[AST] Pack CXXDependentScopeMemberExpr

[riccibruno]

Summary

Use the newly available space in the bit-fields of Stmt. Additionally store FirstQualifierFoundInScope
as a trailing object since it is most of the time null (non-null for 2 of the 35446 CXXDependentScopeMemberExpr
when parsing all of Boost).

It would be possible to move the data for the nested-name-specifier to a trailing object too to save
another 2 pointers, however doing so did actually regress the time taken to parse all of Boost slightly.

This saves 8 bytes + 1 pointer per CXXDependentScopeMemberExpr in the vast majority of cases.

[rjmccall]

Does the regression disappear if you make the scope specifier the first trailing object? Later trailing objects are more expensive to access, and I would imagine that the scope specifier and template arguments are more important than the other fields.

[riccibruno]

In D56367#1347845, @rjmccall wrote:

Does the regression disappear if you make the scope specifier the first trailing object? Later trailing objects are more expensive to access, and I would imagine that the scope specifier and template arguments are more important than the other fields.

That was with the scope specifier first. I would imagine that putting it last would be indeed be more expensive.
I can leave a TODO here saying that it could be in principle be stored as a trailing object, but the that the run-time effect
of doing so should be investigated.

[rjmccall]

Okay. That comment seems reasonable. Glad to hear you're on top of it. :)

[riccibruno]

Okay. That comment seems reasonable. Glad to hear you're on top of it. :)

I guess that means I can land this as is ?

[rjmccall]

Yeah, LGTM.

include/clang/AST/ExprCXX.h

 ///
 /// Like UnresolvedMemberExprs, these can be either implicit or
 /// explicit accesses.  It is only possible to get one of these with
 /// an implicit access if a qualifier is provided.
 class CXXDependentScopeMemberExpr final
     : public Expr,
       private llvm::TrailingObjects<CXXDependentScopeMemberExpr,
                                     ASTTemplateKWAndArgsInfo,
-                                    TemplateArgumentLoc> {
+                                    TemplateArgumentLoc, NamedDecl *> {
+  friend class ASTStmtReader;
+  friend class ASTStmtWriter;
+  friend TrailingObjects;
+
   /// The expression for the base pointer or class reference,
   /// e.g., the \c x in x.f.  Can be null in implicit accesses.
   Stmt *Base;
 
   /// The type of the base expression.  Never null, even for
   /// implicit accesses.
   QualType BaseType;
 
-  /// Whether this member expression used the '->' operator or
-  /// the '.' operator.
-  bool IsArrow : 1;
-
-  /// Whether this member expression has info for explicit template
-  /// keyword and arguments.
-  bool HasTemplateKWAndArgsInfo : 1;
-
-  /// The location of the '->' or '.' operator.
-  SourceLocation OperatorLoc;
-
   /// The nested-name-specifier that precedes the member name, if any.
+  /// FIXME: This could be in principle store as a trailing object.
+  /// However the performance impact of doing so should be investigated first.
   NestedNameSpecifierLoc QualifierLoc;
 
-  /// In a qualified member access expression such as t->Base::f, this
-  /// member stores the resolves of name lookup in the context of the member
-  /// access expression, to be used at instantiation time.
-  ///
-  /// FIXME: This member, along with the QualifierLoc, could
-  /// be stuck into a structure that is optionally allocated at the end of
-  /// the CXXDependentScopeMemberExpr, to save space in the common case.
-  NamedDecl *FirstQualifierFoundInScope;
-
   /// The member to which this member expression refers, which
   /// can be name, overloaded operator, or destructor.
   ///
   /// FIXME: could also be a template-id
   DeclarationNameInfo MemberNameInfo;
 
