lib/MC/MCMachOStreamer.cpp - llvm - Git at Google

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

 #include "llvm/MC/MCStreamer.h"

 #include "llvm/MC/MCAssembler.h"
 #include "llvm/MC/MCContext.h"
 #include "llvm/MC/MCCodeEmitter.h"
 #include "llvm/MC/MCExpr.h"
 #include "llvm/MC/MCInst.h"
 #include "llvm/MC/MCObjectStreamer.h"
 #include "llvm/MC/MCSection.h"
 #include "llvm/MC/MCSymbol.h"
 #include "llvm/MC/MCMachOSymbolFlags.h"
 #include "llvm/MC/MCSectionMachO.h"
 #include "llvm/MC/MCDwarf.h"
 #include "llvm/MC/MCAsmBackend.h"
 #include "llvm/Support/Dwarf.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/raw_ostream.h"

 using namespace llvm;

 namespace {

 class MCMachOStreamer : public MCObjectStreamer {
 private:
   virtual void EmitInstToData(const MCInst &Inst);

 public:
   MCMachOStreamer(MCContext &Context, MCAsmBackend &MAB,
                   raw_ostream &OS, MCCodeEmitter *Emitter)
     : MCObjectStreamer(Context, MAB, OS, Emitter) {}

   /// @name MCStreamer Interface
   /// @{

   virtual void InitSections();
   virtual void EmitLabel(MCSymbol *Symbol);
   virtual void EmitEHSymAttributes(const MCSymbol *Symbol,
                                    MCSymbol *EHSymbol);
   virtual void EmitAssemblerFlag(MCAssemblerFlag Flag);
   virtual void EmitThumbFunc(MCSymbol *Func);
   virtual void EmitAssignment(MCSymbol *Symbol, const MCExpr *Value);
   virtual void EmitSymbolAttribute(MCSymbol *Symbol, MCSymbolAttr Attribute);
   virtual void EmitSymbolDesc(MCSymbol *Symbol, unsigned DescValue);
   virtual void EmitCommonSymbol(MCSymbol *Symbol, uint64_t Size,
                                 unsigned ByteAlignment);
   virtual void BeginCOFFSymbolDef(const MCSymbol *Symbol) {
     assert(0 && "macho doesn't support this directive");
   }
   virtual void EmitCOFFSymbolStorageClass(int StorageClass) {
     assert(0 && "macho doesn't support this directive");
   }
   virtual void EmitCOFFSymbolType(int Type) {
     assert(0 && "macho doesn't support this directive");
   }
   virtual void EndCOFFSymbolDef() {
     assert(0 && "macho doesn't support this directive");
   }
   virtual void EmitELFSize(MCSymbol *Symbol, const MCExpr *Value) {
     assert(0 && "macho doesn't support this directive");
   }
   virtual void EmitLocalCommonSymbol(MCSymbol *Symbol, uint64_t Size,
                                      unsigned ByteAlignment) {
     assert(0 && "macho doesn't support this directive");
   }
   virtual void EmitZerofill(const MCSection *Section, MCSymbol *Symbol = 0,
                             unsigned Size = 0, unsigned ByteAlignment = 0);
   virtual void EmitTBSSSymbol(const MCSection *Section, MCSymbol *Symbol,
                               uint64_t Size, unsigned ByteAlignment = 0);
   virtual void EmitBytes(StringRef Data, unsigned AddrSpace);
   virtual void EmitValueToAlignment(unsigned ByteAlignment, int64_t Value = 0,
                                     unsigned ValueSize = 1,
                                     unsigned MaxBytesToEmit = 0);
   virtual void EmitCodeAlignment(unsigned ByteAlignment,
                                  unsigned MaxBytesToEmit = 0);

   virtual void EmitFileDirective(StringRef Filename) {
     // FIXME: Just ignore the .file; it isn't important enough to fail the
     // entire assembly.

     //report_fatal_error("unsupported directive: '.file'");
   }

   virtual void Finish();

   /// @}
 };

 } // end anonymous namespace.

 void MCMachOStreamer::InitSections() {
   SwitchSection(getContext().getMachOSection("__TEXT", "__text",
                                     MCSectionMachO::S_ATTR_PURE_INSTRUCTIONS,
                                     0, SectionKind::getText()));

