C++ MCValue::getConstant方法代码示例

void X86MachObjectWriter::RecordTLVPRelocation(MachObjectWriter *Writer,
                                               const MCAssembler &Asm,
                                               const MCAsmLayout &Layout,
                                               const MCFragment *Fragment,
                                               const MCFixup &Fixup,
                                               MCValue Target,
                                               uint64_t &FixedValue) {
  assert(Target.getSymA()->getKind() == MCSymbolRefExpr::VK_TLVP &&
         !is64Bit() &&
         "Should only be called with a 32-bit TLVP relocation!");

  unsigned Log2Size = getFixupKindLog2Size(Fixup.getKind());
  uint32_t Value = Layout.getFragmentOffset(Fragment)+Fixup.getOffset();
  unsigned IsPCRel = 0;

  // We're only going to have a second symbol in pic mode and it'll be a
  // subtraction from the picbase. For 32-bit pic the addend is the difference
  // between the picbase and the next address.  For 32-bit static the addend is
  // zero.
  if (Target.getSymB()) {
    // If this is a subtraction then we're pcrel.
    uint32_t FixupAddress =
      Writer->getFragmentAddress(Fragment, Layout) + Fixup.getOffset();
    IsPCRel = 1;
    FixedValue =
        FixupAddress -
        Writer->getSymbolAddress(Target.getSymB()->getSymbol(), Layout) +
        Target.getConstant();
    FixedValue += 1ULL << Log2Size;
  } else {
    FixedValue = 0;
  }

  // struct relocation_info (8 bytes)
  MachO::any_relocation_info MRE;
  MRE.r_word0 = Value;
  MRE.r_word1 =
      (IsPCRel << 24) | (Log2Size << 25) | (MachO::GENERIC_RELOC_TLV << 28);
  Writer->addRelocation(&Target.getSymA()->getSymbol(), Fragment->getParent(),
                        MRE);
}

static bool EvaluateSymbolicAdd(const MCValue &LHS, const MCSymbol *RHS_A, 
                                const MCSymbol *RHS_B, int64_t RHS_Cst,
                                MCValue &Res) {
  // We can't add or subtract two symbols.
  if ((LHS.getSymA() && RHS_A) ||
      (LHS.getSymB() && RHS_B))
    return false;

  const MCSymbol *A = LHS.getSymA() ? LHS.getSymA() : RHS_A;
  const MCSymbol *B = LHS.getSymB() ? LHS.getSymB() : RHS_B;
  if (B) {
    // If we have a negated symbol, then we must have also have a non-negated
    // symbol in order to encode the expression. We can do this check later to
    // permit expressions which eventually fold to a representable form -- such
    // as (a + (0 - b)) -- if necessary.
    if (!A)
      return false;
  }
  Res = MCValue::get(A, B, LHS.getConstant() + RHS_Cst);
  return true;
}

bool MCExpr::EvaluateAsAbsolute(int64_t &Res, const MCAssembler *Asm,
                                const MCAsmLayout *Layout,
                                const SectionAddrMap *Addrs) const {
  MCValue Value;

  // Fast path constants.
  if (const MCConstantExpr *CE = dyn_cast<MCConstantExpr>(this)) {
    Res = CE->getValue();
    return true;
  }

  // FIXME: The use if InSet = Addrs is a hack. Setting InSet causes us
  // absolutize differences across sections and that is what the MachO writer
  // uses Addrs for.
  bool IsRelocatable =
    EvaluateAsRelocatableImpl(Value, Asm, Layout, Addrs, /*InSet*/ Addrs);

  // Record the current value.
  Res = Value.getConstant();

  return IsRelocatable && Value.isAbsolute();
}

static bool getSymbolOffsetImpl(const MCAsmLayout &Layout,
                                const MCSymbolData *SD, bool ReportError,
                                uint64_t &Val) {
  const MCSymbol &S = SD->getSymbol();

  if (!S.isVariable())
    return getLabelOffset(Layout, *SD, ReportError, Val);

  // If SD is a variable, evaluate it.
  MCValue Target;
  if (!S.getVariableValue()->EvaluateAsValue(Target, &Layout, nullptr))
    report_fatal_error("unable to evaluate offset for variable '" +
                       S.getName() + "'");

  uint64_t Offset = Target.getConstant();

  const MCAssembler &Asm = Layout.getAssembler();

  const MCSymbolRefExpr *A = Target.getSymA();
  if (A) {
    uint64_t ValA;
    if (!getLabelOffset(Layout, Asm.getSymbolData(A->getSymbol()), ReportError,
                        ValA))
      return false;
    Offset += ValA;
  }

  const MCSymbolRefExpr *B = Target.getSymB();
  if (B) {
    uint64_t ValB;
    if (!getLabelOffset(Layout, Asm.getSymbolData(B->getSymbol()), ReportError,
                        ValB))
      return false;
    Offset -= ValB;
  }

  Val = Offset;
  return true;
}

uint64_t MachObjectWriter::getSymbolAddress(const MCSymbolData* SD,
                                            const MCAsmLayout &Layout) const {
  const MCSymbol &S = SD->getSymbol();

