llvm-project

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//===- VTTBuilder.cpp - C++ VTT layout builder ----------------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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//
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// This contains code dealing with generation of the layout of virtual table
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// tables (VTT).
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//
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//===----------------------------------------------------------------------===//
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#include "clang/AST/VTTBuilder.h"
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#include "clang/AST/ASTContext.h"
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#include "clang/AST/BaseSubobject.h"
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#include "clang/AST/CharUnits.h"
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#include "clang/AST/Decl.h"
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#include "clang/AST/DeclCXX.h"
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#include "clang/AST/RecordLayout.h"
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#include "clang/AST/Type.h"
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#include "clang/Basic/LLVM.h"
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#include "llvm/Support/Casting.h"
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#include <cassert>
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#include <cstdint>
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using namespace clang;
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#define DUMP_OVERRIDERS 0
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VTTBuilder::VTTBuilder(ASTContext &Ctx,
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                       const CXXRecordDecl *MostDerivedClass,
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                       bool GenerateDefinition)
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    : Ctx(Ctx), MostDerivedClass(MostDerivedClass),
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      MostDerivedClassLayout(Ctx.getASTRecordLayout(MostDerivedClass)),
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      GenerateDefinition(GenerateDefinition) {
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  // Lay out this VTT.
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  LayoutVTT(BaseSubobject(MostDerivedClass, CharUnits::Zero()),
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            /*BaseIsVirtual=*/false);
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}
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void VTTBuilder::AddVTablePointer(BaseSubobject Base, uint64_t VTableIndex,
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                                  const CXXRecordDecl *VTableClass) {
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  // Store the vtable pointer index if we're generating the primary VTT.
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  if (VTableClass == MostDerivedClass) {
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    assert(!SecondaryVirtualPointerIndices.count(Base) &&
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           "A virtual pointer index already exists for this base subobject!");
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    SecondaryVirtualPointerIndices[Base] = VTTComponents.size();
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  }
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  if (!GenerateDefinition) {
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    VTTComponents.push_back(VTTComponent());
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    return;
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  }
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  VTTComponents.push_back(VTTComponent(VTableIndex, Base));
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}
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void VTTBuilder::LayoutSecondaryVTTs(BaseSubobject Base) {
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  const CXXRecordDecl *RD = Base.getBase();
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  for (const auto &I : RD->bases()) {
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    // Don't layout virtual bases.
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    if (I.isVirtual())
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        continue;
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    const auto *BaseDecl =
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        cast<CXXRecordDecl>(I.getType()->castAs<RecordType>()->getDecl());
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    const ASTRecordLayout &Layout = Ctx.getASTRecordLayout(RD);
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    CharUnits BaseOffset = Base.getBaseOffset() +
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      Layout.getBaseClassOffset(BaseDecl);
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    // Layout the VTT for this base.
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    LayoutVTT(BaseSubobject(BaseDecl, BaseOffset), /*BaseIsVirtual=*/false);
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  }
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}
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void
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VTTBuilder::LayoutSecondaryVirtualPointers(BaseSubobject Base,
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                                           bool BaseIsMorallyVirtual,
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                                           uint64_t VTableIndex,
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                                           const CXXRecordDecl *VTableClass,
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                                           VisitedVirtualBasesSetTy &VBases) {
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  const CXXRecordDecl *RD = Base.getBase();
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  // We're not interested in bases that don't have virtual bases, and not
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  // morally virtual bases.
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  if (!RD->getNumVBases() && !BaseIsMorallyVirtual)
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    return;
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  for (const auto &I : RD->bases()) {
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    const auto *BaseDecl =
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        cast<CXXRecordDecl>(I.getType()->castAs<RecordType>()->getDecl());
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    // Itanium C++ ABI 2.6.2:
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    //   Secondary virtual pointers are present for all bases with either
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    //   virtual bases or virtual function declarations overridden along a
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    //   virtual path.
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    //
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    // If the base class is not dynamic, we don't want to add it, nor any
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    // of its base classes.
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    if (!BaseDecl->isDynamicClass())
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      continue;
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    bool BaseDeclIsMorallyVirtual = BaseIsMorallyVirtual;
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    bool BaseDeclIsNonVirtualPrimaryBase = false;
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    CharUnits BaseOffset;
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    if (I.isVirtual()) {
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      // Ignore virtual bases that we've already visited.