-  size_t numTrailingObjects(OverloadToken<ASTTemplateKWAndArgsInfo>) const {
-    return HasTemplateKWAndArgsInfo ? 1 : 0;
+  // CXXDependentScopeMemberExpr is followed by several trailing objects,
+  // some of which optional. They are in order:
+  //
+  // * An optional ASTTemplateKWAndArgsInfo for the explicitly specified
+  //   template keyword and arguments. Present if and only if
+  //   hasTemplateKWAndArgsInfo().
+  //
+  // * An array of getNumTemplateArgs() TemplateArgumentLoc containing location
+  //   information for the explicitly specified template arguments.
+  //
+  // * An optional NamedDecl *. In a qualified member access expression such
+  //   as t->Base::f, this member stores the resolves of name lookup in the
+  //   context of the member access expression, to be used at instantiation
+  //   time. Present if and only if hasFirstQualifierFoundInScope().
+
+  bool hasTemplateKWAndArgsInfo() const {
+    return CXXDependentScopeMemberExprBits.HasTemplateKWAndArgsInfo;
+  }
+
+  bool hasFirstQualifierFoundInScope() const {
+    return CXXDependentScopeMemberExprBits.HasFirstQualifierFoundInScope;
+  }
+
+  unsigned numTrailingObjects(OverloadToken<ASTTemplateKWAndArgsInfo>) const {
+    return hasTemplateKWAndArgsInfo();
+  }
+
+  unsigned numTrailingObjects(OverloadToken<TemplateArgumentLoc>) const {
+    return getNumTemplateArgs();
   }
 
-  CXXDependentScopeMemberExpr(const ASTContext &C, Expr *Base,
+  unsigned numTrailingObjects(OverloadToken<NamedDecl *>) const {
+    return hasFirstQualifierFoundInScope();
+  }
+
+  CXXDependentScopeMemberExpr(const ASTContext &Ctx, Expr *Base,
                               QualType BaseType, bool IsArrow,
                               SourceLocation OperatorLoc,
                               NestedNameSpecifierLoc QualifierLoc,
                               SourceLocation TemplateKWLoc,
                               NamedDecl *FirstQualifierFoundInScope,
                               DeclarationNameInfo MemberNameInfo,
                               const TemplateArgumentListInfo *TemplateArgs);
 
-public:
-  friend class ASTStmtReader;
-  friend class ASTStmtWriter;
-  friend TrailingObjects;
-
-  CXXDependentScopeMemberExpr(const ASTContext &C, Expr *Base,
-                              QualType BaseType, bool IsArrow,
-                              SourceLocation OperatorLoc,
-                              NestedNameSpecifierLoc QualifierLoc,
-                              NamedDecl *FirstQualifierFoundInScope,
-                              DeclarationNameInfo MemberNameInfo);
+  CXXDependentScopeMemberExpr(EmptyShell Empty, bool HasTemplateKWAndArgsInfo,
+                              bool HasFirstQualifierFoundInScope);
 
+public:
   static CXXDependentScopeMemberExpr *
-  Create(const ASTContext &C, Expr *Base, QualType BaseType, bool IsArrow,
+  Create(const ASTContext &Ctx, Expr *Base, QualType BaseType, bool IsArrow,
          SourceLocation OperatorLoc, NestedNameSpecifierLoc QualifierLoc,
          SourceLocation TemplateKWLoc, NamedDecl *FirstQualifierFoundInScope,
          DeclarationNameInfo MemberNameInfo,
          const TemplateArgumentListInfo *TemplateArgs);
 
   static CXXDependentScopeMemberExpr *
-  CreateEmpty(const ASTContext &C, bool HasTemplateKWAndArgsInfo,
-              unsigned NumTemplateArgs);
+  CreateEmpty(const ASTContext &Ctx, bool HasTemplateKWAndArgsInfo,
+              unsigned NumTemplateArgs, bool HasFirstQualifierFoundInScope);
 