 }

 void MCMachOStreamer::EmitEHSymAttributes(const MCSymbol *Symbol,
                                           MCSymbol *EHSymbol) {
   MCSymbolData &SD =
     getAssembler().getOrCreateSymbolData(*Symbol);
   if (SD.isExternal())
     EmitSymbolAttribute(EHSymbol, MCSA_Global);
   if (SD.getFlags() & SF_WeakDefinition)
     EmitSymbolAttribute(EHSymbol, MCSA_WeakDefinition);
   if (SD.isPrivateExtern())
     EmitSymbolAttribute(EHSymbol, MCSA_PrivateExtern);
 }

 void MCMachOStreamer::EmitLabel(MCSymbol *Symbol) {
   assert(Symbol->isUndefined() && "Cannot define a symbol twice!");

   // isSymbolLinkerVisible uses the section.
   Symbol->setSection(*getCurrentSection());
   // We have to create a new fragment if this is an atom defining symbol,
   // fragments cannot span atoms.
   if (getAssembler().isSymbolLinkerVisible(*Symbol))
     new MCDataFragment(getCurrentSectionData());

   MCObjectStreamer::EmitLabel(Symbol);

   MCSymbolData &SD = getAssembler().getSymbolData(*Symbol);
   // This causes the reference type flag to be cleared. Darwin 'as' was "trying"
   // to clear the weak reference and weak definition bits too, but the
   // implementation was buggy. For now we just try to match 'as', for
   // diffability.
   //
   // FIXME: Cleanup this code, these bits should be emitted based on semantic
   // properties, not on the order of definition, etc.
   SD.setFlags(SD.getFlags() & ~SF_ReferenceTypeMask);
 }

 void MCMachOStreamer::EmitAssemblerFlag(MCAssemblerFlag Flag) {
   // Let the target do whatever target specific stuff it needs to do.
   getAssembler().getBackend().HandleAssemblerFlag(Flag);
   // Do any generic stuff we need to do.
   switch (Flag) {
   case MCAF_SyntaxUnified: return; // no-op here.
   case MCAF_Code16: return; // Change parsing mode; no-op here.
   case MCAF_Code32: return; // Change parsing mode; no-op here.
   case MCAF_Code64: return; // Change parsing mode; no-op here.
   case MCAF_SubsectionsViaSymbols:
     getAssembler().setSubsectionsViaSymbols(true);
     return;
   default:
     llvm_unreachable("invalid assembler flag!");
   }
 }

 void MCMachOStreamer::EmitThumbFunc(MCSymbol *Symbol) {
   // FIXME: Flag the function ISA as thumb with DW_AT_APPLE_isa.

   // Remember that the function is a thumb function. Fixup and relocation
   // values will need adjusted.
   getAssembler().setIsThumbFunc(Symbol);

   // Mark the thumb bit on the symbol.
   MCSymbolData &SD = getAssembler().getOrCreateSymbolData(*Symbol);
   SD.setFlags(SD.getFlags() | SF_ThumbFunc);
 }

 void MCMachOStreamer::EmitAssignment(MCSymbol *Symbol, const MCExpr *Value) {
   // TODO: This is exactly the same as WinCOFFStreamer. Consider merging into
   // MCObjectStreamer.
   // FIXME: Lift context changes into super class.
   getAssembler().getOrCreateSymbolData(*Symbol);
   Symbol->setVariableValue(AddValueSymbols(Value));
 }

 void MCMachOStreamer::EmitSymbolAttribute(MCSymbol *Symbol,
                                           MCSymbolAttr Attribute) {
   // Indirect symbols are handled differently, to match how 'as' handles
   // them. This makes writing matching .o files easier.
   if (Attribute == MCSA_IndirectSymbol) {
     // Note that we intentionally cannot use the symbol data here; this is
     // important for matching the string table that 'as' generates.
     IndirectSymbolData ISD;
     ISD.Symbol = Symbol;
     ISD.SectionData = getCurrentSectionData();
     getAssembler().getIndirectSymbols().push_back(ISD);
     return;
   }

   // Adding a symbol attribute always introduces the symbol, note that an
   // important side effect of calling getOrCreateSymbolData here is to register
   // the symbol with the assembler.
   MCSymbolData &SD = getAssembler().getOrCreateSymbolData(*Symbol);