  // If this is a variable, then recursively evaluate now.
  if (S.isVariable()) {
    if (const MCConstantExpr *C =
          dyn_cast<const MCConstantExpr>(S.getVariableValue()))
      return C->getValue();


    MCValue Target;
    if (!S.getVariableValue()->EvaluateAsRelocatable(Target, Layout))
      report_fatal_error("unable to evaluate offset for variable '" +
                         S.getName() + "'");

    // Verify that any used symbols are defined.
    if (Target.getSymA() && Target.getSymA()->getSymbol().isUndefined())
      report_fatal_error("unable to evaluate offset to undefined symbol '" +
                         Target.getSymA()->getSymbol().getName() + "'");
    if (Target.getSymB() && Target.getSymB()->getSymbol().isUndefined())
      report_fatal_error("unable to evaluate offset to undefined symbol '" +
                         Target.getSymB()->getSymbol().getName() + "'");

    uint64_t Address = Target.getConstant();
    if (Target.getSymA())
      Address += getSymbolAddress(&Layout.getAssembler().getSymbolData(
                                    Target.getSymA()->getSymbol()), Layout);
    if (Target.getSymB())
      Address += getSymbolAddress(&Layout.getAssembler().getSymbolData(
                                    Target.getSymB()->getSymbol()), Layout);
    return Address;
  }

  return getSectionAddress(SD->getFragment()->getParent()) +
    Layout.getSymbolOffset(SD);
}

void SVMMemoryLayout::RecordRelocation(const MCAssembler &Asm,
    const MCAsmLayout &Layout, const MCFragment *Fragment,
    const MCFixup &Fixup, MCValue Target, uint64_t &FixedValue)
{
    SVM::Fixups kind = (SVM::Fixups) Fixup.getKind();
    switch (kind) {

        case SVM::fixup_fnstack: {
            /*
             * Function stack adjustment annotation. The adjustment amount is
             * stored in the first argument of a binary op.
             * (See SVMMCCodeEmitter::EncodeInstruction)
             *
             * We store this offset for later, in our FNStackMap, indexed
             * by the address of the FNStack pseudo-op itself.
             */
            FNStackMap[std::make_pair(Fragment->getParent(),
                Layout.getFragmentOffset(Fragment))] = (int)Target.getConstant();
            break;
        }
    
        default: {
            /*
             * All other fixups are applied later, in ApplyLateFixups().
             * This is necessary because other fixup types can depend on
             * the FNStack values collected above.
             */
            SVMLateFixup LF((MCFragment *) Fragment, Fixup, Target);
            LateFixupList.push_back(LF);

            // This gets OR'ed with the fixup later. Zero it.
            FixedValue = 0;
            break;
        }
    }
}

bool MCAssembler::evaluateFixup(const MCAsmLayout &Layout,
                                const MCFixup &Fixup, const MCFragment *DF,
                                MCValue &Target, uint64_t &Value) const {
  ++stats::evaluateFixup;

  // FIXME: This code has some duplication with recordRelocation. We should
  // probably merge the two into a single callback that tries to evaluate a
  // fixup and records a relocation if one is needed.
  const MCExpr *Expr = Fixup.getValue();
  if (!Expr->evaluateAsRelocatable(Target, &Layout, &Fixup))
    getContext().reportFatalError(Fixup.getLoc(), "expected relocatable expression");

  bool IsPCRel = Backend.getFixupKindInfo(
    Fixup.getKind()).Flags & MCFixupKindInfo::FKF_IsPCRel;

  bool IsResolved;
  if (IsPCRel) {
    if (Target.getSymB()) {
      IsResolved = false;
    } else if (!Target.getSymA()) {
      IsResolved = false;
    } else {
      const MCSymbolRefExpr *A = Target.getSymA();
      const MCSymbol &SA = A->getSymbol();
      if (A->getKind() != MCSymbolRefExpr::VK_None || SA.isUndefined()) {
        IsResolved = false;
      } else {
        IsResolved = getWriter().isSymbolRefDifferenceFullyResolvedImpl(
            *this, SA, *DF, false, true);
      }
    }
  } else {
    IsResolved = Target.isAbsolute();
  }

  Value = Target.getConstant();

  if (const MCSymbolRefExpr *A = Target.getSymA()) {
    const MCSymbol &Sym = A->getSymbol();
    if (Sym.isDefined())
      Value += Layout.getSymbolOffset(Sym);
  }
  if (const MCSymbolRefExpr *B = Target.getSymB()) {
    const MCSymbol &Sym = B->getSymbol();
    if (Sym.isDefined())
      Value -= Layout.getSymbolOffset(Sym);
  }


  bool ShouldAlignPC = Backend.getFixupKindInfo(Fixup.getKind()).Flags &
                         MCFixupKindInfo::FKF_IsAlignedDownTo32Bits;
  assert((ShouldAlignPC ? IsPCRel : true) &&
    "FKF_IsAlignedDownTo32Bits is only allowed on PC-relative fixups!");

  if (IsPCRel) {
    uint32_t Offset = Layout.getFragmentOffset(DF) + Fixup.getOffset();