   /// True if this is an implicit access, i.e. one in which the
   /// member being accessed was not written in the source.  The source
   /// location of the operator is invalid in this case.
-  bool isImplicitAccess() const;
+  bool isImplicitAccess() const {
+    if (!Base)
+      return true;
+    return cast<Expr>(Base)->isImplicitCXXThis();
+  }
 
   /// Retrieve the base object of this member expressions,
   /// e.g., the \c x in \c x.m.
   Expr *getBase() const {
     assert(!isImplicitAccess());
     return cast<Expr>(Base);
   }
 
   QualType getBaseType() const { return BaseType; }
 
   /// Determine whether this member expression used the '->'
   /// operator; otherwise, it used the '.' operator.
-  bool isArrow() const { return IsArrow; }
+  bool isArrow() const { return CXXDependentScopeMemberExprBits.IsArrow; }
 
   /// Retrieve the location of the '->' or '.' operator.
-  SourceLocation getOperatorLoc() const { return OperatorLoc; }
+  SourceLocation getOperatorLoc() const {
+    return CXXDependentScopeMemberExprBits.OperatorLoc;
+  }
 
-  /// Retrieve the nested-name-specifier that qualifies the member
-  /// name.
+  /// Retrieve the nested-name-specifier that qualifies the member name.
   NestedNameSpecifier *getQualifier() const {
     return QualifierLoc.getNestedNameSpecifier();
   }
 
   /// Retrieve the nested-name-specifier that qualifies the member
   /// name, with source location information.
   NestedNameSpecifierLoc getQualifierLoc() const { return QualifierLoc; }
 
   /// Retrieve the first part of the nested-name-specifier that was
   /// found in the scope of the member access expression when the member access
   /// was initially parsed.
   ///
   /// This function only returns a useful result when member access expression
   /// uses a qualified member name, e.g., "x.Base::f". Here, the declaration
   /// returned by this function describes what was found by unqualified name
   /// lookup for the identifier "Base" within the scope of the member access
   /// expression itself. At template instantiation time, this information is
   /// combined with the results of name lookup into the type of the object
   /// expression itself (the class type of x).
   NamedDecl *getFirstQualifierFoundInScope() const {
-    return FirstQualifierFoundInScope;
+    if (!hasFirstQualifierFoundInScope())
+      return nullptr;
+    return *getTrailingObjects<NamedDecl *>();
   }
 
-  /// Retrieve the name of the member that this expression
-  /// refers to.
+  /// Retrieve the name of the member that this expression refers to.
   const DeclarationNameInfo &getMemberNameInfo() const {
     return MemberNameInfo;
   }
 
-  /// Retrieve the name of the member that this expression
-  /// refers to.
+  /// Retrieve the name of the member that this expression refers to.
   DeclarationName getMember() const { return MemberNameInfo.getName(); }
 
   // Retrieve the location of the name of the member that this
   // expression refers to.
   SourceLocation getMemberLoc() const { return MemberNameInfo.getLoc(); }
 
   /// Retrieve the location of the template keyword preceding the
   /// member name, if any.
   SourceLocation getTemplateKeywordLoc() const {
-    if (!HasTemplateKWAndArgsInfo) return SourceLocation();
+    if (!hasTemplateKWAndArgsInfo())
+      return SourceLocation();
     return getTrailingObjects<ASTTemplateKWAndArgsInfo>()->TemplateKWLoc;
   }
 
   /// Retrieve the location of the left angle bracket starting the
   /// explicit template argument list following the member name, if any.
   SourceLocation getLAngleLoc() const {
-    if (!HasTemplateKWAndArgsInfo) return SourceLocation();
+    if (!hasTemplateKWAndArgsInfo())
+      return SourceLocation();
     return getTrailingObjects<ASTTemplateKWAndArgsInfo>()->LAngleLoc;
   }
 
   /// Retrieve the location of the right angle bracket ending the
   /// explicit template argument list following the member name, if any.
   SourceLocation getRAngleLoc() const {
-    if (!HasTemplateKWAndArgsInfo) return SourceLocation();
+    if (!hasTemplateKWAndArgsInfo())
+      return SourceLocation();
     return getTrailingObjects<ASTTemplateKWAndArgsInfo>()->RAngleLoc;
   }
 
