lib/CodeGen/CGVTables.cpp

//===--- CGVTables.cpp - Emit LLVM Code for C++ vtables -------------------===//
//
//                     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 virtual tables.
//
//===----------------------------------------------------------------------===//

#include "CodeGenFunction.h"
#include "CGCXXABI.h"
#include "CodeGenModule.h"
#include "clang/AST/CXXInheritance.h"
#include "clang/AST/RecordLayout.h"
#include "clang/Frontend/CodeGenOptions.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Format.h"
#include "llvm/Transforms/Utils/Cloning.h"
#include <algorithm>
#include <cstdio>

using namespace clang;
using namespace CodeGen;

CodeGenVTables::CodeGenVTables(CodeGenModule &CGM)
  : CGM(CGM), VTContext(CGM.getContext()) { }

llvm::Constant *CodeGenModule::GetAddrOfThunk(GlobalDecl GD,
                                              const ThunkInfo &Thunk) {
  const CXXMethodDecl *MD = cast<CXXMethodDecl>(GD.getDecl());

  // Compute the mangled name.
  SmallString<256> Name;
  llvm::raw_svector_ostream Out(Name);
  if (const CXXDestructorDecl* DD = dyn_cast<CXXDestructorDecl>(MD))
    getCXXABI().getMangleContext().mangleCXXDtorThunk(DD, GD.getDtorType(),
                                                      Thunk.This, Out);
  else
    getCXXABI().getMangleContext().mangleThunk(MD, Thunk, Out);
  Out.flush();

  llvm::Type *Ty = getTypes().GetFunctionTypeForVTable(GD);
  return GetOrCreateLLVMFunction(Name, Ty, GD, /*ForVTable=*/true);
}

static llvm::Value *PerformTypeAdjustment(CodeGenFunction &CGF,
                                          llvm::Value *Ptr,
                                          int64_t NonVirtualAdjustment,
                                          int64_t VirtualAdjustment,
                                          bool IsReturnAdjustment) {
  if (!NonVirtualAdjustment && !VirtualAdjustment)
    return Ptr;

  llvm::Type *Int8PtrTy = CGF.Int8PtrTy;
  llvm::Value *V = CGF.Builder.CreateBitCast(Ptr, Int8PtrTy);

  if (NonVirtualAdjustment && !IsReturnAdjustment) {
    // Perform the non-virtual adjustment for a base-to-derived cast.
    V = CGF.Builder.CreateConstInBoundsGEP1_64(V, NonVirtualAdjustment);
  }

  if (VirtualAdjustment) {
    llvm::Type *PtrDiffTy =
      CGF.ConvertType(CGF.getContext().getPointerDiffType());

    // Perform the virtual adjustment.
    llvm::Value *VTablePtrPtr =
      CGF.Builder.CreateBitCast(V, Int8PtrTy->getPointerTo());

    llvm::Value *VTablePtr = CGF.Builder.CreateLoad(VTablePtrPtr);

    llvm::Value *OffsetPtr =
      CGF.Builder.CreateConstInBoundsGEP1_64(VTablePtr, VirtualAdjustment);

    OffsetPtr = CGF.Builder.CreateBitCast(OffsetPtr, PtrDiffTy->getPointerTo());

    // Load the adjustment offset from the vtable.
    llvm::Value *Offset = CGF.Builder.CreateLoad(OffsetPtr);

    // Adjust our pointer.
    V = CGF.Builder.CreateInBoundsGEP(V, Offset);
  }

  if (NonVirtualAdjustment && IsReturnAdjustment) {
    // Perform the non-virtual adjustment for a derived-to-base cast.
    V = CGF.Builder.CreateConstInBoundsGEP1_64(V, NonVirtualAdjustment);
  }

  // Cast back to the original type.
  return CGF.Builder.CreateBitCast(V, Ptr->getType());
}

static void setThunkVisibility(CodeGenModule &CGM, const CXXMethodDecl *MD,
                               const ThunkInfo &Thunk, llvm::Function *Fn) {
  CGM.setGlobalVisibility(Fn, MD);

  if (!CGM.getCodeGenOpts().HiddenWeakVTables)
    return;

  // If the thunk has weak/linkonce linkage, but the function must be
  // emitted in every translation unit that references it, then we can
  // emit its thunks with hidden visibility, since its thunks must be
  // emitted when the function is.

