lib/MC/MCMachOStreamer.cpp - platform/external/llvm - Gitiles

 //===- 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/MCSection.h"
 #include "llvm/MC/MCSymbol.h"
 #include "llvm/MC/MCValue.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/raw_ostream.h"
 using namespace llvm;

 namespace {

 class MCMachOStreamer : public MCStreamer {
   /// SymbolFlags - We store the value for the 'desc' symbol field in the lowest
   /// 16 bits of the implementation defined flags.
   enum SymbolFlags { // See <mach-o/nlist.h>.
     SF_DescFlagsMask                        = 0xFFFF,

     // Reference type flags.
     SF_ReferenceTypeMask                    = 0x0007,
     SF_ReferenceTypeUndefinedNonLazy        = 0x0000,
     SF_ReferenceTypeUndefinedLazy           = 0x0001,
     SF_ReferenceTypeDefined                 = 0x0002,
     SF_ReferenceTypePrivateDefined          = 0x0003,
     SF_ReferenceTypePrivateUndefinedNonLazy = 0x0004,
     SF_ReferenceTypePrivateUndefinedLazy    = 0x0005,

     // Other 'desc' flags.
     SF_NoDeadStrip                          = 0x0020,
     SF_WeakReference                        = 0x0040,
     SF_WeakDefinition                       = 0x0080
   };

 private:
   MCAssembler Assembler;
   MCCodeEmitter *Emitter;
   MCSectionData *CurSectionData;
   DenseMap<const MCSection*, MCSectionData*> SectionMap;
   DenseMap<const MCSymbol*, MCSymbolData*> SymbolMap;

 private:
   MCFragment *getCurrentFragment() const {
     assert(CurSectionData && "No current section!");

     if (!CurSectionData->empty())
       return &CurSectionData->getFragmentList().back();

     return 0;
   }

   MCSectionData &getSectionData(const MCSection &Section) {
     MCSectionData *&Entry = SectionMap[&Section];

     if (!Entry)
       Entry = new MCSectionData(Section, &Assembler);

     return *Entry;
   }

   MCSymbolData &getSymbolData(const MCSymbol &Symbol) {
     MCSymbolData *&Entry = SymbolMap[&Symbol];

     if (!Entry)
       Entry = new MCSymbolData(Symbol, 0, 0, &Assembler);

     return *Entry;
   }

 public:
   MCMachOStreamer(MCContext &Context, raw_ostream &_OS, MCCodeEmitter *_Emitter)
     : MCStreamer(Context), Assembler(Context, _OS), Emitter(_Emitter),
       CurSectionData(0) {}
   ~MCMachOStreamer() {}

   const MCExpr *AddValueSymbols(const MCExpr *Value) {
     switch (Value->getKind()) {
     case MCExpr::Target: assert(0 && "Can't handle target exprs yet!");
     case MCExpr::Constant:
       break;

     case MCExpr::Binary: {
       const MCBinaryExpr *BE = cast<MCBinaryExpr>(Value);
       AddValueSymbols(BE->getLHS());
       AddValueSymbols(BE->getRHS());
       break;
     }

     case MCExpr::SymbolRef:
       getSymbolData(cast<MCSymbolRefExpr>(Value)->getSymbol());
       break;

     case MCExpr::Unary:
       AddValueSymbols(cast<MCUnaryExpr>(Value)->getSubExpr());
       break;
     }

     return Value;
   }

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

   virtual void SwitchSection(const MCSection *Section);
   virtual void EmitLabel(MCSymbol *Symbol);
   virtual void EmitAssemblerFlag(MCAssemblerFlag Flag);
   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 EmitELFSize(MCSymbol *Symbol, const MCExpr *Value) {
     assert(0 && "macho doesn't support this directive");
   }
   virtual void EmitLocalCommonSymbol(MCSymbol *Symbol, uint64_t Size) {
     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 EmitBytes(StringRef Data, unsigned AddrSpace);
   virtual void EmitValue(const MCExpr *Value, unsigned Size,unsigned AddrSpace);
   virtual void EmitGPRel32Value(const MCExpr *Value) {
     assert(0 && "macho doesn't support this directive");
   }
   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 EmitValueToOffset(const MCExpr *Offset,
                                  unsigned char Value = 0);

   virtual void EmitFileDirective(StringRef Filename) {
     errs() << "FIXME: MCMachoStreamer:EmitFileDirective not implemented\n";
   }
   virtual void EmitDwarfFileDirective(unsigned FileNo, StringRef Filename) {
     errs() << "FIXME: MCMachoStreamer:EmitDwarfFileDirective not implemented\n";
   }

   virtual void EmitInstruction(const MCInst &Inst);
   virtual void Finish();

