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diff --git a/clang/lib/AST/ItaniumCXXABI.cpp b/clang/lib/AST/ItaniumCXXABI.cpp
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+++ b/clang/lib/AST/ItaniumCXXABI.cpp
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+//===------- ItaniumCXXABI.cpp - AST support for the Itanium C++ ABI ------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This provides C++ AST support targeting the Itanium C++ ABI, which is
+// documented at:
+//  http://www.codesourcery.com/public/cxx-abi/abi.html
+//  http://www.codesourcery.com/public/cxx-abi/abi-eh.html
+//
+// It also supports the closely-related ARM C++ ABI, documented at:
+// http://infocenter.arm.com/help/topic/com.arm.doc.ihi0041c/IHI0041C_cppabi.pdf
+//
+//===----------------------------------------------------------------------===//
+
+#include "CXXABI.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/AST/DeclCXX.h"
+#include "clang/AST/Mangle.h"
+#include "clang/AST/MangleNumberingContext.h"
+#include "clang/AST/RecordLayout.h"
+#include "clang/AST/Type.h"
+#include "clang/Basic/TargetInfo.h"
+#include "llvm/ADT/FoldingSet.h"
+#include "llvm/ADT/iterator.h"
+
+using namespace clang;
+
+namespace {
+
+/// According to Itanium C++ ABI 5.1.2:
+/// the name of an anonymous union is considered to be
+/// the name of the first named data member found by a pre-order,
+/// depth-first, declaration-order walk of the data members of
+/// the anonymous union.
+/// If there is no such data member (i.e., if all of the data members
+/// in the union are unnamed), then there is no way for a program to
+/// refer to the anonymous union, and there is therefore no need to mangle its name.
+///
+/// Returns the name of anonymous union VarDecl or nullptr if it is not found.
+static const IdentifierInfo *findAnonymousUnionVarDeclName(const VarDecl& VD) {
+  const RecordType *RT = VD.getType()->getAs<RecordType>();
+  assert(RT && "type of VarDecl is expected to be RecordType.");
+  assert(RT->getDecl()->isUnion() && "RecordType is expected to be a union.");
+  if (const FieldDecl *FD = RT->getDecl()->findFirstNamedDataMember()) {
+    return FD->getIdentifier();
+  }
+
+  return nullptr;
+}
+
+/// The name of a decomposition declaration.
+struct DecompositionDeclName {
+  using BindingArray = ArrayRef<const BindingDecl*>;
+
+  /// Representative example of a set of bindings with these names.
+  BindingArray Bindings;
+
+  /// Iterators over the sequence of identifiers in the name.
+  struct Iterator
+      : llvm::iterator_adaptor_base<Iterator, BindingArray::const_iterator,
+                                    std::random_access_iterator_tag,
+                                    const IdentifierInfo *> {
+    Iterator(BindingArray::const_iterator It) : iterator_adaptor_base(It) {}
+    const IdentifierInfo *operator*() const {
+      return (*this->I)->getIdentifier();
+    }
+  };
+  Iterator begin() const { return Iterator(Bindings.begin()); }
+  Iterator end() const { return Iterator(Bindings.end()); }
+};
+}
+
+namespace llvm {
+template<typename T> bool isDenseMapKeyEmpty(T V) {
+  return llvm::DenseMapInfo<T>::isEqual(
+      V, llvm::DenseMapInfo<T>::getEmptyKey());
+}
+template<typename T> bool isDenseMapKeyTombstone(T V) {
+  return llvm::DenseMapInfo<T>::isEqual(
+      V, llvm::DenseMapInfo<T>::getTombstoneKey());
+}
+
+template<typename T>
+Optional<bool> areDenseMapKeysEqualSpecialValues(T LHS, T RHS) {
+  bool LHSEmpty = isDenseMapKeyEmpty(LHS);
+  bool RHSEmpty = isDenseMapKeyEmpty(RHS);
+  if (LHSEmpty || RHSEmpty)
+    return LHSEmpty && RHSEmpty;
+
+  bool LHSTombstone = isDenseMapKeyTombstone(LHS);
+  bool RHSTombstone = isDenseMapKeyTombstone(RHS);
+  if (LHSTombstone || RHSTombstone)
+    return LHSTombstone && RHSTombstone;
+
+  return None;
+}
+
+template<>
+struct DenseMapInfo<DecompositionDeclName> {
+  using ArrayInfo = llvm::DenseMapInfo<ArrayRef<const BindingDecl*>>;
+  static DecompositionDeclName getEmptyKey() {
+    return {ArrayInfo::getEmptyKey()};
+  }
+  static DecompositionDeclName getTombstoneKey() {
+    return {ArrayInfo::getTombstoneKey()};
+  }
+  static unsigned getHashValue(DecompositionDeclName Key) {
+    assert(!isEqual(Key, getEmptyKey()) && !isEqual(Key, getTombstoneKey()));
+    return llvm::hash_combine_range(Key.begin(), Key.end());
+  }
+  static bool isEqual(DecompositionDeclName LHS, DecompositionDeclName RHS) {
+    if (Optional<bool> Result = areDenseMapKeysEqualSpecialValues(
+            LHS.Bindings, RHS.Bindings))
+      return *Result;
+
+    return LHS.Bindings.size() == RHS.Bindings.size() &&
+           std::equal(LHS.begin(), LHS.end(), RHS.begin());
+  }
+};
+}
+
+namespace {
+
+/// Keeps track of the mangled names of lambda expressions and block
+/// literals within a particular context.
