lib/MC/MachObjectWriter.cpp

//===- lib/MC/MachObjectWriter.cpp - Mach-O File Writer -------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//

#include "llvm/MC/MCMachObjectWriter.h"
#include "llvm/ADT/OwningPtr.h"
#include "llvm/ADT/StringMap.h"
#include "llvm/ADT/Twine.h"
#include "llvm/MC/MCAssembler.h"
#include "llvm/MC/MCAsmLayout.h"
#include "llvm/MC/MCExpr.h"
#include "llvm/MC/MCObjectWriter.h"
#include "llvm/MC/MCSectionMachO.h"
#include "llvm/MC/MCSymbol.h"
#include "llvm/MC/MCMachOSymbolFlags.h"
#include "llvm/MC/MCValue.h"
#include "llvm/Object/MachOFormat.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Target/TargetAsmBackend.h"

// FIXME: Gross.
#include "../Target/ARM/ARMFixupKinds.h"
#include "../Target/X86/X86FixupKinds.h"

#include <vector>
using namespace llvm;
using namespace llvm::object;

// FIXME: this has been copied from (or to) X86AsmBackend.cpp
static unsigned getFixupKindLog2Size(unsigned Kind) {
  switch (Kind) {
  default:
    llvm_unreachable("invalid fixup kind!");
  case FK_PCRel_1:
  case FK_Data_1: return 0;
  case FK_PCRel_2:
  case FK_Data_2: return 1;
  case FK_PCRel_4:
    // FIXME: Remove these!!!
  case X86::reloc_riprel_4byte:
  case X86::reloc_riprel_4byte_movq_load:
  case X86::reloc_signed_4byte:
  case FK_Data_4: return 2;
  case FK_Data_8: return 3;
  }
}

static bool doesSymbolRequireExternRelocation(MCSymbolData *SD) {
  // Undefined symbols are always extern.
  if (SD->Symbol->isUndefined())
    return true;

  // References to weak definitions require external relocation entries; the
  // definition may not always be the one in the same object file.
  if (SD->getFlags() & SF_WeakDefinition)
    return true;

  // Otherwise, we can use an internal relocation.
  return false;
}

namespace {

class MachObjectWriter : public MCObjectWriter {
  /// MachSymbolData - Helper struct for containing some precomputed information
  /// on symbols.
  struct MachSymbolData {
    MCSymbolData *SymbolData;
    uint64_t StringIndex;
    uint8_t SectionIndex;

    // Support lexicographic sorting.
    bool operator<(const MachSymbolData &RHS) const {
      return SymbolData->getSymbol().getName() <
             RHS.SymbolData->getSymbol().getName();
    }
  };

  /// The target specific Mach-O writer instance.
  llvm::OwningPtr<MCMachObjectTargetWriter> TargetObjectWriter;

  /// @name Relocation Data
  /// @{

  llvm::DenseMap<const MCSectionData*,
                 std::vector<macho::RelocationEntry> > Relocations;
  llvm::DenseMap<const MCSectionData*, unsigned> IndirectSymBase;

  /// @}
  /// @name Symbol Table Data
  /// @{

  SmallString<256> StringTable;
  std::vector<MachSymbolData> LocalSymbolData;
  std::vector<MachSymbolData> ExternalSymbolData;
  std::vector<MachSymbolData> UndefinedSymbolData;

  /// @}

private:
  /// @name Utility Methods
  /// @{

  bool isFixupKindPCRel(const MCAssembler &Asm, unsigned Kind) {
    const MCFixupKindInfo &FKI = Asm.getBackend().getFixupKindInfo(
      (MCFixupKind) Kind);

    return FKI.Flags & MCFixupKindInfo::FKF_IsPCRel;
  }

  /// @}

  SectionAddrMap SectionAddress;
  uint64_t getSectionAddress(const MCSectionData* SD) const {
    return SectionAddress.lookup(SD);
  }
  uint64_t getSymbolAddress(const MCSymbolData* SD,
                            const MCAsmLayout &Layout) const;

  uint64_t getFragmentAddress(const MCFragment *Fragment,
                            const MCAsmLayout &Layout) const {
    return getSectionAddress(Fragment->getParent()) +
      Layout.getFragmentOffset(Fragment);
  }

  uint64_t getPaddingSize(const MCSectionData *SD,
                          const MCAsmLayout &Layout) const;
public:
  MachObjectWriter(MCMachObjectTargetWriter *MOTW, raw_ostream &_OS,
                   bool _IsLittleEndian)
    : MCObjectWriter(_OS, _IsLittleEndian), TargetObjectWriter(MOTW) {
  }

  /// @name Target Writer Proxy Accessors
  /// @{

  bool is64Bit() const { return TargetObjectWriter->is64Bit(); }
  bool isARM() const {
    uint32_t CPUType = TargetObjectWriter->getCPUType() & ~mach::CTFM_ArchMask;
    return CPUType == mach::CTM_ARM;
  }

  /// @}

  void WriteHeader(unsigned NumLoadCommands, unsigned LoadCommandsSize,
                   bool SubsectionsViaSymbols);

  /// WriteSegmentLoadCommand - Write a segment load command.
  ///
  /// \arg NumSections - The number of sections in this segment.
  /// \arg SectionDataSize - The total size of the sections.
  void WriteSegmentLoadCommand(unsigned NumSections,
                               uint64_t VMSize,
                               uint64_t SectionDataStartOffset,
                               uint64_t SectionDataSize);

  void WriteSection(const MCAssembler &Asm, const MCAsmLayout &Layout,
                    const MCSectionData &SD, uint64_t FileOffset,
                    uint64_t RelocationsStart, unsigned NumRelocations);

  void WriteSymtabLoadCommand(uint32_t SymbolOffset, uint32_t NumSymbols,
                              uint32_t StringTableOffset,
                              uint32_t StringTableSize);

  void WriteDysymtabLoadCommand(uint32_t FirstLocalSymbol,
                                uint32_t NumLocalSymbols,
                                uint32_t FirstExternalSymbol,
                                uint32_t NumExternalSymbols,
                                uint32_t FirstUndefinedSymbol,
                                uint32_t NumUndefinedSymbols,
                                uint32_t IndirectSymbolOffset,
                                uint32_t NumIndirectSymbols);

  void WriteNlist(MachSymbolData &MSD, const MCAsmLayout &Layout);

  // FIXME: We really need to improve the relocation validation. Basically, we
  // want to implement a separate computation which evaluates the relocation
  // entry as the linker would, and verifies that the resultant fixup value is
  // exactly what the encoder wanted. This will catch several classes of
  // problems:
  //
  //  - Relocation entry bugs, the two algorithms are unlikely to have the same
  //    exact bug.
  //
  //  - Relaxation issues, where we forget to relax something.
  //
  //  - Input errors, where something cannot be correctly encoded. 'as' allows
  //    these through in many cases.

  static bool isFixupKindRIPRel(unsigned Kind) {
    return Kind == X86::reloc_riprel_4byte ||
      Kind == X86::reloc_riprel_4byte_movq_load;
  }
  void RecordX86_64Relocation(const MCAssembler &Asm, const MCAsmLayout &Layout,
                              const MCFragment *Fragment,
                              const MCFixup &Fixup, MCValue Target,
                              uint64_t &FixedValue);

  void RecordScatteredRelocation(const MCAssembler &Asm,
                                 const MCAsmLayout &Layout,
                                 const MCFragment *Fragment,
                                 const MCFixup &Fixup, MCValue Target,
                                 unsigned Log2Size,
                                 uint64_t &FixedValue);

  void RecordARMScatteredRelocation(const MCAssembler &Asm,
                                    const MCAsmLayout &Layout,
                                    const MCFragment *Fragment,
                                    const MCFixup &Fixup, MCValue Target,
                                    unsigned Log2Size,
                                    uint64_t &FixedValue);

  void RecordARMMovwMovtRelocation(const MCAssembler &Asm,
                                   const MCAsmLayout &Layout,
                                   const MCFragment *Fragment,
                                   const MCFixup &Fixup, MCValue Target,
                                   uint64_t &FixedValue);

  void RecordTLVPRelocation(const MCAssembler &Asm,
                            const MCAsmLayout &Layout,
                            const MCFragment *Fragment,
                            const MCFixup &Fixup, MCValue Target,
                            uint64_t &FixedValue);

  static bool getARMFixupKindMachOInfo(unsigned Kind, unsigned &RelocType,
                                       unsigned &Log2Size);

  void RecordARMRelocation(const MCAssembler &Asm, const MCAsmLayout &Layout,
                           const MCFragment *Fragment, const MCFixup &Fixup,
                           MCValue Target, uint64_t &FixedValue);

  void RecordRelocation(const MCAssembler &Asm, const MCAsmLayout &Layout,
                        const MCFragment *Fragment, const MCFixup &Fixup,
                        MCValue Target, uint64_t &FixedValue);

  void BindIndirectSymbols(MCAssembler &Asm);

