desc->TSFlags & X86II::FormMask) {
  default:
    llvm_unreachable("Unknown FormMask value in X86 MachineCodeEmitter!");
  case X86II::Pseudo:
    // Remember the current PC offset, this is the PIC relocation
    // base address.
    switch (Opcode) {
    default: 
      llvm_unreachable("psuedo instructions should be removed before code"
                       " emission");
      break;
    case TargetOpcode::INLINEASM:
      // We allow inline assembler nodes with empty bodies - they can
      // implicitly define registers, which is ok for JIT.
      if (MI.getOperand(0).getSymbolName()[0])
        llvm_report_error("JIT does not support inline asm!");
      break;
    case TargetOpcode::PROLOG_LABEL:
    case TargetOpcode::GC_LABEL:
    case TargetOpcode::EH_LABEL:
      MCE.emitLabel(MI.getOperand(0).getMCSymbol());
      break;
        
    case TargetOpcode::IMPLICIT_DEF:
    case TargetOpcode::KILL:
    case X86::FP_REG_KILL:
      break;
    case X86::MOVPC32r: {
      // This emits the "call" portion of this pseudo instruction.
      MCE.emitByte(BaseOpcode);
      emitConstant(0, X86II::getSizeOfImm(Desc->TSFlags));
      // Remember PIC base.
      PICBaseOffset = (intptr_t) MCE.getCurrentPCOffset();
      X86JITInfo *JTI = TM.getJITInfo();
      JTI->setPICBase(MCE.getCurrentPCValue());
      break;
    }
    }
    CurOp = NumOps;
    break;
  case X86II::RawFrm: {
    MCE.emitByte(BaseOpcode);

    if (CurOp == NumOps)
      break;
      
    const MachineOperand &MO = MI.getOperand(CurOp++);

    DEBUG(dbgs() << "RawFrm CurOp " << CurOp << "\n");
    DEBUG(dbgs() << "isMBB " << MO.isMBB() << "\n");
    DEBUG(dbgs() << "isGlobal " << MO.isGlobal() << "\n");
    DEBUG(dbgs() << "isSymbol " << MO.isSymbol() << "\n");
    DEBUG(dbgs() << "isImm " << MO.isImm() << "\n");

    if (MO.isMBB()) {
      emitPCRelativeBlockAddress(MO.getMBB());
      break;
    }
    
    if (MO.isGlobal()) {
      emitGlobalAddress(MO.getGlobal(), X86::reloc_pcrel_word,
                        MO.getOffset(), 0);
      break;
    }
    
    if (MO.isSymbol()) {
      emitExternalSymbolAddress(MO.getSymbolName(), X86::reloc_pcrel_word);
      break;
    }

    // FIXME: Only used by hackish MCCodeEmitter, remove when dead.
    if (MO.isJTI()) {
      emitJumpTableAddress(MO.getIndex(), X86::reloc_pcrel_word);
      break;
    }
    
    assert(MO.isImm() && "Unknown RawFrm operand!");
    if (Opcode == X86::CALLpcrel32 || Opcode == X86::CALL64pcrel32) {
      // Fix up immediate operand for pc relative calls.
      intptr_t Imm = (intptr_t)MO.getImm();
      Imm = Imm - MCE.getCurrentPCValue() - 4;
      emitConstant(Imm, X86II::getSizeOfImm(Desc->TSFlags));
    } else
      emitConstant(MO.getImm(), X86II::getSizeOfImm(Desc->TSFlags));
    break;
  }
      
  case X86II::AddRegFrm: {
    MCE.emitByte(BaseOpcode + getX86RegNum(MI.getOperand(CurOp++).getReg()));
    
    if (CurOp == NumOps)
      break;
      
    const MachineOperand &MO1 = MI.getOperand(CurOp++);
    unsigned Size = X86II::getSizeOfImm(Desc->TSFlags);
    if (MO1.isImm()) {
      emitConstant(MO1.getImm(), Size);
      break;
    }
    