  // This follows CodeGenModule::setTypeVisibility; see the comments
  // there for explanation.

  if ((Fn->getLinkage() != llvm::GlobalVariable::LinkOnceODRLinkage &&
       Fn->getLinkage() != llvm::GlobalVariable::WeakODRLinkage) ||
      Fn->getVisibility() != llvm::GlobalVariable::DefaultVisibility)
    return;

  if (MD->getExplicitVisibility(ValueDecl::VisibilityForValue))
    return;

  switch (MD->getTemplateSpecializationKind()) {
  case TSK_ExplicitInstantiationDefinition:
  case TSK_ExplicitInstantiationDeclaration:
    return;

  case TSK_Undeclared:
    break;

  case TSK_ExplicitSpecialization:
  case TSK_ImplicitInstantiation:
    return;
    break;
  }

  // If there's an explicit definition, and that definition is
  // out-of-line, then we can't assume that all users will have a
  // definition to emit.
  const FunctionDecl *Def = 0;
  if (MD->hasBody(Def) && Def->isOutOfLine())
    return;

  Fn->setVisibility(llvm::GlobalValue::HiddenVisibility);
}

#ifndef NDEBUG
static bool similar(const ABIArgInfo &infoL, CanQualType typeL,
                    const ABIArgInfo &infoR, CanQualType typeR) {
  return (infoL.getKind() == infoR.getKind() &&
          (typeL == typeR ||
           (isa<PointerType>(typeL) && isa<PointerType>(typeR)) ||
           (isa<ReferenceType>(typeL) && isa<ReferenceType>(typeR))));
}
#endif

static RValue PerformReturnAdjustment(CodeGenFunction &CGF,
                                      QualType ResultType, RValue RV,
                                      const ThunkInfo &Thunk) {
  // Emit the return adjustment.
  bool NullCheckValue = !ResultType->isReferenceType();

  llvm::BasicBlock *AdjustNull = 0;
  llvm::BasicBlock *AdjustNotNull = 0;
  llvm::BasicBlock *AdjustEnd = 0;

  llvm::Value *ReturnValue = RV.getScalarVal();

  if (NullCheckValue) {
    AdjustNull = CGF.createBasicBlock("adjust.null");
    AdjustNotNull = CGF.createBasicBlock("adjust.notnull");
    AdjustEnd = CGF.createBasicBlock("adjust.end");

    llvm::Value *IsNull = CGF.Builder.CreateIsNull(ReturnValue);
    CGF.Builder.CreateCondBr(IsNull, AdjustNull, AdjustNotNull);
    CGF.EmitBlock(AdjustNotNull);
  }

  ReturnValue = PerformTypeAdjustment(CGF, ReturnValue,
                                      Thunk.Return.NonVirtual,
                                      Thunk.Return.VBaseOffsetOffset,
                                      /*IsReturnAdjustment*/true);

  if (NullCheckValue) {
    CGF.Builder.CreateBr(AdjustEnd);
    CGF.EmitBlock(AdjustNull);
    CGF.Builder.CreateBr(AdjustEnd);
    CGF.EmitBlock(AdjustEnd);

    llvm::PHINode *PHI = CGF.Builder.CreatePHI(ReturnValue->getType(), 2);
    PHI->addIncoming(ReturnValue, AdjustNotNull);
    PHI->addIncoming(llvm::Constant::getNullValue(ReturnValue->getType()),
                     AdjustNull);
    ReturnValue = PHI;
  }

  return RValue::get(ReturnValue);
}

// This function does roughly the same thing as GenerateThunk, but in a
// very different way, so that va_start and va_end work correctly.
// FIXME: This function assumes "this" is the first non-sret LLVM argument of
//        a function, and that there is an alloca built in the entry block
//        for all accesses to "this".
// FIXME: This function assumes there is only one "ret" statement per function.
// FIXME: Cloning isn't correct in the presence of indirect goto!
// FIXME: This implementation of thunks bloats codesize by duplicating the
//        function definition.  There are alternatives:
//        1. Add some sort of stub support to LLVM for cases where we can
//           do a this adjustment, then a sibcall.
//        2. We could transform the definition to take a va_list instead of an
//           actual variable argument list, then have the thunks (including a
//           no-op thunk for the regular definition) call va_start/va_end.
//           There's a bit of per-call overhead for this solution, but it's
//           better for codesize if the definition is long.
void CodeGenFunction::GenerateVarArgsThunk(
                                      llvm::Function *Fn,
                                      const CGFunctionInfo &FnInfo,
                                      GlobalDecl GD, const ThunkInfo &Thunk) {
  const CXXMethodDecl *MD = cast<CXXMethodDecl>(GD.getDecl());
  const FunctionProtoType *FPT = MD->getType()->getAs<FunctionProtoType>();
  QualType ResultType = FPT->getResultType();

  // Get the original function
  assert(FnInfo.isVariadic());
  llvm::Type *Ty = CGM.getTypes().GetFunctionType(FnInfo);
  llvm::Value *Callee = CGM.GetAddrOfFunction(GD, Ty, /*ForVTable=*/true);
  llvm::Function *BaseFn = cast<llvm::Function>(Callee);