    // A number of ARM fixups in Thumb mode require that the effective PC
    // address be determined as the 32-bit aligned version of the actual offset.
    if (ShouldAlignPC) Offset &= ~0x3;
    Value -= Offset;
  }

  // Let the backend adjust the fixup value if necessary, including whether
  // we need a relocation.
  Backend.processFixupValue(*this, Layout, Fixup, DF, Target, Value,
                            IsResolved);

  return IsResolved;
}

void WinCOFFObjectWriter::RecordRelocation(const MCAssembler &Asm,
                                           const MCAsmLayout &Layout,
                                           const MCFragment *Fragment,
                                           const MCFixup &Fixup,
                                           MCValue Target,
                                           uint64_t &FixedValue) {
  assert(Target.getSymA() != NULL && "Relocation must reference a symbol!");

  const MCSymbol &Symbol = Target.getSymA()->getSymbol();
  const MCSymbol &A = Symbol.AliasedSymbol();
  MCSymbolData &A_SD = Asm.getSymbolData(A);

  MCSectionData const *SectionData = Fragment->getParent();

  // Mark this symbol as requiring an entry in the symbol table.
  assert(SectionMap.find(&SectionData->getSection()) != SectionMap.end() &&
         "Section must already have been defined in ExecutePostLayoutBinding!");
  assert(SymbolMap.find(&A_SD.getSymbol()) != SymbolMap.end() &&
         "Symbol must already have been defined in ExecutePostLayoutBinding!");

  COFFSection *coff_section = SectionMap[&SectionData->getSection()];
  COFFSymbol *coff_symbol = SymbolMap[&A_SD.getSymbol()];
  const MCSymbolRefExpr *SymA = Target.getSymA();
  const MCSymbolRefExpr *SymB = Target.getSymB();
  const bool CrossSection = SymB &&
    &SymA->getSymbol().getSection() != &SymB->getSymbol().getSection();

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

    // Offset of the symbol in the section
    int64_t a = Layout.getSymbolOffset(&B_SD);

    // Ofeset of the relocation in the section
    int64_t b = Layout.getFragmentOffset(Fragment) + Fixup.getOffset();

    FixedValue = b - a;
    // In the case where we have SymbA and SymB, we just need to store the delta
    // between the two symbols.  Update FixedValue to account for the delta, and
    // skip recording the relocation.
    if (!CrossSection)
      return;
  } else {
    FixedValue = Target.getConstant();
  }

  COFFRelocation Reloc;

  Reloc.Data.SymbolTableIndex = 0;
  Reloc.Data.VirtualAddress = Layout.getFragmentOffset(Fragment);

  // Turn relocations for temporary symbols into section relocations.
  if (coff_symbol->MCData->getSymbol().isTemporary() || CrossSection) {
    Reloc.Symb = coff_symbol->Section->Symbol;
    FixedValue += Layout.getFragmentOffset(coff_symbol->MCData->Fragment)
                + coff_symbol->MCData->getOffset();
  } else
    Reloc.Symb = coff_symbol;

  ++Reloc.Symb->Relocations;

  Reloc.Data.VirtualAddress += Fixup.getOffset();
  Reloc.Data.Type = TargetObjectWriter->getRelocType(Target, Fixup,
                                                     CrossSection);

  // FIXME: Can anyone explain what this does other than adjust for the size
  // of the offset?
  if (Reloc.Data.Type == COFF::IMAGE_REL_AMD64_REL32 ||
      Reloc.Data.Type == COFF::IMAGE_REL_I386_REL32)
    FixedValue += 4;

  coff_section->Relocations.push_back(Reloc);
}

void AArch64MachObjectWriter::RecordRelocation(
    MachObjectWriter *Writer, MCAssembler &Asm, const MCAsmLayout &Layout,
    const MCFragment *Fragment, const MCFixup &Fixup, MCValue Target,
    uint64_t &FixedValue) {
    unsigned IsPCRel = Writer->isFixupKindPCRel(Asm, Fixup.getKind());

    // See <reloc.h>.
    uint32_t FixupOffset = Layout.getFragmentOffset(Fragment);
    unsigned Log2Size = 0;
    int64_t Value = 0;
    unsigned Index = 0;
    unsigned Type = 0;
    unsigned Kind = Fixup.getKind();
    const MCSymbolData *RelSymbol = nullptr;

    FixupOffset += Fixup.getOffset();

    // AArch64 pcrel relocation addends do not include the section offset.
    if (IsPCRel)
        FixedValue += FixupOffset;

    // ADRP fixups use relocations for the whole symbol value and only
    // put the addend in the instruction itself. Clear out any value the
    // generic code figured out from the sybmol definition.
    if (Kind == AArch64::fixup_aarch64_pcrel_adrp_imm21)
        FixedValue = 0;

