lib/Target/ARM/MCTargetDesc/ARMMachObjectWriter.cpp - llvm - Git at Google

 //===-- ARMMachObjectWriter.cpp - ARM Mach Object Writer ------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//

 #include "MCTargetDesc/ARMBaseInfo.h"
 #include "MCTargetDesc/ARMFixupKinds.h"
 #include "llvm/ADT/Twine.h"
 #include "llvm/MC/MCAssembler.h"
 #include "llvm/MC/MCAsmLayout.h"
 #include "llvm/MC/MCMachObjectWriter.h"
 #include "llvm/MC/MCContext.h"
 #include "llvm/MC/MCExpr.h"
 #include "llvm/MC/MCFixup.h"
 #include "llvm/MC/MCFixupKindInfo.h"
 #include "llvm/MC/MCMachOSymbolFlags.h"
 #include "llvm/MC/MCValue.h"
 #include "llvm/Object/MachOFormat.h"
 #include "llvm/Support/ErrorHandling.h"
 using namespace llvm;
 using namespace llvm::object;

 namespace {
 class ARMMachObjectWriter : public MCMachObjectTargetWriter {
   void RecordARMScatteredRelocation(MachObjectWriter *Writer,
                                     const MCAssembler &Asm,
                                     const MCAsmLayout &Layout,
                                     const MCFragment *Fragment,
                                     const MCFixup &Fixup,
                                     MCValue Target,
                                     unsigned Log2Size,
                                     uint64_t &FixedValue);
   void RecordARMScatteredHalfRelocation(MachObjectWriter *Writer,
                                         const MCAssembler &Asm,
                                         const MCAsmLayout &Layout,
                                         const MCFragment *Fragment,
                                         const MCFixup &Fixup, MCValue Target,
                                         uint64_t &FixedValue);

 public:
   ARMMachObjectWriter(bool Is64Bit, uint32_t CPUType,
                       uint32_t CPUSubtype)
     : MCMachObjectTargetWriter(Is64Bit, CPUType, CPUSubtype,
                                /*UseAggressiveSymbolFolding=*/true) {}

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

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

   switch (Kind) {
   default:
     return false;

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

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

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

   case ARM::fixup_t2_uncondbranch:
   case ARM::fixup_arm_thumb_bl:
   case ARM::fixup_arm_thumb_blx:
     RelocType = unsigned(macho::RIT_ARM_ThumbBranch22Bit);
     Log2Size = llvm::Log2_32(4);
     return true;

   // For movw/movt r_type relocations they always have a pair following them and
   // the r_length bits are used differently.  The encoding of the r_length is as
   // follows:
   //   low bit of r_length:
   //      0 - :lower16: for movw instructions
   //      1 - :upper16: for movt instructions
   //   high bit of r_length:
   //      0 - arm instructions
   //      1 - thumb instructions
   case ARM::fixup_arm_movt_hi16:
   case ARM::fixup_arm_movt_hi16_pcrel:
     RelocType = unsigned(macho::RIT_ARM_Half);
     Log2Size = 1;
     return true;
   case ARM::fixup_t2_movt_hi16:
   case ARM::fixup_t2_movt_hi16_pcrel:
     RelocType = unsigned(macho::RIT_ARM_Half);
     Log2Size = 3;
     return true;

   case ARM::fixup_arm_movw_lo16:
   case ARM::fixup_arm_movw_lo16_pcrel:
     RelocType = unsigned(macho::RIT_ARM_Half);
     Log2Size = 0;
     return true;
   case ARM::fixup_t2_movw_lo16:
   case ARM::fixup_t2_movw_lo16_pcrel:
     RelocType = unsigned(macho::RIT_ARM_Half);
     Log2Size = 2;
     return true;
   }
 }

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

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

   if (!A_SD->getFragment())
     Asm.getContext().FatalError(Fixup.getLoc(),
                        "symbol '" + A->getName() +
                        "' can not be undefined in a subtraction expression");

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

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

     if (!B_SD->getFragment())
       Asm.getContext().FatalError(Fixup.getLoc(),
                          "symbol '" + B->getSymbol().getName() +
                          "' can not be undefined in a subtraction expression");