  // Clone to thunk.
  llvm::ValueToValueMapTy VMap;
  llvm::Function *NewFn = llvm::CloneFunction(BaseFn, VMap,
                                              /*ModuleLevelChanges=*/false);
  CGM.getModule().getFunctionList().push_back(NewFn);
  Fn->replaceAllUsesWith(NewFn);
  NewFn->takeName(Fn);
  Fn->eraseFromParent();
  Fn = NewFn;

  // "Initialize" CGF (minimally).
  CurFn = Fn;

  // Get the "this" value
  llvm::Function::arg_iterator AI = Fn->arg_begin();
  if (CGM.ReturnTypeUsesSRet(FnInfo))
    ++AI;

  // Find the first store of "this", which will be to the alloca associated
  // with "this".
  llvm::Value *ThisPtr = &*AI;
  llvm::BasicBlock *EntryBB = Fn->begin();
  llvm::Instruction *ThisStore = 0;
  for (llvm::BasicBlock::iterator I = EntryBB->begin(), E = EntryBB->end();
       I != E; I++) {
    if (isa<llvm::StoreInst>(I) && I->getOperand(0) == ThisPtr) {
      ThisStore = cast<llvm::StoreInst>(I);
      break;
    }
  }
  assert(ThisStore && "Store of this should be in entry block?");
  // Adjust "this", if necessary.
  Builder.SetInsertPoint(ThisStore);
  llvm::Value *AdjustedThisPtr =
    PerformTypeAdjustment(*this, ThisPtr,
                          Thunk.This.NonVirtual,
                          Thunk.This.VCallOffsetOffset,
                          /*IsReturnAdjustment*/false);
  ThisStore->setOperand(0, AdjustedThisPtr);

  if (!Thunk.Return.isEmpty()) {
    // Fix up the returned value, if necessary.
    for (llvm::Function::iterator I = Fn->begin(), E = Fn->end(); I != E; I++) {
      llvm::Instruction *T = I->getTerminator();
      if (isa<llvm::ReturnInst>(T)) {
        RValue RV = RValue::get(T->getOperand(0));
        T->eraseFromParent();
        Builder.SetInsertPoint(&*I);
        RV = PerformReturnAdjustment(*this, ResultType, RV, Thunk);
        Builder.CreateRet(RV.getScalarVal());
        break;
      }
    }
  }
}

void CodeGenFunction::GenerateThunk(llvm::Function *Fn,
                                    const CGFunctionInfo &FnInfo,
                                    GlobalDecl GD, const ThunkInfo &Thunk) {
  const CXXMethodDecl *MD = cast<CXXMethodDecl>(GD.getDecl());
  const FunctionProtoType *FPT = MD->getType()->getAs<FunctionProtoType>();
  QualType ResultType = FPT->getResultType();
  QualType ThisType = MD->getThisType(getContext());

  FunctionArgList FunctionArgs;

  // FIXME: It would be nice if more of this code could be shared with
  // CodeGenFunction::GenerateCode.

  // Create the implicit 'this' parameter declaration.
  CurGD = GD;
  CGM.getCXXABI().BuildInstanceFunctionParams(*this, ResultType, FunctionArgs);

  // Add the rest of the parameters.
  for (FunctionDecl::param_const_iterator I = MD->param_begin(),
       E = MD->param_end(); I != E; ++I) {
    ParmVarDecl *Param = *I;

    FunctionArgs.push_back(Param);
  }

  // Initialize debug info if needed.
  maybeInitializeDebugInfo();

  StartFunction(GlobalDecl(), ResultType, Fn, FnInfo, FunctionArgs,
                SourceLocation());

  CGM.getCXXABI().EmitInstanceFunctionProlog(*this);
  CXXThisValue = CXXABIThisValue;

  // Adjust the 'this' pointer if necessary.
  llvm::Value *AdjustedThisPtr =
    PerformTypeAdjustment(*this, LoadCXXThis(),
                          Thunk.This.NonVirtual,
                          Thunk.This.VCallOffsetOffset,
                          /*IsReturnAdjustment*/false);

  CallArgList CallArgs;

  // Add our adjusted 'this' pointer.
  CallArgs.add(RValue::get(AdjustedThisPtr), ThisType);

  // Add the rest of the parameters.
  for (FunctionDecl::param_const_iterator I = MD->param_begin(),
       E = MD->param_end(); I != E; ++I) {
    ParmVarDecl *param = *I;
    EmitDelegateCallArg(CallArgs, param);
  }

  // Get our callee.
  llvm::Type *Ty =
    CGM.getTypes().GetFunctionType(CGM.getTypes().arrangeGlobalDeclaration(GD));
  llvm::Value *Callee = CGM.GetAddrOfFunction(GD, Ty, /*ForVTable=*/true);