   // The implementation of symbol attributes is designed to match 'as', but it
   // leaves much to desired. It doesn't really make sense to arbitrarily add and
   // remove flags, but 'as' allows this (in particular, see .desc).
   //
   // In the future it might be worth trying to make these operations more well
   // defined.
   switch (Attribute) {
   case MCSA_Invalid:
   case MCSA_ELF_TypeFunction:
   case MCSA_ELF_TypeIndFunction:
   case MCSA_ELF_TypeObject:
   case MCSA_ELF_TypeTLS:
   case MCSA_ELF_TypeCommon:
   case MCSA_ELF_TypeNoType:
   case MCSA_ELF_TypeGnuUniqueObject:
   case MCSA_Hidden:
   case MCSA_IndirectSymbol:
   case MCSA_Internal:
   case MCSA_Protected:
   case MCSA_Weak:
   case MCSA_Local:
     assert(0 && "Invalid symbol attribute for Mach-O!");
     break;

   case MCSA_Global:
     SD.setExternal(true);
     // This effectively clears the undefined lazy bit, in Darwin 'as', although
     // it isn't very consistent because it implements this as part of symbol
     // lookup.
     //
     // FIXME: Cleanup this code, these bits should be emitted based on semantic
     // properties, not on the order of definition, etc.
     SD.setFlags(SD.getFlags() & ~SF_ReferenceTypeUndefinedLazy);
     break;

   case MCSA_LazyReference:
     // FIXME: This requires -dynamic.
     SD.setFlags(SD.getFlags() | SF_NoDeadStrip);
     if (Symbol->isUndefined())
       SD.setFlags(SD.getFlags() | SF_ReferenceTypeUndefinedLazy);
     break;

     // Since .reference sets the no dead strip bit, it is equivalent to
     // .no_dead_strip in practice.
   case MCSA_Reference:
   case MCSA_NoDeadStrip:
     SD.setFlags(SD.getFlags() | SF_NoDeadStrip);
     break;

   case MCSA_SymbolResolver:
     SD.setFlags(SD.getFlags() | SF_SymbolResolver);
     break;

   case MCSA_PrivateExtern:
     SD.setExternal(true);
     SD.setPrivateExtern(true);
     break;

   case MCSA_WeakReference:
     // FIXME: This requires -dynamic.
     if (Symbol->isUndefined())
       SD.setFlags(SD.getFlags() | SF_WeakReference);
     break;

   case MCSA_WeakDefinition:
     // FIXME: 'as' enforces that this is defined and global. The manual claims
     // it has to be in a coalesced section, but this isn't enforced.
     SD.setFlags(SD.getFlags() | SF_WeakDefinition);
     break;

   case MCSA_WeakDefAutoPrivate:
     SD.setFlags(SD.getFlags() | SF_WeakDefinition | SF_WeakReference);
     break;
   }
 }

 void MCMachOStreamer::EmitSymbolDesc(MCSymbol *Symbol, unsigned DescValue) {
   // Encode the 'desc' value into the lowest implementation defined bits.
   assert(DescValue == (DescValue & SF_DescFlagsMask) &&
          "Invalid .desc value!");
   getAssembler().getOrCreateSymbolData(*Symbol).setFlags(
     DescValue & SF_DescFlagsMask);
 }

 void MCMachOStreamer::EmitCommonSymbol(MCSymbol *Symbol, uint64_t Size,
                                        unsigned ByteAlignment) {
   // FIXME: Darwin 'as' does appear to allow redef of a .comm by itself.
   assert(Symbol->isUndefined() && "Cannot define a symbol twice!");

   MCSymbolData &SD = getAssembler().getOrCreateSymbolData(*Symbol);
   SD.setExternal(true);
   SD.setCommon(Size, ByteAlignment);
 }

 void MCMachOStreamer::EmitZerofill(const MCSection *Section, MCSymbol *Symbol,
                                    unsigned Size, unsigned ByteAlignment) {
   MCSectionData &SectData = getAssembler().getOrCreateSectionData(*Section);

   // The symbol may not be present, which only creates the section.
   if (!Symbol)
     return;

   // FIXME: Assert that this section has the zerofill type.

   assert(Symbol->isUndefined() && "Cannot define a symbol twice!");

   MCSymbolData &SD = getAssembler().getOrCreateSymbolData(*Symbol);

   // Emit an align fragment if necessary.
   if (ByteAlignment != 1)
     new MCAlignFragment(ByteAlignment, 0, 0, ByteAlignment, &SectData);

   MCFragment *F = new MCFillFragment(0, 0, Size, &SectData);
   SD.setFragment(F);

   Symbol->setSection(*Section);

   // Update the maximum alignment on the zero fill section if necessary.
   if (ByteAlignment > SectData.getAlignment())
     SectData.setAlignment(ByteAlignment);
 }