   /// Determines whether the member name was preceded by the template keyword.
   bool hasTemplateKeyword() const { return getTemplateKeywordLoc().isValid(); }
 
   /// Determines whether this member expression actually had a C++
   /// template argument list explicitly specified, e.g., x.f<int>.

include/clang/AST/Stmt.h

class CXXUnresolvedConstructExprBitfields {
     friend class CXXUnresolvedConstructExpr;
 
     unsigned : NumExprBits;
 
     /// The number of arguments used to construct the type.
     unsigned NumArgs;
   };
 
+  class CXXDependentScopeMemberExprBitfields {
+    friend class ASTStmtReader;
+    friend class CXXDependentScopeMemberExpr;
+
+    unsigned : NumExprBits;
+
+    /// Whether this member expression used the '->' operator or
+    /// the '.' operator.
+    unsigned IsArrow : 1;
+
+    /// Whether this member expression has info for explicit template
+    /// keyword and arguments.
+    unsigned HasTemplateKWAndArgsInfo : 1;
+
+    /// See getFirstQualifierFoundInScope() and the comment listing
+    /// the trailing objects.
+    unsigned HasFirstQualifierFoundInScope : 1;
+
+    /// The location of the '->' or '.' operator.
+    SourceLocation OperatorLoc;
+  };
+
   //===--- C++ Coroutines TS bitfields classes ---===//
 
   class CoawaitExprBitfields {
     friend class CoawaitExpr;
 
     unsigned : NumExprBits;
 
     unsigned IsImplicit : 1;
union {
     CXXDefaultInitExprBitfields CXXDefaultInitExprBits;
     CXXNewExprBitfields CXXNewExprBits;
     CXXDeleteExprBitfields CXXDeleteExprBits;
     TypeTraitExprBitfields TypeTraitExprBits;
     DependentScopeDeclRefExprBitfields DependentScopeDeclRefExprBits;
     CXXConstructExprBitfields CXXConstructExprBits;
     ExprWithCleanupsBitfields ExprWithCleanupsBits;
     CXXUnresolvedConstructExprBitfields CXXUnresolvedConstructExprBits;
+    CXXDependentScopeMemberExprBitfields CXXDependentScopeMemberExprBits;
 
     // C++ Coroutines TS expressions
     CoawaitExprBitfields CoawaitBits;
 
     // Obj-C Expressions
     ObjCIndirectCopyRestoreExprBitfields ObjCIndirectCopyRestoreExprBits;
 
     // Clang Extensions

lib/AST/ExprCXX.cpp

CXXUnresolvedConstructExpr::CreateEmpty(const ASTContext &Context,
   return new (Mem) CXXUnresolvedConstructExpr(EmptyShell(), NumArgs);
 }
 
 SourceLocation CXXUnresolvedConstructExpr::getBeginLoc() const {
   return TSI->getTypeLoc().getBeginLoc();
 }
 