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

   // Relocations are written out in reverse order, so the PAIR comes first.
   // ARM_RELOC_HALF and ARM_RELOC_HALF_SECTDIFF abuse the r_length field:
   //
   // For these two r_type relocations they always have a pair following them and
   // the r_length bits are used differently.  The encoding of the r_length is as
   // follows:
   //   low bit of r_length:
   //      0 - :lower16: for movw instructions
   //      1 - :upper16: for movt instructions
   //   high bit of r_length:
   //      0 - arm instructions
   //      1 - thumb instructions
   // the other half of the relocated expression is in the following pair
   // relocation entry in the the low 16 bits of r_address field.
   unsigned ThumbBit = 0;
   unsigned MovtBit = 0;
   switch ((unsigned)Fixup.getKind()) {
   default: break;
   case ARM::fixup_arm_movt_hi16:
   case ARM::fixup_arm_movt_hi16_pcrel:
     MovtBit = 1;
     // The thumb bit shouldn't be set in the 'other-half' bit of the
     // relocation, but it will be set in FixedValue if the base symbol
     // is a thumb function. Clear it out here.
     if (A_SD->getFlags() & SF_ThumbFunc)
       FixedValue &= 0xfffffffe;
     break;
   case ARM::fixup_t2_movt_hi16:
   case ARM::fixup_t2_movt_hi16_pcrel:
     if (A_SD->getFlags() & SF_ThumbFunc)
       FixedValue &= 0xfffffffe;
     MovtBit = 1;
     // Fallthrough
   case ARM::fixup_t2_movw_lo16:
   case ARM::fixup_t2_movw_lo16_pcrel:
     ThumbBit = 1;
     break;
   }

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

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

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

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

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

   if (!A_SD->getFragment())
     Asm.getContext().FatalError(Fixup.getLoc(),
                        "symbol '" + A->getName() +
                        "' can not be undefined in a subtraction expression");

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

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

     if (!B_SD->getFragment())
       Asm.getContext().FatalError(Fixup.getLoc(),
                          "symbol '" + B->getSymbol().getName() +
                          "' can not be undefined in a subtraction expression");

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

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

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

 void ARMMachObjectWriter::RecordRelocation(MachObjectWriter *Writer,
                                            const MCAssembler &Asm,
                                            const MCAsmLayout &Layout,
                                            const MCFragment *Fragment,
                                            const MCFixup &Fixup,
                                            MCValue Target,
                                            uint64_t &FixedValue) {
   unsigned IsPCRel = Writer->isFixupKindPCRel(Asm, Fixup.getKind());
   unsigned Log2Size;
   unsigned RelocType = macho::RIT_Vanilla;
   if (!getARMFixupKindMachOInfo(Fixup.getKind(), RelocType, Log2Size))
     // If we failed to get fixup kind info, it's because there's no legal
     // relocation type for the fixup kind. This happens when it's a fixup that's
     // expected to always be resolvable at assembly time and not have any
     // relocations needed.
     Asm.getContext().FatalError(Fixup.getLoc(),
                                 "unsupported relocation on symbol");

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

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

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

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

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

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

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

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

   // struct relocation_info (8 bytes)
   macho::RelocationEntry MRE;
   MRE.Word0 = FixupOffset;
   MRE.Word1 = ((Index     <<  0) |
                (IsPCRel   << 24) |
                (Log2Size  << 25) |
                (IsExtern  << 27) |
                (Type      << 28));

   // Even when it's not a scattered relocation, movw/movt always uses
   // a PAIR relocation.
   if (Type == macho::RIT_ARM_Half) {
     // The other-half value only gets populated for the movt relocation.
     uint32_t Value = 0;;
     switch ((unsigned)Fixup.getKind()) {
     default: break;
     case ARM::fixup_arm_movt_hi16:
     case ARM::fixup_arm_movt_hi16_pcrel:
     case ARM::fixup_t2_movt_hi16:
     case ARM::fixup_t2_movt_hi16_pcrel:
       Value = FixedValue;
       break;
     }
     macho::RelocationEntry MREPair;
     MREPair.Word0 = Value;
     MREPair.Word1 = ((0xffffff) |
                      (Log2Size << 25) |
                      (macho::RIT_Pair << 28));

     Writer->addRelocation(Fragment->getParent(), MREPair);
   }

   Writer->addRelocation(Fragment->getParent(), MRE);
 }

 MCObjectWriter *llvm::createARMMachObjectWriter(raw_ostream &OS,
                                                 bool Is64Bit,
                                                 uint32_t CPUType,
                                                 uint32_t CPUSubtype) {
   return createMachObjectWriter(new ARMMachObjectWriter(Is64Bit,
                                                         CPUType,
                                                         CPUSubtype),
                                 OS, /*IsLittleEndian=*/true);
 }
	//===-- ARMMachObjectWriter.cpp - ARM Mach Object Writer ------------------===//
	//
	// The LLVM Compiler Infrastructure
	//
	// This file is distributed under the University of Illinois Open Source
	// License. See LICENSE.TXT for details.
	//
	//===----------------------------------------------------------------------===//