   /// @}
 };

 } // end anonymous namespace.

 void MCMachOStreamer::SwitchSection(const MCSection *Section) {
   assert(Section && "Cannot switch to a null section!");

   // If already in this section, then this is a noop.
   if (Section == CurSection) return;

   CurSection = Section;
   CurSectionData = &getSectionData(*Section);
 }

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

   // FIXME: We should also use offsets into Fill fragments.
   MCDataFragment *F = dyn_cast_or_null<MCDataFragment>(getCurrentFragment());
   if (!F)
     F = new MCDataFragment(CurSectionData);

   MCSymbolData &SD = getSymbolData(*Symbol);
   assert(!SD.getFragment() && "Unexpected fragment on symbol data!");
   SD.setFragment(F);
   SD.setOffset(F->getContents().size());

   // This causes the reference type and weak reference flags to be cleared.
   SD.setFlags(SD.getFlags() & ~(SF_WeakReference | SF_ReferenceTypeMask));

   Symbol->setSection(*CurSection);
 }

 void MCMachOStreamer::EmitAssemblerFlag(MCAssemblerFlag Flag) {
   switch (Flag) {
   case MCAF_SubsectionsViaSymbols:
     Assembler.setSubsectionsViaSymbols(true);
     return;
   }

   assert(0 && "invalid assembler flag!");
 }

 void MCMachOStreamer::EmitAssignment(MCSymbol *Symbol, const MCExpr *Value) {
   // Only absolute symbols can be redefined.
   assert((Symbol->isUndefined() || Symbol->isAbsolute()) &&
          "Cannot define a symbol twice!");

   // FIXME: Lift context changes into super class.
   // FIXME: Set associated section.
   Symbol->setValue(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 = CurSectionData;
     Assembler.getIndirectSymbols().push_back(ISD);
     return;
   }

   // Adding a symbol attribute always introduces the symbol, note that an
   // important side effect of calling getSymbolData here is to register the
   // symbol with the assembler.
   MCSymbolData &SD = getSymbolData(*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_IndirectSymbol:
   case MCSA_Hidden:
   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);
     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_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;
   }
 }

 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!");
   getSymbolData(*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 = getSymbolData(*Symbol);
   SD.setExternal(true);
   SD.setCommon(Size, ByteAlignment);
 }

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

   MCFragment *F = new MCZeroFillFragment(Size, ByteAlignment, &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);
 }

 void MCMachOStreamer::EmitBytes(StringRef Data, unsigned AddrSpace) {
   MCDataFragment *DF = dyn_cast_or_null<MCDataFragment>(getCurrentFragment());
   if (!DF)
     DF = new MCDataFragment(CurSectionData);
   DF->getContents().append(Data.begin(), Data.end());
 }

 void MCMachOStreamer::EmitValue(const MCExpr *Value, unsigned Size,
                                 unsigned AddrSpace) {
   MCDataFragment *DF = dyn_cast_or_null<MCDataFragment>(getCurrentFragment());
   if (!DF)
     DF = new MCDataFragment(CurSectionData);

   // Avoid fixups when possible.
   int64_t AbsValue;
   if (AddValueSymbols(Value)->EvaluateAsAbsolute(AbsValue)) {
     // FIXME: Endianness assumption.
     for (unsigned i = 0; i != Size; ++i)
       DF->getContents().push_back(uint8_t(AbsValue >> (i * 8)));
   } else {
     DF->getFixups().push_back(MCAsmFixup(DF->getContents().size(),
                                          *AddValueSymbols(Value),
                                          MCFixup::getKindForSize(Size)));
     DF->getContents().resize(DF->getContents().size() + Size, 0);
   }
 }

 void MCMachOStreamer::EmitValueToAlignment(unsigned ByteAlignment,
                                            int64_t Value, unsigned ValueSize,
                                            unsigned MaxBytesToEmit) {
   if (MaxBytesToEmit == 0)
     MaxBytesToEmit = ByteAlignment;
   new MCAlignFragment(ByteAlignment, Value, ValueSize, MaxBytesToEmit,
                       false /* EmitNops */, CurSectionData);