 // This should always be called with the thread local bss section.  Like the
 // .zerofill directive this doesn't actually switch sections on us.
 void MCMachOStreamer::EmitTBSSSymbol(const MCSection *Section, MCSymbol *Symbol,
                                      uint64_t Size, unsigned ByteAlignment) {
   EmitZerofill(Section, Symbol, Size, ByteAlignment);
   return;
 }

 void MCMachOStreamer::EmitBytes(StringRef Data, unsigned AddrSpace) {
   // TODO: This is exactly the same as WinCOFFStreamer. Consider merging into
   // MCObjectStreamer.
   getOrCreateDataFragment()->getContents().append(Data.begin(), Data.end());
 }

 void MCMachOStreamer::EmitValueToAlignment(unsigned ByteAlignment,
                                            int64_t Value, unsigned ValueSize,
                                            unsigned MaxBytesToEmit) {
   // TODO: This is exactly the same as WinCOFFStreamer. Consider merging into
   // MCObjectStreamer.
   if (MaxBytesToEmit == 0)
     MaxBytesToEmit = ByteAlignment;
   new MCAlignFragment(ByteAlignment, Value, ValueSize, MaxBytesToEmit,
                       getCurrentSectionData());

   // Update the maximum alignment on the current section if necessary.
   if (ByteAlignment > getCurrentSectionData()->getAlignment())
     getCurrentSectionData()->setAlignment(ByteAlignment);
 }

 void MCMachOStreamer::EmitCodeAlignment(unsigned ByteAlignment,
                                         unsigned MaxBytesToEmit) {
   // TODO: This is exactly the same as WinCOFFStreamer. Consider merging into
   // MCObjectStreamer.
   if (MaxBytesToEmit == 0)
     MaxBytesToEmit = ByteAlignment;
   MCAlignFragment *F = new MCAlignFragment(ByteAlignment, 0, 1, MaxBytesToEmit,
                                            getCurrentSectionData());
   F->setEmitNops(true);

   // Update the maximum alignment on the current section if necessary.
   if (ByteAlignment > getCurrentSectionData()->getAlignment())
     getCurrentSectionData()->setAlignment(ByteAlignment);
 }

 void MCMachOStreamer::EmitInstToData(const MCInst &Inst) {
   MCDataFragment *DF = getOrCreateDataFragment();

   SmallVector<MCFixup, 4> Fixups;
   SmallString<256> Code;
   raw_svector_ostream VecOS(Code);
   getAssembler().getEmitter().EncodeInstruction(Inst, VecOS, Fixups);
   VecOS.flush();

   // Add the fixups and data.
   for (unsigned i = 0, e = Fixups.size(); i != e; ++i) {
     Fixups[i].setOffset(Fixups[i].getOffset() + DF->getContents().size());
     DF->addFixup(Fixups[i]);
   }
   DF->getContents().append(Code.begin(), Code.end());
 }

 void MCMachOStreamer::Finish() {
   EmitFrames(true);

   // We have to set the fragment atom associations so we can relax properly for
   // Mach-O.

   // First, scan the symbol table to build a lookup table from fragments to
   // defining symbols.
   DenseMap<const MCFragment*, MCSymbolData*> DefiningSymbolMap;
   for (MCAssembler::symbol_iterator it = getAssembler().symbol_begin(),
          ie = getAssembler().symbol_end(); it != ie; ++it) {
     if (getAssembler().isSymbolLinkerVisible(it->getSymbol()) &&
         it->getFragment()) {
       // An atom defining symbol should never be internal to a fragment.
       assert(it->getOffset() == 0 && "Invalid offset in atom defining symbol!");
       DefiningSymbolMap[it->getFragment()] = it;
     }
   }

   // Set the fragment atom associations by tracking the last seen atom defining
   // symbol.
   for (MCAssembler::iterator it = getAssembler().begin(),
          ie = getAssembler().end(); it != ie; ++it) {
     MCSymbolData *CurrentAtom = 0;
     for (MCSectionData::iterator it2 = it->begin(),
            ie2 = it->end(); it2 != ie2; ++it2) {
       if (MCSymbolData *SD = DefiningSymbolMap.lookup(it2))
         CurrentAtom = SD;
       it2->setAtom(CurrentAtom);
     }
   }

   this->MCObjectStreamer::Finish();
 }

 MCStreamer *llvm::createMachOStreamer(MCContext &Context, MCAsmBackend &MAB,
                                       raw_ostream &OS, MCCodeEmitter *CE,
                                       bool RelaxAll) {
   MCMachOStreamer *S = new MCMachOStreamer(Context, MAB, OS, CE);
   if (RelaxAll)
     S->getAssembler().setRelaxAll(true);
   return S;
 }
	//===- lib/MC/MCMachOStreamer.cpp - Mach-O Object Output ------------===//
	//
	// The LLVM Compiler Infrastructure
	//
	// This file is distributed under the University of Illinois Open Source
	// License. See LICENSE.TXT for details.
	//
	//===----------------------------------------------------------------------===//