#ifndef NDEBUG
  const CGFunctionInfo &CallFnInfo =
    CGM.getTypes().arrangeCXXMethodCall(CallArgs, FPT,
                                       RequiredArgs::forPrototypePlus(FPT, 1));
  assert(CallFnInfo.getRegParm() == FnInfo.getRegParm() &&
         CallFnInfo.isNoReturn() == FnInfo.isNoReturn() &&
         CallFnInfo.getCallingConvention() == FnInfo.getCallingConvention());
  assert(isa<CXXDestructorDecl>(MD) || // ignore dtor return types
         similar(CallFnInfo.getReturnInfo(), CallFnInfo.getReturnType(),
                 FnInfo.getReturnInfo(), FnInfo.getReturnType()));
  assert(CallFnInfo.arg_size() == FnInfo.arg_size());
  for (unsigned i = 0, e = FnInfo.arg_size(); i != e; ++i)
    assert(similar(CallFnInfo.arg_begin()[i].info,
                   CallFnInfo.arg_begin()[i].type,
                   FnInfo.arg_begin()[i].info, FnInfo.arg_begin()[i].type));
#endif

  // Determine whether we have a return value slot to use.
  ReturnValueSlot Slot;
  if (!ResultType->isVoidType() &&
      FnInfo.getReturnInfo().getKind() == ABIArgInfo::Indirect &&
      !hasScalarEvaluationKind(CurFnInfo->getReturnType()))
    Slot = ReturnValueSlot(ReturnValue, ResultType.isVolatileQualified());

  // Now emit our call.
  RValue RV = EmitCall(FnInfo, Callee, Slot, CallArgs, MD);

  if (!Thunk.Return.isEmpty())
    RV = PerformReturnAdjustment(*this, ResultType, RV, Thunk);

  if (!ResultType->isVoidType() && Slot.isNull())
    CGM.getCXXABI().EmitReturnFromThunk(*this, RV, ResultType);

  // Disable the final ARC autorelease.
  AutoreleaseResult = false;

  FinishFunction();

  // Set the right linkage.
  CGM.setFunctionLinkage(MD, Fn);

  // Set the right visibility.
  setThunkVisibility(CGM, MD, Thunk, Fn);
}

void CodeGenVTables::EmitThunk(GlobalDecl GD, const ThunkInfo &Thunk,
                               bool UseAvailableExternallyLinkage)
{
  const CGFunctionInfo &FnInfo = CGM.getTypes().arrangeGlobalDeclaration(GD);

  // FIXME: re-use FnInfo in this computation.
  llvm::Constant *Entry = CGM.GetAddrOfThunk(GD, Thunk);

  // Strip off a bitcast if we got one back.
  if (llvm::ConstantExpr *CE = dyn_cast<llvm::ConstantExpr>(Entry)) {
    assert(CE->getOpcode() == llvm::Instruction::BitCast);
    Entry = CE->getOperand(0);
  }

  // There's already a declaration with the same name, check if it has the same
  // type or if we need to replace it.
  if (cast<llvm::GlobalValue>(Entry)->getType()->getElementType() !=
      CGM.getTypes().GetFunctionTypeForVTable(GD)) {
    llvm::GlobalValue *OldThunkFn = cast<llvm::GlobalValue>(Entry);

    // If the types mismatch then we have to rewrite the definition.
    assert(OldThunkFn->isDeclaration() &&
           "Shouldn't replace non-declaration");

    // Remove the name from the old thunk function and get a new thunk.
    OldThunkFn->setName(StringRef());
    Entry = CGM.GetAddrOfThunk(GD, Thunk);

    // If needed, replace the old thunk with a bitcast.
    if (!OldThunkFn->use_empty()) {
      llvm::Constant *NewPtrForOldDecl =
        llvm::ConstantExpr::getBitCast(Entry, OldThunkFn->getType());
      OldThunkFn->replaceAllUsesWith(NewPtrForOldDecl);
    }

    // Remove the old thunk.
    OldThunkFn->eraseFromParent();
  }

  llvm::Function *ThunkFn = cast<llvm::Function>(Entry);

  if (!ThunkFn->isDeclaration()) {
    if (UseAvailableExternallyLinkage) {
      // There is already a thunk emitted for this function, do nothing.
      return;
    }

    // If a function has a body, it should have available_externally linkage.
    assert(ThunkFn->hasAvailableExternallyLinkage() &&
           "Function should have available_externally linkage!");

    // Change the linkage.
    CGM.setFunctionLinkage(cast<CXXMethodDecl>(GD.getDecl()), ThunkFn);
    return;
  }