 CXXDependentScopeMemberExpr::CXXDependentScopeMemberExpr(
-    const ASTContext &C, Expr *Base, QualType BaseType, bool IsArrow,
+    const ASTContext &Ctx, Expr *Base, QualType BaseType, bool IsArrow,
     SourceLocation OperatorLoc, NestedNameSpecifierLoc QualifierLoc,
     SourceLocation TemplateKWLoc, NamedDecl *FirstQualifierFoundInScope,
     DeclarationNameInfo MemberNameInfo,
     const TemplateArgumentListInfo *TemplateArgs)
-    : Expr(CXXDependentScopeMemberExprClass, C.DependentTy, VK_LValue,
+    : Expr(CXXDependentScopeMemberExprClass, Ctx.DependentTy, VK_LValue,
            OK_Ordinary, true, true, true,
            ((Base && Base->containsUnexpandedParameterPack()) ||
-            (QualifierLoc &&
-             QualifierLoc.getNestedNameSpecifier()
+            (QualifierLoc && QualifierLoc.getNestedNameSpecifier()
                                  ->containsUnexpandedParameterPack()) ||
             MemberNameInfo.containsUnexpandedParameterPack())),
-      Base(Base), BaseType(BaseType), IsArrow(IsArrow),
-      HasTemplateKWAndArgsInfo(TemplateArgs != nullptr ||
-                               TemplateKWLoc.isValid()),
-      OperatorLoc(OperatorLoc), QualifierLoc(QualifierLoc),
-      FirstQualifierFoundInScope(FirstQualifierFoundInScope),
+      Base(Base), BaseType(BaseType), QualifierLoc(QualifierLoc),
       MemberNameInfo(MemberNameInfo) {
+  CXXDependentScopeMemberExprBits.IsArrow = IsArrow;
+  CXXDependentScopeMemberExprBits.HasTemplateKWAndArgsInfo =
+      (TemplateArgs != nullptr) || TemplateKWLoc.isValid();
+  CXXDependentScopeMemberExprBits.HasFirstQualifierFoundInScope =
+      FirstQualifierFoundInScope != nullptr;
+  CXXDependentScopeMemberExprBits.OperatorLoc = OperatorLoc;
+
   if (TemplateArgs) {
     bool Dependent = true;
     bool InstantiationDependent = true;
     bool ContainsUnexpandedParameterPack = false;
     getTrailingObjects<ASTTemplateKWAndArgsInfo>()->initializeFrom(
         TemplateKWLoc, *TemplateArgs, getTrailingObjects<TemplateArgumentLoc>(),
         Dependent, InstantiationDependent, ContainsUnexpandedParameterPack);
     if (ContainsUnexpandedParameterPack)
       ExprBits.ContainsUnexpandedParameterPack = true;
   } else if (TemplateKWLoc.isValid()) {
     getTrailingObjects<ASTTemplateKWAndArgsInfo>()->initializeFrom(
         TemplateKWLoc);
   }
+
+  if (hasFirstQualifierFoundInScope())
+    *getTrailingObjects<NamedDecl *>() = FirstQualifierFoundInScope;
 }
 
-CXXDependentScopeMemberExpr *
-CXXDependentScopeMemberExpr::Create(const ASTContext &C,
-                                Expr *Base, QualType BaseType, bool IsArrow,
-                                SourceLocation OperatorLoc,
-                                NestedNameSpecifierLoc QualifierLoc,
-                                SourceLocation TemplateKWLoc,
-                                NamedDecl *FirstQualifierFoundInScope,
-                                DeclarationNameInfo MemberNameInfo,
-                                const TemplateArgumentListInfo *TemplateArgs) {
-  bool HasTemplateKWAndArgsInfo = TemplateArgs || TemplateKWLoc.isValid();
+CXXDependentScopeMemberExpr::CXXDependentScopeMemberExpr(
+    EmptyShell Empty, bool HasTemplateKWAndArgsInfo,
+    bool HasFirstQualifierFoundInScope)
+    : Expr(CXXDependentScopeMemberExprClass, Empty) {
+  CXXDependentScopeMemberExprBits.HasTemplateKWAndArgsInfo =
+      HasTemplateKWAndArgsInfo;
+  CXXDependentScopeMemberExprBits.HasFirstQualifierFoundInScope =
+      HasFirstQualifierFoundInScope;
+}
+
+CXXDependentScopeMemberExpr *CXXDependentScopeMemberExpr::Create(
+    const ASTContext &Ctx, Expr *Base, QualType BaseType, bool IsArrow,
+    SourceLocation OperatorLoc, NestedNameSpecifierLoc QualifierLoc,
+    SourceLocation TemplateKWLoc, NamedDecl *FirstQualifierFoundInScope,
+  bool HasTemplateKWAndArgsInfo =
+      (TemplateArgs != nullptr) || TemplateKWLoc.isValid();
   unsigned NumTemplateArgs = TemplateArgs ? TemplateArgs->size() : 0;
-  std::size_t Size =
-      totalSizeToAlloc<ASTTemplateKWAndArgsInfo, TemplateArgumentLoc>(
-          HasTemplateKWAndArgsInfo, NumTemplateArgs);
+  bool HasFirstQualifierFoundInScope = FirstQualifierFoundInScope != nullptr;
 