	#include "MCTargetDesc/ARMBaseInfo.h"
	#include "MCTargetDesc/ARMFixupKinds.h"
	#include "llvm/ADT/Twine.h"
	#include "llvm/MC/MCAssembler.h"
	#include "llvm/MC/MCAsmLayout.h"
	#include "llvm/MC/MCMachObjectWriter.h"
	#include "llvm/MC/MCContext.h"
	#include "llvm/MC/MCExpr.h"
	#include "llvm/MC/MCFixup.h"
	#include "llvm/MC/MCFixupKindInfo.h"
	#include "llvm/MC/MCMachOSymbolFlags.h"
	#include "llvm/MC/MCValue.h"
	#include "llvm/Object/MachOFormat.h"
	#include "llvm/Support/ErrorHandling.h"
	using namespace llvm;
	using namespace llvm::object;

	namespace {
	class ARMMachObjectWriter : public MCMachObjectTargetWriter {
	void RecordARMScatteredRelocation(MachObjectWriter *Writer,
	const MCAssembler &Asm,
	const MCAsmLayout &Layout,
	const MCFragment *Fragment,
	const MCFixup &Fixup,
	MCValue Target,
	unsigned Log2Size,
	uint64_t &FixedValue);
	void RecordARMScatteredHalfRelocation(MachObjectWriter *Writer,
	const MCAssembler &Asm,
	const MCAsmLayout &Layout,
	const MCFragment *Fragment,
	const MCFixup &Fixup, MCValue Target,
	uint64_t &FixedValue);

	public:
	ARMMachObjectWriter(bool Is64Bit, uint32_t CPUType,
	uint32_t CPUSubtype)
	: MCMachObjectTargetWriter(Is64Bit, CPUType, CPUSubtype,
	/UseAggressiveSymbolFolding=/true) {}

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

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

	switch (Kind) {
	default:
	return false;

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

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

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

	case ARM::fixup_t2_uncondbranch:
	case ARM::fixup_arm_thumb_bl:
	case ARM::fixup_arm_thumb_blx:
	RelocType = unsigned(macho::RIT_ARM_ThumbBranch22Bit);
	Log2Size = llvm::Log2_32(4);
	return true;

	// For movw/movt r_type relocations they always have a pair following them and
	// the r_length bits are used differently. The encoding of the r_length is as
	// follows:
	// low bit of r_length:
	// 0 - :lower16: for movw instructions
	// 1 - :upper16: for movt instructions
	// high bit of r_length:
	// 0 - arm instructions
	// 1 - thumb instructions
	case ARM::fixup_arm_movt_hi16:
	case ARM::fixup_arm_movt_hi16_pcrel:
	RelocType = unsigned(macho::RIT_ARM_Half);
	Log2Size = 1;
	return true;
	case ARM::fixup_t2_movt_hi16:
	case ARM::fixup_t2_movt_hi16_pcrel:
	RelocType = unsigned(macho::RIT_ARM_Half);
	Log2Size = 3;
	return true;

	case ARM::fixup_arm_movw_lo16:
	case ARM::fixup_arm_movw_lo16_pcrel:
	RelocType = unsigned(macho::RIT_ARM_Half);
	Log2Size = 0;
	return true;
	case ARM::fixup_t2_movw_lo16:
	case ARM::fixup_t2_movw_lo16_pcrel:
	RelocType = unsigned(macho::RIT_ARM_Half);
	Log2Size = 2;
	return true;
	}
	}

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

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

	if (!A_SD->getFragment())
	Asm.getContext().FatalError(Fixup.getLoc(),
	"symbol '" + A->getName() +
	"' can not be undefined in a subtraction expression");

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

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

	if (!B_SD->getFragment())
	Asm.getContext().FatalError(Fixup.getLoc(),
	"symbol '" + B->getSymbol().getName() +
	"' can not be undefined in a subtraction expression");

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

	// Relocations are written out in reverse order, so the PAIR comes first.
	// ARM_RELOC_HALF and ARM_RELOC_HALF_SECTDIFF abuse the r_length field:
	//
	// For these two r_type relocations they always have a pair following them and
	// the r_length bits are used differently. The encoding of the r_length is as
	// follows:
	// low bit of r_length:
	// 0 - :lower16: for movw instructions
	// 1 - :upper16: for movt instructions
	// high bit of r_length:
	// 0 - arm instructions
	// 1 - thumb instructions
	// the other half of the relocated expression is in the following pair
	// relocation entry in the the low 16 bits of r_address field.
	unsigned ThumbBit = 0;
	unsigned MovtBit = 0;
	switch ((unsigned)Fixup.getKind()) {
	default: break;
	case ARM::fixup_arm_movt_hi16:
	case ARM::fixup_arm_movt_hi16_pcrel:
	MovtBit = 1;
	// The thumb bit shouldn't be set in the 'other-half' bit of the
	// relocation, but it will be set in FixedValue if the base symbol
	// is a thumb function. Clear it out here.
	if (A_SD->getFlags() & SF_ThumbFunc)
	FixedValue &= 0xfffffffe;
	break;
	case ARM::fixup_t2_movt_hi16:
	case ARM::fixup_t2_movt_hi16_pcrel:
	if (A_SD->getFlags() & SF_ThumbFunc)
	FixedValue &= 0xfffffffe;
	MovtBit = 1;
	// Fallthrough
	case ARM::fixup_t2_movw_lo16:
	case ARM::fixup_t2_movw_lo16_pcrel:
	ThumbBit = 1;
	break;
	}