  CGM.SetLLVMFunctionAttributesForDefinition(GD.getDecl(), ThunkFn);

  if (ThunkFn->isVarArg()) {
    // Varargs thunks are special; we can't just generate a call because
    // we can't copy the varargs.  Our implementation is rather
    // expensive/sucky at the moment, so don't generate the thunk unless
    // we have to.
    // FIXME: Do something better here; GenerateVarArgsThunk is extremely ugly.
    if (!UseAvailableExternallyLinkage)
      CodeGenFunction(CGM).GenerateVarArgsThunk(ThunkFn, FnInfo, GD, Thunk);
  } else {
    // Normal thunk body generation.
    CodeGenFunction(CGM).GenerateThunk(ThunkFn, FnInfo, GD, Thunk);
    if (UseAvailableExternallyLinkage)
      ThunkFn->setLinkage(llvm::GlobalValue::AvailableExternallyLinkage);
  }
}

void CodeGenVTables::MaybeEmitThunkAvailableExternally(GlobalDecl GD,
                                                       const ThunkInfo &Thunk) {
  // We only want to do this when building with optimizations.
  if (!CGM.getCodeGenOpts().OptimizationLevel)
    return;

  // We can't emit thunks for member functions with incomplete types.
  const CXXMethodDecl *MD = cast<CXXMethodDecl>(GD.getDecl());
  if (!CGM.getTypes().isFuncTypeConvertible(
                                cast<FunctionType>(MD->getType().getTypePtr())))
    return;

  EmitThunk(GD, Thunk, /*UseAvailableExternallyLinkage=*/true);
}

void CodeGenVTables::EmitThunks(GlobalDecl GD)
{
  const CXXMethodDecl *MD =
    cast<CXXMethodDecl>(GD.getDecl())->getCanonicalDecl();

  // We don't need to generate thunks for the base destructor.
  if (isa<CXXDestructorDecl>(MD) && GD.getDtorType() == Dtor_Base)
    return;

  const VTableContext::ThunkInfoVectorTy *ThunkInfoVector =
    VTContext.getThunkInfo(MD);
  if (!ThunkInfoVector)
    return;

  for (unsigned I = 0, E = ThunkInfoVector->size(); I != E; ++I)
    EmitThunk(GD, (*ThunkInfoVector)[I],
              /*UseAvailableExternallyLinkage=*/false);
}

llvm::Constant *
CodeGenVTables::CreateVTableInitializer(const CXXRecordDecl *RD,
                                        const VTableComponent *Components,
                                        unsigned NumComponents,
                                const VTableLayout::VTableThunkTy *VTableThunks,
                                        unsigned NumVTableThunks) {
  SmallVector<llvm::Constant *, 64> Inits;

  llvm::Type *Int8PtrTy = CGM.Int8PtrTy;

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

  QualType ClassType = CGM.getContext().getTagDeclType(RD);
  llvm::Constant *RTTI = CGM.GetAddrOfRTTIDescriptor(ClassType);

  unsigned NextVTableThunkIndex = 0;

  llvm::Constant *PureVirtualFn = 0, *DeletedVirtualFn = 0;

  for (unsigned I = 0; I != NumComponents; ++I) {
    VTableComponent Component = Components[I];

    llvm::Constant *Init = 0;

    switch (Component.getKind()) {
    case VTableComponent::CK_VCallOffset:
      Init = llvm::ConstantInt::get(PtrDiffTy,
                                    Component.getVCallOffset().getQuantity());
      Init = llvm::ConstantExpr::getIntToPtr(Init, Int8PtrTy);
      break;
    case VTableComponent::CK_VBaseOffset:
      Init = llvm::ConstantInt::get(PtrDiffTy,
                                    Component.getVBaseOffset().getQuantity());
      Init = llvm::ConstantExpr::getIntToPtr(Init, Int8PtrTy);
      break;
    case VTableComponent::CK_OffsetToTop:
      Init = llvm::ConstantInt::get(PtrDiffTy,
                                    Component.getOffsetToTop().getQuantity());
      Init = llvm::ConstantExpr::getIntToPtr(Init, Int8PtrTy);
      break;
    case VTableComponent::CK_RTTI:
      Init = llvm::ConstantExpr::getBitCast(RTTI, Int8PtrTy);
      break;
    case VTableComponent::CK_FunctionPointer:
    case VTableComponent::CK_CompleteDtorPointer:
    case VTableComponent::CK_DeletingDtorPointer: {
      GlobalDecl GD;

      // Get the right global decl.
      switch (Component.getKind()) {
      default:
        llvm_unreachable("Unexpected vtable component kind");
      case VTableComponent::CK_FunctionPointer:
        GD = Component.getFunctionDecl();
        break;
      case VTableComponent::CK_CompleteDtorPointer:
        GD = GlobalDecl(Component.getDestructorDecl(), Dtor_Complete);
        break;
      case VTableComponent::CK_DeletingDtorPointer:
        GD = GlobalDecl(Component.getDestructorDecl(), Dtor_Deleting);
        break;
      }