  /// ComputeSymbolTable - Compute the symbol table data
  ///
  /// \param StringTable [out] - The string table data.
  /// \param StringIndexMap [out] - Map from symbol names to offsets in the
  /// string table.
  void ComputeSymbolTable(MCAssembler &Asm, SmallString<256> &StringTable,
                          std::vector<MachSymbolData> &LocalSymbolData,
                          std::vector<MachSymbolData> &ExternalSymbolData,
                          std::vector<MachSymbolData> &UndefinedSymbolData);

  void computeSectionAddresses(const MCAssembler &Asm,
                               const MCAsmLayout &Layout);

  void ExecutePostLayoutBinding(MCAssembler &Asm, const MCAsmLayout &Layout);

  virtual bool IsSymbolRefDifferenceFullyResolvedImpl(const MCAssembler &Asm,
                                                      const MCSymbolData &DataA,
                                                      const MCFragment &FB,
                                                      bool InSet,
                                                      bool IsPCRel) const;

  void WriteObject(MCAssembler &Asm, const MCAsmLayout &Layout);
};

} // end anonymous namespace

uint64_t MachObjectWriter::getSymbolAddress(const MCSymbolData* SD,
                                            const MCAsmLayout &Layout) const {
  const MCSymbol &S = SD->getSymbol();

  // If this is a variable, then recursively evaluate now.
  if (S.isVariable()) {
    MCValue Target;
    if (!S.getVariableValue()->EvaluateAsRelocatable(Target, Layout))
      report_fatal_error("unable to evaluate offset for variable '" +
                         S.getName() + "'");

    // Verify that any used symbols are defined.
    if (Target.getSymA() && Target.getSymA()->getSymbol().isUndefined())
      report_fatal_error("unable to evaluate offset to undefined symbol '" +
                         Target.getSymA()->getSymbol().getName() + "'");
    if (Target.getSymB() && Target.getSymB()->getSymbol().isUndefined())
      report_fatal_error("unable to evaluate offset to undefined symbol '" +
                         Target.getSymB()->getSymbol().getName() + "'");

    uint64_t Address = Target.getConstant();
    if (Target.getSymA())
      Address += getSymbolAddress(&Layout.getAssembler().getSymbolData(
                                    Target.getSymA()->getSymbol()), Layout);
    if (Target.getSymB())
      Address += getSymbolAddress(&Layout.getAssembler().getSymbolData(
                                    Target.getSymB()->getSymbol()), Layout);
    return Address;
  }

  return getSectionAddress(SD->getFragment()->getParent()) +
    Layout.getSymbolOffset(SD);
}

uint64_t MachObjectWriter::getPaddingSize(const MCSectionData *SD,
                                          const MCAsmLayout &Layout) const {
  uint64_t EndAddr = getSectionAddress(SD) + Layout.getSectionAddressSize(SD);
  unsigned Next = SD->getLayoutOrder() + 1;
  if (Next >= Layout.getSectionOrder().size())
    return 0;

  const MCSectionData &NextSD = *Layout.getSectionOrder()[Next];
  if (NextSD.getSection().isVirtualSection())
    return 0;
  return OffsetToAlignment(EndAddr, NextSD.getAlignment());
}

void MachObjectWriter::WriteHeader(unsigned NumLoadCommands,
                                   unsigned LoadCommandsSize,
                                   bool SubsectionsViaSymbols) {
  uint32_t Flags = 0;

  if (SubsectionsViaSymbols)
    Flags |= macho::HF_SubsectionsViaSymbols;

  // struct mach_header (28 bytes) or
  // struct mach_header_64 (32 bytes)

  uint64_t Start = OS.tell();
  (void) Start;

  Write32(is64Bit() ? macho::HM_Object64 : macho::HM_Object32);

  Write32(TargetObjectWriter->getCPUType());
  Write32(TargetObjectWriter->getCPUSubtype());

  Write32(macho::HFT_Object);
  Write32(NumLoadCommands);
  Write32(LoadCommandsSize);
  Write32(Flags);
  if (is64Bit())
    Write32(0); // reserved

  assert(OS.tell() - Start ==
         (is64Bit() ? macho::Header64Size : macho::Header32Size));
}

/// WriteSegmentLoadCommand - Write a segment load command.
///
/// \arg NumSections - The number of sections in this segment.
/// \arg SectionDataSize - The total size of the sections.
void MachObjectWriter::WriteSegmentLoadCommand(unsigned NumSections,
                                               uint64_t VMSize,
                                               uint64_t SectionDataStartOffset,
                                               uint64_t SectionDataSize) {
  // struct segment_command (56 bytes) or
  // struct segment_command_64 (72 bytes)

  uint64_t Start = OS.tell();
  (void) Start;

  unsigned SegmentLoadCommandSize =
    is64Bit() ? macho::SegmentLoadCommand64Size:
    macho::SegmentLoadCommand32Size;
  Write32(is64Bit() ? macho::LCT_Segment64 : macho::LCT_Segment);
  Write32(SegmentLoadCommandSize +
          NumSections * (is64Bit() ? macho::Section64Size :
                         macho::Section32Size));

  WriteBytes("", 16);
  if (is64Bit()) {
    Write64(0); // vmaddr
    Write64(VMSize); // vmsize
    Write64(SectionDataStartOffset); // file offset
    Write64(SectionDataSize); // file size
  } else {
    Write32(0); // vmaddr
    Write32(VMSize); // vmsize
    Write32(SectionDataStartOffset); // file offset
    Write32(SectionDataSize); // file size
  }
  Write32(0x7); // maxprot
  Write32(0x7); // initprot
  Write32(NumSections);
  Write32(0); // flags

  assert(OS.tell() - Start == SegmentLoadCommandSize);
}

void MachObjectWriter::WriteSection(const MCAssembler &Asm,
                                    const MCAsmLayout &Layout,
                                    const MCSectionData &SD,
                                    uint64_t FileOffset,
                                    uint64_t RelocationsStart,
                                    unsigned NumRelocations) {
  uint64_t SectionSize = Layout.getSectionAddressSize(&SD);

  // The offset is unused for virtual sections.
  if (SD.getSection().isVirtualSection()) {
    assert(Layout.getSectionFileSize(&SD) == 0 && "Invalid file size!");
    FileOffset = 0;
  }

  // struct section (68 bytes) or
  // struct section_64 (80 bytes)

  uint64_t Start = OS.tell();
  (void) Start;

  const MCSectionMachO &Section = cast<MCSectionMachO>(SD.getSection());
  WriteBytes(Section.getSectionName(), 16);
  WriteBytes(Section.getSegmentName(), 16);
  if (is64Bit()) {
    Write64(getSectionAddress(&SD)); // address
    Write64(SectionSize); // size
  } else {
    Write32(getSectionAddress(&SD)); // address
    Write32(SectionSize); // size
  }
  Write32(FileOffset);

  unsigned Flags = Section.getTypeAndAttributes();
  if (SD.hasInstructions())
    Flags |= MCSectionMachO::S_ATTR_SOME_INSTRUCTIONS;

  assert(isPowerOf2_32(SD.getAlignment()) && "Invalid alignment!");
  Write32(Log2_32(SD.getAlignment()));
  Write32(NumRelocations ? RelocationsStart : 0);
  Write32(NumRelocations);
  Write32(Flags);
  Write32(IndirectSymBase.lookup(&SD)); // reserved1
  Write32(Section.getStubSize()); // reserved2
  if (is64Bit())
    Write32(0); // reserved3

  assert(OS.tell() - Start == (is64Bit() ? macho::Section64Size :
                               macho::Section32Size));
}

void MachObjectWriter::WriteSymtabLoadCommand(uint32_t SymbolOffset,
                                              uint32_t NumSymbols,
                                              uint32_t StringTableOffset,
                                              uint32_t StringTableSize) {
  // struct symtab_command (24 bytes)

  uint64_t Start = OS.tell();
  (void) Start;

  Write32(macho::LCT_Symtab);
  Write32(macho::SymtabLoadCommandSize);
  Write32(SymbolOffset);
  Write32(NumSymbols);
  Write32(StringTableOffset);
  Write32(StringTableSize);

  assert(OS.tell() - Start == macho::SymtabLoadCommandSize);
}

void MachObjectWriter::WriteDysymtabLoadCommand(uint32_t FirstLocalSymbol,
                                                uint32_t NumLocalSymbols,
                                                uint32_t FirstExternalSymbol,
                                                uint32_t NumExternalSymbols,
                                                uint32_t FirstUndefinedSymbol,
                                                uint32_t NumUndefinedSymbols,
                                                uint32_t IndirectSymbolOffset,
                                                uint32_t NumIndirectSymbols) {
  // struct dysymtab_command (80 bytes)

  uint64_t Start = OS.tell();
  (void) Start;

