lib/CodeGen/CGRTTI.cpp

//===--- CGCXXRTTI.cpp - Emit LLVM Code for C++ RTTI descriptors ----------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This contains code dealing with C++ code generation of RTTI descriptors.
//
//===----------------------------------------------------------------------===//

#include "CodeGenModule.h"
#include "CGCXXABI.h"
#include "clang/AST/RecordLayout.h"
#include "clang/AST/Type.h"
#include "clang/Frontend/CodeGenOptions.h"
#include "CGObjCRuntime.h"

using namespace clang;
using namespace CodeGen;

namespace {
class RTTIBuilder {
  CodeGenModule &CGM;  // Per-module state.
  llvm::LLVMContext &VMContext;

  llvm::Type *Int8PtrTy;

  /// Fields - The fields of the RTTI descriptor currently being built.
  SmallVector<llvm::Constant *, 16> Fields;

  /// GetAddrOfTypeName - Returns the mangled type name of the given type.
  llvm::GlobalVariable *
  GetAddrOfTypeName(QualType Ty, llvm::GlobalVariable::LinkageTypes Linkage);

  /// GetAddrOfExternalRTTIDescriptor - Returns the constant for the RTTI
  /// descriptor of the given type.
  llvm::Constant *GetAddrOfExternalRTTIDescriptor(QualType Ty);

  /// BuildVTablePointer - Build the vtable pointer for the given type.
  void BuildVTablePointer(const Type *Ty);

  /// BuildSIClassTypeInfo - Build an abi::__si_class_type_info, used for single
  /// inheritance, according to the Itanium C++ ABI, 2.9.5p6b.
  void BuildSIClassTypeInfo(const CXXRecordDecl *RD);

  /// BuildVMIClassTypeInfo - Build an abi::__vmi_class_type_info, used for
  /// classes with bases that do not satisfy the abi::__si_class_type_info
  /// constraints, according ti the Itanium C++ ABI, 2.9.5p5c.
  void BuildVMIClassTypeInfo(const CXXRecordDecl *RD);

  /// BuildPointerTypeInfo - Build an abi::__pointer_type_info struct, used
  /// for pointer types.
  void BuildPointerTypeInfo(QualType PointeeTy);

  /// BuildObjCObjectTypeInfo - Build the appropriate kind of
  /// type_info for an object type.
  void BuildObjCObjectTypeInfo(const ObjCObjectType *Ty);

  /// BuildPointerToMemberTypeInfo - Build an abi::__pointer_to_member_type_info
  /// struct, used for member pointer types.
  void BuildPointerToMemberTypeInfo(const MemberPointerType *Ty);

public:
  RTTIBuilder(CodeGenModule &CGM) : CGM(CGM),
    VMContext(CGM.getModule().getContext()),
    Int8PtrTy(llvm::Type::getInt8PtrTy(VMContext)) { }

  // Pointer type info flags.
  enum {
    /// PTI_Const - Type has const qualifier.
    PTI_Const = 0x1,

    /// PTI_Volatile - Type has volatile qualifier.
    PTI_Volatile = 0x2,

    /// PTI_Restrict - Type has restrict qualifier.
    PTI_Restrict = 0x4,

    /// PTI_Incomplete - Type is incomplete.
    PTI_Incomplete = 0x8,

    /// PTI_ContainingClassIncomplete - Containing class is incomplete.
    /// (in pointer to member).
    PTI_ContainingClassIncomplete = 0x10
  };

  // VMI type info flags.
  enum {
    /// VMI_NonDiamondRepeat - Class has non-diamond repeated inheritance.
    VMI_NonDiamondRepeat = 0x1,

    /// VMI_DiamondShaped - Class is diamond shaped.
    VMI_DiamondShaped = 0x2
  };

  // Base class type info flags.
  enum {
    /// BCTI_Virtual - Base class is virtual.
    BCTI_Virtual = 0x1,

    /// BCTI_Public - Base class is public.
    BCTI_Public = 0x2
  };

  /// BuildTypeInfo - Build the RTTI type info struct for the given type.
  ///
  /// \param Force - true to force the creation of this RTTI value
  /// \param ForEH - true if this is for exception handling
  llvm::Constant *BuildTypeInfo(QualType Ty, bool Force = false);
};
}

llvm::GlobalVariable *
RTTIBuilder::GetAddrOfTypeName(QualType Ty,
                               llvm::GlobalVariable::LinkageTypes Linkage) {
  llvm::SmallString<256> OutName;
  llvm::raw_svector_ostream Out(OutName);
  CGM.getCXXABI().getMangleContext().mangleCXXRTTIName(Ty, Out);
  Out.flush();
  StringRef Name = OutName.str();

  // We know that the mangled name of the type starts at index 4 of the
  // mangled name of the typename, so we can just index into it in order to
  // get the mangled name of the type.
  llvm::Constant *Init = llvm::ConstantArray::get(VMContext, Name.substr(4));

  llvm::GlobalVariable *GV =
    CGM.CreateOrReplaceCXXRuntimeVariable(Name, Init->getType(), Linkage);

