MC/Mach-O: Move to MachObjectWriter.{h,cpp}.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@98952 91177308-0d34-0410-b5e6-96231b3b80d8

4 files changed, 900 insertions, 821 deletions

diff --git a/lib/MC/CMakeLists.txt b/lib/MC/CMakeLists.txt
index 682fe24..dba0e14 100644
--- a/lib/MC/CMakeLists.txt
+++ b/lib/MC/CMakeLists.txt
@@ -19,5 +19,6 @@ add_llvm_library(LLVMMC
   MCStreamer.cpp
   MCSymbol.cpp
   MCValue.cpp
+  MachObjectWriter.cpp
   TargetAsmBackend.cpp
   )
diff --git a/lib/MC/MCAssembler.cpp b/lib/MC/MCAssembler.cpp
index f1847df..6e5f73f 100644
--- a/lib/MC/MCAssembler.cpp
+++ b/lib/MC/MCAssembler.cpp
@@ -12,17 +12,13 @@
 #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/MCValue.h"
-#include "llvm/ADT/DenseMap.h"
-#include "llvm/ADT/SmallString.h"
+#include "llvm/MC/MachObjectWriter.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/ADT/StringExtras.h"
-#include "llvm/ADT/StringMap.h"
 #include "llvm/ADT/Twine.h"
 #include "llvm/Support/ErrorHandling.h"
-#include "llvm/Support/MachO.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Target/TargetRegistry.h"
@@ -34,8 +30,6 @@
 #include <vector>
 using namespace llvm;
 
-class MachObjectWriter;
-
 STATISTIC(EmittedFragments, "Number of emitted assembler fragments");
 
 // FIXME FIXME FIXME: There are number of places in this file where we convert
@@ -43,19 +37,6 @@ STATISTIC(EmittedFragments, "Number of emitted assembler fragments");
 // object file, which may truncate it. We should detect that truncation where
 // invalid and report errors back.
 
-static unsigned getFixupKindLog2Size(unsigned Kind) {
-  switch (Kind) {
-  default: llvm_unreachable("invalid fixup kind!");
-  case X86::reloc_pcrel_1byte:
-  case FK_Data_1: return 0;
-  case FK_Data_2: return 1;
-  case X86::reloc_pcrel_4byte:
-  case X86::reloc_riprel_4byte:
-  case FK_Data_4: return 2;
-  case FK_Data_8: return 3;
-  }
-}
-
 static bool isFixupKindPCRel(unsigned Kind) {
   switch (Kind) {
   default:
@@ -67,806 +48,6 @@ static bool isFixupKindPCRel(unsigned Kind) {
   }
 }
 
-class MachObjectWriter : public MCObjectWriter {
-  // See <mach-o/loader.h>.
-  enum {
-    Header_Magic32 = 0xFEEDFACE,
-    Header_Magic64 = 0xFEEDFACF
-  };
-
-  enum {
-    Header32Size = 28,
-    Header64Size = 32,
-    SegmentLoadCommand32Size = 56,
-    SegmentLoadCommand64Size = 72,
-    Section32Size = 68,
-    Section64Size = 80,
-    SymtabLoadCommandSize = 24,
-    DysymtabLoadCommandSize = 80,
-    Nlist32Size = 12,
-    Nlist64Size = 16,
-    RelocationInfoSize = 8
-  };
-
-  enum HeaderFileType {
-    HFT_Object = 0x1
-  };
-
-  enum HeaderFlags {
-    HF_SubsectionsViaSymbols = 0x2000
-  };
-
-  enum LoadCommandType {
-    LCT_Segment = 0x1,
-    LCT_Symtab = 0x2,
-    LCT_Dysymtab = 0xb,
-    LCT_Segment64 = 0x19
-  };
-
-  // See <mach-o/nlist.h>.
-  enum SymbolTypeType {
-    STT_Undefined = 0x00,
-    STT_Absolute  = 0x02,
-    STT_Section   = 0x0e
-  };
-
-  enum SymbolTypeFlags {
-    // If any of these bits are set, then the entry is a stab entry number (see
-    // <mach-o/stab.h>. Otherwise the other masks apply.
-    STF_StabsEntryMask = 0xe0,
-
-    STF_TypeMask       = 0x0e,
-    STF_External       = 0x01,
-    STF_PrivateExtern  = 0x10
-  };
-
-  /// IndirectSymbolFlags - Flags for encoding special values in the indirect
-  /// symbol entry.
-  enum IndirectSymbolFlags {
-    ISF_Local    = 0x80000000,
-    ISF_Absolute = 0x40000000
-  };
-
-  /// RelocationFlags - Special flags for addresses.