  Write32(macho::LCT_Dysymtab);
  Write32(macho::DysymtabLoadCommandSize);
  Write32(FirstLocalSymbol);
  Write32(NumLocalSymbols);
  Write32(FirstExternalSymbol);
  Write32(NumExternalSymbols);
  Write32(FirstUndefinedSymbol);
  Write32(NumUndefinedSymbols);
  Write32(0); // tocoff
  Write32(0); // ntoc
  Write32(0); // modtaboff
  Write32(0); // nmodtab
  Write32(0); // extrefsymoff
  Write32(0); // nextrefsyms
  Write32(IndirectSymbolOffset);
  Write32(NumIndirectSymbols);
  Write32(0); // extreloff
  Write32(0); // nextrel
  Write32(0); // locreloff
  Write32(0); // nlocrel

  assert(OS.tell() - Start == macho::DysymtabLoadCommandSize);
}

void MachObjectWriter::WriteNlist(MachSymbolData &MSD,
                                  const MCAsmLayout &Layout) {
  MCSymbolData &Data = *MSD.SymbolData;
  const MCSymbol &Symbol = Data.getSymbol();
  uint8_t Type = 0;
  uint16_t Flags = Data.getFlags();
  uint32_t Address = 0;

  // Set the N_TYPE bits. See <mach-o/nlist.h>.
  //
  // FIXME: Are the prebound or indirect fields possible here?
  if (Symbol.isUndefined())
    Type = macho::STT_Undefined;
  else if (Symbol.isAbsolute())
    Type = macho::STT_Absolute;
  else
    Type = macho::STT_Section;

  // FIXME: Set STAB bits.

  if (Data.isPrivateExtern())
    Type |= macho::STF_PrivateExtern;

  // Set external bit.
  if (Data.isExternal() || Symbol.isUndefined())
    Type |= macho::STF_External;

  // Compute the symbol address.
  if (Symbol.isDefined()) {
    if (Symbol.isAbsolute()) {
      Address = cast<MCConstantExpr>(Symbol.getVariableValue())->getValue();
    } else {
      Address = getSymbolAddress(&Data, Layout);
    }
  } else if (Data.isCommon()) {
    // Common symbols are encoded with the size in the address
    // field, and their alignment in the flags.
    Address = Data.getCommonSize();

    // Common alignment is packed into the 'desc' bits.
    if (unsigned Align = Data.getCommonAlignment()) {
      unsigned Log2Size = Log2_32(Align);
      assert((1U << Log2Size) == Align && "Invalid 'common' alignment!");
      if (Log2Size > 15)
        report_fatal_error("invalid 'common' alignment '" +
                           Twine(Align) + "'");
      // FIXME: Keep this mask with the SymbolFlags enumeration.
      Flags = (Flags & 0xF0FF) | (Log2Size << 8);
    }
  }

  // struct nlist (12 bytes)

  Write32(MSD.StringIndex);
  Write8(Type);
  Write8(MSD.SectionIndex);

  // The Mach-O streamer uses the lowest 16-bits of the flags for the 'desc'
  // value.
  Write16(Flags);
  if (is64Bit())
    Write64(Address);
  else
    Write32(Address);
}

void MachObjectWriter::RecordX86_64Relocation(const MCAssembler &Asm,
                                              const MCAsmLayout &Layout,
                                              const MCFragment *Fragment,
                                              const MCFixup &Fixup,
                                              MCValue Target,
                                              uint64_t &FixedValue) {
  unsigned IsPCRel = isFixupKindPCRel(Asm, Fixup.getKind());
  unsigned IsRIPRel = isFixupKindRIPRel(Fixup.getKind());
  unsigned Log2Size = getFixupKindLog2Size(Fixup.getKind());

  // See <reloc.h>.
  uint32_t FixupOffset =
    Layout.getFragmentOffset(Fragment) + Fixup.getOffset();
  uint32_t FixupAddress =
    getFragmentAddress(Fragment, Layout) + Fixup.getOffset();
  int64_t Value = 0;
  unsigned Index = 0;
  unsigned IsExtern = 0;
  unsigned Type = 0;

  Value = Target.getConstant();

  if (IsPCRel) {
    // Compensate for the relocation offset, Darwin x86_64 relocations only have
    // the addend and appear to have attempted to define it to be the actual
    // expression addend without the PCrel bias. However, instructions with data
    // following the relocation are not accommodated for (see comment below
    // regarding SIGNED{1,2,4}), so it isn't exactly that either.
    Value += 1LL << Log2Size;
  }

  if (Target.isAbsolute()) { // constant
    // SymbolNum of 0 indicates the absolute section.
    Type = macho::RIT_X86_64_Unsigned;
    Index = 0;

    // FIXME: I believe this is broken, I don't think the linker can understand
    // it. I think it would require a local relocation, but I'm not sure if that
    // would work either. The official way to get an absolute PCrel relocation
    // is to use an absolute symbol (which we don't support yet).
    if (IsPCRel) {
      IsExtern = 1;
      Type = macho::RIT_X86_64_Branch;
    }
  } else if (Target.getSymB()) { // A - B + constant
    const MCSymbol *A = &Target.getSymA()->getSymbol();
    MCSymbolData &A_SD = Asm.getSymbolData(*A);
    const MCSymbolData *A_Base = Asm.getAtom(&A_SD);

    const MCSymbol *B = &Target.getSymB()->getSymbol();
    MCSymbolData &B_SD = Asm.getSymbolData(*B);
    const MCSymbolData *B_Base = Asm.getAtom(&B_SD);

    // Neither symbol can be modified.
    if (Target.getSymA()->getKind() != MCSymbolRefExpr::VK_None ||
        Target.getSymB()->getKind() != MCSymbolRefExpr::VK_None)
      report_fatal_error("unsupported relocation of modified symbol");

    // We don't support PCrel relocations of differences. Darwin 'as' doesn't
    // implement most of these correctly.
    if (IsPCRel)
      report_fatal_error("unsupported pc-relative relocation of difference");

    // The support for the situation where one or both of the symbols would
    // require a local relocation is handled just like if the symbols were
    // external.  This is certainly used in the case of debug sections where the
    // section has only temporary symbols and thus the symbols don't have base
    // symbols.  This is encoded using the section ordinal and non-extern
    // relocation entries.

    // Darwin 'as' doesn't emit correct relocations for this (it ends up with a
    // single SIGNED relocation); reject it for now.  Except the case where both
    // symbols don't have a base, equal but both NULL.
    if (A_Base == B_Base && A_Base)
      report_fatal_error("unsupported relocation with identical base");

    Value += getSymbolAddress(&A_SD, Layout) -
      (A_Base == NULL ? 0 : getSymbolAddress(A_Base, Layout));
    Value -= getSymbolAddress(&B_SD, Layout) -
      (B_Base == NULL ? 0 : getSymbolAddress(B_Base, Layout));

    if (A_Base) {
      Index = A_Base->getIndex();
      IsExtern = 1;
    }
    else {
      Index = A_SD.getFragment()->getParent()->getOrdinal() + 1;
      IsExtern = 0;
    }
    Type = macho::RIT_X86_64_Unsigned;

    macho::RelocationEntry MRE;
    MRE.Word0 = FixupOffset;
    MRE.Word1 = ((Index     <<  0) |
                 (IsPCRel   << 24) |
                 (Log2Size  << 25) |
                 (IsExtern  << 27) |
                 (Type      << 28));
    Relocations[Fragment->getParent()].push_back(MRE);

    if (B_Base) {
      Index = B_Base->getIndex();
      IsExtern = 1;
    }
    else {
      Index = B_SD.getFragment()->getParent()->getOrdinal() + 1;
      IsExtern = 0;
    }
    Type = macho::RIT_X86_64_Subtractor;
  } else {
    const MCSymbol *Symbol = &Target.getSymA()->getSymbol();
    MCSymbolData &SD = Asm.getSymbolData(*Symbol);
    const MCSymbolData *Base = Asm.getAtom(&SD);

    // Relocations inside debug sections always use local relocations when
    // possible. This seems to be done because the debugger doesn't fully
    // understand x86_64 relocation entries, and expects to find values that
    // have already been fixed up.
    if (Symbol->isInSection()) {
      const MCSectionMachO &Section = static_cast<const MCSectionMachO&>(
        Fragment->getParent()->getSection());
      if (Section.hasAttribute(MCSectionMachO::S_ATTR_DEBUG))
        Base = 0;
    }

    // x86_64 almost always uses external relocations, except when there is no
    // symbol to use as a base address (a local symbol with no preceding
    // non-local symbol).
    if (Base) {
      Index = Base->getIndex();
      IsExtern = 1;