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

 void MCMachOStreamer::EmitCodeAlignment(unsigned ByteAlignment,
                                         unsigned MaxBytesToEmit) {
   if (MaxBytesToEmit == 0)
     MaxBytesToEmit = ByteAlignment;
   // FIXME the 0x90 is the default x86 1 byte nop opcode.
   new MCAlignFragment(ByteAlignment, 0x90, 1, MaxBytesToEmit,
                       true /* EmitNops */, CurSectionData);

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

 void MCMachOStreamer::EmitValueToOffset(const MCExpr *Offset,
                                         unsigned char Value) {
   new MCOrgFragment(*Offset, Value, CurSectionData);
 }

 void MCMachOStreamer::EmitInstruction(const MCInst &Inst) {
   // Scan for values.
   for (unsigned i = 0; i != Inst.getNumOperands(); ++i)
     if (Inst.getOperand(i).isExpr())
       AddValueSymbols(Inst.getOperand(i).getExpr());

   if (!Emitter)
     llvm_unreachable("no code emitter available!");

   CurSectionData->setHasInstructions(true);

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

   // Add the fixups and data.
   MCDataFragment *DF = dyn_cast_or_null<MCDataFragment>(getCurrentFragment());
   if (!DF)
     DF = new MCDataFragment(CurSectionData);
   for (unsigned i = 0, e = Fixups.size(); i != e; ++i) {
     MCFixup &F = Fixups[i];
     DF->getFixups().push_back(MCAsmFixup(DF->getContents().size()+F.getOffset(),
                                          *F.getValue(),
                                          F.getKind()));
   }
   DF->getContents().append(Code.begin(), Code.end());
 }

 void MCMachOStreamer::Finish() {
   Assembler.Finish();
 }

 MCStreamer *llvm::createMachOStreamer(MCContext &Context, raw_ostream &OS,
                                       MCCodeEmitter *CE) {
   return new MCMachOStreamer(Context, OS, CE);
 }
	//===- 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/MCSection.h"
	#include "llvm/MC/MCSymbol.h"
	#include "llvm/MC/MCValue.h"
	#include "llvm/Support/ErrorHandling.h"
	#include "llvm/Support/raw_ostream.h"
	using namespace llvm;

	namespace {

	class MCMachOStreamer : public MCStreamer {
	/// SymbolFlags - We store the value for the 'desc' symbol field in the lowest
	/// 16 bits of the implementation defined flags.
	enum SymbolFlags { // See <mach-o/nlist.h>.
	SF_DescFlagsMask = 0xFFFF,

	// Reference type flags.
	SF_ReferenceTypeMask = 0x0007,
	SF_ReferenceTypeUndefinedNonLazy = 0x0000,
	SF_ReferenceTypeUndefinedLazy = 0x0001,
	SF_ReferenceTypeDefined = 0x0002,
	SF_ReferenceTypePrivateDefined = 0x0003,
	SF_ReferenceTypePrivateUndefinedNonLazy = 0x0004,
	SF_ReferenceTypePrivateUndefinedLazy = 0x0005,

	// Other 'desc' flags.
	SF_NoDeadStrip = 0x0020,
	SF_WeakReference = 0x0040,
	SF_WeakDefinition = 0x0080
	};

	private:
	MCAssembler Assembler;
	MCCodeEmitter *Emitter;
	MCSectionData *CurSectionData;
	DenseMap<const MCSection, MCSectionData> SectionMap;
	DenseMap<const MCSymbol, MCSymbolData> SymbolMap;

	private:
	MCFragment *getCurrentFragment() const {
	assert(CurSectionData && "No current section!");

	if (!CurSectionData->empty())
	return &CurSectionData->getFragmentList().back();

	return 0;
	}

	MCSectionData &getSectionData(const MCSection &Section) {
	MCSectionData *&Entry = SectionMap[&Section];

	if (!Entry)
	Entry = new MCSectionData(Section, &Assembler);

	return *Entry;
	}

	MCSymbolData &getSymbolData(const MCSymbol &Symbol) {
	MCSymbolData *&Entry = SymbolMap[&Symbol];