  GV->setInitializer(Init);

  return GV;
}

llvm::Constant *RTTIBuilder::GetAddrOfExternalRTTIDescriptor(QualType Ty) {
  // Mangle the RTTI name.
  llvm::SmallString<256> OutName;
  llvm::raw_svector_ostream Out(OutName);
  CGM.getCXXABI().getMangleContext().mangleCXXRTTI(Ty, Out);
  Out.flush();
  StringRef Name = OutName.str();

  // Look for an existing global.
  llvm::GlobalVariable *GV = CGM.getModule().getNamedGlobal(Name);

  if (!GV) {
    // Create a new global variable.
    GV = new llvm::GlobalVariable(CGM.getModule(), Int8PtrTy, /*Constant=*/true,
                                  llvm::GlobalValue::ExternalLinkage, 0, Name);
  }

  return llvm::ConstantExpr::getBitCast(GV, Int8PtrTy);
}

/// TypeInfoIsInStandardLibrary - Given a builtin type, returns whether the type
/// info for that type is defined in the standard library.
static bool TypeInfoIsInStandardLibrary(const BuiltinType *Ty) {
  // Itanium C++ ABI 2.9.2:
  //   Basic type information (e.g. for "int", "bool", etc.) will be kept in
  //   the run-time support library. Specifically, the run-time support
  //   library should contain type_info objects for the types X, X* and
  //   X const*, for every X in: void, std::nullptr_t, bool, wchar_t, char,
  //   unsigned char, signed char, short, unsigned short, int, unsigned int,
  //   long, unsigned long, long long, unsigned long long, float, double,
  //   long double, char16_t, char32_t, and the IEEE 754r decimal and
  //   half-precision floating point types.
  switch (Ty->getKind()) {
    case BuiltinType::Void:
    case BuiltinType::NullPtr:
    case BuiltinType::Bool:
    case BuiltinType::WChar_S:
    case BuiltinType::WChar_U:
    case BuiltinType::Char_U:
    case BuiltinType::Char_S:
    case BuiltinType::UChar:
    case BuiltinType::SChar:
    case BuiltinType::Short:
    case BuiltinType::UShort:
    case BuiltinType::Int:
    case BuiltinType::UInt:
    case BuiltinType::Long:
    case BuiltinType::ULong:
    case BuiltinType::LongLong:
    case BuiltinType::ULongLong:
    case BuiltinType::Half:
    case BuiltinType::Float:
    case BuiltinType::Double:
    case BuiltinType::LongDouble:
    case BuiltinType::Char16:
    case BuiltinType::Char32:
    case BuiltinType::Int128:
    case BuiltinType::UInt128:
      return true;

    case BuiltinType::Dependent:
#define BUILTIN_TYPE(Id, SingletonId)
#define PLACEHOLDER_TYPE(Id, SingletonId) \
    case BuiltinType::Id:
#include "clang/AST/BuiltinTypes.def"
      llvm_unreachable("asking for RRTI for a placeholder type!");

    case BuiltinType::ObjCId:
    case BuiltinType::ObjCClass:
    case BuiltinType::ObjCSel:
      llvm_unreachable("FIXME: Objective-C types are unsupported!");
  }

  // Silent gcc.
  return false;
}

static bool TypeInfoIsInStandardLibrary(const PointerType *PointerTy) {
  QualType PointeeTy = PointerTy->getPointeeType();
  const BuiltinType *BuiltinTy = dyn_cast<BuiltinType>(PointeeTy);
  if (!BuiltinTy)
    return false;

  // Check the qualifiers.
  Qualifiers Quals = PointeeTy.getQualifiers();
  Quals.removeConst();

  if (!Quals.empty())
    return false;

  return TypeInfoIsInStandardLibrary(BuiltinTy);
}

/// IsStandardLibraryRTTIDescriptor - Returns whether the type
/// information for the given type exists in the standard library.
static bool IsStandardLibraryRTTIDescriptor(QualType Ty) {
  // Type info for builtin types is defined in the standard library.
  if (const BuiltinType *BuiltinTy = dyn_cast<BuiltinType>(Ty))
    return TypeInfoIsInStandardLibrary(BuiltinTy);

  // Type info for some pointer types to builtin types is defined in the
  // standard library.
  if (const PointerType *PointerTy = dyn_cast<PointerType>(Ty))
    return TypeInfoIsInStandardLibrary(PointerTy);

  return false;
}

/// ShouldUseExternalRTTIDescriptor - Returns whether the type information for
/// the given type exists somewhere else, and that we should not emit the type
/// information in this translation unit.  Assumes that it is not a
/// standard-library type.
static bool ShouldUseExternalRTTIDescriptor(CodeGenModule &CGM, QualType Ty) {
  ASTContext &Context = CGM.getContext();

  // If RTTI is disabled, don't consider key functions.
  if (!Context.getLangOptions().RTTI) return false;

  if (const RecordType *RecordTy = dyn_cast<RecordType>(Ty)) {
    const CXXRecordDecl *RD = cast<CXXRecordDecl>(RecordTy->getDecl());
    if (!RD->hasDefinition())
      return false;

    if (!RD->isDynamicClass())
      return false;

    return !CGM.getVTables().ShouldEmitVTableInThisTU(RD);
  }

  return false;
}

/// IsIncompleteClassType - Returns whether the given record type is incomplete.
static bool IsIncompleteClassType(const RecordType *RecordTy) {
  return !RecordTy->getDecl()->isCompleteDefinition();
}