	#include "llvm/MC/MCStreamer.h"

	#include "llvm/MC/MCAssembler.h"
	#include "llvm/MC/MCContext.h"
	#include "llvm/MC/MCCodeEmitter.h"
	#include "llvm/MC/MCExpr.h"
	#include "llvm/MC/MCInst.h"
	#include "llvm/MC/MCObjectStreamer.h"
	#include "llvm/MC/MCSection.h"
	#include "llvm/MC/MCSymbol.h"
	#include "llvm/MC/MCMachOSymbolFlags.h"
	#include "llvm/MC/MCSectionMachO.h"
	#include "llvm/MC/MCDwarf.h"
	#include "llvm/MC/MCAsmBackend.h"
	#include "llvm/Support/Dwarf.h"
	#include "llvm/Support/ErrorHandling.h"
	#include "llvm/Support/raw_ostream.h"

	using namespace llvm;

	namespace {

	class MCMachOStreamer : public MCObjectStreamer {
	private:
	virtual void EmitInstToData(const MCInst &Inst);

	public:
	MCMachOStreamer(MCContext &Context, MCAsmBackend &MAB,
	raw_ostream &OS, MCCodeEmitter *Emitter)
	: MCObjectStreamer(Context, MAB, OS, Emitter) {}

	/// @name MCStreamer Interface
	/// @{

	virtual void InitSections();
	virtual void EmitLabel(MCSymbol *Symbol);
	virtual void EmitEHSymAttributes(const MCSymbol *Symbol,
	MCSymbol *EHSymbol);
	virtual void EmitAssemblerFlag(MCAssemblerFlag Flag);
	virtual void EmitThumbFunc(MCSymbol *Func);
	virtual void EmitAssignment(MCSymbol Symbol, const MCExpr Value);
	virtual void EmitSymbolAttribute(MCSymbol *Symbol, MCSymbolAttr Attribute);
	virtual void EmitSymbolDesc(MCSymbol *Symbol, unsigned DescValue);
	virtual void EmitCommonSymbol(MCSymbol *Symbol, uint64_t Size,
	unsigned ByteAlignment);
	virtual void BeginCOFFSymbolDef(const MCSymbol *Symbol) {
	assert(0 && "macho doesn't support this directive");
	}
	virtual void EmitCOFFSymbolStorageClass(int StorageClass) {
	assert(0 && "macho doesn't support this directive");
	}
	virtual void EmitCOFFSymbolType(int Type) {
	assert(0 && "macho doesn't support this directive");
	}
	virtual void EndCOFFSymbolDef() {
	assert(0 && "macho doesn't support this directive");
	}
	virtual void EmitELFSize(MCSymbol Symbol, const MCExpr Value) {
	assert(0 && "macho doesn't support this directive");
	}
	virtual void EmitLocalCommonSymbol(MCSymbol *Symbol, uint64_t Size,
	unsigned ByteAlignment) {
	assert(0 && "macho doesn't support this directive");
	}
	virtual void EmitZerofill(const MCSection Section, MCSymbol Symbol = 0,
	unsigned Size = 0, unsigned ByteAlignment = 0);
	virtual void EmitTBSSSymbol(const MCSection Section, MCSymbol Symbol,
	uint64_t Size, unsigned ByteAlignment = 0);
	virtual void EmitBytes(StringRef Data, unsigned AddrSpace);
	virtual void EmitValueToAlignment(unsigned ByteAlignment, int64_t Value = 0,
	unsigned ValueSize = 1,
	unsigned MaxBytesToEmit = 0);
	virtual void EmitCodeAlignment(unsigned ByteAlignment,
	unsigned MaxBytesToEmit = 0);

	virtual void EmitFileDirective(StringRef Filename) {
	// FIXME: Just ignore the .file; it isn't important enough to fail the
	// entire assembly.