-  enum RelocationFlags {
-    RF_Scattered = 0x80000000
-  };
-
-  enum RelocationInfoType {
-    RIT_Vanilla             = 0,
-    RIT_Pair                = 1,
-    RIT_Difference          = 2,
-    RIT_PreboundLazyPointer = 3,
-    RIT_LocalDifference     = 4
-  };
-
-  /// 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 {
-      const std::string &Name = SymbolData->getSymbol().getName();
-      return Name < RHS.SymbolData->getSymbol().getName();
-    }
-  };
-
-  unsigned Is64Bit : 1;
-
-  /// @name Relocation Data
-  /// @{
-
-  struct MachRelocationEntry {
-    uint32_t Word0;
-    uint32_t Word1;
-  };
-
-  llvm::DenseMap<const MCSectionData*,
-                 std::vector<MachRelocationEntry> > Relocations;
-
-  /// @}
-  /// @name Symbol Table Data
-
-  SmallString<256> StringTable;
-  std::vector<MachSymbolData> LocalSymbolData;
-  std::vector<MachSymbolData> ExternalSymbolData;
-  std::vector<MachSymbolData> UndefinedSymbolData;
-
-  /// @}
-
-public:
-  MachObjectWriter(raw_ostream &_OS, bool _Is64Bit, bool _IsLittleEndian = true)
-    : MCObjectWriter(_OS, _IsLittleEndian), Is64Bit(_Is64Bit) {
-  }
-
-  void WriteHeader(unsigned NumLoadCommands, unsigned LoadCommandsSize,
-                   bool SubsectionsViaSymbols) {
-    uint32_t Flags = 0;
-
-    if (SubsectionsViaSymbols)
-      Flags |= HF_SubsectionsViaSymbols;
-
-    // struct mach_header (28 bytes) or
-    // struct mach_header_64 (32 bytes)
-
-    uint64_t Start = OS.tell();
-    (void) Start;
-
-    Write32(Is64Bit ? Header_Magic64 : Header_Magic32);
-
-    // FIXME: Support cputype.
-    Write32(Is64Bit ? MachO::CPUTypeX86_64 : MachO::CPUTypeI386);
-    // FIXME: Support cpusubtype.
-    Write32(MachO::CPUSubType_I386_ALL);
-    Write32(HFT_Object);
-    Write32(NumLoadCommands);    // Object files have a single load command, the
-                                 // segment.
-    Write32(LoadCommandsSize);
-    Write32(Flags);
-    if (Is64Bit)
-      Write32(0); // reserved
-
-    assert(OS.tell() - Start == Is64Bit ? Header64Size : 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 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 ? SegmentLoadCommand64Size :
-      SegmentLoadCommand32Size;
-    Write32(Is64Bit ? LCT_Segment64 : LCT_Segment);
-    Write32(SegmentLoadCommandSize +
-            NumSections * (Is64Bit ? Section64Size : 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 WriteSection(const MCAssembler &Asm, const MCSectionData &SD,
-                    uint64_t FileOffset, uint64_t RelocationsStart,
-                    unsigned NumRelocations) {
-    // The offset is unused for virtual sections.
-    if (Asm.getBackend().isVirtualSection(SD.getSection())) {
-      assert(SD.getFileSize() == 0 && "Invalid file size!");
-      FileOffset = 0;
-    }
-
-    // struct section (68 bytes) or
-    // struct section_64 (80 bytes)
-
-    uint64_t Start = OS.tell();
-    (void) Start;
-
-    // FIXME: cast<> support!
-    const MCSectionMachO &Section =
-      static_cast<const MCSectionMachO&>(SD.getSection());
-    WriteBytes(Section.getSectionName(), 16);
-    WriteBytes(Section.getSegmentName(), 16);
-    if (Is64Bit) {
-      Write64(SD.getAddress()); // address
-      Write64(SD.getSize()); // size
-    } else {
-      Write32(SD.getAddress()); // address
-      Write32(SD.getSize()); // 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(0); // reserved1
-    Write32(Section.getStubSize()); // reserved2
-    if (Is64Bit)
-      Write32(0); // reserved3
-
-    assert(OS.tell() - Start == Is64Bit ? Section64Size : Section32Size);
-  }
-
-  void 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(LCT_Symtab);
-    Write32(SymtabLoadCommandSize);
-    Write32(SymbolOffset);
-    Write32(NumSymbols);
-    Write32(StringTableOffset);
-    Write32(StringTableSize);
-
-    assert(OS.tell() - Start == SymtabLoadCommandSize);
-  }
-
-  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) {
-    // struct dysymtab_command (80 bytes)
-
-    uint64_t Start = OS.tell();
-    (void) Start;
-
-    Write32(LCT_Dysymtab);
-    Write32(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 == DysymtabLoadCommandSize);
-  }
-
-  void WriteNlist(MachSymbolData &MSD) {
-    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 = STT_Undefined;
-    else if (Symbol.isAbsolute())
-      Type = STT_Absolute;
-    else
-      Type = STT_Section;
-
-    // FIXME: Set STAB bits.