    // imm19 relocations are for conditional branches, which require
    // assembler local symbols. If we got here, that's not what we have,
    // so complain loudly.
    if (Kind == AArch64::fixup_aarch64_pcrel_branch19) {
        Asm.getContext().FatalError(Fixup.getLoc(),
                                    "conditional branch requires assembler-local"
                                    " label. '" +
                                    Target.getSymA()->getSymbol().getName() +
                                    "' is external.");
        return;
    }

    // 14-bit branch relocations should only target internal labels, and so
    // should never get here.
    if (Kind == AArch64::fixup_aarch64_pcrel_branch14) {
        Asm.getContext().FatalError(Fixup.getLoc(),
                                    "Invalid relocation on conditional branch!");
        return;
    }

    if (!getAArch64FixupKindMachOInfo(Fixup, Type, Target.getSymA(), Log2Size,
                                      Asm)) {
        Asm.getContext().FatalError(Fixup.getLoc(), "unknown AArch64 fixup kind!");
        return;
    }

    Value = Target.getConstant();

    if (Target.isAbsolute()) { // constant
        // FIXME: Should this always be extern?
        // SymbolNum of 0 indicates the absolute section.
        Type = MachO::ARM64_RELOC_UNSIGNED;

        if (IsPCRel) {
            Asm.getContext().FatalError(Fixup.getLoc(),
                                        "PC relative absolute relocation!");

            // FIXME: x86_64 sets the type to a branch reloc here. Should we do
            // something similar?
        }
    } else if (Target.getSymB()) { // A - B + constant
        const MCSymbol *A = &Target.getSymA()->getSymbol();
        const MCSymbolData &A_SD = Asm.getSymbolData(*A);
        const MCSymbolData *A_Base = Asm.getAtom(&A_SD);

        const MCSymbol *B = &Target.getSymB()->getSymbol();
        const MCSymbolData &B_SD = Asm.getSymbolData(*B);
        const MCSymbolData *B_Base = Asm.getAtom(&B_SD);

        // Check for "[email protected] - .", which comes through here as:
        // Ltmp0:
        //    ... [email protected] - Ltmp0
        if (Target.getSymA()->getKind() == MCSymbolRefExpr::VK_GOT &&
                Target.getSymB()->getKind() == MCSymbolRefExpr::VK_None &&
                Layout.getSymbolOffset(&B_SD) ==
                Layout.getFragmentOffset(Fragment) + Fixup.getOffset()) {
            // SymB is the PC, so use a PC-rel pointer-to-GOT relocation.
            Type = MachO::ARM64_RELOC_POINTER_TO_GOT;
            IsPCRel = 1;
            MachO::any_relocation_info MRE;
            MRE.r_word0 = FixupOffset;
            MRE.r_word1 = (IsPCRel << 24) | (Log2Size << 25) | (Type << 28);
            Writer->addRelocation(A_Base, Fragment->getParent(), MRE);
            return;
        } else if (Target.getSymA()->getKind() != MCSymbolRefExpr::VK_None ||
                   Target.getSymB()->getKind() != MCSymbolRefExpr::VK_None)
            // Otherwise, neither symbol can be modified.
            Asm.getContext().FatalError(Fixup.getLoc(),
                                        "unsupported relocation of modified symbol");

        // We don't support PCrel relocations of differences.
        if (IsPCRel)
            Asm.getContext().FatalError(Fixup.getLoc(),
//.........这里部分代码省略.........

bool MCExpr::EvaluateAsRelocatableImpl(MCValue &Res,
                                       const MCAssembler *Asm,
                                       const MCAsmLayout *Layout,
                                       const SectionAddrMap *Addrs,
                                       bool InSet) const {
  ++stats::MCExprEvaluate;

  switch (getKind()) {
  case Target:
    return cast<MCTargetExpr>(this)->EvaluateAsRelocatableImpl(Res, Layout);

  case Constant:
    Res = MCValue::get(cast<MCConstantExpr>(this)->getValue());
    return true;

  case SymbolRef: {
    const MCSymbolRefExpr *SRE = cast<MCSymbolRefExpr>(this);
    const MCSymbol &Sym = SRE->getSymbol();

    // Evaluate recursively if this is a variable.
    if (Sym.isVariable() && SRE->getKind() == MCSymbolRefExpr::VK_None) {
      bool Ret = Sym.getVariableValue()->EvaluateAsRelocatableImpl(Res, Asm,
                                                                   Layout,
                                                                   Addrs,
                                                                   true);
      // If we failed to simplify this to a constant, let the target
      // handle it.
      if (Ret && !Res.getSymA() && !Res.getSymB())
        return true;
    }