	//report_fatal_error("unsupported directive: '.file'");
	}

	virtual void Finish();

	/// @}
	};

	} // end anonymous namespace.

	void MCMachOStreamer::InitSections() {
	SwitchSection(getContext().getMachOSection("__TEXT", "__text",
	MCSectionMachO::S_ATTR_PURE_INSTRUCTIONS,
	0, SectionKind::getText()));

	}

	void MCMachOStreamer::EmitEHSymAttributes(const MCSymbol *Symbol,
	MCSymbol *EHSymbol) {
	MCSymbolData &SD =
	getAssembler().getOrCreateSymbolData(*Symbol);
	if (SD.isExternal())
	EmitSymbolAttribute(EHSymbol, MCSA_Global);
	if (SD.getFlags() & SF_WeakDefinition)
	EmitSymbolAttribute(EHSymbol, MCSA_WeakDefinition);
	if (SD.isPrivateExtern())
	EmitSymbolAttribute(EHSymbol, MCSA_PrivateExtern);
	}

	void MCMachOStreamer::EmitLabel(MCSymbol *Symbol) {
	assert(Symbol->isUndefined() && "Cannot define a symbol twice!");

	// isSymbolLinkerVisible uses the section.
	Symbol->setSection(*getCurrentSection());
	// We have to create a new fragment if this is an atom defining symbol,
	// fragments cannot span atoms.
	if (getAssembler().isSymbolLinkerVisible(*Symbol))
	new MCDataFragment(getCurrentSectionData());

	MCObjectStreamer::EmitLabel(Symbol);

	MCSymbolData &SD = getAssembler().getSymbolData(*Symbol);
	// This causes the reference type flag to be cleared. Darwin 'as' was "trying"
	// to clear the weak reference and weak definition bits too, but the
	// implementation was buggy. For now we just try to match 'as', for
	// diffability.
	//
	// FIXME: Cleanup this code, these bits should be emitted based on semantic
	// properties, not on the order of definition, etc.
	SD.setFlags(SD.getFlags() & ~SF_ReferenceTypeMask);
	}

	void MCMachOStreamer::EmitAssemblerFlag(MCAssemblerFlag Flag) {
	// Let the target do whatever target specific stuff it needs to do.
	getAssembler().getBackend().HandleAssemblerFlag(Flag);
	// Do any generic stuff we need to do.
	switch (Flag) {
	case MCAF_SyntaxUnified: return; // no-op here.
	case MCAF_Code16: return; // Change parsing mode; no-op here.
	case MCAF_Code32: return; // Change parsing mode; no-op here.
	case MCAF_Code64: return; // Change parsing mode; no-op here.
	case MCAF_SubsectionsViaSymbols:
	getAssembler().setSubsectionsViaSymbols(true);
	return;
	default:
	llvm_unreachable("invalid assembler flag!");
	}
	}

	void MCMachOStreamer::EmitThumbFunc(MCSymbol *Symbol) {
	// FIXME: Flag the function ISA as thumb with DW_AT_APPLE_isa.

	// Remember that the function is a thumb function. Fixup and relocation
	// values will need adjusted.
	getAssembler().setIsThumbFunc(Symbol);

	// Mark the thumb bit on the symbol.
	MCSymbolData &SD = getAssembler().getOrCreateSymbolData(*Symbol);
	SD.setFlags(SD.getFlags() \| SF_ThumbFunc);
	}

	void MCMachOStreamer::EmitAssignment(MCSymbol Symbol, const MCExpr Value) {
	// TODO: This is exactly the same as WinCOFFStreamer. Consider merging into
	// MCObjectStreamer.
	// FIXME: Lift context changes into super class.
	getAssembler().getOrCreateSymbolData(*Symbol);
	Symbol->setVariableValue(AddValueSymbols(Value));
	}

	void MCMachOStreamer::EmitSymbolAttribute(MCSymbol *Symbol,
	MCSymbolAttr Attribute) {
	// Indirect symbols are handled differently, to match how 'as' handles
	// them. This makes writing matching .o files easier.
	if (Attribute == MCSA_IndirectSymbol) {
	// Note that we intentionally cannot use the symbol data here; this is
	// important for matching the string table that 'as' generates.
	IndirectSymbolData ISD;
	ISD.Symbol = Symbol;
	ISD.SectionData = getCurrentSectionData();
	getAssembler().getIndirectSymbols().push_back(ISD);
	return;
	}

	// Adding a symbol attribute always introduces the symbol, note that an
	// important side effect of calling getOrCreateSymbolData here is to register
	// the symbol with the assembler.
	MCSymbolData &SD = getAssembler().getOrCreateSymbolData(*Symbol);