      // Add the local offset, if needed.
      if (Base != &SD)
        Value += Layout.getSymbolOffset(&SD) - Layout.getSymbolOffset(Base);
    } else if (Symbol->isInSection() && !Symbol->isVariable()) {
      // The index is the section ordinal (1-based).
      Index = SD.getFragment()->getParent()->getOrdinal() + 1;
      IsExtern = 0;
      Value += getSymbolAddress(&SD, Layout);

      if (IsPCRel)
        Value -= FixupAddress + (1 << Log2Size);
    } else if (Symbol->isVariable()) {
      const MCExpr *Value = Symbol->getVariableValue();
      int64_t Res;
      bool isAbs = Value->EvaluateAsAbsolute(Res, Layout, SectionAddress);
      if (isAbs) {
        FixedValue = Res;
        return;
      } else {
        report_fatal_error("unsupported relocation of variable '" +
                           Symbol->getName() + "'");
      }
    } else {
      report_fatal_error("unsupported relocation of undefined symbol '" +
                         Symbol->getName() + "'");
    }

    MCSymbolRefExpr::VariantKind Modifier = Target.getSymA()->getKind();
    if (IsPCRel) {
      if (IsRIPRel) {
        if (Modifier == MCSymbolRefExpr::VK_GOTPCREL) {
          // x86_64 distinguishes movq foo@GOTPCREL so that the linker can
          // rewrite the movq to an leaq at link time if the symbol ends up in
          // the same linkage unit.
          if (unsigned(Fixup.getKind()) == X86::reloc_riprel_4byte_movq_load)
            Type = macho::RIT_X86_64_GOTLoad;
          else
            Type = macho::RIT_X86_64_GOT;
        }  else if (Modifier == MCSymbolRefExpr::VK_TLVP) {
          Type = macho::RIT_X86_64_TLV;
        }  else if (Modifier != MCSymbolRefExpr::VK_None) {
          report_fatal_error("unsupported symbol modifier in relocation");
        } else {
          Type = macho::RIT_X86_64_Signed;

          // The Darwin x86_64 relocation format has a problem where it cannot
          // encode an address (L<foo> + <constant>) which is outside the atom
          // containing L<foo>. Generally, this shouldn't occur but it does
          // happen when we have a RIPrel instruction with data following the
          // relocation entry (e.g., movb $012, L0(%rip)). Even with the PCrel
          // adjustment Darwin x86_64 uses, the offset is still negative and the
          // linker has no way to recognize this.
          //
          // To work around this, Darwin uses several special relocation types
          // to indicate the offsets. However, the specification or
          // implementation of these seems to also be incomplete; they should
          // adjust the addend as well based on the actual encoded instruction
          // (the additional bias), but instead appear to just look at the final
          // offset.
          switch (-(Target.getConstant() + (1LL << Log2Size))) {
          case 1: Type = macho::RIT_X86_64_Signed1; break;
          case 2: Type = macho::RIT_X86_64_Signed2; break;
          case 4: Type = macho::RIT_X86_64_Signed4; break;
          }
        }
      } else {
        if (Modifier != MCSymbolRefExpr::VK_None)
          report_fatal_error("unsupported symbol modifier in branch "
                             "relocation");

        Type = macho::RIT_X86_64_Branch;
      }
    } else {
      if (Modifier == MCSymbolRefExpr::VK_GOT) {
        Type = macho::RIT_X86_64_GOT;
      } else if (Modifier == MCSymbolRefExpr::VK_GOTPCREL) {
        // GOTPCREL is allowed as a modifier on non-PCrel instructions, in which
        // case all we do is set the PCrel bit in the relocation entry; this is
        // used with exception handling, for example. The source is required to
        // include any necessary offset directly.
        Type = macho::RIT_X86_64_GOT;
        IsPCRel = 1;
      } else if (Modifier == MCSymbolRefExpr::VK_TLVP) {
        report_fatal_error("TLVP symbol modifier should have been rip-rel");
      } else if (Modifier != MCSymbolRefExpr::VK_None)
        report_fatal_error("unsupported symbol modifier in relocation");
      else
        Type = macho::RIT_X86_64_Unsigned;
    }
  }

  // x86_64 always writes custom values into the fixups.
  FixedValue = Value;

  // struct relocation_info (8 bytes)
  macho::RelocationEntry MRE;
  MRE.Word0 = FixupOffset;
  MRE.Word1 = ((Index     <<  0) |
               (IsPCRel   << 24) |
               (Log2Size  << 25) |
               (IsExtern  << 27) |
               (Type      << 28));
  Relocations[Fragment->getParent()].push_back(MRE);
}

void MachObjectWriter::RecordScatteredRelocation(const MCAssembler &Asm,
                                                 const MCAsmLayout &Layout,
                                                 const MCFragment *Fragment,
                                                 const MCFixup &Fixup,
                                                 MCValue Target,
                                                 unsigned Log2Size,
                                                 uint64_t &FixedValue) {
  uint32_t FixupOffset = Layout.getFragmentOffset(Fragment)+Fixup.getOffset();
  unsigned IsPCRel = isFixupKindPCRel(Asm, Fixup.getKind());
  unsigned Type = macho::RIT_Vanilla;

  // See <reloc.h>.
  const MCSymbol *A = &Target.getSymA()->getSymbol();
  MCSymbolData *A_SD = &Asm.getSymbolData(*A);

  if (!A_SD->getFragment())
    report_fatal_error("symbol '" + A->getName() +
                       "' can not be undefined in a subtraction expression");

  uint32_t Value = getSymbolAddress(A_SD, Layout);
  uint64_t SecAddr = getSectionAddress(A_SD->getFragment()->getParent());
  FixedValue += SecAddr;
  uint32_t Value2 = 0;

  if (const MCSymbolRefExpr *B = Target.getSymB()) {
    MCSymbolData *B_SD = &Asm.getSymbolData(B->getSymbol());

    if (!B_SD->getFragment())
      report_fatal_error("symbol '" + B->getSymbol().getName() +
                         "' can not be undefined in a subtraction expression");

    // Select the appropriate difference relocation type.
    //
    // Note that there is no longer any semantic difference between these two
    // relocation types from the linkers point of view, this is done solely for
    // pedantic compatibility with 'as'.
    Type = A_SD->isExternal() ? (unsigned)macho::RIT_Difference :
      (unsigned)macho::RIT_Generic_LocalDifference;
    Value2 = getSymbolAddress(B_SD, Layout);
    FixedValue -= getSectionAddress(B_SD->getFragment()->getParent());
  }

  // Relocations are written out in reverse order, so the PAIR comes first.
  if (Type == macho::RIT_Difference ||
      Type == macho::RIT_Generic_LocalDifference) {
    macho::RelocationEntry MRE;
    MRE.Word0 = ((0         <<  0) |
                 (macho::RIT_Pair  << 24) |
                 (Log2Size  << 28) |
                 (IsPCRel   << 30) |
                 macho::RF_Scattered);
    MRE.Word1 = Value2;
    Relocations[Fragment->getParent()].push_back(MRE);
  }

  macho::RelocationEntry MRE;
  MRE.Word0 = ((FixupOffset <<  0) |
               (Type        << 24) |
               (Log2Size    << 28) |
               (IsPCRel     << 30) |
               macho::RF_Scattered);
  MRE.Word1 = Value;
  Relocations[Fragment->getParent()].push_back(MRE);
}

void MachObjectWriter::RecordARMScatteredRelocation(const MCAssembler &Asm,
                                                    const MCAsmLayout &Layout,
                                                    const MCFragment *Fragment,
                                                    const MCFixup &Fixup,
                                                    MCValue Target,
                                                    unsigned Log2Size,
                                                    uint64_t &FixedValue) {
  uint32_t FixupOffset = Layout.getFragmentOffset(Fragment)+Fixup.getOffset();
  unsigned IsPCRel = isFixupKindPCRel(Asm, Fixup.getKind());
  unsigned Type = macho::RIT_Vanilla;

  // See <reloc.h>.
  const MCSymbol *A = &Target.getSymA()->getSymbol();
  MCSymbolData *A_SD = &Asm.getSymbolData(*A);

  if (!A_SD->getFragment())
    report_fatal_error("symbol '" + A->getName() +
                       "' can not be undefined in a subtraction expression");

  uint32_t Value = getSymbolAddress(A_SD, Layout);
  uint64_t SecAddr = getSectionAddress(A_SD->getFragment()->getParent());
  FixedValue += SecAddr;
  uint32_t Value2 = 0;

  if (const MCSymbolRefExpr *B = Target.getSymB()) {
    MCSymbolData *B_SD = &Asm.getSymbolData(B->getSymbol());

    if (!B_SD->getFragment())
      report_fatal_error("symbol '" + B->getSymbol().getName() +
                         "' can not be undefined in a subtraction expression");