-
-    if (Data.isPrivateExtern())
-      Type |= STF_PrivateExtern;
-
-    // Set external bit.
-    if (Data.isExternal() || Symbol.isUndefined())
-      Type |= STF_External;
-
-    // Compute the symbol address.
-    if (Symbol.isDefined()) {
-      if (Symbol.isAbsolute()) {
-        llvm_unreachable("FIXME: Not yet implemented!");
-      } else {
-        Address = Data.getAddress();
-      }
-    } 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)
-          llvm_report_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 RecordScatteredRelocation(const MCAssembler &Asm,
-                                 const MCFragment &Fragment,
-                                 const MCAsmFixup &Fixup, MCValue Target,
-                                 uint64_t &FixedValue) {
-    uint32_t Address = Fragment.getOffset() + Fixup.Offset;
-    unsigned IsPCRel = isFixupKindPCRel(Fixup.Kind);
-    unsigned Log2Size = getFixupKindLog2Size(Fixup.Kind);
-    unsigned Type = RIT_Vanilla;
-
-    // See <reloc.h>.
-    const MCSymbol *A = &Target.getSymA()->getSymbol();
-    MCSymbolData *A_SD = &Asm.getSymbolData(*A);
-
-    if (!A_SD->getFragment())
-      llvm_report_error("symbol '" + A->getName() +
-                        "' can not be undefined in a subtraction expression");
-
-    uint32_t Value = A_SD->getAddress();
-    uint32_t Value2 = 0;
-
-    if (const MCSymbolRefExpr *B = Target.getSymB()) {
-      MCSymbolData *B_SD = &Asm.getSymbolData(B->getSymbol());
-
-      if (!B_SD->getFragment())
-        llvm_report_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() ? RIT_Difference : RIT_LocalDifference;
-      Value2 = B_SD->getAddress();
-    }
-
-    // Relocations are written out in reverse order, so the PAIR comes first.
-    if (Type == RIT_Difference || Type == RIT_LocalDifference) {
-      MachRelocationEntry MRE;
-      MRE.Word0 = ((0         <<  0) |
-                   (RIT_Pair  << 24) |
-                   (Log2Size  << 28) |
-                   (IsPCRel   << 30) |
-                   RF_Scattered);
-      MRE.Word1 = Value2;
-      Relocations[Fragment.getParent()].push_back(MRE);
-    }
-
-    MachRelocationEntry MRE;
-    MRE.Word0 = ((Address   <<  0) |
-                 (Type      << 24) |
-                 (Log2Size  << 28) |
-                 (IsPCRel   << 30) |
-                 RF_Scattered);
-    MRE.Word1 = Value;
-    Relocations[Fragment.getParent()].push_back(MRE);
-  }
-
-  virtual void RecordRelocation(const MCAssembler &Asm,
-                                const MCDataFragment &Fragment,
-                                const MCAsmFixup &Fixup, MCValue Target,
-                                uint64_t &FixedValue) {
-    unsigned IsPCRel = isFixupKindPCRel(Fixup.Kind);
-    unsigned Log2Size = getFixupKindLog2Size(Fixup.Kind);
-
-    // If this is a difference or a defined symbol plus an offset, then we need
-    // a scattered relocation entry.
-    uint32_t Offset = Target.getConstant();
-    if (IsPCRel)
-      Offset += 1 << Log2Size;
-    if (Target.getSymB() ||
-        (Target.getSymA() && !Target.getSymA()->getSymbol().isUndefined() &&
-         Offset)) {
-      RecordScatteredRelocation(Asm, Fragment, Fixup, Target, FixedValue);
-      return;
-    }
-
-    // See <reloc.h>.
-    uint32_t Address = Fragment.getOffset() + Fixup.Offset;
-    uint32_t Value = 0;
-    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 = RIT_Vanilla;
-      Value = 0;
-    } else {
-      const MCSymbol *Symbol = &Target.getSymA()->getSymbol();
-      MCSymbolData *SD = &Asm.getSymbolData(*Symbol);
-
-      if (Symbol->isUndefined()) {
-        IsExtern = 1;
-        Index = SD->getIndex();
-        Value = 0;
-      } else {
-        // The index is the section ordinal.
-        //
-        // FIXME: O(N)
-        Index = 1;
-        MCAssembler::const_iterator it = Asm.begin(), ie = Asm.end();
-        for (; it != ie; ++it, ++Index)
-          if (&*it == SD->getFragment()->getParent())
-            break;
-        assert(it != ie && "Unable to find section index!");
-        Value = SD->getAddress();
-      }
-
-      Type = RIT_Vanilla;
-    }
-
-    // struct relocation_info (8 bytes)
-    MachRelocationEntry MRE;
-    MRE.Word0 = Address;
-    MRE.Word1 = ((Index     <<  0) |
-                 (IsPCRel   << 24) |
-                 (Log2Size  << 25) |
-                 (IsExtern  << 27) |
-                 (Type      << 28));
-    Relocations[Fragment.getParent()].push_back(MRE);
-  }
-
-  void 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.