    Res = MCValue::get(SRE, 0, 0);
    return true;
  }

  case Unary: {
    const MCUnaryExpr *AUE = cast<MCUnaryExpr>(this);
    MCValue Value;

    if (!AUE->getSubExpr()->EvaluateAsRelocatableImpl(Value, Asm, Layout,
                                                      Addrs, InSet))
      return false;

    switch (AUE->getOpcode()) {
    case MCUnaryExpr::LNot:
      if (!Value.isAbsolute())
        return false;
      Res = MCValue::get(!Value.getConstant());
      break;
    case MCUnaryExpr::Minus:
      /// -(a - b + const) ==> (b - a - const)
      if (Value.getSymA() && !Value.getSymB())
        return false;
      Res = MCValue::get(Value.getSymB(), Value.getSymA(),
                         -Value.getConstant());
      break;
    case MCUnaryExpr::Not:
      if (!Value.isAbsolute())
        return false;
      Res = MCValue::get(~Value.getConstant());
      break;
    case MCUnaryExpr::Plus:
      Res = Value;
      break;
    }

    return true;
  }

  case Binary: {
    const MCBinaryExpr *ABE = cast<MCBinaryExpr>(this);
    MCValue LHSValue, RHSValue;

    if (!ABE->getLHS()->EvaluateAsRelocatableImpl(LHSValue, Asm, Layout,
                                                  Addrs, InSet) ||
        !ABE->getRHS()->EvaluateAsRelocatableImpl(RHSValue, Asm, Layout,
                                                  Addrs, InSet))
      return false;

    // We only support a few operations on non-constant expressions, handle
    // those first.
    if (!LHSValue.isAbsolute() || !RHSValue.isAbsolute()) {
      switch (ABE->getOpcode()) {
      default:
        return false;
      case MCBinaryExpr::Sub:
        // Negate RHS and add.
        return EvaluateSymbolicAdd(Asm, Layout, Addrs, InSet, LHSValue,
                                   RHSValue.getSymB(), RHSValue.getSymA(),
                                   -RHSValue.getConstant(),
                                   Res);

      case MCBinaryExpr::Add:
        return EvaluateSymbolicAdd(Asm, Layout, Addrs, InSet, LHSValue,
                                   RHSValue.getSymA(), RHSValue.getSymB(),
                                   RHSValue.getConstant(),
                                   Res);
      }
    }

//.........这里部分代码省略.........

void WinCOFFObjectWriter::RecordRelocation(const MCAssembler &Asm,
                                           const MCAsmLayout &Layout,
                                           const MCFragment *Fragment,
                                           const MCFixup &Fixup,
                                           MCValue Target,
                                           uint64_t &FixedValue) {
  assert(Target.getSymA() != NULL && "Relocation must reference a symbol!");

  const MCSymbol *A = &Target.getSymA()->getSymbol();
  MCSymbolData &A_SD = Asm.getSymbolData(*A);

  MCSectionData const *SectionData = Fragment->getParent();

  // Mark this symbol as requiring an entry in the symbol table.
  assert(SectionMap.find(&SectionData->getSection()) != SectionMap.end() &&
         "Section must already have been defined in ExecutePostLayoutBinding!");
  assert(SymbolMap.find(&A_SD.getSymbol()) != SymbolMap.end() &&
         "Symbol must already have been defined in ExecutePostLayoutBinding!");

  COFFSection *coff_section = SectionMap[&SectionData->getSection()];
  COFFSymbol *coff_symbol = SymbolMap[&A_SD.getSymbol()];

  if (Target.getSymB()) {
    if (&Target.getSymA()->getSymbol().getSection()
     != &Target.getSymB()->getSymbol().getSection()) {
      llvm_unreachable("Symbol relative relocations are only allowed between "
                       "symbols in the same section");
    }
    const MCSymbol *B = &Target.getSymB()->getSymbol();
    MCSymbolData &B_SD = Asm.getSymbolData(*B);

    FixedValue = Layout.getSymbolOffset(&A_SD) - Layout.getSymbolOffset(&B_SD);

    // In the case where we have SymbA and SymB, we just need to store the delta
    // between the two symbols.  Update FixedValue to account for the delta, and
    // skip recording the relocation.
    return;
  } else {
    FixedValue = Target.getConstant();
  }

  COFFRelocation Reloc;

  Reloc.Data.SymbolTableIndex = 0;
  Reloc.Data.VirtualAddress = Layout.getFragmentOffset(Fragment);

  // Turn relocations for temporary symbols into section relocations.
  if (coff_symbol->MCData->getSymbol().isTemporary()) {
    Reloc.Symb = coff_symbol->Section->Symbol;
    FixedValue += Layout.getFragmentOffset(coff_symbol->MCData->Fragment)
                + coff_symbol->MCData->getOffset();
  } else
    Reloc.Symb = coff_symbol;

  ++Reloc.Symb->Relocations;