      if (cast<CXXMethodDecl>(GD.getDecl())->isPure()) {
        // We have a pure virtual member function.
        if (!PureVirtualFn) {
          llvm::FunctionType *Ty =
            llvm::FunctionType::get(CGM.VoidTy, /*isVarArg=*/false);
          StringRef PureCallName = CGM.getCXXABI().GetPureVirtualCallName();
          PureVirtualFn = CGM.CreateRuntimeFunction(Ty, PureCallName);
          PureVirtualFn = llvm::ConstantExpr::getBitCast(PureVirtualFn,
                                                         CGM.Int8PtrTy);
        }
        Init = PureVirtualFn;
      } else if (cast<CXXMethodDecl>(GD.getDecl())->isDeleted()) {
        if (!DeletedVirtualFn) {
          llvm::FunctionType *Ty =
            llvm::FunctionType::get(CGM.VoidTy, /*isVarArg=*/false);
          StringRef DeletedCallName =
            CGM.getCXXABI().GetDeletedVirtualCallName();
          DeletedVirtualFn = CGM.CreateRuntimeFunction(Ty, DeletedCallName);
          DeletedVirtualFn = llvm::ConstantExpr::getBitCast(DeletedVirtualFn,
                                                         CGM.Int8PtrTy);
        }
        Init = DeletedVirtualFn;
      } else {
        // Check if we should use a thunk.
        if (NextVTableThunkIndex < NumVTableThunks &&
            VTableThunks[NextVTableThunkIndex].first == I) {
          const ThunkInfo &Thunk = VTableThunks[NextVTableThunkIndex].second;

          MaybeEmitThunkAvailableExternally(GD, Thunk);
          Init = CGM.GetAddrOfThunk(GD, Thunk);

          NextVTableThunkIndex++;
        } else {
          llvm::Type *Ty = CGM.getTypes().GetFunctionTypeForVTable(GD);

          Init = CGM.GetAddrOfFunction(GD, Ty, /*ForVTable=*/true);
        }

        Init = llvm::ConstantExpr::getBitCast(Init, Int8PtrTy);
      }
      break;
    }

    case VTableComponent::CK_UnusedFunctionPointer:
      Init = llvm::ConstantExpr::getNullValue(Int8PtrTy);
      break;
    };

    Inits.push_back(Init);
  }

  llvm::ArrayType *ArrayType = llvm::ArrayType::get(Int8PtrTy, NumComponents);
  return llvm::ConstantArray::get(ArrayType, Inits);
}

llvm::GlobalVariable *CodeGenVTables::GetAddrOfVTable(const CXXRecordDecl *RD) {
  llvm::GlobalVariable *&VTable = VTables[RD];
  if (VTable)
    return VTable;

  // Queue up this v-table for possible deferred emission.
  CGM.addDeferredVTable(RD);

  SmallString<256> OutName;
  llvm::raw_svector_ostream Out(OutName);
  CGM.getCXXABI().getMangleContext().mangleCXXVTable(RD, Out);
  Out.flush();
  StringRef Name = OutName.str();

  llvm::ArrayType *ArrayType =
    llvm::ArrayType::get(CGM.Int8PtrTy,
                        VTContext.getVTableLayout(RD).getNumVTableComponents());

  VTable =
    CGM.CreateOrReplaceCXXRuntimeVariable(Name, ArrayType,
                                          llvm::GlobalValue::ExternalLinkage);
  VTable->setUnnamedAddr(true);
  return VTable;
}

void
CodeGenVTables::EmitVTableDefinition(llvm::GlobalVariable *VTable,
                                     llvm::GlobalVariable::LinkageTypes Linkage,
                                     const CXXRecordDecl *RD) {
  const VTableLayout &VTLayout = VTContext.getVTableLayout(RD);

  // Create and set the initializer.
  llvm::Constant *Init =
    CreateVTableInitializer(RD,
                            VTLayout.vtable_component_begin(),
                            VTLayout.getNumVTableComponents(),
                            VTLayout.vtable_thunk_begin(),
                            VTLayout.getNumVTableThunks());
  VTable->setInitializer(Init);

  // Set the correct linkage.
  VTable->setLinkage(Linkage);

  // Set the right visibility.
  CGM.setTypeVisibility(VTable, RD, CodeGenModule::TVK_ForVTable);
}

llvm::GlobalVariable *
CodeGenVTables::GenerateConstructionVTable(const CXXRecordDecl *RD,
                                      const BaseSubobject &Base,
                                      bool BaseIsVirtual,
                                   llvm::GlobalVariable::LinkageTypes Linkage,
                                      VTableAddressPointsMapTy& AddressPoints) {
  OwningPtr<VTableLayout> VTLayout(
    VTContext.createConstructionVTableLayout(Base.getBase(),
                                             Base.getBaseOffset(),
                                             BaseIsVirtual, RD));

  // Add the address points.
  AddressPoints = VTLayout->getAddressPoints();