/// ContainsIncompleteClassType - Returns whether the given type contains an
/// incomplete class type. This is true if
///
///   * The given type is an incomplete class type.
///   * The given type is a pointer type whose pointee type contains an
///     incomplete class type.
///   * The given type is a member pointer type whose class is an incomplete
///     class type.
///   * The given type is a member pointer type whoise pointee type contains an
///     incomplete class type.
/// is an indirect or direct pointer to an incomplete class type.
static bool ContainsIncompleteClassType(QualType Ty) {
  if (const RecordType *RecordTy = dyn_cast<RecordType>(Ty)) {
    if (IsIncompleteClassType(RecordTy))
      return true;
  }

  if (const PointerType *PointerTy = dyn_cast<PointerType>(Ty))
    return ContainsIncompleteClassType(PointerTy->getPointeeType());

  if (const MemberPointerType *MemberPointerTy =
      dyn_cast<MemberPointerType>(Ty)) {
    // Check if the class type is incomplete.
    const RecordType *ClassType = cast<RecordType>(MemberPointerTy->getClass());
    if (IsIncompleteClassType(ClassType))
      return true;

    return ContainsIncompleteClassType(MemberPointerTy->getPointeeType());
  }

  return false;
}

/// getTypeInfoLinkage - Return the linkage that the type info and type info
/// name constants should have for the given type.
static llvm::GlobalVariable::LinkageTypes
getTypeInfoLinkage(CodeGenModule &CGM, QualType Ty) {
  // Itanium C++ ABI 2.9.5p7:
  //   In addition, it and all of the intermediate abi::__pointer_type_info
  //   structs in the chain down to the abi::__class_type_info for the
  //   incomplete class type must be prevented from resolving to the
  //   corresponding type_info structs for the complete class type, possibly
  //   by making them local static objects. Finally, a dummy class RTTI is
  //   generated for the incomplete type that will not resolve to the final
  //   complete class RTTI (because the latter need not exist), possibly by
  //   making it a local static object.
  if (ContainsIncompleteClassType(Ty))
    return llvm::GlobalValue::InternalLinkage;

  switch (Ty->getLinkage()) {
  case NoLinkage:
  case InternalLinkage:
  case UniqueExternalLinkage:
    return llvm::GlobalValue::InternalLinkage;

  case ExternalLinkage:
    if (!CGM.getLangOptions().RTTI) {
      // RTTI is not enabled, which means that this type info struct is going
      // to be used for exception handling. Give it linkonce_odr linkage.
      return llvm::GlobalValue::LinkOnceODRLinkage;
    }

    if (const RecordType *Record = dyn_cast<RecordType>(Ty)) {
      const CXXRecordDecl *RD = cast<CXXRecordDecl>(Record->getDecl());
      if (RD->isDynamicClass())
        return CGM.getVTableLinkage(RD);
    }

    return llvm::GlobalValue::LinkOnceODRLinkage;
  }

  return llvm::GlobalValue::LinkOnceODRLinkage;
}

// CanUseSingleInheritance - Return whether the given record decl has a "single,
// public, non-virtual base at offset zero (i.e. the derived class is dynamic
// iff the base is)", according to Itanium C++ ABI, 2.95p6b.
static bool CanUseSingleInheritance(const CXXRecordDecl *RD) {
  // Check the number of bases.
  if (RD->getNumBases() != 1)
    return false;

  // Get the base.
  CXXRecordDecl::base_class_const_iterator Base = RD->bases_begin();

  // Check that the base is not virtual.
  if (Base->isVirtual())
    return false;

  // Check that the base is public.
  if (Base->getAccessSpecifier() != AS_public)
    return false;

  // Check that the class is dynamic iff the base is.
  const CXXRecordDecl *BaseDecl =
    cast<CXXRecordDecl>(Base->getType()->getAs<RecordType>()->getDecl());
  if (!BaseDecl->isEmpty() &&
      BaseDecl->isDynamicClass() != RD->isDynamicClass())
    return false;

  return true;
}

void RTTIBuilder::BuildVTablePointer(const Type *Ty) {
  // abi::__class_type_info.
  static const char * const ClassTypeInfo =
    "_ZTVN10__cxxabiv117__class_type_infoE";
  // abi::__si_class_type_info.
  static const char * const SIClassTypeInfo =
    "_ZTVN10__cxxabiv120__si_class_type_infoE";
  // abi::__vmi_class_type_info.
  static const char * const VMIClassTypeInfo =
    "_ZTVN10__cxxabiv121__vmi_class_type_infoE";

  const char *VTableName = 0;

  switch (Ty->getTypeClass()) {
#define TYPE(Class, Base)
#define ABSTRACT_TYPE(Class, Base)
#define NON_CANONICAL_UNLESS_DEPENDENT_TYPE(Class, Base) case Type::Class:
#define NON_CANONICAL_TYPE(Class, Base) case Type::Class:
#define DEPENDENT_TYPE(Class, Base) case Type::Class:
#include "clang/AST/TypeNodes.def"
    llvm_unreachable("Non-canonical and dependent types shouldn't get here");