-  void *Mem = C.Allocate(Size, alignof(CXXDependentScopeMemberExpr));
-  return new (Mem) CXXDependentScopeMemberExpr(C, Base, BaseType,
-                                               IsArrow, OperatorLoc,
-                                               QualifierLoc,
-                                               TemplateKWLoc,
-                                               FirstQualifierFoundInScope,
-                                               MemberNameInfo, TemplateArgs);
-}
+  unsigned Size = totalSizeToAlloc<ASTTemplateKWAndArgsInfo,
+                                   TemplateArgumentLoc, NamedDecl *>(
+      HasTemplateKWAndArgsInfo, NumTemplateArgs, HasFirstQualifierFoundInScope);
 
-CXXDependentScopeMemberExpr *
-CXXDependentScopeMemberExpr::CreateEmpty(const ASTContext &C,
-                                         bool HasTemplateKWAndArgsInfo,
-                                         unsigned NumTemplateArgs) {
-  assert(NumTemplateArgs == 0 || HasTemplateKWAndArgsInfo);
-  std::size_t Size =
-      totalSizeToAlloc<ASTTemplateKWAndArgsInfo, TemplateArgumentLoc>(
-          HasTemplateKWAndArgsInfo, NumTemplateArgs);
-  void *Mem = C.Allocate(Size, alignof(CXXDependentScopeMemberExpr));
-  auto *E =
-      new (Mem) CXXDependentScopeMemberExpr(C, nullptr, QualType(),
-                                            false, SourceLocation(),
-                                            NestedNameSpecifierLoc(),
-                                            SourceLocation(), nullptr,
-                                            DeclarationNameInfo(), nullptr);
-  E->HasTemplateKWAndArgsInfo = HasTemplateKWAndArgsInfo;
-  return E;
+  void *Mem = Ctx.Allocate(Size, alignof(CXXDependentScopeMemberExpr));
+  return new (Mem) CXXDependentScopeMemberExpr(
+      Ctx, Base, BaseType, IsArrow, OperatorLoc, QualifierLoc, TemplateKWLoc,
+      FirstQualifierFoundInScope, MemberNameInfo, TemplateArgs);
 }
 
-bool CXXDependentScopeMemberExpr::isImplicitAccess() const {
-  if (!Base)
-    return true;
+CXXDependentScopeMemberExpr *CXXDependentScopeMemberExpr::CreateEmpty(
+    const ASTContext &Ctx, bool HasTemplateKWAndArgsInfo,
+    unsigned NumTemplateArgs, bool HasFirstQualifierFoundInScope) {
+  assert(NumTemplateArgs == 0 || HasTemplateKWAndArgsInfo);
 
-  return cast<Expr>(Base)->isImplicitCXXThis();
+  unsigned Size = totalSizeToAlloc<ASTTemplateKWAndArgsInfo,
+                                   TemplateArgumentLoc, NamedDecl *>(
+      HasTemplateKWAndArgsInfo, NumTemplateArgs, HasFirstQualifierFoundInScope);
+
+  void *Mem = Ctx.Allocate(Size, alignof(CXXDependentScopeMemberExpr));
+  return new (Mem) CXXDependentScopeMemberExpr(
+      EmptyShell(), HasTemplateKWAndArgsInfo, HasFirstQualifierFoundInScope);
 }
 