+class ItaniumNumberingContext : public MangleNumberingContext {
+  ItaniumMangleContext *Mangler;
+  llvm::StringMap<unsigned> LambdaManglingNumbers;
+  unsigned BlockManglingNumber = 0;
+  llvm::DenseMap<const IdentifierInfo *, unsigned> VarManglingNumbers;
+  llvm::DenseMap<const IdentifierInfo *, unsigned> TagManglingNumbers;
+  llvm::DenseMap<DecompositionDeclName, unsigned>
+      DecompsitionDeclManglingNumbers;
+
+public:
+  ItaniumNumberingContext(ItaniumMangleContext *Mangler) : Mangler(Mangler) {}
+
+  unsigned getManglingNumber(const CXXMethodDecl *CallOperator) override {
+    const CXXRecordDecl *Lambda = CallOperator->getParent();
+    assert(Lambda->isLambda());
+
+    // Computation of the <lambda-sig> is non-trivial and subtle. Rather than
+    // duplicating it here, just mangle the <lambda-sig> directly.
+    llvm::SmallString<128> LambdaSig;
+    llvm::raw_svector_ostream Out(LambdaSig);
+    Mangler->mangleLambdaSig(Lambda, Out);
+
+    return ++LambdaManglingNumbers[LambdaSig];
+  }
+
+  unsigned getManglingNumber(const BlockDecl *BD) override {
+    return ++BlockManglingNumber;
+  }
+
+  unsigned getStaticLocalNumber(const VarDecl *VD) override {
+    return 0;
+  }
+
+  /// Variable decls are numbered by identifier.
+  unsigned getManglingNumber(const VarDecl *VD, unsigned) override {
+    if (auto *DD = dyn_cast<DecompositionDecl>(VD)) {
+      DecompositionDeclName Name{DD->bindings()};
+      return ++DecompsitionDeclManglingNumbers[Name];
+    }
+
+    const IdentifierInfo *Identifier = VD->getIdentifier();
+    if (!Identifier) {
+      // VarDecl without an identifier represents an anonymous union
+      // declaration.
+      Identifier = findAnonymousUnionVarDeclName(*VD);
+    }
+    return ++VarManglingNumbers[Identifier];
+  }
+
+  unsigned getManglingNumber(const TagDecl *TD, unsigned) override {
+    return ++TagManglingNumbers[TD->getIdentifier()];
+  }
+};
+
+class ItaniumCXXABI : public CXXABI {
+private:
+  std::unique_ptr<MangleContext> Mangler;
+protected:
+  ASTContext &Context;
+public:
+  ItaniumCXXABI(ASTContext &Ctx)
+      : Mangler(Ctx.createMangleContext()), Context(Ctx) {}
+
+  MemberPointerInfo
+  getMemberPointerInfo(const MemberPointerType *MPT) const override {
+    const TargetInfo &Target = Context.getTargetInfo();
+    TargetInfo::IntType PtrDiff = Target.getPtrDiffType(0);
+    MemberPointerInfo MPI;
+    MPI.Width = Target.getTypeWidth(PtrDiff);
+    MPI.Align = Target.getTypeAlign(PtrDiff);
+    MPI.HasPadding = false;
+    if (MPT->isMemberFunctionPointer())
+      MPI.Width *= 2;
+    return MPI;
+  }
+
+  CallingConv getDefaultMethodCallConv(bool isVariadic) const override {
+    const llvm::Triple &T = Context.getTargetInfo().getTriple();
+    if (!isVariadic && T.isWindowsGNUEnvironment() &&
+        T.getArch() == llvm::Triple::x86)
+      return CC_X86ThisCall;
+    return Context.getTargetInfo().getDefaultCallingConv();
+  }
+
+  // We cheat and just check that the class has a vtable pointer, and that it's
+  // only big enough to have a vtable pointer and nothing more (or less).
+  bool isNearlyEmpty(const CXXRecordDecl *RD) const override {
+
+    // Check that the class has a vtable pointer.
+    if (!RD->isDynamicClass())
+      return false;
+
+    const ASTRecordLayout &Layout = Context.getASTRecordLayout(RD);
+    CharUnits PointerSize =
+      Context.toCharUnitsFromBits(Context.getTargetInfo().getPointerWidth(0));
+    return Layout.getNonVirtualSize() == PointerSize;
+  }
+
+  const CXXConstructorDecl *
+  getCopyConstructorForExceptionObject(CXXRecordDecl *RD) override {
+    return nullptr;
+  }
+
+  void addCopyConstructorForExceptionObject(CXXRecordDecl *RD,
+                                            CXXConstructorDecl *CD) override {}
+
+  void addTypedefNameForUnnamedTagDecl(TagDecl *TD,
+                                       TypedefNameDecl *DD) override {}
+
+  TypedefNameDecl *getTypedefNameForUnnamedTagDecl(const TagDecl *TD) override {
+    return nullptr;
+  }
+
+  void addDeclaratorForUnnamedTagDecl(TagDecl *TD,
+                                      DeclaratorDecl *DD) override {}
+
+  DeclaratorDecl *getDeclaratorForUnnamedTagDecl(const TagDecl *TD) override {
+    return nullptr;
+  }
+
+  std::unique_ptr<MangleNumberingContext>
+  createMangleNumberingContext() const override {
+    return std::make_unique<ItaniumNumberingContext>(
+        cast<ItaniumMangleContext>(Mangler.get()));
+  }
+};
+}
+
+CXXABI *clang::CreateItaniumCXXABI(ASTContext &Ctx) {
+  return new ItaniumCXXABI(Ctx);
+}