  case Type::LValueReference:
  case Type::RValueReference:
    llvm_unreachable("References shouldn't get here");

  case Type::Builtin:
  // GCC treats vector and complex types as fundamental types.
  case Type::Vector:
  case Type::ExtVector:
  case Type::Complex:
  case Type::Atomic:
  // FIXME: GCC treats block pointers as fundamental types?!
  case Type::BlockPointer:
    // abi::__fundamental_type_info.
    VTableName = "_ZTVN10__cxxabiv123__fundamental_type_infoE";
    break;

  case Type::ConstantArray:
  case Type::IncompleteArray:
  case Type::VariableArray:
    // abi::__array_type_info.
    VTableName = "_ZTVN10__cxxabiv117__array_type_infoE";
    break;

  case Type::FunctionNoProto:
  case Type::FunctionProto:
    // abi::__function_type_info.
    VTableName = "_ZTVN10__cxxabiv120__function_type_infoE";
    break;

  case Type::Enum:
    // abi::__enum_type_info.
    VTableName = "_ZTVN10__cxxabiv116__enum_type_infoE";
    break;

  case Type::Record: {
    const CXXRecordDecl *RD =
      cast<CXXRecordDecl>(cast<RecordType>(Ty)->getDecl());

    if (!RD->hasDefinition() || !RD->getNumBases()) {
      VTableName = ClassTypeInfo;
    } else if (CanUseSingleInheritance(RD)) {
      VTableName = SIClassTypeInfo;
    } else {
      VTableName = VMIClassTypeInfo;
    }

    break;
  }

  case Type::ObjCObject:
    // Ignore protocol qualifiers.
    Ty = cast<ObjCObjectType>(Ty)->getBaseType().getTypePtr();

    // Handle id and Class.
    if (isa<BuiltinType>(Ty)) {
      VTableName = ClassTypeInfo;
      break;
    }

    assert(isa<ObjCInterfaceType>(Ty));
    // Fall through.

  case Type::ObjCInterface:
    if (cast<ObjCInterfaceType>(Ty)->getDecl()->getSuperClass()) {
      VTableName = SIClassTypeInfo;
    } else {
      VTableName = ClassTypeInfo;
    }
    break;

  case Type::ObjCObjectPointer:
  case Type::Pointer:
    // abi::__pointer_type_info.
    VTableName = "_ZTVN10__cxxabiv119__pointer_type_infoE";
    break;

  case Type::MemberPointer:
    // abi::__pointer_to_member_type_info.
    VTableName = "_ZTVN10__cxxabiv129__pointer_to_member_type_infoE";
    break;
  }

  llvm::Constant *VTable =
    CGM.getModule().getOrInsertGlobal(VTableName, Int8PtrTy);

  llvm::Type *PtrDiffTy =
    CGM.getTypes().ConvertType(CGM.getContext().getPointerDiffType());

  // The vtable address point is 2.
  llvm::Constant *Two = llvm::ConstantInt::get(PtrDiffTy, 2);
  VTable = llvm::ConstantExpr::getInBoundsGetElementPtr(VTable, Two);
  VTable = llvm::ConstantExpr::getBitCast(VTable, Int8PtrTy);

  Fields.push_back(VTable);
}

// maybeUpdateRTTILinkage - Will update the linkage of the RTTI data structures
// from available_externally to the correct linkage if necessary. An example of
// this is:
//
//   struct A {
//     virtual void f();
//   };
//
//   const std::type_info &g() {
//     return typeid(A);
//   }
//
//   void A::f() { }
//
// When we're generating the typeid(A) expression, we do not yet know that
// A's key function is defined in this translation unit, so we will give the
// typeinfo and typename structures available_externally linkage. When A::f
// forces the vtable to be generated, we need to change the linkage of the
// typeinfo and typename structs, otherwise we'll end up with undefined
// externals when linking.
static void
maybeUpdateRTTILinkage(CodeGenModule &CGM, llvm::GlobalVariable *GV,
                       QualType Ty) {
  // We're only interested in globals with available_externally linkage.
  if (!GV->hasAvailableExternallyLinkage())
    return;

  // Get the real linkage for the type.
  llvm::GlobalVariable::LinkageTypes Linkage = getTypeInfoLinkage(CGM, Ty);

  // If variable is supposed to have available_externally linkage, we don't
  // need to do anything.
  if (Linkage == llvm::GlobalVariable::AvailableExternallyLinkage)
    return;

  // Update the typeinfo linkage.
  GV->setLinkage(Linkage);

  // Get the typename global.
  llvm::SmallString<256> OutName;
  llvm::raw_svector_ostream Out(OutName);
  CGM.getCXXABI().getMangleContext().mangleCXXRTTIName(Ty, Out);
  Out.flush();
  StringRef Name = OutName.str();

  llvm::GlobalVariable *TypeNameGV = CGM.getModule().getNamedGlobal(Name);

  assert(TypeNameGV->hasAvailableExternallyLinkage() &&
         "Type name has different linkage from type info!");