  Reloc.Data.VirtualAddress += Fixup.getOffset();

  switch ((unsigned)Fixup.getKind()) {
  case FK_PCRel_4:
  case X86::reloc_riprel_4byte:
  case X86::reloc_riprel_4byte_movq_load:
    Reloc.Data.Type = Is64Bit ? COFF::IMAGE_REL_AMD64_REL32
                              : COFF::IMAGE_REL_I386_REL32;
    // FIXME: Can anyone explain what this does other than adjust for the size
    // of the offset?
    FixedValue += 4;
    break;
  case FK_Data_4:
  case X86::reloc_signed_4byte:
    Reloc.Data.Type = Is64Bit ? COFF::IMAGE_REL_AMD64_ADDR32
                              : COFF::IMAGE_REL_I386_DIR32;
    break;
  case FK_Data_8:
    if (Is64Bit)
      Reloc.Data.Type = COFF::IMAGE_REL_AMD64_ADDR64;
    else
      llvm_unreachable("unsupported relocation type");
    break;
  default:
    llvm_unreachable("unsupported relocation type");
  }

  coff_section->Relocations.push_back(Reloc);
}

bool AsmExpr::EvaluateAsRelocatable(MCContext &Ctx, MCValue &Res) const {
  switch (getKind()) {
  default:
    assert(0 && "Invalid assembly expression kind!");

  case Constant:
    Res = MCValue::get(cast<AsmConstantExpr>(this)->getValue());
    return true;

  case SymbolRef: {
    MCSymbol *Sym = cast<AsmSymbolRefExpr>(this)->getSymbol();
    if (const MCValue *Value = Ctx.GetSymbolValue(Sym))
      Res = *Value;
    else
      Res = MCValue::get(Sym, 0, 0);
    return true;
  }

  case Unary: {
    const AsmUnaryExpr *AUE = cast<AsmUnaryExpr>(this);
    MCValue Value;

    if (!AUE->getSubExpr()->EvaluateAsRelocatable(Ctx, Value))
      return false;

    switch (AUE->getOpcode()) {
    case AsmUnaryExpr::LNot:
      if (!Value.isAbsolute())
        return false;
      Res = MCValue::get(!Value.getConstant());
      break;
    case AsmUnaryExpr::Minus:
      /// -(a - b + const) ==> (b - a - const)
      if (Value.getSymA() && !Value.getSymB())
        return false;
      Res = MCValue::get(Value.getSymB(), Value.getSymA(), 
                         -Value.getConstant()); 
      break;
    case AsmUnaryExpr::Not:
      if (!Value.isAbsolute())
        return false;
      Res = MCValue::get(~Value.getConstant()); 
      break;
    case AsmUnaryExpr::Plus:
      Res = Value;
      break;
    }

    return true;
  }

  case Binary: {
    const AsmBinaryExpr *ABE = cast<AsmBinaryExpr>(this);
    MCValue LHSValue, RHSValue;
    
    if (!ABE->getLHS()->EvaluateAsRelocatable(Ctx, LHSValue) ||
        !ABE->getRHS()->EvaluateAsRelocatable(Ctx, RHSValue))
      return false;

    // We only support a few operations on non-constant expressions, handle
    // those first.
    if (!LHSValue.isAbsolute() || !RHSValue.isAbsolute()) {
      switch (ABE->getOpcode()) {
      default:
        return false;
      case AsmBinaryExpr::Sub:
        // Negate RHS and add.
        return EvaluateSymbolicAdd(LHSValue,
                                   RHSValue.getSymB(), RHSValue.getSymA(),
                                   -RHSValue.getConstant(),
                                   Res);

      case AsmBinaryExpr::Add:
        return EvaluateSymbolicAdd(LHSValue,
                                   RHSValue.getSymA(), RHSValue.getSymB(),
                                   RHSValue.getConstant(),
                                   Res);
      }
    }

    // FIXME: We need target hooks for the evaluation. It may be limited in
    // width, and gas defines the result of comparisons differently from Apple
    // as (the result is sign extended).
    int64_t LHS = LHSValue.getConstant(), RHS = RHSValue.getConstant();
    int64_t Result = 0;
    switch (ABE->getOpcode()) {
    case AsmBinaryExpr::Add:  Result = LHS + RHS; break;
    case AsmBinaryExpr::And:  Result = LHS & RHS; break;
    case AsmBinaryExpr::Div:  Result = LHS / RHS; break;
    case AsmBinaryExpr::EQ:   Result = LHS == RHS; break;
    case AsmBinaryExpr::GT:   Result = LHS > RHS; break;
    case AsmBinaryExpr::GTE:  Result = LHS >= RHS; break;
    case AsmBinaryExpr::LAnd: Result = LHS && RHS; break;
    case AsmBinaryExpr::LOr:  Result = LHS || RHS; break;
    case AsmBinaryExpr::LT:   Result = LHS < RHS; break;
    case AsmBinaryExpr::LTE:  Result = LHS <= RHS; break;
    case AsmBinaryExpr::Mod:  Result = LHS % RHS; break;
    case AsmBinaryExpr::Mul:  Result = LHS * RHS; break;
    case AsmBinaryExpr::NE:   Result = LHS != RHS; break;
    case AsmBinaryExpr::Or:   Result = LHS | RHS; break;
//.........这里部分代码省略.........

