C++ MCValue类代码示例

const MCSymbol *MCAsmLayout::getBaseSymbol(const MCSymbol &Symbol) const {
  if (!Symbol.isVariable())
    return &Symbol;

  const MCExpr *Expr = Symbol.getVariableValue();
  MCValue Value;
  if (!Expr->evaluateAsValue(Value, *this))
    llvm_unreachable("Invalid Expression");

  const MCSymbolRefExpr *RefB = Value.getSymB();
  if (RefB)
    Assembler.getContext().reportFatalError(
        SMLoc(), Twine("symbol '") + RefB->getSymbol().getName() +
                     "' could not be evaluated in a subtraction expression");

  const MCSymbolRefExpr *A = Value.getSymA();
  if (!A)
    return nullptr;

  const MCSymbol &ASym = A->getSymbol();
  const MCAssembler &Asm = getAssembler();
  if (ASym.isCommon()) {
    // FIXME: we should probably add a SMLoc to MCExpr.
    Asm.getContext().reportFatalError(SMLoc(),
                                "Common symbol " + ASym.getName() +
                                    " cannot be used in assignment expr");
  }

  return &ASym;
}

void AMDGPUAsmBackend::processFixupValue(const MCAssembler &Asm,
                                         const MCAsmLayout &Layout,
                                         const MCFixup &Fixup, const MCFragment *DF,
                                         const MCValue &Target, uint64_t &Value,
                                         bool &IsResolved) {
  MCValue Res;

  // When we have complex expressions like: BB0_1 + (BB0_2 - 4), which are
  // used for long branches, this function will be called with
  // IsResolved = false and Value set to some pre-computed value.  In
  // the example above, the value would be:
  // (BB0_1 + (BB0_2 - 4)) - CurrentOffsetFromStartOfFunction.
  // This is not what we want.  We just want the expression computation
  // only.  The reason the MC layer subtracts the current offset from the
  // expression is because the fixup is of kind FK_PCRel_4.
  // For these scenarios, evaluateAsValue gives us the computation that we
  // want.
  if (!IsResolved && Fixup.getValue()->evaluateAsValue(Res, Layout) &&
      Res.isAbsolute()) {
    Value = Res.getConstant();
    IsResolved = true;

  }
  if (IsResolved)
    Value = adjustFixupValue(Fixup, Value, &Asm.getContext());
}

bool WinCOFFObjectWriter::IsFixupFullyResolved(const MCAssembler &Asm,
                                               const MCValue Target,
                                               bool IsPCRel,
                                               const MCFragment *DF) const {
  // If this is a PCrel relocation, find the section this fixup value is
  // relative to.
  const MCSection *BaseSection = 0;
  if (IsPCRel) {
    BaseSection = &DF->getParent()->getSection();
    assert(BaseSection);
  }

  const MCSection *SectionA = 0;
  const MCSymbol *SymbolA = 0;
  if (const MCSymbolRefExpr *A = Target.getSymA()) {
    SymbolA = &A->getSymbol();
    SectionA = &SymbolA->getSection();
  }

  const MCSection *SectionB = 0;
  if (const MCSymbolRefExpr *B = Target.getSymB()) {
    SectionB = &B->getSymbol().getSection();
  }

  if (!BaseSection)
    return SectionA == SectionB;

  return !SectionB && BaseSection == SectionA;
}

const MCFixup *RISCVMCExpr::getPCRelHiFixup() const {
  MCValue AUIPCLoc;
  if (!getSubExpr()->evaluateAsRelocatable(AUIPCLoc, nullptr, nullptr))
    return nullptr;

  const MCSymbolRefExpr *AUIPCSRE = AUIPCLoc.getSymA();
  if (!AUIPCSRE)
    return nullptr;

  const auto *DF =
      dyn_cast_or_null<MCDataFragment>(AUIPCSRE->findAssociatedFragment());
  if (!DF)
    return nullptr;

  const MCSymbol *AUIPCSymbol = &AUIPCSRE->getSymbol();
  for (const MCFixup &F : DF->getFixups()) {
    if (F.getOffset() != AUIPCSymbol->getOffset())
      continue;

    switch ((unsigned)F.getKind()) {
    default:
      continue;
    case RISCV::fixup_riscv_pcrel_hi20:
      return &F;
    }
  }

  return nullptr;
}

static bool getSymbolOffsetImpl(const MCAsmLayout &Layout, const MCSymbol &S,
                                bool ReportError, uint64_t &Val) {
  if (!S.isVariable())
    return getLabelOffset(Layout, S, ReportError, Val);

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

  uint64_t Offset = Target.getConstant();

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

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

  Val = Offset;
  return true;
}

bool MCAssembler::isThumbFunc(const MCSymbol *Symbol) const {
  if (ThumbFuncs.count(Symbol))
    return true;

  if (!Symbol->isVariable())
    return false;

  const MCExpr *Expr = Symbol->getVariableValue();