	// The implementation of symbol attributes is designed to match 'as', but it
	// leaves much to desired. It doesn't really make sense to arbitrarily add and
	// remove flags, but 'as' allows this (in particular, see .desc).
	//
	// In the future it might be worth trying to make these operations more well
	// defined.
	switch (Attribute) {
	case MCSA_Invalid:
	case MCSA_ELF_TypeFunction:
	case MCSA_ELF_TypeIndFunction:
	case MCSA_ELF_TypeObject:
	case MCSA_ELF_TypeTLS:
	case MCSA_ELF_TypeCommon:
	case MCSA_ELF_TypeNoType:
	case MCSA_ELF_TypeGnuUniqueObject:
	case MCSA_Hidden:
	case MCSA_IndirectSymbol:
	case MCSA_Internal:
	case MCSA_Protected:
	case MCSA_Weak:
	case MCSA_Local:
	assert(0 && "Invalid symbol attribute for Mach-O!");
	break;

	case MCSA_Global:
	SD.setExternal(true);
	// This effectively clears the undefined lazy bit, in Darwin 'as', although
	// it isn't very consistent because it implements this as part of symbol
	// lookup.
	//
	// FIXME: Cleanup this code, these bits should be emitted based on semantic
	// properties, not on the order of definition, etc.
	SD.setFlags(SD.getFlags() & ~SF_ReferenceTypeUndefinedLazy);
	break;

	case MCSA_LazyReference:
	// FIXME: This requires -dynamic.
	SD.setFlags(SD.getFlags() \| SF_NoDeadStrip);
	if (Symbol->isUndefined())
	SD.setFlags(SD.getFlags() \| SF_ReferenceTypeUndefinedLazy);
	break;

	// Since .reference sets the no dead strip bit, it is equivalent to
	// .no_dead_strip in practice.
	case MCSA_Reference:
	case MCSA_NoDeadStrip:
	SD.setFlags(SD.getFlags() \| SF_NoDeadStrip);
	break;

	case MCSA_SymbolResolver:
	SD.setFlags(SD.getFlags() \| SF_SymbolResolver);
	break;

	case MCSA_PrivateExtern:
	SD.setExternal(true);
	SD.setPrivateExtern(true);
	break;

	case MCSA_WeakReference:
	// FIXME: This requires -dynamic.
	if (Symbol->isUndefined())
	SD.setFlags(SD.getFlags() \| SF_WeakReference);
	break;

	case MCSA_WeakDefinition:
	// FIXME: 'as' enforces that this is defined and global. The manual claims
	// it has to be in a coalesced section, but this isn't enforced.
	SD.setFlags(SD.getFlags() \| SF_WeakDefinition);
	break;

	case MCSA_WeakDefAutoPrivate:
	SD.setFlags(SD.getFlags() \| SF_WeakDefinition \| SF_WeakReference);
	break;
	}
	}

	void MCMachOStreamer::EmitSymbolDesc(MCSymbol *Symbol, unsigned DescValue) {
	// Encode the 'desc' value into the lowest implementation defined bits.
	assert(DescValue == (DescValue & SF_DescFlagsMask) &&
	"Invalid .desc value!");
	getAssembler().getOrCreateSymbolData(*Symbol).setFlags(
	DescValue & SF_DescFlagsMask);
	}

	void MCMachOStreamer::EmitCommonSymbol(MCSymbol *Symbol, uint64_t Size,
	unsigned ByteAlignment) {
	// FIXME: Darwin 'as' does appear to allow redef of a .comm by itself.
	assert(Symbol->isUndefined() && "Cannot define a symbol twice!");

	MCSymbolData &SD = getAssembler().getOrCreateSymbolData(*Symbol);
	SD.setExternal(true);
	SD.setCommon(Size, ByteAlignment);
	}

	void MCMachOStreamer::EmitZerofill(const MCSection Section, MCSymbol Symbol,
	unsigned Size, unsigned ByteAlignment) {
	MCSectionData &SectData = getAssembler().getOrCreateSectionData(*Section);

	// The symbol may not be present, which only creates the section.
	if (!Symbol)
	return;

	// FIXME: Assert that this section has the zerofill type.

	assert(Symbol->isUndefined() && "Cannot define a symbol twice!");

	MCSymbolData &SD = getAssembler().getOrCreateSymbolData(*Symbol);

	// Emit an align fragment if necessary.
	if (ByteAlignment != 1)
	new MCAlignFragment(ByteAlignment, 0, 0, ByteAlignment, &SectData);

	MCFragment *F = new MCFillFragment(0, 0, Size, &SectData);
	SD.setFragment(F);

	Symbol->setSection(*Section);

	// Update the maximum alignment on the zero fill section if necessary.
	if (ByteAlignment > SectData.getAlignment())
	SectData.setAlignment(ByteAlignment);
	}