  // And update its linkage.
  TypeNameGV->setLinkage(Linkage);
}

llvm::Constant *RTTIBuilder::BuildTypeInfo(QualType Ty, bool Force) {
  // We want to operate on the canonical type.
  Ty = CGM.getContext().getCanonicalType(Ty);

  // Check if we've already emitted an RTTI descriptor for this type.
  llvm::SmallString<256> OutName;
  llvm::raw_svector_ostream Out(OutName);
  CGM.getCXXABI().getMangleContext().mangleCXXRTTI(Ty, Out);
  Out.flush();
  StringRef Name = OutName.str();

  llvm::GlobalVariable *OldGV = CGM.getModule().getNamedGlobal(Name);
  if (OldGV && !OldGV->isDeclaration()) {
    maybeUpdateRTTILinkage(CGM, OldGV, Ty);

    return llvm::ConstantExpr::getBitCast(OldGV, Int8PtrTy);
  }

  // Check if there is already an external RTTI descriptor for this type.
  bool IsStdLib = IsStandardLibraryRTTIDescriptor(Ty);
  if (!Force && (IsStdLib || ShouldUseExternalRTTIDescriptor(CGM, Ty)))
    return GetAddrOfExternalRTTIDescriptor(Ty);

  // Emit the standard library with external linkage.
  llvm::GlobalVariable::LinkageTypes Linkage;
  if (IsStdLib)
    Linkage = llvm::GlobalValue::ExternalLinkage;
  else
    Linkage = getTypeInfoLinkage(CGM, Ty);

  // Add the vtable pointer.
  BuildVTablePointer(cast<Type>(Ty));

  // And the name.
  llvm::GlobalVariable *TypeName = GetAddrOfTypeName(Ty, Linkage);

  llvm::Type *Int8PtrTy = llvm::Type::getInt8PtrTy(VMContext);
  Fields.push_back(llvm::ConstantExpr::getBitCast(TypeName, Int8PtrTy));

  switch (Ty->getTypeClass()) {
#define TYPE(Class, Base)
#define ABSTRACT_TYPE(Class, Base)
#define NON_CANONICAL_UNLESS_DEPENDENT_TYPE(Class, Base) case Type::Class:
#define NON_CANONICAL_TYPE(Class, Base) case Type::Class:
#define DEPENDENT_TYPE(Class, Base) case Type::Class:
#include "clang/AST/TypeNodes.def"
    llvm_unreachable("Non-canonical and dependent types shouldn't get here");

  // GCC treats vector types as fundamental types.
  case Type::Builtin:
  case Type::Vector:
  case Type::ExtVector:
  case Type::Complex:
  case Type::BlockPointer:
    // Itanium C++ ABI 2.9.5p4:
    // abi::__fundamental_type_info adds no data members to std::type_info.
    break;

  case Type::LValueReference:
  case Type::RValueReference:
    llvm_unreachable("References shouldn't get here");

  case Type::ConstantArray:
  case Type::IncompleteArray:
  case Type::VariableArray:
    // Itanium C++ ABI 2.9.5p5:
    // abi::__array_type_info adds no data members to std::type_info.
    break;

  case Type::FunctionNoProto:
  case Type::FunctionProto:
    // Itanium C++ ABI 2.9.5p5:
    // abi::__function_type_info adds no data members to std::type_info.
    break;

  case Type::Enum:
    // Itanium C++ ABI 2.9.5p5:
    // abi::__enum_type_info adds no data members to std::type_info.
    break;

  case Type::Record: {
    const CXXRecordDecl *RD =
      cast<CXXRecordDecl>(cast<RecordType>(Ty)->getDecl());
    if (!RD->hasDefinition() || !RD->getNumBases()) {
      // We don't need to emit any fields.
      break;
    }

    if (CanUseSingleInheritance(RD))
      BuildSIClassTypeInfo(RD);
    else
      BuildVMIClassTypeInfo(RD);

    break;
  }

  case Type::ObjCObject:
  case Type::ObjCInterface:
    BuildObjCObjectTypeInfo(cast<ObjCObjectType>(Ty));
    break;

  case Type::ObjCObjectPointer:
    BuildPointerTypeInfo(cast<ObjCObjectPointerType>(Ty)->getPointeeType());
    break;

  case Type::Pointer:
    BuildPointerTypeInfo(cast<PointerType>(Ty)->getPointeeType());
    break;

  case Type::MemberPointer:
    BuildPointerToMemberTypeInfo(cast<MemberPointerType>(Ty));
    break;

  case Type::Atomic:
    // No fields, at least for the moment.
    break;
  }

  llvm::Constant *Init = llvm::ConstantStruct::getAnon(Fields);

  llvm::GlobalVariable *GV =
    new llvm::GlobalVariable(CGM.getModule(), Init->getType(),
                             /*Constant=*/true, Linkage, Init, Name);

  // If there's already an old global variable, replace it with the new one.
  if (OldGV) {
    GV->takeName(OldGV);
    llvm::Constant *NewPtr =
      llvm::ConstantExpr::getBitCast(GV, OldGV->getType());
    OldGV->replaceAllUsesWith(NewPtr);
    OldGV->eraseFromParent();
  }

