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/MCAsmBackend.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 <vector>
using namespace llvm;
using namespace llvm::object;

bool MachObjectWriter::
doesSymbolRequireExternRelocation(const 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;
}

bool MachObjectWriter::
MachSymbolData::operator<(const MachSymbolData &RHS) const {
  return SymbolData->getSymbol().getName() <
    RHS.SymbolData->getSymbol().getName();
}

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

  return FKI.Flags & MCFixupKindInfo::FKF_IsPCRel;
}

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

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();
  uint64_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::RecordRelocation(const MCAssembler &Asm,
                                        const MCAsmLayout &Layout,
                                        const MCFragment *Fragment,
                                        const MCFixup &Fixup,
                                        MCValue Target,
                                        uint64_t &FixedValue) {
  TargetObjectWriter->RecordRelocation(this, Asm, Layout, Fragment, Fixup,
                                       Target, FixedValue);
}

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 the file isn't using sub-sections-via-symbols, we can make the
    // same assumptions about any symbol that we normally make about
    // assembler locals.

    if (!Asm.getBackend().hasReliableSymbolDifference()) {
      if ((!SA.isTemporary() && Asm.getSubsectionsViaSymbols()) ||
           !SA.isInSection() || &SecA != &SecB)
        return false;
      return true;
    }
    // For Darwin x86_64, there is one special case when the reference IsPCRel.
    // If the fragment with the reference does not have a base symbol but meets
    // the simple way of dealing with this, in that it is a temporary symbol in
    // the same atom then it is assumed to be fully resolved.  This is needed so
    // a relocation entry is not created and so the static linker does not
    // mess up the reference later.
    else if(!FB.getAtom() &&
            SA.isTemporary() && SA.isInSection() && &SecA == &SecB){
      return true;
    }
  } else {
    if (!TargetObjectWriter->useAggressiveSymbolFolding())
      return false;
  }

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

  // Bail if the symbol has no fragment.
  if (!FA)
    return false;

  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);
}