 static bool hasOnlyNonStaticMemberFunctions(UnresolvedSetIterator begin,
                                             UnresolvedSetIterator end) {
   do {
     NamedDecl *decl = *begin;
     if (isa<UnresolvedUsingValueDecl>(decl))
       return false;

lib/Serialization/ASTReaderStmt.cpp

void ASTStmtReader::VisitExprWithCleanups(ExprWithCleanups *E) {
   for (unsigned i = 0; i != NumObjects; ++i)
     E->getTrailingObjects<BlockDecl *>()[i] =
         ReadDeclAs<BlockDecl>();
 
   E->ExprWithCleanupsBits.CleanupsHaveSideEffects = Record.readInt();
   E->SubExpr = Record.readSubExpr();
 }
 
-void
-ASTStmtReader::VisitCXXDependentScopeMemberExpr(CXXDependentScopeMemberExpr *E){
+void ASTStmtReader::VisitCXXDependentScopeMemberExpr(
+    CXXDependentScopeMemberExpr *E) {
   VisitExpr(E);
 
-  if (Record.readInt()) // HasTemplateKWAndArgsInfo
+  bool HasTemplateKWAndArgsInfo = Record.readInt();
+  unsigned NumTemplateArgs = Record.readInt();
+  bool HasFirstQualifierFoundInScope = Record.readInt();
+
+  assert((HasTemplateKWAndArgsInfo == E->hasTemplateKWAndArgsInfo()) &&
+         "Wrong HasTemplateKWAndArgsInfo!");
+  assert(
+      (HasFirstQualifierFoundInScope == E->hasFirstQualifierFoundInScope()) &&
+      "Wrong HasFirstQualifierFoundInScope!");
+
+  if (HasTemplateKWAndArgsInfo)
     ReadTemplateKWAndArgsInfo(
         *E->getTrailingObjects<ASTTemplateKWAndArgsInfo>(),
-        E->getTrailingObjects<TemplateArgumentLoc>(),
-        /*NumTemplateArgs=*/Record.readInt());
+        E->getTrailingObjects<TemplateArgumentLoc>(), NumTemplateArgs);
 
-  E->Base = Record.readSubExpr();
+  assert((NumTemplateArgs == E->getNumTemplateArgs()) &&
+         "Wrong NumTemplateArgs!");
+
+  E->CXXDependentScopeMemberExprBits.IsArrow = Record.readInt();
+  E->CXXDependentScopeMemberExprBits.OperatorLoc = ReadSourceLocation();
   E->BaseType = Record.readType();
-  E->IsArrow = Record.readInt();
-  E->OperatorLoc = ReadSourceLocation();
   E->QualifierLoc = Record.readNestedNameSpecifierLoc();
-  E->FirstQualifierFoundInScope = ReadDeclAs<NamedDecl>();
+  E->Base = Record.readSubExpr();
+
+  if (HasFirstQualifierFoundInScope)
+    *E->getTrailingObjects<NamedDecl *>() = ReadDeclAs<NamedDecl>();
+
   ReadDeclarationNameInfo(E->MemberNameInfo);
 }
 
 void
 ASTStmtReader::VisitDependentScopeDeclRefExpr(DependentScopeDeclRefExpr *E) {
   VisitExpr(E);
 
   if (Record.readInt()) // HasTemplateKWAndArgsInfo
case EXPR_CXX_PSEUDO_DESTRUCTOR:
       break;
 
     case EXPR_EXPR_WITH_CLEANUPS:
       S = ExprWithCleanups::Create(Context, Empty,
                                    Record[ASTStmtReader::NumExprFields]);
       break;
 