-    for (MCAssembler::indirect_symbol_iterator it = Asm.indirect_symbol_begin(),
-           ie = Asm.indirect_symbol_end(); it != ie; ++it) {
-      // FIXME: cast<> support!
-      const MCSectionMachO &Section =
-        static_cast<const MCSectionMachO&>(it->SectionData->getSection());
-
-      if (Section.getType() != MCSectionMachO::S_NON_LAZY_SYMBOL_POINTERS)
-        continue;
-
-      Asm.getOrCreateSymbolData(*it->Symbol);
-    }
-
-    // Then lazy symbol pointers and symbol stubs.
-    for (MCAssembler::indirect_symbol_iterator it = Asm.indirect_symbol_begin(),
-           ie = Asm.indirect_symbol_end(); it != ie; ++it) {
-      // FIXME: cast<> support!
-      const MCSectionMachO &Section =
-        static_cast<const MCSectionMachO&>(it->SectionData->getSection());
-
-      if (Section.getType() != MCSectionMachO::S_LAZY_SYMBOL_POINTERS &&
-          Section.getType() != MCSectionMachO::S_SYMBOL_STUBS)
-        continue;
-
-      // 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 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))
-        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))
-        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';
-  }
-
-  virtual void ExecutePostLayoutBinding(MCAssembler &Asm) {
-    // Create symbol data for any indirect symbols.
-    BindIndirectSymbols(Asm);
-
-    // Compute symbol table information and bind symbol indices.
-    ComputeSymbolTable(Asm, StringTable, LocalSymbolData, ExternalSymbolData,
-                       UndefinedSymbolData);
-  }
-
-  virtual void WriteObject(const MCAssembler &Asm) {
-    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 ?
-      SegmentLoadCommand64Size + NumSections * Section64Size :
-      SegmentLoadCommand32Size + NumSections * Section32Size;
-
-    // Add the symbol table load command sizes, if used.
-    unsigned NumSymbols = LocalSymbolData.size() + ExternalSymbolData.size() +
-      UndefinedSymbolData.size();
-    if (NumSymbols) {
-      NumLoadCommands += 2;
-      LoadCommandsSize += SymtabLoadCommandSize + DysymtabLoadCommandSize;
-    }
-
-    // Compute the total size of the section data, as well as its file size and
-    // vm size.
-    uint64_t SectionDataStart = (Is64Bit ? Header64Size : 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;
-
-      VMSize = std::max(VMSize, SD.getAddress() + SD.getSize());
-
-      if (Asm.getBackend().isVirtualSection(SD.getSection()))
-        continue;
-
-      SectionDataSize = std::max(SectionDataSize,
-                                 SD.getAddress() + SD.getSize());
-      SectionDataFileSize = std::max(SectionDataFileSize,
-                                     SD.getAddress() + SD.getFileSize());
-    }
-
-    // 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<MachRelocationEntry> &Relocs = Relocations[it];
-      unsigned NumRelocs = Relocs.size();
-      uint64_t SectionStart = SectionDataStart + it->getAddress();
-      WriteSection(Asm, *it, SectionStart, RelocTableEnd, NumRelocs);
-      RelocTableEnd += NumRelocs * 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 ? Nlist64Size :
-                                                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, this);
-
-    // 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<MachRelocationEntry> &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 = ISF_Local;
-            if (it->Symbol->isAbsolute())
-              Flags |= 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]);
-      for (unsigned i = 0, e = ExternalSymbolData.size(); i != e; ++i)
-        WriteNlist(ExternalSymbolData[i]);
-      for (unsigned i = 0, e = UndefinedSymbolData.size(); i != e; ++i)
-        WriteNlist(UndefinedSymbolData[i]);
-
-      // Write the string table.
-      OS << StringTable.str();
-    }
-  }
-};
-
 /* *** */
 
 MCFragment::MCFragment() : Kind(FragmentType(~0)) {
diff --git a/lib/MC/MCObjectWriter.cpp b/lib/MC/MCObjectWriter.cpp
index ab85e31..d117e82 100644
--- a/lib/MC/MCObjectWriter.cpp
+++ b/lib/MC/MCObjectWriter.cpp
@@ -1,4 +1,4 @@
-//===- lib/MC/MCObjetWriter.cpp - MCObjetWriter implementation ------------===//
+//===- lib/MC/MCObjectWriter.cpp - MCObjectWriter implementation ----------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
diff --git a/lib/MC/MachObjectWriter.cpp b/lib/MC/MachObjectWriter.cpp
new file mode 100644
index 0000000..4b0077e
--- /dev/null
+++ b/lib/MC/MachObjectWriter.cpp
@@ -0,0 +1,897 @@
+//===- 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/MachObjectWriter.h"
+#include "llvm/ADT/StringMap.h"
+#include "llvm/ADT/Twine.h"
+#include "llvm/MC/MCAssembler.h"
+#include "llvm/MC/MCExpr.h"
+#include "llvm/MC/MCObjectWriter.h"
+#include "llvm/MC/MCSectionMachO.h"
+#include "llvm/MC/MCSymbol.h"
+#include "llvm/MC/MCValue.h"
+#include "llvm/Support/ErrorHandling.h"
+#include "llvm/Support/MachO.h"
+#include "llvm/Target/TargetAsmBackend.h"
+
+// FIXME: Gross.