  // Get the mangled construction vtable name.
  SmallString<256> OutName;
  llvm::raw_svector_ostream Out(OutName);
  CGM.getCXXABI().getMangleContext().
    mangleCXXCtorVTable(RD, Base.getBaseOffset().getQuantity(), Base.getBase(),
                        Out);
  Out.flush();
  StringRef Name = OutName.str();

  llvm::ArrayType *ArrayType =
    llvm::ArrayType::get(CGM.Int8PtrTy, VTLayout->getNumVTableComponents());

  // Construction vtable symbols are not part of the Itanium ABI, so we cannot
  // guarantee that they actually will be available externally. Instead, when
  // emitting an available_externally VTT, we provide references to an internal
  // linkage construction vtable. The ABI only requires complete-object vtables
  // to be the same for all instances of a type, not construction vtables.
  if (Linkage == llvm::GlobalVariable::AvailableExternallyLinkage)
    Linkage = llvm::GlobalVariable::InternalLinkage;

  // Create the variable that will hold the construction vtable.
  llvm::GlobalVariable *VTable =
    CGM.CreateOrReplaceCXXRuntimeVariable(Name, ArrayType, Linkage);
  CGM.setTypeVisibility(VTable, RD, CodeGenModule::TVK_ForConstructionVTable);

  // V-tables are always unnamed_addr.
  VTable->setUnnamedAddr(true);

  // Create and set the initializer.
  llvm::Constant *Init =
    CreateVTableInitializer(Base.getBase(),
                            VTLayout->vtable_component_begin(),
                            VTLayout->getNumVTableComponents(),
                            VTLayout->vtable_thunk_begin(),
                            VTLayout->getNumVTableThunks());
  VTable->setInitializer(Init);

  return VTable;
}

/// Compute the required linkage of the v-table for the given class.
///
/// Note that we only call this at the end of the translation unit.
llvm::GlobalVariable::LinkageTypes
CodeGenModule::getVTableLinkage(const CXXRecordDecl *RD) {
  if (RD->getLinkage() != ExternalLinkage)
    return llvm::GlobalVariable::InternalLinkage;

  // We're at the end of the translation unit, so the current key
  // function is fully correct.
  if (const CXXMethodDecl *keyFunction = Context.getCurrentKeyFunction(RD)) {
    // If this class has a key function, use that to determine the
    // linkage of the vtable.
    const FunctionDecl *def = 0;
    if (keyFunction->hasBody(def))
      keyFunction = cast<CXXMethodDecl>(def);

    switch (keyFunction->getTemplateSpecializationKind()) {
      case TSK_Undeclared:
      case TSK_ExplicitSpecialization:
        // When compiling with optimizations turned on, we emit all vtables,
        // even if the key function is not defined in the current translation
        // unit. If this is the case, use available_externally linkage.
        if (!def && CodeGenOpts.OptimizationLevel)
          return llvm::GlobalVariable::AvailableExternallyLinkage;

        if (keyFunction->isInlined())
          return !Context.getLangOpts().AppleKext ?
                   llvm::GlobalVariable::LinkOnceODRLinkage :
                   llvm::Function::InternalLinkage;

        return llvm::GlobalVariable::ExternalLinkage;

      case TSK_ImplicitInstantiation:
        return !Context.getLangOpts().AppleKext ?
                 llvm::GlobalVariable::LinkOnceODRLinkage :
                 llvm::Function::InternalLinkage;

      case TSK_ExplicitInstantiationDefinition:
        return !Context.getLangOpts().AppleKext ?
                 llvm::GlobalVariable::WeakODRLinkage :
                 llvm::Function::InternalLinkage;

      case TSK_ExplicitInstantiationDeclaration:
        return !Context.getLangOpts().AppleKext ?
                 llvm::GlobalVariable::AvailableExternallyLinkage :
                 llvm::Function::InternalLinkage;
    }
  }

  // -fapple-kext mode does not support weak linkage, so we must use
  // internal linkage.
  if (Context.getLangOpts().AppleKext)
    return llvm::Function::InternalLinkage;

  switch (RD->getTemplateSpecializationKind()) {
  case TSK_Undeclared:
  case TSK_ExplicitSpecialization:
  case TSK_ImplicitInstantiation:
    return llvm::GlobalVariable::LinkOnceODRLinkage;

  case TSK_ExplicitInstantiationDeclaration:
    return llvm::GlobalVariable::AvailableExternallyLinkage;

  case TSK_ExplicitInstantiationDefinition:
      return llvm::GlobalVariable::WeakODRLinkage;
  }

  llvm_unreachable("Invalid TemplateSpecializationKind!");
}

/// This is a callback from Sema to tell us that it believes that a
/// particular v-table is required to be emitted in this translation
/// unit.
///
/// The reason we don't simply trust this callback is because Sema
/// will happily report that something is used even when it's used
/// only in code that we don't actually have to emit.
///
/// \param isRequired - if true, the v-table is mandatory, e.g.
///   because the translation unit defines the key function
void CodeGenModule::EmitVTable(CXXRecordDecl *theClass, bool isRequired) {
  if (!isRequired) return;