  MCValue V;
  if (!Expr->evaluateAsRelocatable(V, nullptr, nullptr))
    return false;

  if (V.getSymB() || V.getRefKind() != MCSymbolRefExpr::VK_None)
    return false;

  const MCSymbolRefExpr *Ref = V.getSymA();
  if (!Ref)
    return false;

  if (Ref->getKind() != MCSymbolRefExpr::VK_None)
    return false;

  const MCSymbol &Sym = Ref->getSymbol();
  if (!isThumbFunc(&Sym))
    return false;

  ThumbFuncs.insert(Symbol); // Cache it.
  return true;
}

unsigned X86WinCOFFObjectWriter::getRelocType(const MCValue &Target,
                                              const MCFixup &Fixup,
                                              bool IsCrossSection,
                                              const MCAsmBackend &MAB) const {
  unsigned FixupKind = IsCrossSection ? FK_PCRel_4 : Fixup.getKind();

  MCSymbolRefExpr::VariantKind Modifier = Target.isAbsolute() ?
    MCSymbolRefExpr::VK_None : Target.getSymA()->getKind();

  if (getMachine() == COFF::IMAGE_FILE_MACHINE_AMD64) {
    switch (FixupKind) {
    case FK_PCRel_4:
    case X86::reloc_riprel_4byte:
    case X86::reloc_riprel_4byte_movq_load:
    case X86::reloc_riprel_4byte_relax:
    case X86::reloc_riprel_4byte_relax_rex:
      return COFF::IMAGE_REL_AMD64_REL32;
    case FK_Data_4:
    case X86::reloc_signed_4byte:
    case X86::reloc_signed_4byte_relax:
      if (Modifier == MCSymbolRefExpr::VK_COFF_IMGREL32)
        return COFF::IMAGE_REL_AMD64_ADDR32NB;
      if (Modifier == MCSymbolRefExpr::VK_SECREL)
        return COFF::IMAGE_REL_AMD64_SECREL;
      return COFF::IMAGE_REL_AMD64_ADDR32;
    case FK_Data_8:
      return COFF::IMAGE_REL_AMD64_ADDR64;
    case FK_SecRel_2:
      return COFF::IMAGE_REL_AMD64_SECTION;
    case FK_SecRel_4:
      return COFF::IMAGE_REL_AMD64_SECREL;
    default:
      llvm_unreachable("unsupported relocation type");
    }
  } else if (getMachine() == COFF::IMAGE_FILE_MACHINE_I386) {
    switch (FixupKind) {
    case FK_PCRel_4:
    case X86::reloc_riprel_4byte:
    case X86::reloc_riprel_4byte_movq_load:
      return COFF::IMAGE_REL_I386_REL32;
    case FK_Data_4:
    case X86::reloc_signed_4byte:
    case X86::reloc_signed_4byte_relax:
      if (Modifier == MCSymbolRefExpr::VK_COFF_IMGREL32)
        return COFF::IMAGE_REL_I386_DIR32NB;
      if (Modifier == MCSymbolRefExpr::VK_SECREL)
        return COFF::IMAGE_REL_AMD64_SECREL;
      return COFF::IMAGE_REL_I386_DIR32;
    case FK_SecRel_2:
      return COFF::IMAGE_REL_I386_SECTION;
    case FK_SecRel_4:
      return COFF::IMAGE_REL_I386_SECREL;
    default:
      llvm_unreachable("unsupported relocation type");
    }
  } else
    llvm_unreachable("Unsupported COFF machine type.");
}

/// NOTE: It is really important to have both the Asm and Layout arguments.
/// They might look redundant, but this function can be used before layout
/// is done (see the object streamer for example) and having the Asm argument
/// lets us avoid relaxations early.
static bool EvaluateSymbolicAdd(const MCAssembler *Asm,
                                const MCAsmLayout *Layout,
                                const SectionAddrMap *Addrs,
                                bool InSet,
                                const MCValue &LHS,const MCSymbolRefExpr *RHS_A,
                                const MCSymbolRefExpr *RHS_B, int64_t RHS_Cst,
                                MCValue &Res) {
  // FIXME: This routine (and other evaluation parts) are *incredibly* sloppy
  // about dealing with modifiers. This will ultimately bite us, one day.
  const MCSymbolRefExpr *LHS_A = LHS.getSymA();
  const MCSymbolRefExpr *LHS_B = LHS.getSymB();
  int64_t LHS_Cst = LHS.getConstant();

  // Fold the result constant immediately.
  int64_t Result_Cst = LHS_Cst + RHS_Cst;

  assert((!Layout || Asm) &&
         "Must have an assembler object if layout is given!");

  // If we have a layout, we can fold resolved differences.
  if (Asm) {
    // First, fold out any differences which are fully resolved. By
    // reassociating terms in
    //   Result = (LHS_A - LHS_B + LHS_Cst) + (RHS_A - RHS_B + RHS_Cst).
    // we have the four possible differences:
    //   (LHS_A - LHS_B),
    //   (LHS_A - RHS_B),
    //   (RHS_A - LHS_B),
    //   (RHS_A - RHS_B).
    // Since we are attempting to be as aggressive as possible about folding, we
    // attempt to evaluate each possible alternative.
    AttemptToFoldSymbolOffsetDifference(Asm, Layout, Addrs, InSet, LHS_A, LHS_B,
                                        Result_Cst);
    AttemptToFoldSymbolOffsetDifference(Asm, Layout, Addrs, InSet, LHS_A, RHS_B,
                                        Result_Cst);
    AttemptToFoldSymbolOffsetDifference(Asm, Layout, Addrs, InSet, RHS_A, LHS_B,
                                        Result_Cst);
    AttemptToFoldSymbolOffsetDifference(Asm, Layout, Addrs, InSet, RHS_A, RHS_B,
                                        Result_Cst);
  }