+#include "../Target/X86/X86FixupKinds.h"
+
+#include <vector>
+using namespace llvm;
+
+static unsigned getFixupKindLog2Size(unsigned Kind) {
+  switch (Kind) {
+  default: llvm_unreachable("invalid fixup kind!");
+  case X86::reloc_pcrel_1byte:
+  case FK_Data_1: return 0;
+  case FK_Data_2: return 1;
+  case X86::reloc_pcrel_4byte:
+  case X86::reloc_riprel_4byte:
+  case FK_Data_4: return 2;
+  case FK_Data_8: return 3;
+  }
+}
+
+static bool isFixupKindPCRel(unsigned Kind) {
+  switch (Kind) {
+  default:
+    return false;
+  case X86::reloc_pcrel_1byte:
+  case X86::reloc_pcrel_4byte:
+  case X86::reloc_riprel_4byte:
+    return true;
+  }
+}
+
+namespace {
+
+class MachObjectWriterImpl {
+  // See <mach-o/loader.h>.
+  enum {
+    Header_Magic32 = 0xFEEDFACE,
+    Header_Magic64 = 0xFEEDFACF
+  };
+
+  enum {
+    Header32Size = 28,
+    Header64Size = 32,
+    SegmentLoadCommand32Size = 56,
+    SegmentLoadCommand64Size = 72,
+    Section32Size = 68,
+    Section64Size = 80,
+    SymtabLoadCommandSize = 24,
+    DysymtabLoadCommandSize = 80,
+    Nlist32Size = 12,
+    Nlist64Size = 16,
+    RelocationInfoSize = 8
+  };
+
+  enum HeaderFileType {
+    HFT_Object = 0x1
+  };
+
+  enum HeaderFlags {
+    HF_SubsectionsViaSymbols = 0x2000
+  };
+
+  enum LoadCommandType {
+    LCT_Segment = 0x1,
+    LCT_Symtab = 0x2,
+    LCT_Dysymtab = 0xb,
+    LCT_Segment64 = 0x19
+  };
+
+  // See <mach-o/nlist.h>.
+  enum SymbolTypeType {
+    STT_Undefined = 0x00,
+    STT_Absolute  = 0x02,
+    STT_Section   = 0x0e
+  };
+
+  enum SymbolTypeFlags {
+    // If any of these bits are set, then the entry is a stab entry number (see
+    // <mach-o/stab.h>. Otherwise the other masks apply.
+    STF_StabsEntryMask = 0xe0,
+
+    STF_TypeMask       = 0x0e,
+    STF_External       = 0x01,
+    STF_PrivateExtern  = 0x10
+  };
+
+  /// IndirectSymbolFlags - Flags for encoding special values in the indirect
+  /// symbol entry.
+  enum IndirectSymbolFlags {
+    ISF_Local    = 0x80000000,
+    ISF_Absolute = 0x40000000
+  };
+
+  /// RelocationFlags - Special flags for addresses.