void WinCOFFObjectWriter::RecordRelocation(const MCAssembler &Asm,
                                           const MCAsmLayout &Layout,
                                           const MCFragment *Fragment,
                                           const MCFixup &Fixup,
                                           MCValue Target,
                                           bool &IsPCRel,
                                           uint64_t &FixedValue) {
  assert(Target.getSymA() && "Relocation must reference a symbol!");

  const MCSymbol &Symbol = Target.getSymA()->getSymbol();
  const MCSymbol &A = Symbol.AliasedSymbol();
  if (!Asm.hasSymbolData(A))
    Asm.getContext().FatalError(
        Fixup.getLoc(),
        Twine("symbol '") + A.getName() + "' can not be undefined");

  const MCSymbolData &A_SD = Asm.getSymbolData(A);

  MCSectionData const *SectionData = Fragment->getParent();

  // Mark this symbol as requiring an entry in the symbol table.
  assert(SectionMap.find(&SectionData->getSection()) != SectionMap.end() &&
         "Section must already have been defined in ExecutePostLayoutBinding!");
  assert(SymbolMap.find(&A_SD.getSymbol()) != SymbolMap.end() &&
         "Symbol must already have been defined in ExecutePostLayoutBinding!");

  COFFSection *coff_section = SectionMap[&SectionData->getSection()];
  COFFSymbol *coff_symbol = SymbolMap[&A_SD.getSymbol()];
  const MCSymbolRefExpr *SymB = Target.getSymB();
  bool CrossSection = false;

  if (SymB) {
    const MCSymbol *B = &SymB->getSymbol();
    const MCSymbolData &B_SD = Asm.getSymbolData(*B);
    if (!B_SD.getFragment())
      Asm.getContext().FatalError(
          Fixup.getLoc(),
          Twine("symbol '") + B->getName() +
              "' can not be undefined in a subtraction expression");

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

    CrossSection = &Symbol.getSection() != &B->getSection();

    // Offset of the symbol in the section
    int64_t a = Layout.getSymbolOffset(&B_SD);

    // Ofeset of the relocation in the section
    int64_t b = Layout.getFragmentOffset(Fragment) + Fixup.getOffset();

    FixedValue = b - a;
    // In the case where we have SymbA and SymB, we just need to store the delta
    // between the two symbols.  Update FixedValue to account for the delta, and
    // skip recording the relocation.
    if (!CrossSection)
      return;
  } else {
    FixedValue = Target.getConstant();
  }

  COFFRelocation Reloc;

  Reloc.Data.SymbolTableIndex = 0;
  Reloc.Data.VirtualAddress = Layout.getFragmentOffset(Fragment);

  // Turn relocations for temporary symbols into section relocations.
  if (coff_symbol->MCData->getSymbol().isTemporary() || CrossSection) {
    Reloc.Symb = coff_symbol->Section->Symbol;
    FixedValue += Layout.getFragmentOffset(coff_symbol->MCData->Fragment)
                + coff_symbol->MCData->getOffset();
  } else
    Reloc.Symb = coff_symbol;

  ++Reloc.Symb->Relocations;

  Reloc.Data.VirtualAddress += Fixup.getOffset();
  Reloc.Data.Type = TargetObjectWriter->getRelocType(Target, Fixup,
                                                     CrossSection);

  // FIXME: Can anyone explain what this does other than adjust for the size
  // of the offset?
  if ((Header.Machine == COFF::IMAGE_FILE_MACHINE_AMD64 &&
       Reloc.Data.Type == COFF::IMAGE_REL_AMD64_REL32) ||
      (Header.Machine == COFF::IMAGE_FILE_MACHINE_I386 &&
       Reloc.Data.Type == COFF::IMAGE_REL_I386_REL32))
    FixedValue += 4;

  if (Header.Machine == COFF::IMAGE_FILE_MACHINE_ARMNT) {
    switch (Reloc.Data.Type) {
    case COFF::IMAGE_REL_ARM_ABSOLUTE:
    case COFF::IMAGE_REL_ARM_ADDR32:
    case COFF::IMAGE_REL_ARM_ADDR32NB:
    case COFF::IMAGE_REL_ARM_TOKEN:
    case COFF::IMAGE_REL_ARM_SECTION:
    case COFF::IMAGE_REL_ARM_SECREL:
      break;
//.........这里部分代码省略.........

void X86MachObjectWriter::RecordX86Relocation(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 = getFixupKindLog2Size(Fixup.getKind());

  // If this is a 32-bit TLVP reloc it's handled a bit differently.
  if (Target.getSymA() &&
      Target.getSymA()->getKind() == MCSymbolRefExpr::VK_TLVP) {
    RecordTLVPRelocation(Writer, Asm, Layout, Fragment, Fixup, Target,
                         FixedValue);
    return;
  }

  // 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()) {
    RecordScatteredRelocation(Writer, Asm, Layout, Fragment, Fixup,
                              Target, Log2Size, FixedValue);
    return;
  }

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

  // If this is an internal relocation with an offset, it also needs a scattered
  // relocation entry.
  uint32_t Offset = Target.getConstant();
  if (IsPCRel)
    Offset += 1 << Log2Size;
  // Try to record the scattered relocation if needed. Fall back to non
  // scattered if necessary (see comments in RecordScatteredRelocation()
  // for details).
  if (Offset && SD && !Writer->doesSymbolRequireExternRelocation(SD) &&
      RecordScatteredRelocation(Writer, Asm, Layout, Fragment, Fixup,
                                Target, Log2Size, FixedValue))
    return;