	if (!Entry)
	Entry = new MCSymbolData(Symbol, 0, 0, &Assembler);

	return *Entry;
	}

	public:
	MCMachOStreamer(MCContext &Context, raw_ostream &_OS, MCCodeEmitter *_Emitter)
	: MCStreamer(Context), Assembler(Context, _OS), Emitter(_Emitter),
	CurSectionData(0) {}
	~MCMachOStreamer() {}

	const MCExpr AddValueSymbols(const MCExpr Value) {
	switch (Value->getKind()) {
	case MCExpr::Target: assert(0 && "Can't handle target exprs yet!");
	case MCExpr::Constant:
	break;

	case MCExpr::Binary: {
	const MCBinaryExpr *BE = cast<MCBinaryExpr>(Value);
	AddValueSymbols(BE->getLHS());
	AddValueSymbols(BE->getRHS());
	break;
	}

	case MCExpr::SymbolRef:
	getSymbolData(cast<MCSymbolRefExpr>(Value)->getSymbol());
	break;

	case MCExpr::Unary:
	AddValueSymbols(cast<MCUnaryExpr>(Value)->getSubExpr());
	break;
	}

	return Value;
	}

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

	virtual void SwitchSection(const MCSection *Section);
	virtual void EmitLabel(MCSymbol *Symbol);
	virtual void EmitAssemblerFlag(MCAssemblerFlag Flag);
	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 EmitELFSize(MCSymbol Symbol, const MCExpr Value) {
	assert(0 && "macho doesn't support this directive");
	}
	virtual void EmitLocalCommonSymbol(MCSymbol *Symbol, uint64_t Size) {
	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 EmitBytes(StringRef Data, unsigned AddrSpace);
	virtual void EmitValue(const MCExpr *Value, unsigned Size,unsigned AddrSpace);
	virtual void EmitGPRel32Value(const MCExpr *Value) {
	assert(0 && "macho doesn't support this directive");
	}
	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 EmitValueToOffset(const MCExpr *Offset,
	unsigned char Value = 0);

	virtual void EmitFileDirective(StringRef Filename) {
	errs() << "FIXME: MCMachoStreamer:EmitFileDirective not implemented\n";
	}
	virtual void EmitDwarfFileDirective(unsigned FileNo, StringRef Filename) {
	errs() << "FIXME: MCMachoStreamer:EmitDwarfFileDirective not implemented\n";
	}

	virtual void EmitInstruction(const MCInst &Inst);
	virtual void Finish();

	/// @}
	};

	} // end anonymous namespace.

	void MCMachOStreamer::SwitchSection(const MCSection *Section) {
	assert(Section && "Cannot switch to a null section!");

	// If already in this section, then this is a noop.
	if (Section == CurSection) return;

	CurSection = Section;
	CurSectionData = &getSectionData(*Section);
	}

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

	// FIXME: We should also use offsets into Fill fragments.
	MCDataFragment *F = dyn_cast_or_null<MCDataFragment>(getCurrentFragment());
	if (!F)
	F = new MCDataFragment(CurSectionData);

	MCSymbolData &SD = getSymbolData(*Symbol);
	assert(!SD.getFragment() && "Unexpected fragment on symbol data!");
	SD.setFragment(F);
	SD.setOffset(F->getContents().size());

	// This causes the reference type and weak reference flags to be cleared.
	SD.setFlags(SD.getFlags() & ~(SF_WeakReference \| SF_ReferenceTypeMask));

	Symbol->setSection(*CurSection);
	}

	void MCMachOStreamer::EmitAssemblerFlag(MCAssemblerFlag Flag) {
	switch (Flag) {
	case MCAF_SubsectionsViaSymbols:
	Assembler.setSubsectionsViaSymbols(true);
	return;
	}

	assert(0 && "invalid assembler flag!");
	}

	void MCMachOStreamer::EmitAssignment(MCSymbol Symbol, const MCExpr Value) {
	// Only absolute symbols can be redefined.
	assert((Symbol->isUndefined() \|\| Symbol->isAbsolute()) &&
	"Cannot define a symbol twice!");

	// FIXME: Lift context changes into super class.
	// FIXME: Set associated section.
	Symbol->setValue(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 = CurSectionData;
	Assembler.getIndirectSymbols().push_back(ISD);
	return;
	}