  // GCC only relies on the uniqueness of the type names, not the
  // type_infos themselves, so we can emit these as hidden symbols.
  // But don't do this if we're worried about strict visibility
  // compatibility.
  if (const RecordType *RT = dyn_cast<RecordType>(Ty)) {
    const CXXRecordDecl *RD = cast<CXXRecordDecl>(RT->getDecl());

    CGM.setTypeVisibility(GV, RD, CodeGenModule::TVK_ForRTTI);
    CGM.setTypeVisibility(TypeName, RD, CodeGenModule::TVK_ForRTTIName);
  } else {
    Visibility TypeInfoVisibility = DefaultVisibility;
    if (CGM.getCodeGenOpts().HiddenWeakVTables &&
        Linkage == llvm::GlobalValue::LinkOnceODRLinkage)
      TypeInfoVisibility = HiddenVisibility;

    // The type name should have the same visibility as the type itself.
    Visibility ExplicitVisibility = Ty->getVisibility();
    TypeName->setVisibility(CodeGenModule::
                            GetLLVMVisibility(ExplicitVisibility));

    TypeInfoVisibility = minVisibility(TypeInfoVisibility, Ty->getVisibility());
    GV->setVisibility(CodeGenModule::GetLLVMVisibility(TypeInfoVisibility));
  }

  GV->setUnnamedAddr(true);

  return llvm::ConstantExpr::getBitCast(GV, Int8PtrTy);
}

/// ComputeQualifierFlags - Compute the pointer type info flags from the
/// given qualifier.
static unsigned ComputeQualifierFlags(Qualifiers Quals) {
  unsigned Flags = 0;

  if (Quals.hasConst())
    Flags |= RTTIBuilder::PTI_Const;
  if (Quals.hasVolatile())
    Flags |= RTTIBuilder::PTI_Volatile;
  if (Quals.hasRestrict())
    Flags |= RTTIBuilder::PTI_Restrict;

  return Flags;
}

/// BuildObjCObjectTypeInfo - Build the appropriate kind of type_info
/// for the given Objective-C object type.
void RTTIBuilder::BuildObjCObjectTypeInfo(const ObjCObjectType *OT) {
  // Drop qualifiers.
  const Type *T = OT->getBaseType().getTypePtr();
  assert(isa<BuiltinType>(T) || isa<ObjCInterfaceType>(T));

  // The builtin types are abi::__class_type_infos and don't require
  // extra fields.
  if (isa<BuiltinType>(T)) return;

  ObjCInterfaceDecl *Class = cast<ObjCInterfaceType>(T)->getDecl();
  ObjCInterfaceDecl *Super = Class->getSuperClass();

  // Root classes are also __class_type_info.
  if (!Super) return;

  QualType SuperTy = CGM.getContext().getObjCInterfaceType(Super);

  // Everything else is single inheritance.
  llvm::Constant *BaseTypeInfo = RTTIBuilder(CGM).BuildTypeInfo(SuperTy);
  Fields.push_back(BaseTypeInfo);
}

/// BuildSIClassTypeInfo - Build an abi::__si_class_type_info, used for single
/// inheritance, according to the Itanium C++ ABI, 2.95p6b.
void RTTIBuilder::BuildSIClassTypeInfo(const CXXRecordDecl *RD) {
  // Itanium C++ ABI 2.9.5p6b:
  // It adds to abi::__class_type_info a single member pointing to the
  // type_info structure for the base type,
  llvm::Constant *BaseTypeInfo =
    RTTIBuilder(CGM).BuildTypeInfo(RD->bases_begin()->getType());
  Fields.push_back(BaseTypeInfo);
}

namespace {
  /// SeenBases - Contains virtual and non-virtual bases seen when traversing
  /// a class hierarchy.
  struct SeenBases {
    llvm::SmallPtrSet<const CXXRecordDecl *, 16> NonVirtualBases;
    llvm::SmallPtrSet<const CXXRecordDecl *, 16> VirtualBases;
  };
}

/// ComputeVMIClassTypeInfoFlags - Compute the value of the flags member in
/// abi::__vmi_class_type_info.
///
static unsigned ComputeVMIClassTypeInfoFlags(const CXXBaseSpecifier *Base,
                                             SeenBases &Bases) {

  unsigned Flags = 0;

  const CXXRecordDecl *BaseDecl =
    cast<CXXRecordDecl>(Base->getType()->getAs<RecordType>()->getDecl());

  if (Base->isVirtual()) {
    if (Bases.VirtualBases.count(BaseDecl)) {
      // If this virtual base has been seen before, then the class is diamond
      // shaped.
      Flags |= RTTIBuilder::VMI_DiamondShaped;
    } else {
      if (Bases.NonVirtualBases.count(BaseDecl))
        Flags |= RTTIBuilder::VMI_NonDiamondRepeat;