+  enum RelocationFlags {
+    RF_Scattered = 0x80000000
+  };
+
+  enum RelocationInfoType {
+    RIT_Vanilla             = 0,
+    RIT_Pair                = 1,
+    RIT_Difference          = 2,
+    RIT_PreboundLazyPointer = 3,
+    RIT_LocalDifference     = 4
+  };
+
+  /// 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 {
+      const std::string &Name = SymbolData->getSymbol().getName();
+      return Name < RHS.SymbolData->getSymbol().getName();
+    }
+  };
+
+  /// @name Relocation Data
+  /// @{
+
+  struct MachRelocationEntry {
+    uint32_t Word0;
+    uint32_t Word1;
+  };
+
+  llvm::DenseMap<const MCSectionData*,
+                 std::vector<MachRelocationEntry> > Relocations;
+
+  /// @}
+  /// @name Symbol Table Data
+  /// @{
+
+  SmallString<256> StringTable;
+  std::vector<MachSymbolData> LocalSymbolData;
+  std::vector<MachSymbolData> ExternalSymbolData;
+  std::vector<MachSymbolData> UndefinedSymbolData;
+
+  /// @}
+
+  MachObjectWriter *Writer;
+
+  raw_ostream &OS;
+
+  unsigned Is64Bit : 1;
+
+public:
+  MachObjectWriterImpl(MachObjectWriter *_Writer, bool _Is64Bit)
+    : Writer(_Writer), OS(Writer->getStream()), Is64Bit(_Is64Bit) {
+  }
+
+  void Write8(uint8_t Value) { Writer->Write8(Value); }
+  void Write16(uint16_t Value) { Writer->Write16(Value); }
+  void Write32(uint32_t Value) { Writer->Write32(Value); }
+  void Write64(uint64_t Value) { Writer->Write64(Value); }
+  void WriteZeros(unsigned N) { Writer->WriteZeros(N); }
+  void WriteBytes(StringRef Str, unsigned ZeroFillSize = 0) {
+    Writer->WriteBytes(Str, ZeroFillSize);
+  }
+
+  void WriteHeader(unsigned NumLoadCommands, unsigned LoadCommandsSize,
+                   bool SubsectionsViaSymbols) {
+    uint32_t Flags = 0;
+
+    if (SubsectionsViaSymbols)
+      Flags |= HF_SubsectionsViaSymbols;
+
+    // struct mach_header (28 bytes) or
+    // struct mach_header_64 (32 bytes)
+
+    uint64_t Start = OS.tell();
+    (void) Start;
+
+    Write32(Is64Bit ? Header_Magic64 : Header_Magic32);
+
+    // FIXME: Support cputype.
+    Write32(Is64Bit ? MachO::CPUTypeX86_64 : MachO::CPUTypeI386);
+    // FIXME: Support cpusubtype.
+    Write32(MachO::CPUSubType_I386_ALL);
+    Write32(HFT_Object);
+    Write32(NumLoadCommands);    // Object files have a single load command, the
+                                 // segment.
+    Write32(LoadCommandsSize);
+    Write32(Flags);
+    if (Is64Bit)
+      Write32(0); // reserved
+
+    assert(OS.tell() - Start == Is64Bit ? Header64Size : 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 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 ? SegmentLoadCommand64Size :
+      SegmentLoadCommand32Size;
+    Write32(Is64Bit ? LCT_Segment64 : LCT_Segment);
+    Write32(SegmentLoadCommandSize +
+            NumSections * (Is64Bit ? Section64Size : 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 WriteSection(const MCAssembler &Asm, const MCSectionData &SD,
+                    uint64_t FileOffset, uint64_t RelocationsStart,
+                    unsigned NumRelocations) {
+    // The offset is unused for virtual sections.
+    if (Asm.getBackend().isVirtualSection(SD.getSection())) {
+      assert(SD.getFileSize() == 0 && "Invalid file size!");
+      FileOffset = 0;
+    }
+
+    // struct section (68 bytes) or
+    // struct section_64 (80 bytes)
+
+    uint64_t Start = OS.tell();
+    (void) Start;
+
+    // FIXME: cast<> support!
+    const MCSectionMachO &Section =
+      static_cast<const MCSectionMachO&>(SD.getSection());
+    WriteBytes(Section.getSectionName(), 16);
+    WriteBytes(Section.getSegmentName(), 16);
+    if (Is64Bit) {
+      Write64(SD.getAddress()); // address
+      Write64(SD.getSize()); // size
+    } else {
+      Write32(SD.getAddress()); // address
+      Write32(SD.getSize()); // 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(0); // reserved1
+    Write32(Section.getStubSize()); // reserved2
+    if (Is64Bit)
+      Write32(0); // reserved3
+
+    assert(OS.tell() - Start == Is64Bit ? Section64Size : Section32Size);
+  }
+
+  void 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(LCT_Symtab);
+    Write32(SymtabLoadCommandSize);
+    Write32(SymbolOffset);
+    Write32(NumSymbols);
+    Write32(StringTableOffset);
+    Write32(StringTableSize);
+
+    assert(OS.tell() - Start == SymtabLoadCommandSize);
+  }
+
+  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) {
+    // struct dysymtab_command (80 bytes)
+
+    uint64_t Start = OS.tell();
+    (void) Start;
+
+    Write32(LCT_Dysymtab);
+    Write32(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 == DysymtabLoadCommandSize);
+  }
+
+  void WriteNlist(MachSymbolData &MSD) {
+    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 = STT_Undefined;
+    else if (Symbol.isAbsolute())
+      Type = STT_Absolute;
+    else
+      Type = STT_Section;
+
+    // FIXME: Set STAB bits.
+
+    if (Data.isPrivateExtern())
+      Type |= STF_PrivateExtern;
+
+    // Set external bit.
+    if (Data.isExternal() || Symbol.isUndefined())
+      Type |= STF_External;
+
+    // Compute the symbol address.