  // We can't represent the addition or subtraction of two symbols.
  if ((LHS_A && RHS_A) || (LHS_B && RHS_B))
    return false;

  // At this point, we have at most one additive symbol and one subtractive
  // symbol -- find them.
  const MCSymbolRefExpr *A = LHS_A ? LHS_A : RHS_A;
  const MCSymbolRefExpr *B = LHS_B ? LHS_B : RHS_B;

  // If we have a negated symbol, then we must have also have a non-negated
  // symbol in order to encode the expression.
  if (B && !A)
    return false;

  Res = MCValue::get(A, B, Result_Cst);
  return true;
}

bool AsmExpr::EvaluateAsAbsolute(MCContext &Ctx, int64_t &Res) const {
  MCValue Value;
  
  if (!EvaluateAsRelocatable(Ctx, Value) || !Value.isAbsolute())
    return false;

  Res = Value.getConstant();
  return true;
}

bool MCExpr::EvaluateAsAbsolute(int64_t &Res) const {
  MCValue Value;
  
  if (!EvaluateAsRelocatable(Value) || !Value.isAbsolute())
    return false;

  Res = Value.getConstant();
  return true;
}

bool AArch64MCExpr::EvaluateAsRelocatableImpl(MCValue &Res,
                                            const MCAsmLayout *Layout,
					    const MCFixup *Fixup) const {
  if (!getSubExpr()->EvaluateAsRelocatable(Res, Layout, Fixup))
    return false;

  Res =
      MCValue::get(Res.getSymA(), Res.getSymB(), Res.getConstant(), getKind());

  return true;
}

bool
PPCMCExpr::evaluateAsConstant(int64_t &Res) const {
  MCValue Value;

  if (!getSubExpr()->evaluateAsRelocatable(Value, nullptr, nullptr))
    return false;

  if (!Value.isAbsolute())
    return false;

  Res = evaluateAsInt64(Value.getConstant());
  return true;
}

static MCSymbolRefExpr::VariantKind getAccessVariant(const MCValue &Target,
                                                     const MCFixup &Fixup) {
  const MCExpr *Expr = Fixup.getValue();

  if (Expr->getKind() != MCExpr::Target)
    return Target.getAccessVariant();

  switch (cast<PPCMCExpr>(Expr)->getKind()) {
  case PPCMCExpr::VK_PPC_None:
    return MCSymbolRefExpr::VK_None;
  case PPCMCExpr::VK_PPC_LO:
    return MCSymbolRefExpr::VK_PPC_LO;
  case PPCMCExpr::VK_PPC_HI:
    return MCSymbolRefExpr::VK_PPC_HI;
  case PPCMCExpr::VK_PPC_HA:
    return MCSymbolRefExpr::VK_PPC_HA;
  case PPCMCExpr::VK_PPC_HIGHERA:
    return MCSymbolRefExpr::VK_PPC_HIGHERA;
  case PPCMCExpr::VK_PPC_HIGHER:
    return MCSymbolRefExpr::VK_PPC_HIGHER;
  case PPCMCExpr::VK_PPC_HIGHEST:
    return MCSymbolRefExpr::VK_PPC_HIGHEST;
  case PPCMCExpr::VK_PPC_HIGHESTA:
    return MCSymbolRefExpr::VK_PPC_HIGHESTA;
  }
  llvm_unreachable("unknown PPCMCExpr kind");
}

bool RISCVMCExpr::evaluateAsConstant(int64_t &Res) const {
  MCValue Value;

  if (Kind == VK_RISCV_PCREL_HI || Kind == VK_RISCV_PCREL_LO ||
      Kind == VK_RISCV_CALL)
    return false;

  if (!getSubExpr()->evaluateAsRelocatable(Value, nullptr, nullptr))
    return false;

  if (!Value.isAbsolute())
    return false;

  Res = evaluateAsInt64(Value.getConstant());
  return true;
}

void MachObjectWriter::markAbsoluteVariableSymbols(MCAssembler &Asm,
                                                   const MCAsmLayout &Layout) {
  for (MCSymbolData &SD : Asm.symbols()) {
    if (!SD.getSymbol().isVariable())
      continue;

    // Is the variable is a symbol difference (SA - SB + C) expression,
    // and neither symbol is external, mark the variable as absolute.
    const MCExpr *Expr = SD.getSymbol().getVariableValue();
    MCValue Value;
    if (Expr->EvaluateAsRelocatable(Value, &Layout, nullptr)) {
      if (Value.getSymA() && Value.getSymB())
        const_cast<MCSymbol*>(&SD.getSymbol())->setAbsolute();
    }
  }
}