      // Mark the virtual base as seen.
      Bases.VirtualBases.insert(BaseDecl);
    }
  } else {
    if (Bases.NonVirtualBases.count(BaseDecl)) {
      // If this non-virtual base has been seen before, then the class has non-
      // diamond shaped repeated inheritance.
      Flags |= RTTIBuilder::VMI_NonDiamondRepeat;
    } else {
      if (Bases.VirtualBases.count(BaseDecl))
        Flags |= RTTIBuilder::VMI_NonDiamondRepeat;

      // Mark the non-virtual base as seen.
      Bases.NonVirtualBases.insert(BaseDecl);
    }
  }

  // Walk all bases.
  for (CXXRecordDecl::base_class_const_iterator I = BaseDecl->bases_begin(),
       E = BaseDecl->bases_end(); I != E; ++I)
    Flags |= ComputeVMIClassTypeInfoFlags(I, Bases);

  return Flags;
}

static unsigned ComputeVMIClassTypeInfoFlags(const CXXRecordDecl *RD) {
  unsigned Flags = 0;
  SeenBases Bases;

  // Walk all bases.
  for (CXXRecordDecl::base_class_const_iterator I = RD->bases_begin(),
       E = RD->bases_end(); I != E; ++I)
    Flags |= ComputeVMIClassTypeInfoFlags(I, Bases);

  return Flags;
}

/// BuildVMIClassTypeInfo - Build an abi::__vmi_class_type_info, used for
/// classes with bases that do not satisfy the abi::__si_class_type_info
/// constraints, according ti the Itanium C++ ABI, 2.9.5p5c.
void RTTIBuilder::BuildVMIClassTypeInfo(const CXXRecordDecl *RD) {
  llvm::Type *UnsignedIntLTy =
    CGM.getTypes().ConvertType(CGM.getContext().UnsignedIntTy);

  // Itanium C++ ABI 2.9.5p6c:
  //   __flags is a word with flags describing details about the class
  //   structure, which may be referenced by using the __flags_masks
  //   enumeration. These flags refer to both direct and indirect bases.
  unsigned Flags = ComputeVMIClassTypeInfoFlags(RD);
  Fields.push_back(llvm::ConstantInt::get(UnsignedIntLTy, Flags));

  // Itanium C++ ABI 2.9.5p6c:
  //   __base_count is a word with the number of direct proper base class
  //   descriptions that follow.
  Fields.push_back(llvm::ConstantInt::get(UnsignedIntLTy, RD->getNumBases()));

  if (!RD->getNumBases())
    return;

  llvm::Type *LongLTy =
    CGM.getTypes().ConvertType(CGM.getContext().LongTy);

  // Now add the base class descriptions.

  // Itanium C++ ABI 2.9.5p6c:
  //   __base_info[] is an array of base class descriptions -- one for every
  //   direct proper base. Each description is of the type:
  //
  //   struct abi::__base_class_type_info {
  //   public:
  //     const __class_type_info *__base_type;
  //     long __offset_flags;
  //
  //     enum __offset_flags_masks {
  //       __virtual_mask = 0x1,
  //       __public_mask = 0x2,
  //       __offset_shift = 8
  //     };
  //   };
  for (CXXRecordDecl::base_class_const_iterator I = RD->bases_begin(),
       E = RD->bases_end(); I != E; ++I) {
    const CXXBaseSpecifier *Base = I;

    // The __base_type member points to the RTTI for the base type.
    Fields.push_back(RTTIBuilder(CGM).BuildTypeInfo(Base->getType()));

    const CXXRecordDecl *BaseDecl =
      cast<CXXRecordDecl>(Base->getType()->getAs<RecordType>()->getDecl());

    int64_t OffsetFlags = 0;

    // All but the lower 8 bits of __offset_flags are a signed offset.
    // For a non-virtual base, this is the offset in the object of the base
    // subobject. For a virtual base, this is the offset in the virtual table of
    // the virtual base offset for the virtual base referenced (negative).
    CharUnits Offset;
    if (Base->isVirtual())
      Offset =
        CGM.getVTableContext().getVirtualBaseOffsetOffset(RD, BaseDecl);
    else {
      const ASTRecordLayout &Layout = CGM.getContext().getASTRecordLayout(RD);
      Offset = Layout.getBaseClassOffset(BaseDecl);
    };

    OffsetFlags = Offset.getQuantity() << 8;

    // The low-order byte of __offset_flags contains flags, as given by the
    // masks from the enumeration __offset_flags_masks.
    if (Base->isVirtual())
      OffsetFlags |= BCTI_Virtual;
    if (Base->getAccessSpecifier() == AS_public)
      OffsetFlags |= BCTI_Public;

    Fields.push_back(llvm::ConstantInt::get(LongLTy, OffsetFlags));
  }
}

/// BuildPointerTypeInfo - Build an abi::__pointer_type_info struct,
/// used for pointer types.
void RTTIBuilder::BuildPointerTypeInfo(QualType PointeeTy) {
  Qualifiers Quals;
  QualType UnqualifiedPointeeTy =
    CGM.getContext().getUnqualifiedArrayType(PointeeTy, Quals);

  // Itanium C++ ABI 2.9.5p7:
  //   __flags is a flag word describing the cv-qualification and other
  //   attributes of the type pointed to
  unsigned Flags = ComputeQualifierFlags(Quals);