  VTables.GenerateClassData(theClass);
}

void
CodeGenVTables::GenerateClassData(const CXXRecordDecl *RD) {
  // First off, check whether we've already emitted the v-table and
  // associated stuff.
  llvm::GlobalVariable *VTable = GetAddrOfVTable(RD);
  if (VTable->hasInitializer())
    return;

  llvm::GlobalVariable::LinkageTypes Linkage = CGM.getVTableLinkage(RD);
  EmitVTableDefinition(VTable, Linkage, RD);

  if (RD->getNumVBases()) {
    if (!CGM.getTarget().getCXXABI().isMicrosoft()) {
      llvm::GlobalVariable *VTT = GetAddrOfVTT(RD);
      EmitVTTDefinition(VTT, Linkage, RD);
    } else {
      // FIXME: Emit vbtables here.
    }
  }

  // If this is the magic class __cxxabiv1::__fundamental_type_info,
  // we will emit the typeinfo for the fundamental types. This is the
  // same behaviour as GCC.
  const DeclContext *DC = RD->getDeclContext();
  if (RD->getIdentifier() &&
      RD->getIdentifier()->isStr("__fundamental_type_info") &&
      isa<NamespaceDecl>(DC) &&
      cast<NamespaceDecl>(DC)->getIdentifier() &&
      cast<NamespaceDecl>(DC)->getIdentifier()->isStr("__cxxabiv1") &&
      DC->getParent()->isTranslationUnit())
    CGM.EmitFundamentalRTTIDescriptors();
}

/// At this point in the translation unit, does it appear that can we
/// rely on the vtable being defined elsewhere in the program?
///
/// The response is really only definitive when called at the end of
/// the translation unit.
///
/// The only semantic restriction here is that the object file should
/// not contain a v-table definition when that v-table is defined
/// strongly elsewhere.  Otherwise, we'd just like to avoid emitting
/// v-tables when unnecessary.
bool CodeGenVTables::isVTableExternal(const CXXRecordDecl *RD) {
  assert(RD->isDynamicClass() && "Non dynamic classes have no VTable.");

  // If we have an explicit instantiation declaration (and not a
  // definition), the v-table is defined elsewhere.
  TemplateSpecializationKind TSK = RD->getTemplateSpecializationKind();
  if (TSK == TSK_ExplicitInstantiationDeclaration)
    return true;

  // Otherwise, if the class is an instantiated template, the
  // v-table must be defined here.
  if (TSK == TSK_ImplicitInstantiation ||
      TSK == TSK_ExplicitInstantiationDefinition)
    return false;

  // Otherwise, if the class doesn't have a key function (possibly
  // anymore), the v-table must be defined here.
  const CXXMethodDecl *keyFunction = CGM.getContext().getCurrentKeyFunction(RD);
  if (!keyFunction)
    return false;

  // Otherwise, if we don't have a definition of the key function, the
  // v-table must be defined somewhere else.
  return !keyFunction->hasBody();
}

/// Given that we're currently at the end of the translation unit, and
/// we've emitted a reference to the v-table for this class, should
/// we define that v-table?
static bool shouldEmitVTableAtEndOfTranslationUnit(CodeGenModule &CGM,
                                                   const CXXRecordDecl *RD) {
  // If we're building with optimization, we always emit v-tables
  // since that allows for virtual function calls to be devirtualized.
  // If the v-table is defined strongly elsewhere, this definition
  // will be emitted available_externally.
  //
  // However, we don't want to do this in -fapple-kext mode, because
  // kext mode does not permit devirtualization.
  if (CGM.getCodeGenOpts().OptimizationLevel && !CGM.getLangOpts().AppleKext)
    return true;

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

/// Given that at some point we emitted a reference to one or more
/// v-tables, and that we are now at the end of the translation unit,
/// decide whether we should emit them.
void CodeGenModule::EmitDeferredVTables() {
#ifndef NDEBUG
  // Remember the size of DeferredVTables, because we're going to assume
  // that this entire operation doesn't modify it.
  size_t savedSize = DeferredVTables.size();
#endif

  typedef std::vector<const CXXRecordDecl *>::const_iterator const_iterator;
  for (const_iterator i = DeferredVTables.begin(),
                      e = DeferredVTables.end(); i != e; ++i) {
    const CXXRecordDecl *RD = *i;
    if (shouldEmitVTableAtEndOfTranslationUnit(*this, RD))
      VTables.GenerateClassData(RD);
  }

  assert(savedSize == DeferredVTables.size() &&
         "deferred extra v-tables during v-table emission?");
  DeferredVTables.clear();
}