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      if (!VBases.insert(BaseDecl).second)
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        continue;
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      BaseOffset = MostDerivedClassLayout.getVBaseClassOffset(BaseDecl);
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      BaseDeclIsMorallyVirtual = true;
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    } else {
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      const ASTRecordLayout &Layout = Ctx.getASTRecordLayout(RD);
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      BaseOffset = Base.getBaseOffset() +
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        Layout.getBaseClassOffset(BaseDecl);
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      if (!Layout.isPrimaryBaseVirtual() &&
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          Layout.getPrimaryBase() == BaseDecl)
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        BaseDeclIsNonVirtualPrimaryBase = true;
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    }
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    // Itanium C++ ABI 2.6.2:
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    //   Secondary virtual pointers: for each base class X which (a) has virtual
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    //   bases or is reachable along a virtual path from D, and (b) is not a
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    //   non-virtual primary base, the address of the virtual table for X-in-D
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    //   or an appropriate construction virtual table.
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    if (!BaseDeclIsNonVirtualPrimaryBase &&
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        (BaseDecl->getNumVBases() || BaseDeclIsMorallyVirtual)) {
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      // Add the vtable pointer.
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      AddVTablePointer(BaseSubobject(BaseDecl, BaseOffset), VTableIndex,
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                       VTableClass);
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    }
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    // And lay out the secondary virtual pointers for the base class.
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    LayoutSecondaryVirtualPointers(BaseSubobject(BaseDecl, BaseOffset),
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                                   BaseDeclIsMorallyVirtual, VTableIndex,
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                                   VTableClass, VBases);
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  }
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}
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void
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VTTBuilder::LayoutSecondaryVirtualPointers(BaseSubobject Base,
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                                           uint64_t VTableIndex) {
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  VisitedVirtualBasesSetTy VBases;
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  LayoutSecondaryVirtualPointers(Base, /*BaseIsMorallyVirtual=*/false,
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                                 VTableIndex, Base.getBase(), VBases);
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}
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void VTTBuilder::LayoutVirtualVTTs(const CXXRecordDecl *RD,
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                                   VisitedVirtualBasesSetTy &VBases) {
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  for (const auto &I : RD->bases()) {
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    const auto *BaseDecl =
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        cast<CXXRecordDecl>(I.getType()->castAs<RecordType>()->getDecl());
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    // Check if this is a virtual base.
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    if (I.isVirtual()) {
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      // Check if we've seen this base before.
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      if (!VBases.insert(BaseDecl).second)
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        continue;
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      CharUnits BaseOffset =
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        MostDerivedClassLayout.getVBaseClassOffset(BaseDecl);
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      LayoutVTT(BaseSubobject(BaseDecl, BaseOffset), /*BaseIsVirtual=*/true);
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    }
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    // We only need to layout virtual VTTs for this base if it actually has
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    // virtual bases.
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    if (BaseDecl->getNumVBases())
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      LayoutVirtualVTTs(BaseDecl, VBases);
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  }
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}
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void VTTBuilder::LayoutVTT(BaseSubobject Base, bool BaseIsVirtual) {
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  const CXXRecordDecl *RD = Base.getBase();
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  // Itanium C++ ABI 2.6.2:
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  //   An array of virtual table addresses, called the VTT, is declared for
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  //   each class type that has indirect or direct virtual base classes.
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  if (RD->getNumVBases() == 0)
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    return;
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  bool IsPrimaryVTT = Base.getBase() == MostDerivedClass;
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  if (!IsPrimaryVTT) {
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    // Remember the sub-VTT index.
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    SubVTTIndices[Base] = VTTComponents.size();
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  }
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  uint64_t VTableIndex = VTTVTables.size();
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  VTTVTables.push_back(VTTVTable(Base, BaseIsVirtual));
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  // Add the primary vtable pointer.
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  AddVTablePointer(Base, VTableIndex, RD);
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  // Add the secondary VTTs.
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  LayoutSecondaryVTTs(Base);
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  // Add the secondary virtual pointers.
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  LayoutSecondaryVirtualPointers(Base, VTableIndex);
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  // If this is the primary VTT, we want to lay out virtual VTTs as well.
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  if (IsPrimaryVTT) {
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    VisitedVirtualBasesSetTy VBases;
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    LayoutVirtualVTTs(Base.getBase(), VBases);
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  }
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}
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