	// This should always be called with the thread local bss section. Like the
	// .zerofill directive this doesn't actually switch sections on us.
	void MCMachOStreamer::EmitTBSSSymbol(const MCSection Section, MCSymbol Symbol,
	uint64_t Size, unsigned ByteAlignment) {
	EmitZerofill(Section, Symbol, Size, ByteAlignment);
	return;
	}

	void MCMachOStreamer::EmitBytes(StringRef Data, unsigned AddrSpace) {
	// TODO: This is exactly the same as WinCOFFStreamer. Consider merging into
	// MCObjectStreamer.
	getOrCreateDataFragment()->getContents().append(Data.begin(), Data.end());
	}

	void MCMachOStreamer::EmitValueToAlignment(unsigned ByteAlignment,
	int64_t Value, unsigned ValueSize,
	unsigned MaxBytesToEmit) {
	// TODO: This is exactly the same as WinCOFFStreamer. Consider merging into
	// MCObjectStreamer.
	if (MaxBytesToEmit == 0)
	MaxBytesToEmit = ByteAlignment;
	new MCAlignFragment(ByteAlignment, Value, ValueSize, MaxBytesToEmit,
	getCurrentSectionData());

	// Update the maximum alignment on the current section if necessary.
	if (ByteAlignment > getCurrentSectionData()->getAlignment())
	getCurrentSectionData()->setAlignment(ByteAlignment);
	}

	void MCMachOStreamer::EmitCodeAlignment(unsigned ByteAlignment,
	unsigned MaxBytesToEmit) {
	// TODO: This is exactly the same as WinCOFFStreamer. Consider merging into
	// MCObjectStreamer.
	if (MaxBytesToEmit == 0)
	MaxBytesToEmit = ByteAlignment;
	MCAlignFragment *F = new MCAlignFragment(ByteAlignment, 0, 1, MaxBytesToEmit,
	getCurrentSectionData());
	F->setEmitNops(true);

	// Update the maximum alignment on the current section if necessary.
	if (ByteAlignment > getCurrentSectionData()->getAlignment())
	getCurrentSectionData()->setAlignment(ByteAlignment);
	}

	void MCMachOStreamer::EmitInstToData(const MCInst &Inst) {
	MCDataFragment *DF = getOrCreateDataFragment();

	SmallVector<MCFixup, 4> Fixups;
	SmallString<256> Code;
	raw_svector_ostream VecOS(Code);
	getAssembler().getEmitter().EncodeInstruction(Inst, VecOS, Fixups);
	VecOS.flush();

	// Add the fixups and data.
	for (unsigned i = 0, e = Fixups.size(); i != e; ++i) {
	Fixups[i].setOffset(Fixups[i].getOffset() + DF->getContents().size());
	DF->addFixup(Fixups[i]);
	}
	DF->getContents().append(Code.begin(), Code.end());
	}

	void MCMachOStreamer::Finish() {
	EmitFrames(true);

	// We have to set the fragment atom associations so we can relax properly for
	// Mach-O.

	// First, scan the symbol table to build a lookup table from fragments to
	// defining symbols.
	DenseMap<const MCFragment, MCSymbolData> DefiningSymbolMap;
	for (MCAssembler::symbol_iterator it = getAssembler().symbol_begin(),
	ie = getAssembler().symbol_end(); it != ie; ++it) {
	if (getAssembler().isSymbolLinkerVisible(it->getSymbol()) &&
	it->getFragment()) {
	// An atom defining symbol should never be internal to a fragment.
	assert(it->getOffset() == 0 && "Invalid offset in atom defining symbol!");
	DefiningSymbolMap[it->getFragment()] = it;
	}
	}

	// Set the fragment atom associations by tracking the last seen atom defining
	// symbol.
	for (MCAssembler::iterator it = getAssembler().begin(),
	ie = getAssembler().end(); it != ie; ++it) {
	MCSymbolData *CurrentAtom = 0;
	for (MCSectionData::iterator it2 = it->begin(),
	ie2 = it->end(); it2 != ie2; ++it2) {
	if (MCSymbolData *SD = DefiningSymbolMap.lookup(it2))
	CurrentAtom = SD;
	it2->setAtom(CurrentAtom);
	}
	}

	this->MCObjectStreamer::Finish();
	}

	MCStreamer *llvm::createMachOStreamer(MCContext &Context, MCAsmBackend &MAB,
	raw_ostream &OS, MCCodeEmitter *CE,
	bool RelaxAll) {
	MCMachOStreamer *S = new MCMachOStreamer(Context, MAB, OS, CE);
	if (RelaxAll)
	S->getAssembler().setRelaxAll(true);
	return S;
	}