+    if (Symbol.isDefined()) {
+      if (Symbol.isAbsolute()) {
+        llvm_unreachable("FIXME: Not yet implemented!");
+      } else {
+        Address = Data.getAddress();
+      }
+    } 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)
+          llvm_report_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 RecordScatteredRelocation(const MCAssembler &Asm,
+                                 const MCFragment &Fragment,
+                                 const MCAsmFixup &Fixup, MCValue Target,
+                                 uint64_t &FixedValue) {
+    uint32_t Address = Fragment.getOffset() + Fixup.Offset;
+    unsigned IsPCRel = isFixupKindPCRel(Fixup.Kind);
+    unsigned Log2Size = getFixupKindLog2Size(Fixup.Kind);
+    unsigned Type = RIT_Vanilla;
+
+    // See <reloc.h>.
+    const MCSymbol *A = &Target.getSymA()->getSymbol();
+    MCSymbolData *A_SD = &Asm.getSymbolData(*A);
+
+    if (!A_SD->getFragment())
+      llvm_report_error("symbol '" + A->getName() +
+                        "' can not be undefined in a subtraction expression");
+
+    uint32_t Value = A_SD->getAddress();
+    uint32_t Value2 = 0;
+
+    if (const MCSymbolRefExpr *B = Target.getSymB()) {
+      MCSymbolData *B_SD = &Asm.getSymbolData(B->getSymbol());
+
+      if (!B_SD->getFragment())
+        llvm_report_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() ? RIT_Difference : RIT_LocalDifference;
+      Value2 = B_SD->getAddress();
+    }
+
+    // Relocations are written out in reverse order, so the PAIR comes first.
+    if (Type == RIT_Difference || Type == RIT_LocalDifference) {
+      MachRelocationEntry MRE;
+      MRE.Word0 = ((0         <<  0) |
+                   (RIT_Pair  << 24) |
+                   (Log2Size  << 28) |
+                   (IsPCRel   << 30) |
+                   RF_Scattered);
+      MRE.Word1 = Value2;
+      Relocations[Fragment.getParent()].push_back(MRE);
+    }
+
+    MachRelocationEntry MRE;
+    MRE.Word0 = ((Address   <<  0) |
+                 (Type      << 24) |
+                 (Log2Size  << 28) |
+                 (IsPCRel   << 30) |
+                 RF_Scattered);
+    MRE.Word1 = Value;
+    Relocations[Fragment.getParent()].push_back(MRE);
+  }
+
+  virtual void RecordRelocation(const MCAssembler &Asm,
+                                const MCDataFragment &Fragment,
+                                const MCAsmFixup &Fixup, MCValue Target,
+                                uint64_t &FixedValue) {
+    unsigned IsPCRel = isFixupKindPCRel(Fixup.Kind);
+    unsigned Log2Size = getFixupKindLog2Size(Fixup.Kind);
+
+    // If this is a difference or a defined symbol plus an offset, then we need
+    // a scattered relocation entry.
+    uint32_t Offset = Target.getConstant();
+    if (IsPCRel)
+      Offset += 1 << Log2Size;
+    if (Target.getSymB() ||
+        (Target.getSymA() && !Target.getSymA()->getSymbol().isUndefined() &&
+         Offset)) {
+      RecordScatteredRelocation(Asm, Fragment, Fixup, Target, FixedValue);
+      return;
+    }
+
+    // See <reloc.h>.
+    uint32_t Address = Fragment.getOffset() + Fixup.Offset;
+    uint32_t Value = 0;
+    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 = RIT_Vanilla;
+      Value = 0;
+    } else {
+      const MCSymbol *Symbol = &Target.getSymA()->getSymbol();
+      MCSymbolData *SD = &Asm.getSymbolData(*Symbol);
+
+      if (Symbol->isUndefined()) {
+        IsExtern = 1;
+        Index = SD->getIndex();
+        Value = 0;
+      } else {
+        // The index is the section ordinal.
+        //
+        // FIXME: O(N)
+        Index = 1;
+        MCAssembler::const_iterator it = Asm.begin(), ie = Asm.end();
+        for (; it != ie; ++it, ++Index)
+          if (&*it == SD->getFragment()->getParent())
+            break;
+        assert(it != ie && "Unable to find section index!");
+        Value = SD->getAddress();
+      }
+
+      Type = RIT_Vanilla;
+    }
+
+    // struct relocation_info (8 bytes)
+    MachRelocationEntry MRE;
+    MRE.Word0 = Address;
+    MRE.Word1 = ((Index     <<  0) |
+                 (IsPCRel   << 24) |
+                 (Log2Size  << 25) |
+                 (IsExtern  << 27) |
+                 (Type      << 28));
+    Relocations[Fragment.getParent()].push_back(MRE);
+  }
+
+  void 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.
+    for (MCAssembler::indirect_symbol_iterator it = Asm.indirect_symbol_begin(),
+           ie = Asm.indirect_symbol_end(); it != ie; ++it) {
+      // FIXME: cast<> support!