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

	macho::RelocationEntry MRE;
	MRE.Word0 = ((OtherHalf << 0) \|
	(macho::RIT_Pair << 24) \|
	(MovtBit << 28) \|
	(ThumbBit << 29) \|
	(IsPCRel << 30) \|
	macho::RF_Scattered);
	MRE.Word1 = Value2;
	Writer->addRelocation(Fragment->getParent(), MRE);
	}

	macho::RelocationEntry MRE;
	MRE.Word0 = ((FixupOffset << 0) \|
	(Type << 24) \|
	(MovtBit << 28) \|
	(ThumbBit << 29) \|
	(IsPCRel << 30) \|
	macho::RF_Scattered);
	MRE.Word1 = Value;
	Writer->addRelocation(Fragment->getParent(), MRE);
	}

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

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

	if (!A_SD->getFragment())
	Asm.getContext().FatalError(Fixup.getLoc(),
	"symbol '" + A->getName() +
	"' can not be undefined in a subtraction expression");

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

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

	if (!B_SD->getFragment())
	Asm.getContext().FatalError(Fixup.getLoc(),
	"symbol '" + B->getSymbol().getName() +
	"' can not be undefined in a subtraction expression");

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

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

	macho::RelocationEntry MRE;
	MRE.Word0 = ((FixupOffset << 0) \|
	(Type << 24) \|
	(Log2Size << 28) \|
	(IsPCRel << 30) \|
	macho::RF_Scattered);
	MRE.Word1 = Value;
	Writer->addRelocation(Fragment->getParent(), MRE);
	}

	void ARMMachObjectWriter::RecordRelocation(MachObjectWriter *Writer,
	const MCAssembler &Asm,
	const MCAsmLayout &Layout,
	const MCFragment *Fragment,
	const MCFixup &Fixup,
	MCValue Target,
	uint64_t &FixedValue) {
	unsigned IsPCRel = Writer->isFixupKindPCRel(Asm, Fixup.getKind());
	unsigned Log2Size;
	unsigned RelocType = macho::RIT_Vanilla;
	if (!getARMFixupKindMachOInfo(Fixup.getKind(), RelocType, Log2Size))
	// If we failed to get fixup kind info, it's because there's no legal
	// relocation type for the fixup kind. This happens when it's a fixup that's
	// expected to always be resolvable at assembly time and not have any
	// relocations needed.
	Asm.getContext().FatalError(Fixup.getLoc(),
	"unsupported relocation on symbol");

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

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

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

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

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

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

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

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

	// struct relocation_info (8 bytes)
	macho::RelocationEntry MRE;
	MRE.Word0 = FixupOffset;
	MRE.Word1 = ((Index << 0) \|
	(IsPCRel << 24) \|
	(Log2Size << 25) \|
	(IsExtern << 27) \|
	(Type << 28));

	// Even when it's not a scattered relocation, movw/movt always uses
	// a PAIR relocation.
	if (Type == macho::RIT_ARM_Half) {
	// The other-half value only gets populated for the movt relocation.
	uint32_t Value = 0;;
	switch ((unsigned)Fixup.getKind()) {
	default: break;
	case ARM::fixup_arm_movt_hi16:
	case ARM::fixup_arm_movt_hi16_pcrel:
	case ARM::fixup_t2_movt_hi16:
	case ARM::fixup_t2_movt_hi16_pcrel:
	Value = FixedValue;
	break;
	}
	macho::RelocationEntry MREPair;
	MREPair.Word0 = Value;
	MREPair.Word1 = ((0xffffff) \|
	(Log2Size << 25) \|
	(macho::RIT_Pair << 28));

	Writer->addRelocation(Fragment->getParent(), MREPair);
	}

	Writer->addRelocation(Fragment->getParent(), MRE);
	}

	MCObjectWriter *llvm::createARMMachObjectWriter(raw_ostream &OS,
	bool Is64Bit,
	uint32_t CPUType,
	uint32_t CPUSubtype) {
	return createMachObjectWriter(new ARMMachObjectWriter(Is64Bit,
	CPUType,
	CPUSubtype),
	OS, /IsLittleEndian=/true);
	}