DR1672, DR1813, DR1881, DR2120: Implement recent fixes to "standard

layout" rules.

The new rules say that a standard-layout struct has its first non-static
data member and all base classes at offset 0, and consider a class to
not be standard-layout if that would result in multiple subobjects of a
single type having the same address.

We track "is C++11 standard-layout class" separately from "is
standard-layout class" so that the ABIs that need this information can
still use it.

Differential Revision: https://reviews.llvm.org/D45176

llvm-svn: 329332

clang/docs/ReleaseNotes.rst

@@ -66,6 +66,11 @@ Non-comprehensive list of changes in this release
   For example, the ``clang`` binary will be called ``clang-7``
   instead of ``clang-7.0``.
 
+- Clang implements a collection of recent fixes to the C++ standard's definition
+  of "standard-layout". In particular, a class is only considered to be
+  standard-layout if all base classes and the first data member (or bit-field)
+  can be laid out at offset zero.
+
 - ...
 
 New Compiler Flags

clang/include/clang/AST/DeclCXX.h

@@ -348,7 +348,12 @@ class CXXRecordDecl : public RecordDecl {
     /// one pure virtual function, (that can come from a base class).
     unsigned Abstract : 1;
 
-    /// \brief True when this class has standard layout.
+    /// \brief True when this class is standard-layout, per the applicable
+    /// language rules (including DRs).
+    unsigned IsStandardLayout : 1;
+
+    /// \brief True when this class was standard-layout under the C++11
+    /// definition.
     ///
     /// C++11 [class]p7.  A standard-layout class is a class that:
     /// * has no non-static data members of type non-standard-layout class (or
@@ -362,13 +367,19 @@ class CXXRecordDecl : public RecordDecl {
     ///   classes with non-static data members, and
     /// * has no base classes of the same type as the first non-static data
     ///   member.
-    unsigned IsStandardLayout : 1;
+    unsigned IsCXX11StandardLayout : 1;
 
-    /// \brief True when there are no non-empty base classes.
-    ///
+    /// \brief True when any base class has any declared non-static data
+    /// members or bit-fields.
     /// This is a helper bit of state used to implement IsStandardLayout more
     /// efficiently.
-    unsigned HasNoNonEmptyBases : 1;
+    unsigned HasBasesWithFields : 1;
+
+    /// \brief True when any base class has any declared non-static data
+    /// members.
+    /// This is a helper bit of state used to implement IsCXX11StandardLayout
+    /// more efficiently.
+    unsigned HasBasesWithNonStaticDataMembers : 1;
 
     /// \brief True when there are private non-static data members.
     unsigned HasPrivateFields : 1;
@@ -696,6 +707,12 @@ class CXXRecordDecl : public RecordDecl {
   /// deserializing the friends from an external AST source.
   FriendDecl *getFirstFriend() const;
 
+  /// Determine whether this class has an empty base class subobject of type X
+  /// or of one of the types that might be at offset 0 within X (per the C++
+  /// "standard layout" rules).
+  bool hasSubobjectAtOffsetZeroOfEmptyBaseType(ASTContext &Ctx,
+                                               const CXXRecordDecl *X);
+
 protected:
   CXXRecordDecl(Kind K, TagKind TK, const ASTContext &C, DeclContext *DC,
                 SourceLocation StartLoc, SourceLocation IdLoc,
@@ -1301,10 +1318,14 @@ class CXXRecordDecl : public RecordDecl {
   /// not overridden.
   bool isAbstract() const { return data().Abstract; }
 
-  /// \brief Determine whether this class has standard layout per 
-  /// (C++ [class]p7)
+  /// \brief Determine whether this class is standard-layout per 
+  /// C++ [class]p7.
   bool isStandardLayout() const { return data().IsStandardLayout; }
 
+  /// \brief Determine whether this class was standard-layout per 
+  /// C++11 [class]p7, specifically using the C++11 rules without any DRs.
+  bool isCXX11StandardLayout() const { return data().IsCXX11StandardLayout; }
+
   /// \brief Determine whether this class, or any of its class subobjects,
   /// contains a mutable field.
   bool hasMutableFields() const { return data().HasMutableFields; }

clang/include/clang/AST/Type.h

@@ -798,7 +798,8 @@ class QualType {
 
   /// Return true if this is a POD type according to the more relaxed rules
   /// of the C++11 standard, regardless of the current compilation's language.
-  /// (C++0x [basic.types]p9)
+  /// (C++0x [basic.types]p9). Note that, unlike
+  /// CXXRecordDecl::isCXX11StandardLayout, this takes DRs into account.
   bool isCXX11PODType(const ASTContext &Context) const;
 
   /// Return true if this is a trivial type per (C++0x [basic.types]p9)

clang/lib/AST/ASTImporter.cpp

@@ -1071,7 +1071,10 @@ bool ASTNodeImporter::ImportDefinition(RecordDecl *From, RecordDecl *To,
     ToData.Polymorphic = FromData.Polymorphic;
     ToData.Abstract = FromData.Abstract;
     ToData.IsStandardLayout = FromData.IsStandardLayout;
-    ToData.HasNoNonEmptyBases = FromData.HasNoNonEmptyBases;
+    ToData.IsCXX11StandardLayout = FromData.IsCXX11StandardLayout;
+    ToData.HasBasesWithFields = FromData.HasBasesWithFields;
+    ToData.HasBasesWithNonStaticDataMembers =
+        FromData.HasBasesWithNonStaticDataMembers;
     ToData.HasPrivateFields = FromData.HasPrivateFields;
     ToData.HasProtectedFields = FromData.HasProtectedFields;
     ToData.HasPublicFields = FromData.HasPublicFields;