    // Select the appropriate difference relocation type.
    Type = macho::RIT_Difference;
    Value2 = getSymbolAddress(B_SD, Layout);
    FixedValue -= getSectionAddress(B_SD->getFragment()->getParent());
  }

  // Relocations are written out in reverse order, so the PAIR comes first.
  if (Type == macho::RIT_Difference ||
      Type == macho::RIT_Generic_LocalDifference) {
    macho::RelocationEntry MRE;
    MRE.Word0 = ((0         <<  0) |
                 (macho::RIT_Pair  << 24) |
                 (Log2Size  << 28) |
                 (IsPCRel   << 30) |
                 macho::RF_Scattered);
    MRE.Word1 = Value2;
    Relocations[Fragment->getParent()].push_back(MRE);
  }

  macho::RelocationEntry MRE;
  MRE.Word0 = ((FixupOffset <<  0) |
               (Type        << 24) |
               (Log2Size    << 28) |
               (IsPCRel     << 30) |
               macho::RF_Scattered);
  MRE.Word1 = Value;
  Relocations[Fragment->getParent()].push_back(MRE);
}

void MachObjectWriter::RecordARMMovwMovtRelocation(const MCAssembler &Asm,
                                                   const MCAsmLayout &Layout,
                                                   const MCFragment *Fragment,
                                                   const MCFixup &Fixup,
                                                   MCValue Target,
                                                   uint64_t &FixedValue) {
  uint32_t FixupOffset = Layout.getFragmentOffset(Fragment)+Fixup.getOffset();
  unsigned IsPCRel = isFixupKindPCRel(Asm, Fixup.getKind());
  unsigned Type = macho::RIT_ARM_Half;

  // See <reloc.h>.
  const MCSymbol *A = &Target.getSymA()->getSymbol();
  MCSymbolData *A_SD = &Asm.getSymbolData(*A);

  if (!A_SD->getFragment())
    report_fatal_error("symbol '" + A->getName() +
                       "' can not be undefined in a subtraction expression");

  uint32_t Value = getSymbolAddress(A_SD, Layout);
  uint32_t Value2 = 0;
  uint64_t SecAddr = getSectionAddress(A_SD->getFragment()->getParent());
  FixedValue += SecAddr;

  if (const MCSymbolRefExpr *B = Target.getSymB()) {
    MCSymbolData *B_SD = &Asm.getSymbolData(B->getSymbol());

    if (!B_SD->getFragment())
      report_fatal_error("symbol '" + B->getSymbol().getName() +
                         "' can not be undefined in a subtraction expression");

    // Select the appropriate difference relocation type.
    Type = macho::RIT_ARM_HalfDifference;
    Value2 = getSymbolAddress(B_SD, Layout);
    FixedValue -= getSectionAddress(B_SD->getFragment()->getParent());
  }

  // Relocations are written out in reverse order, so the PAIR comes first.
  // ARM_RELOC_HALF and ARM_RELOC_HALF_SECTDIFF abuse the r_length field:
  //
  // For these two r_type relocations they always have a pair following them and
  // the r_length bits are used differently.  The encoding of the r_length is as
  // follows:
  //   low bit of r_length:
  //      0 - :lower16: for movw instructions
  //      1 - :upper16: for movt instructions
  //   high bit of r_length:
  //      0 - arm instructions
  //      1 - thumb instructions
  // the other half of the relocated expression is in the following pair
  // relocation entry in the the low 16 bits of r_address field.
  unsigned ThumbBit = 0;
  unsigned MovtBit = 0;
  switch ((unsigned)Fixup.getKind()) {
  default: break;
  case ARM::fixup_arm_movt_hi16:
  case ARM::fixup_arm_movt_hi16_pcrel:
    MovtBit = 1;
    break;
  case ARM::fixup_t2_movt_hi16:
  case ARM::fixup_t2_movt_hi16_pcrel:
    MovtBit = 1;
    // Fallthrough
  case ARM::fixup_t2_movw_lo16:
  case ARM::fixup_t2_movw_lo16_pcrel:
    ThumbBit = 1;
    break;
  }


  if (Type == macho::RIT_ARM_HalfDifference) {
    uint32_t OtherHalf = MovtBit
      ? (FixedValue & 0xffff) : ((FixedValue & 0xffff0000) >> 16);

    macho::RelocationEntry MRE;
    MRE.Word0 = ((OtherHalf       <<  0) |
                 (macho::RIT_Pair << 24) |
                 (MovtBit         << 28) |
                 (ThumbBit        << 29) |
                 (IsPCRel         << 30) |
                 macho::RF_Scattered);
    MRE.Word1 = Value2;
    Relocations[Fragment->getParent()].push_back(MRE);
  }

  macho::RelocationEntry MRE;
  MRE.Word0 = ((FixupOffset <<  0) |
               (Type        << 24) |
               (MovtBit     << 28) |
               (ThumbBit    << 29) |
               (IsPCRel     << 30) |
               macho::RF_Scattered);
  MRE.Word1 = Value;
  Relocations[Fragment->getParent()].push_back(MRE);
}

void MachObjectWriter::RecordTLVPRelocation(const MCAssembler &Asm,
                                            const MCAsmLayout &Layout,
                                            const MCFragment *Fragment,
                                            const MCFixup &Fixup,
                                            MCValue Target,
                                            uint64_t &FixedValue) {
  assert(Target.getSymA()->getKind() == MCSymbolRefExpr::VK_TLVP &&
         !is64Bit() &&
         "Should only be called with a 32-bit TLVP relocation!");

  unsigned Log2Size = getFixupKindLog2Size(Fixup.getKind());
  uint32_t Value = Layout.getFragmentOffset(Fragment)+Fixup.getOffset();
  unsigned IsPCRel = 0;

  // Get the symbol data.
  MCSymbolData *SD_A = &Asm.getSymbolData(Target.getSymA()->getSymbol());
  unsigned Index = SD_A->getIndex();

  // We're only going to have a second symbol in pic mode and it'll be a
  // subtraction from the picbase. For 32-bit pic the addend is the difference
  // between the picbase and the next address.  For 32-bit static the addend is
  // zero.
  if (Target.getSymB()) {
    // If this is a subtraction then we're pcrel.
    uint32_t FixupAddress =
      getFragmentAddress(Fragment, Layout) + Fixup.getOffset();
    MCSymbolData *SD_B = &Asm.getSymbolData(Target.getSymB()->getSymbol());
    IsPCRel = 1;
    FixedValue = (FixupAddress - getSymbolAddress(SD_B, Layout) +
                  Target.getConstant());
    FixedValue += 1ULL << Log2Size;
  } else {
    FixedValue = 0;
  }

  // struct relocation_info (8 bytes)
  macho::RelocationEntry MRE;
  MRE.Word0 = Value;
  MRE.Word1 = ((Index                  <<  0) |
               (IsPCRel                << 24) |
               (Log2Size               << 25) |
               (1                      << 27) | // Extern
               (macho::RIT_Generic_TLV << 28)); // Type
  Relocations[Fragment->getParent()].push_back(MRE);
}

bool MachObjectWriter::getARMFixupKindMachOInfo(unsigned Kind,
                                                unsigned &RelocType,
                                                unsigned &Log2Size) {
  RelocType = unsigned(macho::RIT_Vanilla);
  Log2Size = ~0U;

  switch (Kind) {
  default:
    return false;

  case FK_Data_1:
    Log2Size = llvm::Log2_32(1);
    return true;
  case FK_Data_2:
    Log2Size = llvm::Log2_32(2);
    return true;
  case FK_Data_4:
    Log2Size = llvm::Log2_32(4);
    return true;
  case FK_Data_8:
    Log2Size = llvm::Log2_32(8);
    return true;

    // Handle 24-bit branch kinds.
  case ARM::fixup_arm_ldst_pcrel_12:
  case ARM::fixup_arm_pcrel_10:
  case ARM::fixup_arm_adr_pcrel_12:
  case ARM::fixup_arm_condbranch:
  case ARM::fixup_arm_uncondbranch:
    RelocType = unsigned(macho::RIT_ARM_Branch24Bit);
    // Report as 'long', even though that is not quite accurate.
    Log2Size = llvm::Log2_32(4);
    return true;