    unsigned rt = Is64BitMode ? X86::reloc_pcrel_word
      : (IsPIC ? X86::reloc_picrel_word : X86::reloc_absolute_word);
    if (Opcode == X86::MOV64ri64i32)
      rt = X86::reloc_absolute_word;  // FIXME: add X86II flag?
    // This should not occur on Darwin for relocatable objects.
    if (Opcode == X86::MOV64ri)
      rt = X86::reloc_absolute_dword;  // FIXME: add X86II flag?
    if (MO1.isGlobal()) {
      bool Indirect = gvNeedsNonLazyPtr(MO1, TM);
      emitGlobalAddress(MO1.getGlobal(), rt, MO1.getOffset(), 0,
                        Indirect);
    } else if (MO1.isSymbol())
      emitExternalSymbolAddress(MO1.getSymbolName(), rt);
    else if (MO1.isCPI())
      emitConstPoolAddress(MO1.getIndex(), rt);
    else if (MO1.isJTI())
      emitJumpTableAddress(MO1.getIndex(), rt);
    break;
  }

  case X86II::MRMDestReg: {
    MCE.emitByte(BaseOpcode);
    emitRegModRMByte(MI.getOperand(CurOp).getReg(),
                     getX86RegNum(MI.getOperand(CurOp+1).getReg()));
    CurOp += 2;
    if (CurOp != NumOps)
      emitConstant(MI.getOperand(CurOp++).getImm(),
                   X86II::getSizeOfImm(Desc->TSFlags));
    break;
  }
  case X86II::MRMDestMem: {
    MCE.emitByte(BaseOpcode);
    emitMemModRMByte(MI, CurOp,
                     getX86RegNum(MI.getOperand(CurOp + X86AddrNumOperands)
                                  .getReg()));
    CurOp +=  X86AddrNumOperands + 1;
    if (CurOp != NumOps)
      emitConstant(MI.getOperand(CurOp++).getImm(),
                   X86II::getSizeOfImm(Desc->TSFlags));
    break;
  }

  case X86II::MRMSrcReg:
    MCE.emitByte(BaseOpcode);
    emitRegModRMByte(MI.getOperand(CurOp+1).getReg(),
                     getX86RegNum(MI.getOperand(CurOp).getReg()));
    CurOp += 2;
    if (CurOp != NumOps)
      emitConstant(MI.getOperand(CurOp++).getImm(),
                   X86II::getSizeOfImm(Desc->TSFlags));
    break;

  case X86II::MRMSrcMem: {
    // FIXME: Maybe lea should have its own form?
    int AddrOperands;
    if (Opcode == X86::LEA64r || Opcode == X86::LEA64_32r ||
        Opcode == X86::LEA16r || Opcode == X86::LEA32r)
      AddrOperands = X86AddrNumOperands - 1; // No segment register
    else
      AddrOperands = X86AddrNumOperands;

    intptr_t PCAdj = (CurOp + AddrOperands + 1 != NumOps) ?
      X86II::getSizeOfImm(Desc->TSFlags) : 0;

    MCE.emitByte(BaseOpcode);
    emitMemModRMByte(MI, CurOp+1, getX86RegNum(MI.getOperand(CurOp).getReg()),
                     PCAdj);
    CurOp += AddrOperands + 1;
    if (CurOp != NumOps)
      emitConstant(MI.getOperand(CurOp++).getImm(),
                   X86II::getSizeOfImm(Desc->TSFlags));
    break;
  }

  case X86II::MRM0r: case X86II::MRM1r:
  case X86II::MRM2r: case X86II::MRM3r:
  case X86II::MRM4r: case X86II::MRM5r:
  case X86II::MRM6r: case X86II::MRM7r: {
    MCE.emitByte(BaseOpcode);
    emitRegModRMByte(MI.getOperand(CurOp++).getReg(),
                     (Desc->TSFlags & X86II::FormMask)-X86II::MRM0r);

    if (CurOp == NumOps)
      break;
    
    const MachineOperand &MO1 = MI.getOperand(CurOp++);
    unsigned Size = X86II::getSizeOfImm(Desc->TSFlags);
    if (MO1.isImm()) {
      emitConstant(MO1.getImm(), Size);
      break;
    }
    