+      const MCSectionMachO &Section =
+        static_cast<const MCSectionMachO&>(it->SectionData->getSection());
+
+      if (Section.getType() != MCSectionMachO::S_NON_LAZY_SYMBOL_POINTERS)
+        continue;
+
+      Asm.getOrCreateSymbolData(*it->Symbol);
+    }
+
+    // Then lazy symbol pointers and symbol stubs.
+    for (MCAssembler::indirect_symbol_iterator it = Asm.indirect_symbol_begin(),
+           ie = Asm.indirect_symbol_end(); it != ie; ++it) {
+      // FIXME: cast<> support!
+      const MCSectionMachO &Section =
+        static_cast<const MCSectionMachO&>(it->SectionData->getSection());
+
+      if (Section.getType() != MCSectionMachO::S_LAZY_SYMBOL_POINTERS &&
+          Section.getType() != MCSectionMachO::S_SYMBOL_STUBS)
+        continue;
+
+      // 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 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))
+        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))
+        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';
+  }
+
+  virtual void ExecutePostLayoutBinding(MCAssembler &Asm) {
+    // Create symbol data for any indirect symbols.
+    BindIndirectSymbols(Asm);
+
+    // Compute symbol table information and bind symbol indices.
+    ComputeSymbolTable(Asm, StringTable, LocalSymbolData, ExternalSymbolData,
+                       UndefinedSymbolData);
+  }
+
+  virtual void WriteObject(const MCAssembler &Asm) {
+    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 ?
+      SegmentLoadCommand64Size + NumSections * Section64Size :
+      SegmentLoadCommand32Size + NumSections * Section32Size;
+
+    // Add the symbol table load command sizes, if used.
+    unsigned NumSymbols = LocalSymbolData.size() + ExternalSymbolData.size() +
+      UndefinedSymbolData.size();
+    if (NumSymbols) {
+      NumLoadCommands += 2;
+      LoadCommandsSize += SymtabLoadCommandSize + DysymtabLoadCommandSize;
+    }
+
+    // Compute the total size of the section data, as well as its file size and
+    // vm size.
+    uint64_t SectionDataStart = (Is64Bit ? Header64Size : 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;
+
+      VMSize = std::max(VMSize, SD.getAddress() + SD.getSize());
+
+      if (Asm.getBackend().isVirtualSection(SD.getSection()))
+        continue;
+
+      SectionDataSize = std::max(SectionDataSize,
+                                 SD.getAddress() + SD.getSize());
+      SectionDataFileSize = std::max(SectionDataFileSize,
+                                     SD.getAddress() + SD.getFileSize());
+    }
+
+    // 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<MachRelocationEntry> &Relocs = Relocations[it];
+      unsigned NumRelocs = Relocs.size();
+      uint64_t SectionStart = SectionDataStart + it->getAddress();
+      WriteSection(Asm, *it, SectionStart, RelocTableEnd, NumRelocs);
+      RelocTableEnd += NumRelocs * 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 ? Nlist64Size :
+                                                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, Writer);
+
+    // 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<MachRelocationEntry> &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 = ISF_Local;
+            if (it->Symbol->isAbsolute())
+              Flags |= 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]);
+      for (unsigned i = 0, e = ExternalSymbolData.size(); i != e; ++i)
+        WriteNlist(ExternalSymbolData[i]);
+      for (unsigned i = 0, e = UndefinedSymbolData.size(); i != e; ++i)
+        WriteNlist(UndefinedSymbolData[i]);
+
+      // Write the string table.
+      OS << StringTable.str();
+    }
+  }
+};
+
+}
+
+MachObjectWriter::MachObjectWriter(raw_ostream &OS,
+                                   bool Is64Bit,
+                                   bool IsLittleEndian)
+  : MCObjectWriter(OS, IsLittleEndian)
+{
+  Impl = new MachObjectWriterImpl(this, Is64Bit);
+}
+
+MachObjectWriter::~MachObjectWriter() {
+  delete (MachObjectWriterImpl*) Impl;
+}
+
+void MachObjectWriter::ExecutePostLayoutBinding(MCAssembler &Asm) {
+  ((MachObjectWriterImpl*) Impl)->ExecutePostLayoutBinding(Asm);
+}
+
+void MachObjectWriter::RecordRelocation(const MCAssembler &Asm,
+                                        const MCDataFragment &Fragment,
+                                        const MCAsmFixup &Fixup, MCValue Target,
+                                        uint64_t &FixedValue) {
+  ((MachObjectWriterImpl*) Impl)->RecordRelocation(Asm, Fragment, Fixup,
+                                                   Target, FixedValue);
+}
+
+void MachObjectWriter::WriteObject(const MCAssembler &Asm) {
+  ((MachObjectWriterImpl*) Impl)->WriteObject(Asm);
+}