    // Handle Thumb branches.
  case ARM::fixup_arm_thumb_br:
    RelocType = unsigned(macho::RIT_ARM_ThumbBranch22Bit);
    Log2Size = llvm::Log2_32(2);
    return true;
      
  case ARM::fixup_arm_thumb_bl:
  case ARM::fixup_arm_thumb_blx:
    RelocType = unsigned(macho::RIT_ARM_ThumbBranch22Bit);
    Log2Size = llvm::Log2_32(4);
    return true;

  case ARM::fixup_arm_movt_hi16:
  case ARM::fixup_arm_movt_hi16_pcrel:
  case ARM::fixup_t2_movt_hi16:
  case ARM::fixup_t2_movt_hi16_pcrel:
    RelocType = unsigned(macho::RIT_ARM_HalfDifference);
    // Report as 'long', even though that is not quite accurate.
    Log2Size = llvm::Log2_32(4);
    return true;

  case ARM::fixup_arm_movw_lo16:
  case ARM::fixup_arm_movw_lo16_pcrel:
  case ARM::fixup_t2_movw_lo16:
  case ARM::fixup_t2_movw_lo16_pcrel:
    RelocType = unsigned(macho::RIT_ARM_Half);
    // Report as 'long', even though that is not quite accurate.
    Log2Size = llvm::Log2_32(4);
    return true;
  }
}
void MachObjectWriter::RecordARMRelocation(const MCAssembler &Asm,
                                           const MCAsmLayout &Layout,
                                           const MCFragment *Fragment,
                                           const MCFixup &Fixup,
                                           MCValue Target,
                                           uint64_t &FixedValue) {
  unsigned IsPCRel = isFixupKindPCRel(Asm, Fixup.getKind());
  unsigned Log2Size;
  unsigned RelocType = macho::RIT_Vanilla;
  if (!getARMFixupKindMachOInfo(Fixup.getKind(), RelocType, Log2Size)) {
    report_fatal_error("unknown ARM fixup kind!");
    return;
  }

  // If this is a difference or a defined symbol plus an offset, then we need a
  // scattered relocation entry.  Differences always require scattered
  // relocations.
  if (Target.getSymB()) {
    if (RelocType == macho::RIT_ARM_Half ||
        RelocType == macho::RIT_ARM_HalfDifference)
      return RecordARMMovwMovtRelocation(Asm, Layout, Fragment, Fixup,
                                         Target, FixedValue);
    return RecordARMScatteredRelocation(Asm, Layout, Fragment, Fixup,
                                        Target, Log2Size, FixedValue);
  }

  // Get the symbol data, if any.
  MCSymbolData *SD = 0;
  if (Target.getSymA())
    SD = &Asm.getSymbolData(Target.getSymA()->getSymbol());

  // FIXME: For other platforms, we need to use scattered relocations for
  // internal relocations with offsets.  If this is an internal relocation with
  // an offset, it also needs a scattered relocation entry.
  //
  // Is this right for ARM?
  uint32_t Offset = Target.getConstant();
  if (IsPCRel && RelocType == macho::RIT_Vanilla)
    Offset += 1 << Log2Size;
  if (Offset && SD && !doesSymbolRequireExternRelocation(SD))
    return RecordARMScatteredRelocation(Asm, Layout, Fragment, Fixup, Target,
                                        Log2Size, FixedValue);

  // See <reloc.h>.
  uint32_t FixupOffset = Layout.getFragmentOffset(Fragment)+Fixup.getOffset();
  unsigned Index = 0;
  unsigned IsExtern = 0;
  unsigned Type = 0;

  if (Target.isAbsolute()) { // constant
    // FIXME!
    report_fatal_error("FIXME: relocations to absolute targets "
                       "not yet implemented");
  } else {
    // Resolve constant variables.
    if (SD->getSymbol().isVariable()) {
      int64_t Res;
      if (SD->getSymbol().getVariableValue()->EvaluateAsAbsolute(
            Res, Layout, SectionAddress)) {
        FixedValue = Res;
        return;
      }
    }

    // Check whether we need an external or internal relocation.
    if (doesSymbolRequireExternRelocation(SD)) {
      IsExtern = 1;
      Index = SD->getIndex();

      // For external relocations, make sure to offset the fixup value to
      // compensate for the addend of the symbol address, if it was
      // undefined. This occurs with weak definitions, for example.
      if (!SD->Symbol->isUndefined())
        FixedValue -= Layout.getSymbolOffset(SD);
    } else {
      // The index is the section ordinal (1-based).
      const MCSectionData &SymSD = Asm.getSectionData(
        SD->getSymbol().getSection());
      Index = SymSD.getOrdinal() + 1;
      FixedValue += getSectionAddress(&SymSD);
    }
    if (IsPCRel)
      FixedValue -= getSectionAddress(Fragment->getParent());

    // The type is determined by the fixup kind.
    Type = RelocType;
  }

  // struct relocation_info (8 bytes)
  macho::RelocationEntry MRE;
  MRE.Word0 = FixupOffset;
  MRE.Word1 = ((Index     <<  0) |
               (IsPCRel   << 24) |
               (Log2Size  << 25) |
               (IsExtern  << 27) |
               (Type      << 28));
  Relocations[Fragment->getParent()].push_back(MRE);
}

void MachObjectWriter::RecordRelocation(const MCAssembler &Asm,
                                        const MCAsmLayout &Layout,
                                        const MCFragment *Fragment,
                                        const MCFixup &Fixup,
                                        MCValue Target,
                                        uint64_t &FixedValue) {
  // FIXME: These needs to be factored into the target Mach-O writer.
  if (isARM()) {
    RecordARMRelocation(Asm, Layout, Fragment, Fixup, Target, FixedValue);
    return;
  }
  if (is64Bit()) {
    RecordX86_64Relocation(Asm, Layout, Fragment, Fixup, Target, FixedValue);
    return;
  }

  unsigned IsPCRel = isFixupKindPCRel(Asm, Fixup.getKind());
  unsigned Log2Size = getFixupKindLog2Size(Fixup.getKind());

  // If this is a 32-bit TLVP reloc it's handled a bit differently.
  if (Target.getSymA() &&
      Target.getSymA()->getKind() == MCSymbolRefExpr::VK_TLVP) {
    RecordTLVPRelocation(Asm, Layout, Fragment, Fixup, Target, FixedValue);
    return;
  }

  // If this is a difference or a defined symbol plus an offset, then we need a
  // scattered relocation entry. Differences always require scattered
  // relocations.
  if (Target.getSymB())
    return RecordScatteredRelocation(Asm, Layout, Fragment, Fixup,
                                     Target, Log2Size, FixedValue);

  // Get the symbol data, if any.
  MCSymbolData *SD = 0;
  if (Target.getSymA())
    SD = &Asm.getSymbolData(Target.getSymA()->getSymbol());

  // If this is an internal relocation with an offset, it also needs a scattered
  // relocation entry.
  uint32_t Offset = Target.getConstant();
  if (IsPCRel)
    Offset += 1 << Log2Size;
  if (Offset && SD && !doesSymbolRequireExternRelocation(SD))
    return RecordScatteredRelocation(Asm, Layout, Fragment, Fixup,
                                     Target, Log2Size, FixedValue);

  // See <reloc.h>.
  uint32_t FixupOffset = Layout.getFragmentOffset(Fragment)+Fixup.getOffset();
  unsigned Index = 0;
  unsigned IsExtern = 0;
  unsigned Type = 0;

  if (Target.isAbsolute()) { // constant
    // SymbolNum of 0 indicates the absolute section.
    //
    // FIXME: Currently, these are never generated (see code below). I cannot
    // find a case where they are actually emitted.
    Type = macho::RIT_Vanilla;
  } else {
    // Resolve constant variables.
    if (SD->getSymbol().isVariable()) {
      int64_t Res;
      if (SD->getSymbol().getVariableValue()->EvaluateAsAbsolute(
            Res, Layout, SectionAddress)) {
        FixedValue = Res;
        return;
      }
    }

    // Check whether we need an external or internal relocation.
    if (doesSymbolRequireExternRelocation(SD)) {
      IsExtern = 1;
      Index = SD->getIndex();
      // For external relocations, make sure to offset the fixup value to
      // compensate for the addend of the symbol address, if it was
      // undefined. This occurs with weak definitions, for example.
      if (!SD->Symbol->isUndefined())
        FixedValue -= Layout.getSymbolOffset(SD);
    } else {
      // The index is the section ordinal (1-based).
      const MCSectionData &SymSD = Asm.getSectionData(
        SD->getSymbol().getSection());
      Index = SymSD.getOrdinal() + 1;
      FixedValue += getSectionAddress(&SymSD);
    }
    if (IsPCRel)
      FixedValue -= getSectionAddress(Fragment->getParent());

    Type = macho::RIT_Vanilla;
  }

  // struct relocation_info (8 bytes)
  macho::RelocationEntry MRE;
  MRE.Word0 = FixupOffset;
  MRE.Word1 = ((Index     <<  0) |
               (IsPCRel   << 24) |
               (Log2Size  << 25) |
               (IsExtern  << 27) |
               (Type      << 28));
  Relocations[Fragment->getParent()].push_back(MRE);
}

void MachObjectWriter::BindIndirectSymbols(MCAssembler &Asm) {
  // This is the point where 'as' creates actual symbols for indirect symbols
  // (in the following two passes). It would be easier for us to do this sooner
  // when we see the attribute, but that makes getting the order in the symbol
  // table much more complicated than it is worth.
  //
  // FIXME: Revisit this when the dust settles.