    unsigned rt = Is64BitMode ? X86::reloc_pcrel_word
      : (IsPIC ? X86::reloc_picrel_word : X86::reloc_absolute_word);
    if (Opcode == X86::MOV64ri32)
      rt = X86::reloc_absolute_word_sext;  // FIXME: add X86II flag?
    if (MO1.isGlobal()) {
      bool Indirect = gvNeedsNonLazyPtr(MO1, TM);
      emitGlobalAddress(MO1.getGlobal(), rt, MO1.getOffset(), 0,
                        Indirect);
    } else if (MO1.isSymbol())
      emitExternalSymbolAddress(MO1.getSymbolName(), rt);
    else if (MO1.isCPI())
      emitConstPoolAddress(MO1.getIndex(), rt);
    else if (MO1.isJTI())
      emitJumpTableAddress(MO1.getIndex(), rt);
    break;
  }

  case X86II::MRM0m: case X86II::MRM1m:
  case X86II::MRM2m: case X86II::MRM3m:
  case X86II::MRM4m: case X86II::MRM5m:
  case X86II::MRM6m: case X86II::MRM7m: {
    intptr_t PCAdj = (CurOp + X86AddrNumOperands != NumOps) ?
      (MI.getOperand(CurOp+X86AddrNumOperands).isImm() ? 
          X86II::getSizeOfImm(Desc->TSFlags) : 4) : 0;

    MCE.emitByte(BaseOpcode);
    emitMemModRMByte(MI, CurOp, (Desc->TSFlags & X86II::FormMask)-X86II::MRM0m,
                     PCAdj);
    CurOp += X86AddrNumOperands;

    if (CurOp == NumOps)
      break;
    
    const MachineOperand &MO = MI.getOperand(CurOp++);
    unsigned Size = X86II::getSizeOfImm(Desc->TSFlags);
    if (MO.isImm()) {
      emitConstant(MO.getImm(), Size);
      break;
    }
    
    unsigned rt = Is64BitMode ? X86::reloc_pcrel_word
      : (IsPIC ? X86::reloc_picrel_word : X86::reloc_absolute_word);
    if (Opcode == X86::MOV64mi32)
      rt = X86::reloc_absolute_word_sext;  // FIXME: add X86II flag?
    if (MO.isGlobal()) {
      bool Indirect = gvNeedsNonLazyPtr(MO, TM);
      emitGlobalAddress(MO.getGlobal(), rt, MO.getOffset(), 0,
                        Indirect);
    } else if (MO.isSymbol())
      emitExternalSymbolAddress(MO.getSymbolName(), rt);
    else if (MO.isCPI())
      emitConstPoolAddress(MO.getIndex(), rt);
    else if (MO.isJTI())
      emitJumpTableAddress(MO.getIndex(), rt);
    break;
  }

  case X86II::MRMInitReg:
    MCE.emitByte(BaseOpcode);
    // Duplicate register, used by things like MOV8r0 (aka xor reg,reg).
    emitRegModRMByte(MI.getOperand(CurOp).getReg(),
                     getX86RegNum(MI.getOperand(CurOp).getReg()));
    ++CurOp;
    break;
      
  case X86II::MRM_C1:
    MCE.emitByte(BaseOpcode);
    MCE.emitByte(0xC1);
    break;
  case X86II::MRM_C8:
    MCE.emitByte(BaseOpcode);
    MCE.emitByte(0xC8);
    break;
  case X86II::MRM_C9:
    MCE.emitByte(BaseOpcode);
    MCE.emitByte(0xC9);
    break;
  case X86II::MRM_E8:
    MCE.emitByte(BaseOpcode);
    MCE.emitByte(0xE8);
    break;
  case X86II::MRM_F0:
    MCE.emitByte(BaseOpcode);
    MCE.emitByte(0xF0);
    break;
  }

  if (!Desc->isVariadic() && CurOp != NumOps) {
#ifndef NDEBUG
    dbgs() << "Cannot encode all operands of: " << MI << "\n";
#endif
    llvm_unreachable(0);
  }

  MCE.processDebugLoc(MI.getDebugLoc(), false);
}