     case EXPR_CXX_DEPENDENT_SCOPE_MEMBER:
-      S = CXXDependentScopeMemberExpr::CreateEmpty(Context,
+      S = CXXDependentScopeMemberExpr::CreateEmpty(
+          Context,
           /*HasTemplateKWAndArgsInfo=*/Record[ASTStmtReader::NumExprFields],
-                  /*NumTemplateArgs=*/Record[ASTStmtReader::NumExprFields]
-                                   ? Record[ASTStmtReader::NumExprFields + 1]
-                                   : 0);
+          /*NumTemplateArgs=*/Record[ASTStmtReader::NumExprFields + 1],
+          /*HasFirstQualifierFoundInScope=*/
+          Record[ASTStmtReader::NumExprFields + 2]);
       break;
 
     case EXPR_CXX_DEPENDENT_SCOPE_DECL_REF:
       S = DependentScopeDeclRefExpr::CreateEmpty(Context,
          /*HasTemplateKWAndArgsInfo=*/Record[ASTStmtReader::NumExprFields],
                   /*NumTemplateArgs=*/Record[ASTStmtReader::NumExprFields]
                                    ? Record[ASTStmtReader::NumExprFields + 1]
                                    : 0);

lib/Serialization/ASTWriterStmt.cpp

void ASTStmtWriter::VisitExprWithCleanups(ExprWithCleanups *E) {
   for (unsigned i = 0, e = E->getNumObjects(); i != e; ++i)
     Record.AddDeclRef(E->getObject(i));
 
   Record.push_back(E->cleanupsHaveSideEffects());
   Record.AddStmt(E->getSubExpr());
   Code = serialization::EXPR_EXPR_WITH_CLEANUPS;
 }
 
-void
-ASTStmtWriter::VisitCXXDependentScopeMemberExpr(CXXDependentScopeMemberExpr *E){
+void ASTStmtWriter::VisitCXXDependentScopeMemberExpr(
+    CXXDependentScopeMemberExpr *E) {
   VisitExpr(E);
 
-  // Don't emit anything here, HasTemplateKWAndArgsInfo must be
-  // emitted first.
+  // Don't emit anything here (or if you do you will have to update
+  // the corresponding deserialization function).
 
-  Record.push_back(E->HasTemplateKWAndArgsInfo);
-  if (E->HasTemplateKWAndArgsInfo) {
+  Record.push_back(E->hasTemplateKWAndArgsInfo());
+  Record.push_back(E->getNumTemplateArgs());
+  Record.push_back(E->hasFirstQualifierFoundInScope());
+
+  if (E->hasTemplateKWAndArgsInfo()) {
     const ASTTemplateKWAndArgsInfo &ArgInfo =
         *E->getTrailingObjects<ASTTemplateKWAndArgsInfo>();
-    Record.push_back(ArgInfo.NumTemplateArgs);
     AddTemplateKWAndArgsInfo(ArgInfo,
                              E->getTrailingObjects<TemplateArgumentLoc>());
   }
 
+  Record.push_back(E->isArrow());
+  Record.AddSourceLocation(E->getOperatorLoc());
+  Record.AddTypeRef(E->getBaseType());
+  Record.AddNestedNameSpecifierLoc(E->getQualifierLoc());
   if (!E->isImplicitAccess())
     Record.AddStmt(E->getBase());
   else
     Record.AddStmt(nullptr);
-  Record.AddTypeRef(E->getBaseType());
-  Record.push_back(E->isArrow());
-  Record.AddSourceLocation(E->getOperatorLoc());
-  Record.AddNestedNameSpecifierLoc(E->getQualifierLoc());
+
+  if (E->hasFirstQualifierFoundInScope())
     Record.AddDeclRef(E->getFirstQualifierFoundInScope());
+
   Record.AddDeclarationNameInfo(E->MemberNameInfo);
   Code = serialization::EXPR_CXX_DEPENDENT_SCOPE_MEMBER;
 }
 
 void
 ASTStmtWriter::VisitDependentScopeDeclRefExpr(DependentScopeDeclRefExpr *E) {
   VisitExpr(E);