  // See <reloc.h>.
  uint32_t FixupOffset = Layout.getFragmentOffset(Fragment)+Fixup.getOffset();
  unsigned Index = 0;
  unsigned Type = 0;
  const MCSymbol *RelSymbol = nullptr;

  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 = MachO::GENERIC_RELOC_VANILLA;
  } 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)) {
      RelSymbol = &SD->getSymbol();
      // 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->getSymbol().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());

    Type = MachO::GENERIC_RELOC_VANILLA;
  }

  // struct relocation_info (8 bytes)
  MachO::any_relocation_info MRE;
  MRE.r_word0 = FixupOffset;
  MRE.r_word1 =
      (Index << 0) | (IsPCRel << 24) | (Log2Size << 25) | (Type << 28);
  Writer->addRelocation(RelSymbol, Fragment->getParent(), MRE);
}

bool MCExpr::EvaluateAsRelocatable(MCValue &Res) const {
  switch (getKind()) {
  case Target:
    return cast<MCTargetExpr>(this)->EvaluateAsRelocatableImpl(Res);
      
  case Constant:
    Res = MCValue::get(cast<MCConstantExpr>(this)->getValue());
    return true;

  case SymbolRef: {
    const MCSymbol &Sym = cast<MCSymbolRefExpr>(this)->getSymbol();

    // Evaluate recursively if this is a variable.
    if (Sym.isVariable())
      return Sym.getValue()->EvaluateAsRelocatable(Res);

    Res = MCValue::get(&Sym, 0, 0);
    return true;
  }

  case Unary: {
    const MCUnaryExpr *AUE = cast<MCUnaryExpr>(this);
    MCValue Value;

    if (!AUE->getSubExpr()->EvaluateAsRelocatable(Value))
      return false;

    switch (AUE->getOpcode()) {
    case MCUnaryExpr::LNot:
      if (!Value.isAbsolute())
        return false;
      Res = MCValue::get(!Value.getConstant());
      break;
    case MCUnaryExpr::Minus:
      /// -(a - b + const) ==> (b - a - const)
      if (Value.getSymA() && !Value.getSymB())
        return false;
      Res = MCValue::get(Value.getSymB(), Value.getSymA(), 
                         -Value.getConstant()); 
      break;
    case MCUnaryExpr::Not:
      if (!Value.isAbsolute())
        return false;
      Res = MCValue::get(~Value.getConstant()); 
      break;
    case MCUnaryExpr::Plus:
      Res = Value;
      break;
    }

    return true;
  }

  case Binary: {
    const MCBinaryExpr *ABE = cast<MCBinaryExpr>(this);
    MCValue LHSValue, RHSValue;
    
    if (!ABE->getLHS()->EvaluateAsRelocatable(LHSValue) ||
        !ABE->getRHS()->EvaluateAsRelocatable(RHSValue))
      return false;

    // We only support a few operations on non-constant expressions, handle
    // those first.
    if (!LHSValue.isAbsolute() || !RHSValue.isAbsolute()) {
      switch (ABE->getOpcode()) {
      default:
        return false;
      case MCBinaryExpr::Sub:
        // Negate RHS and add.
        return EvaluateSymbolicAdd(LHSValue,
                                   RHSValue.getSymB(), RHSValue.getSymA(),
                                   -RHSValue.getConstant(),
                                   Res);

      case MCBinaryExpr::Add:
        return EvaluateSymbolicAdd(LHSValue,
                                   RHSValue.getSymA(), RHSValue.getSymB(),
                                   RHSValue.getConstant(),
                                   Res);
      }
    }

    // FIXME: We need target hooks for the evaluation. It may be limited in
    // width, and gas defines the result of comparisons and right shifts
    // differently from Apple as.
    int64_t LHS = LHSValue.getConstant(), RHS = RHSValue.getConstant();
    int64_t Result = 0;
    switch (ABE->getOpcode()) {
    case MCBinaryExpr::Add:  Result = LHS + RHS; break;
    case MCBinaryExpr::And:  Result = LHS & RHS; break;
    case MCBinaryExpr::Div:  Result = LHS / RHS; break;
    case MCBinaryExpr::EQ:   Result = LHS == RHS; break;
    case MCBinaryExpr::GT:   Result = LHS > RHS; break;
    case MCBinaryExpr::GTE:  Result = LHS >= RHS; break;
    case MCBinaryExpr::LAnd: Result = LHS && RHS; break;
    case MCBinaryExpr::LOr:  Result = LHS || RHS; break;
    case MCBinaryExpr::LT:   Result = LHS < RHS; break;
    case MCBinaryExpr::LTE:  Result = LHS <= RHS; break;
    case MCBinaryExpr::Mod:  Result = LHS % RHS; break;
    case MCBinaryExpr::Mul:  Result = LHS * RHS; break;
//.........这里部分代码省略.........