  // Bind non lazy symbol pointers first.
  unsigned IndirectIndex = 0;
  for (MCAssembler::indirect_symbol_iterator it = Asm.indirect_symbol_begin(),
         ie = Asm.indirect_symbol_end(); it != ie; ++it, ++IndirectIndex) {
    const MCSectionMachO &Section =
      cast<MCSectionMachO>(it->SectionData->getSection());

    if (Section.getType() != MCSectionMachO::S_NON_LAZY_SYMBOL_POINTERS)
      continue;

    // Initialize the section indirect symbol base, if necessary.
    if (!IndirectSymBase.count(it->SectionData))
      IndirectSymBase[it->SectionData] = IndirectIndex;

    Asm.getOrCreateSymbolData(*it->Symbol);
  }

  // Then lazy symbol pointers and symbol stubs.
  IndirectIndex = 0;
  for (MCAssembler::indirect_symbol_iterator it = Asm.indirect_symbol_begin(),
         ie = Asm.indirect_symbol_end(); it != ie; ++it, ++IndirectIndex) {
    const MCSectionMachO &Section =
      cast<MCSectionMachO>(it->SectionData->getSection());

    if (Section.getType() != MCSectionMachO::S_LAZY_SYMBOL_POINTERS &&
        Section.getType() != MCSectionMachO::S_SYMBOL_STUBS)
      continue;

    // Initialize the section indirect symbol base, if necessary.
    if (!IndirectSymBase.count(it->SectionData))
      IndirectSymBase[it->SectionData] = IndirectIndex;

    // Set the symbol type to undefined lazy, but only on construction.
    //
    // FIXME: Do not hardcode.
    bool Created;
    MCSymbolData &Entry = Asm.getOrCreateSymbolData(*it->Symbol, &Created);
    if (Created)
      Entry.setFlags(Entry.getFlags() | 0x0001);
  }
}

/// ComputeSymbolTable - Compute the symbol table data
///
/// \param StringTable [out] - The string table data.
/// \param StringIndexMap [out] - Map from symbol names to offsets in the
/// string table.
void MachObjectWriter::
ComputeSymbolTable(MCAssembler &Asm, SmallString<256> &StringTable,
                   std::vector<MachSymbolData> &LocalSymbolData,
                   std::vector<MachSymbolData> &ExternalSymbolData,
                   std::vector<MachSymbolData> &UndefinedSymbolData) {
  // Build section lookup table.
  DenseMap<const MCSection*, uint8_t> SectionIndexMap;
  unsigned Index = 1;
  for (MCAssembler::iterator it = Asm.begin(),
         ie = Asm.end(); it != ie; ++it, ++Index)
    SectionIndexMap[&it->getSection()] = Index;
  assert(Index <= 256 && "Too many sections!");

  // Index 0 is always the empty string.
  StringMap<uint64_t> StringIndexMap;
  StringTable += '\x00';

  // Build the symbol arrays and the string table, but only for non-local
  // symbols.
  //
  // The particular order that we collect the symbols and create the string
  // table, then sort the symbols is chosen to match 'as'. Even though it
  // doesn't matter for correctness, this is important for letting us diff .o
  // files.
  for (MCAssembler::symbol_iterator it = Asm.symbol_begin(),
         ie = Asm.symbol_end(); it != ie; ++it) {
    const MCSymbol &Symbol = it->getSymbol();

    // Ignore non-linker visible symbols.
    if (!Asm.isSymbolLinkerVisible(it->getSymbol()))
      continue;

    if (!it->isExternal() && !Symbol.isUndefined())
      continue;

    uint64_t &Entry = StringIndexMap[Symbol.getName()];
    if (!Entry) {
      Entry = StringTable.size();
      StringTable += Symbol.getName();
      StringTable += '\x00';
    }

    MachSymbolData MSD;
    MSD.SymbolData = it;
    MSD.StringIndex = Entry;

    if (Symbol.isUndefined()) {
      MSD.SectionIndex = 0;
      UndefinedSymbolData.push_back(MSD);
    } else if (Symbol.isAbsolute()) {
      MSD.SectionIndex = 0;
      ExternalSymbolData.push_back(MSD);
    } else {
      MSD.SectionIndex = SectionIndexMap.lookup(&Symbol.getSection());
      assert(MSD.SectionIndex && "Invalid section index!");
      ExternalSymbolData.push_back(MSD);
    }
  }

  // Now add the data for local symbols.
  for (MCAssembler::symbol_iterator it = Asm.symbol_begin(),
         ie = Asm.symbol_end(); it != ie; ++it) {
    const MCSymbol &Symbol = it->getSymbol();

    // Ignore non-linker visible symbols.
    if (!Asm.isSymbolLinkerVisible(it->getSymbol()))
      continue;

    if (it->isExternal() || Symbol.isUndefined())
      continue;

    uint64_t &Entry = StringIndexMap[Symbol.getName()];
    if (!Entry) {
      Entry = StringTable.size();
      StringTable += Symbol.getName();
      StringTable += '\x00';
    }

    MachSymbolData MSD;
    MSD.SymbolData = it;
    MSD.StringIndex = Entry;

    if (Symbol.isAbsolute()) {
      MSD.SectionIndex = 0;
      LocalSymbolData.push_back(MSD);
    } else {
      MSD.SectionIndex = SectionIndexMap.lookup(&Symbol.getSection());
      assert(MSD.SectionIndex && "Invalid section index!");
      LocalSymbolData.push_back(MSD);
    }
  }

  // External and undefined symbols are required to be in lexicographic order.
  std::sort(ExternalSymbolData.begin(), ExternalSymbolData.end());
  std::sort(UndefinedSymbolData.begin(), UndefinedSymbolData.end());

  // Set the symbol indices.
  Index = 0;
  for (unsigned i = 0, e = LocalSymbolData.size(); i != e; ++i)
    LocalSymbolData[i].SymbolData->setIndex(Index++);
  for (unsigned i = 0, e = ExternalSymbolData.size(); i != e; ++i)
    ExternalSymbolData[i].SymbolData->setIndex(Index++);
  for (unsigned i = 0, e = UndefinedSymbolData.size(); i != e; ++i)
    UndefinedSymbolData[i].SymbolData->setIndex(Index++);

  // The string table is padded to a multiple of 4.
  while (StringTable.size() % 4)
    StringTable += '\x00';
}

void MachObjectWriter::computeSectionAddresses(const MCAssembler &Asm,
                                               const MCAsmLayout &Layout) {
  uint64_t StartAddress = 0;
  const SmallVectorImpl<MCSectionData*> &Order = Layout.getSectionOrder();
  for (int i = 0, n = Order.size(); i != n ; ++i) {
    const MCSectionData *SD = Order[i];
    StartAddress = RoundUpToAlignment(StartAddress, SD->getAlignment());
    SectionAddress[SD] = StartAddress;
    StartAddress += Layout.getSectionAddressSize(SD);

    // Explicitly pad the section to match the alignment requirements of the
    // following one. This is for 'gas' compatibility, it shouldn't
    /// strictly be necessary.
    StartAddress += getPaddingSize(SD, Layout);
  }
}

void MachObjectWriter::ExecutePostLayoutBinding(MCAssembler &Asm,
                                                const MCAsmLayout &Layout) {
  computeSectionAddresses(Asm, Layout);

  // Create symbol data for any indirect symbols.
  BindIndirectSymbols(Asm);

  // Compute symbol table information and bind symbol indices.
  ComputeSymbolTable(Asm, StringTable, LocalSymbolData, ExternalSymbolData,
                     UndefinedSymbolData);
}

bool MachObjectWriter::
IsSymbolRefDifferenceFullyResolvedImpl(const MCAssembler &Asm,
                                       const MCSymbolData &DataA,
                                       const MCFragment &FB,
                                       bool InSet,
                                       bool IsPCRel) const {
  if (InSet)
    return true;

  // The effective address is
  //     addr(atom(A)) + offset(A)
  //   - addr(atom(B)) - offset(B)
  // and the offsets are not relocatable, so the fixup is fully resolved when
  //  addr(atom(A)) - addr(atom(B)) == 0.
  const MCSymbolData *A_Base = 0, *B_Base = 0;

  const MCSymbol &SA = DataA.getSymbol().AliasedSymbol();
  const MCSection &SecA = SA.getSection();
  const MCSection &SecB = FB.getParent()->getSection();

  if (IsPCRel) {
    // The simple (Darwin, except on x86_64) way of dealing with this was to
    // assume that any reference to a temporary symbol *must* be a temporary
    // symbol in the same atom, unless the sections differ. Therefore, any PCrel
    // relocation to a temporary symbol (in the same section) is fully
    // resolved. This also works in conjunction with absolutized .set, which
    // requires the compiler to use .set to absolutize the differences between
    // symbols which the compiler knows to be assembly time constants, so we
    // don't need to worry about considering symbol differences fully resolved.