  // Itanium C++ ABI 2.9.5p7:
  //   When the abi::__pbase_type_info is for a direct or indirect pointer to an
  //   incomplete class type, the incomplete target type flag is set.
  if (ContainsIncompleteClassType(UnqualifiedPointeeTy))
    Flags |= PTI_Incomplete;

  llvm::Type *UnsignedIntLTy =
    CGM.getTypes().ConvertType(CGM.getContext().UnsignedIntTy);
  Fields.push_back(llvm::ConstantInt::get(UnsignedIntLTy, Flags));

  // Itanium C++ ABI 2.9.5p7:
  //  __pointee is a pointer to the std::type_info derivation for the
  //  unqualified type being pointed to.
  llvm::Constant *PointeeTypeInfo =
    RTTIBuilder(CGM).BuildTypeInfo(UnqualifiedPointeeTy);
  Fields.push_back(PointeeTypeInfo);
}

/// BuildPointerToMemberTypeInfo - Build an abi::__pointer_to_member_type_info
/// struct, used for member pointer types.
void RTTIBuilder::BuildPointerToMemberTypeInfo(const MemberPointerType *Ty) {
  QualType PointeeTy = Ty->getPointeeType();

  Qualifiers Quals;
  QualType UnqualifiedPointeeTy =
    CGM.getContext().getUnqualifiedArrayType(PointeeTy, Quals);

  // Itanium C++ ABI 2.9.5p7:
  //   __flags is a flag word describing the cv-qualification and other
  //   attributes of the type pointed to.
  unsigned Flags = ComputeQualifierFlags(Quals);

  const RecordType *ClassType = cast<RecordType>(Ty->getClass());

  // Itanium C++ ABI 2.9.5p7:
  //   When the abi::__pbase_type_info is for a direct or indirect pointer to an
  //   incomplete class type, the incomplete target type flag is set.
  if (ContainsIncompleteClassType(UnqualifiedPointeeTy))
    Flags |= PTI_Incomplete;

  if (IsIncompleteClassType(ClassType))
    Flags |= PTI_ContainingClassIncomplete;

  llvm::Type *UnsignedIntLTy =
    CGM.getTypes().ConvertType(CGM.getContext().UnsignedIntTy);
  Fields.push_back(llvm::ConstantInt::get(UnsignedIntLTy, Flags));

  // Itanium C++ ABI 2.9.5p7:
  //   __pointee is a pointer to the std::type_info derivation for the
  //   unqualified type being pointed to.
  llvm::Constant *PointeeTypeInfo =
    RTTIBuilder(CGM).BuildTypeInfo(UnqualifiedPointeeTy);
  Fields.push_back(PointeeTypeInfo);

  // Itanium C++ ABI 2.9.5p9:
  //   __context is a pointer to an abi::__class_type_info corresponding to the
  //   class type containing the member pointed to
  //   (e.g., the "A" in "int A::*").
  Fields.push_back(RTTIBuilder(CGM).BuildTypeInfo(QualType(ClassType, 0)));
}

llvm::Constant *CodeGenModule::GetAddrOfRTTIDescriptor(QualType Ty,
                                                       bool ForEH) {
  // Return a bogus pointer if RTTI is disabled, unless it's for EH.
  // FIXME: should we even be calling this method if RTTI is disabled
  // and it's not for EH?
  if (!ForEH && !getContext().getLangOptions().RTTI) {
    llvm::Type *Int8PtrTy = llvm::Type::getInt8PtrTy(VMContext);
    return llvm::Constant::getNullValue(Int8PtrTy);
  }

  if (ForEH && Ty->isObjCObjectPointerType() && !Features.NeXTRuntime) {
    return ObjCRuntime->GetEHType(Ty);
  }

  return RTTIBuilder(*this).BuildTypeInfo(Ty);
}

void CodeGenModule::EmitFundamentalRTTIDescriptor(QualType Type) {
  QualType PointerType = Context.getPointerType(Type);
  QualType PointerTypeConst = Context.getPointerType(Type.withConst());
  RTTIBuilder(*this).BuildTypeInfo(Type, true);
  RTTIBuilder(*this).BuildTypeInfo(PointerType, true);
  RTTIBuilder(*this).BuildTypeInfo(PointerTypeConst, true);
}

void CodeGenModule::EmitFundamentalRTTIDescriptors() {
  QualType FundamentalTypes[] = { Context.VoidTy, Context.NullPtrTy,
                                  Context.BoolTy, Context.WCharTy,
                                  Context.CharTy, Context.UnsignedCharTy,
                                  Context.SignedCharTy, Context.ShortTy,
                                  Context.UnsignedShortTy, Context.IntTy,
                                  Context.UnsignedIntTy, Context.LongTy,
                                  Context.UnsignedLongTy, Context.LongLongTy,
                                  Context.UnsignedLongLongTy, Context.FloatTy,
                                  Context.DoubleTy, Context.LongDoubleTy,
                                  Context.Char16Ty, Context.Char32Ty };
  for (unsigned i = 0; i < sizeof(FundamentalTypes)/sizeof(QualType); ++i)
    EmitFundamentalRTTIDescriptor(FundamentalTypes[i]);
}