	// Adding a symbol attribute always introduces the symbol, note that an
	// important side effect of calling getSymbolData here is to register the
	// symbol with the assembler.
	MCSymbolData &SD = getSymbolData(*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_IndirectSymbol:
	case MCSA_Hidden:
	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);
	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_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;
	}
	}

	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!");
	getSymbolData(*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 = getSymbolData(*Symbol);
	SD.setExternal(true);
	SD.setCommon(Size, ByteAlignment);
	}

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

	MCFragment *F = new MCZeroFillFragment(Size, ByteAlignment, &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);
	}

	void MCMachOStreamer::EmitBytes(StringRef Data, unsigned AddrSpace) {
	MCDataFragment *DF = dyn_cast_or_null<MCDataFragment>(getCurrentFragment());
	if (!DF)
	DF = new MCDataFragment(CurSectionData);
	DF->getContents().append(Data.begin(), Data.end());
	}

	void MCMachOStreamer::EmitValue(const MCExpr *Value, unsigned Size,
	unsigned AddrSpace) {
	MCDataFragment *DF = dyn_cast_or_null<MCDataFragment>(getCurrentFragment());
	if (!DF)
	DF = new MCDataFragment(CurSectionData);

	// Avoid fixups when possible.
	int64_t AbsValue;
	if (AddValueSymbols(Value)->EvaluateAsAbsolute(AbsValue)) {
	// FIXME: Endianness assumption.
	for (unsigned i = 0; i != Size; ++i)
	DF->getContents().push_back(uint8_t(AbsValue >> (i * 8)));
	} else {
	DF->getFixups().push_back(MCAsmFixup(DF->getContents().size(),
	*AddValueSymbols(Value),
	MCFixup::getKindForSize(Size)));
	DF->getContents().resize(DF->getContents().size() + Size, 0);
	}
	}

	void MCMachOStreamer::EmitValueToAlignment(unsigned ByteAlignment,
	int64_t Value, unsigned ValueSize,
	unsigned MaxBytesToEmit) {
	if (MaxBytesToEmit == 0)
	MaxBytesToEmit = ByteAlignment;
	new MCAlignFragment(ByteAlignment, Value, ValueSize, MaxBytesToEmit,
	false /* EmitNops */, CurSectionData);

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

	void MCMachOStreamer::EmitCodeAlignment(unsigned ByteAlignment,
	unsigned MaxBytesToEmit) {
	if (MaxBytesToEmit == 0)
	MaxBytesToEmit = ByteAlignment;
	// FIXME the 0x90 is the default x86 1 byte nop opcode.
	new MCAlignFragment(ByteAlignment, 0x90, 1, MaxBytesToEmit,
	true /* EmitNops */, CurSectionData);

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

	void MCMachOStreamer::EmitValueToOffset(const MCExpr *Offset,
	unsigned char Value) {
	new MCOrgFragment(*Offset, Value, CurSectionData);
	}

	void MCMachOStreamer::EmitInstruction(const MCInst &Inst) {
	// Scan for values.
	for (unsigned i = 0; i != Inst.getNumOperands(); ++i)
	if (Inst.getOperand(i).isExpr())
	AddValueSymbols(Inst.getOperand(i).getExpr());

	if (!Emitter)
	llvm_unreachable("no code emitter available!");

	CurSectionData->setHasInstructions(true);

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

	// Add the fixups and data.
	MCDataFragment *DF = dyn_cast_or_null<MCDataFragment>(getCurrentFragment());
	if (!DF)
	DF = new MCDataFragment(CurSectionData);
	for (unsigned i = 0, e = Fixups.size(); i != e; ++i) {
	MCFixup &F = Fixups[i];
	DF->getFixups().push_back(MCAsmFixup(DF->getContents().size()+F.getOffset(),
	*F.getValue(),
	F.getKind()));
	}
	DF->getContents().append(Code.begin(), Code.end());
	}

	void MCMachOStreamer::Finish() {
	Assembler.Finish();
	}

	MCStreamer *llvm::createMachOStreamer(MCContext &Context, raw_ostream &OS,
	MCCodeEmitter *CE) {
	return new MCMachOStreamer(Context, OS, CE);
	}