    if (!Asm.getBackend().hasReliableSymbolDifference()) {
      if (!SA.isTemporary() || !SA.isInSection() || &SecA != &SecB)
        return false;
      return true;
    }
  } else {
    if (!TargetObjectWriter->useAggressiveSymbolFolding())
      return false;
  }

  const MCFragment &FA = *Asm.getSymbolData(SA).getFragment();

  A_Base = FA.getAtom();
  if (!A_Base)
    return false;

  B_Base = FB.getAtom();
  if (!B_Base)
    return false;

  // If the atoms are the same, they are guaranteed to have the same address.
  if (A_Base == B_Base)
    return true;

  // Otherwise, we can't prove this is fully resolved.
  return false;
}

void MachObjectWriter::WriteObject(MCAssembler &Asm, const MCAsmLayout &Layout) {
  unsigned NumSections = Asm.size();

  // The section data starts after the header, the segment load command (and
  // section headers) and the symbol table.
  unsigned NumLoadCommands = 1;
  uint64_t LoadCommandsSize = is64Bit() ?
    macho::SegmentLoadCommand64Size + NumSections * macho::Section64Size :
    macho::SegmentLoadCommand32Size + NumSections * macho::Section32Size;

  // Add the symbol table load command sizes, if used.
  unsigned NumSymbols = LocalSymbolData.size() + ExternalSymbolData.size() +
    UndefinedSymbolData.size();
  if (NumSymbols) {
    NumLoadCommands += 2;
    LoadCommandsSize += (macho::SymtabLoadCommandSize +
                         macho::DysymtabLoadCommandSize);
  }

  // Compute the total size of the section data, as well as its file size and vm
  // size.
  uint64_t SectionDataStart = (is64Bit() ? macho::Header64Size :
                               macho::Header32Size) + LoadCommandsSize;
  uint64_t SectionDataSize = 0;
  uint64_t SectionDataFileSize = 0;
  uint64_t VMSize = 0;
  for (MCAssembler::const_iterator it = Asm.begin(),
         ie = Asm.end(); it != ie; ++it) {
    const MCSectionData &SD = *it;
    uint64_t Address = getSectionAddress(&SD);
    uint64_t Size = Layout.getSectionAddressSize(&SD);
    uint64_t FileSize = Layout.getSectionFileSize(&SD);
    FileSize += getPaddingSize(&SD, Layout);

    VMSize = std::max(VMSize, Address + Size);

    if (SD.getSection().isVirtualSection())
      continue;

    SectionDataSize = std::max(SectionDataSize, Address + Size);
    SectionDataFileSize = std::max(SectionDataFileSize, Address + FileSize);
  }

  // The section data is padded to 4 bytes.
  //
  // FIXME: Is this machine dependent?
  unsigned SectionDataPadding = OffsetToAlignment(SectionDataFileSize, 4);
  SectionDataFileSize += SectionDataPadding;

  // Write the prolog, starting with the header and load command...
  WriteHeader(NumLoadCommands, LoadCommandsSize,
              Asm.getSubsectionsViaSymbols());
  WriteSegmentLoadCommand(NumSections, VMSize,
                          SectionDataStart, SectionDataSize);

  // ... and then the section headers.
  uint64_t RelocTableEnd = SectionDataStart + SectionDataFileSize;
  for (MCAssembler::const_iterator it = Asm.begin(),
         ie = Asm.end(); it != ie; ++it) {
    std::vector<macho::RelocationEntry> &Relocs = Relocations[it];
    unsigned NumRelocs = Relocs.size();
    uint64_t SectionStart = SectionDataStart + getSectionAddress(it);
    WriteSection(Asm, Layout, *it, SectionStart, RelocTableEnd, NumRelocs);
    RelocTableEnd += NumRelocs * macho::RelocationInfoSize;
  }

  // Write the symbol table load command, if used.
  if (NumSymbols) {
    unsigned FirstLocalSymbol = 0;
    unsigned NumLocalSymbols = LocalSymbolData.size();
    unsigned FirstExternalSymbol = FirstLocalSymbol + NumLocalSymbols;
    unsigned NumExternalSymbols = ExternalSymbolData.size();
    unsigned FirstUndefinedSymbol = FirstExternalSymbol + NumExternalSymbols;
    unsigned NumUndefinedSymbols = UndefinedSymbolData.size();
    unsigned NumIndirectSymbols = Asm.indirect_symbol_size();
    unsigned NumSymTabSymbols =
      NumLocalSymbols + NumExternalSymbols + NumUndefinedSymbols;
    uint64_t IndirectSymbolSize = NumIndirectSymbols * 4;
    uint64_t IndirectSymbolOffset = 0;

    // If used, the indirect symbols are written after the section data.
    if (NumIndirectSymbols)
      IndirectSymbolOffset = RelocTableEnd;

    // The symbol table is written after the indirect symbol data.
    uint64_t SymbolTableOffset = RelocTableEnd + IndirectSymbolSize;

    // The string table is written after symbol table.
    uint64_t StringTableOffset =
      SymbolTableOffset + NumSymTabSymbols * (is64Bit() ? macho::Nlist64Size :
                                              macho::Nlist32Size);
    WriteSymtabLoadCommand(SymbolTableOffset, NumSymTabSymbols,
                           StringTableOffset, StringTable.size());

    WriteDysymtabLoadCommand(FirstLocalSymbol, NumLocalSymbols,
                             FirstExternalSymbol, NumExternalSymbols,
                             FirstUndefinedSymbol, NumUndefinedSymbols,
                             IndirectSymbolOffset, NumIndirectSymbols);
  }

  // Write the actual section data.
  for (MCAssembler::const_iterator it = Asm.begin(),
         ie = Asm.end(); it != ie; ++it) {
    Asm.WriteSectionData(it, Layout);

    uint64_t Pad = getPaddingSize(it, Layout);
    for (unsigned int i = 0; i < Pad; ++i)
      Write8(0);
  }

  // Write the extra padding.
  WriteZeros(SectionDataPadding);

  // Write the relocation entries.
  for (MCAssembler::const_iterator it = Asm.begin(),
         ie = Asm.end(); it != ie; ++it) {
    // Write the section relocation entries, in reverse order to match 'as'
    // (approximately, the exact algorithm is more complicated than this).
    std::vector<macho::RelocationEntry> &Relocs = Relocations[it];
    for (unsigned i = 0, e = Relocs.size(); i != e; ++i) {
      Write32(Relocs[e - i - 1].Word0);
      Write32(Relocs[e - i - 1].Word1);
    }
  }

  // Write the symbol table data, if used.
  if (NumSymbols) {
    // Write the indirect symbol entries.
    for (MCAssembler::const_indirect_symbol_iterator
           it = Asm.indirect_symbol_begin(),
           ie = Asm.indirect_symbol_end(); it != ie; ++it) {
      // Indirect symbols in the non lazy symbol pointer section have some
      // special handling.
      const MCSectionMachO &Section =
        static_cast<const MCSectionMachO&>(it->SectionData->getSection());
      if (Section.getType() == MCSectionMachO::S_NON_LAZY_SYMBOL_POINTERS) {
        // If this symbol is defined and internal, mark it as such.
        if (it->Symbol->isDefined() &&
            !Asm.getSymbolData(*it->Symbol).isExternal()) {
          uint32_t Flags = macho::ISF_Local;
          if (it->Symbol->isAbsolute())
            Flags |= macho::ISF_Absolute;
          Write32(Flags);
          continue;
        }
      }

      Write32(Asm.getSymbolData(*it->Symbol).getIndex());
    }

    // FIXME: Check that offsets match computed ones.

    // Write the symbol table entries.
    for (unsigned i = 0, e = LocalSymbolData.size(); i != e; ++i)
      WriteNlist(LocalSymbolData[i], Layout);
    for (unsigned i = 0, e = ExternalSymbolData.size(); i != e; ++i)
      WriteNlist(ExternalSymbolData[i], Layout);
    for (unsigned i = 0, e = UndefinedSymbolData.size(); i != e; ++i)
      WriteNlist(UndefinedSymbolData[i], Layout);

    // Write the string table.
    OS << StringTable.str();
  }
}

MCObjectWriter *llvm::createMachObjectWriter(MCMachObjectTargetWriter *MOTW,
                                             raw_ostream &OS,
                                             bool IsLittleEndian) {
  return new MachObjectWriter(MOTW, OS, IsLittleEndian);
}