Recommit "Implement [[msvc::no_unique_address]] (#65675)"

clang/include/clang/Basic/Attr.td

@@ -450,6 +450,12 @@ def TargetELF : TargetSpec {
 def TargetSupportsInitPriority : TargetSpec {
   let CustomCode = [{ !Target.getTriple().isOSzOS() }];
 }
+
+class TargetSpecificSpelling<TargetSpec target, list<Spelling> spellings> {
+  TargetSpec Target = target;
+  list<Spelling> Spellings = spellings;
+}
+
 // Attribute subject match rules that are used for #pragma clang attribute.
 //
 // A instance of AttrSubjectMatcherRule represents an individual match rule.
@@ -576,6 +582,8 @@ class Attr {
   list<Argument> Args = [];
   // Accessors which should be generated for the attribute.
   list<Accessor> Accessors = [];
+  // Specify targets for spellings.
+  list<TargetSpecificSpelling> TargetSpecificSpellings = [];
   // Set to true for attributes with arguments which require delayed parsing.
   bit LateParsed = 0;
   // Set to false to prevent an attribute from being propagated from a template
@@ -1798,11 +1806,14 @@ def ArmMveStrictPolymorphism : TypeAttr, TargetSpecificAttr<TargetARM> {
   let Documentation = [ArmMveStrictPolymorphismDocs];
 }
 
-def NoUniqueAddress : InheritableAttr, TargetSpecificAttr<TargetItaniumCXXABI> {
-  let Spellings = [CXX11<"", "no_unique_address", 201803>];
+def NoUniqueAddress : InheritableAttr {
   let Subjects = SubjectList<[NonBitField], ErrorDiag>;
+  let Spellings = [CXX11<"", "no_unique_address", 201803>, CXX11<"msvc", "no_unique_address", 201803>];
+  let TargetSpecificSpellings = [
+    TargetSpecificSpelling<TargetItaniumCXXABI, [CXX11<"", "no_unique_address", 201803>]>,
+    TargetSpecificSpelling<TargetMicrosoftCXXABI, [CXX11<"msvc", "no_unique_address", 201803>]>,
+  ];
   let Documentation = [NoUniqueAddressDocs];
-  let SimpleHandler = 1;
 }
 
 def ReturnsTwice : InheritableAttr {

clang/include/clang/Basic/AttrDocs.td

@@ -1405,6 +1405,10 @@ Example usage:
 
 ``[[no_unique_address]]`` is a standard C++20 attribute. Clang supports its use
 in C++11 onwards.
+
+On MSVC targets, ``[[no_unique_address]]`` is ignored; use
+``[[msvc::no_unique_address]]`` instead. Currently there is no guarantee of ABI
+compatibility or stability with MSVC.
   }];
 }
 

clang/include/clang/Basic/ParsedAttrInfo.h

@@ -116,6 +116,14 @@ struct ParsedAttrInfo {
 
   /// Check if this attribute is allowed when compiling for the given target.
   virtual bool existsInTarget(const TargetInfo &Target) const { return true; }
+
+  /// Check if this attribute's spelling is allowed when compiling for the given
+  /// target.
+  virtual bool spellingExistsInTarget(const TargetInfo &Target,
+                                      const unsigned SpellingListIndex) const {
+    return true;
+  }
+
   /// Convert the spelling index of Attr to a semantic spelling enum value.
   virtual unsigned
   spellingIndexToSemanticSpelling(const ParsedAttr &Attr) const {

clang/lib/AST/Decl.cpp

@@ -4507,9 +4507,14 @@ bool FieldDecl::isZeroSize(const ASTContext &Ctx) const {
 
   // Otherwise, [...] the circumstances under which the object has zero size
   // are implementation-defined.
-  // FIXME: This might be Itanium ABI specific; we don't yet know what the MS
-  // ABI will do.
-  return true;
+  if (!Ctx.getTargetInfo().getCXXABI().isMicrosoft())
+    return true;
+
+  // MS ABI: has nonzero size if it is a class type with class type fields,
+  // whether or not they have nonzero size
+  return !llvm::any_of(CXXRD->fields(), [](const FieldDecl *Field) {
+    return Field->getType()->getAs<RecordType>();
+  });
 }
 
 bool FieldDecl::isPotentiallyOverlapping() const {

clang/lib/AST/RecordLayoutBuilder.cpp

@@ -2545,7 +2545,10 @@ struct MicrosoftRecordLayoutBuilder {
     CharUnits Alignment;
   };
   typedef llvm::DenseMap<const CXXRecordDecl *, CharUnits> BaseOffsetsMapTy;
-  MicrosoftRecordLayoutBuilder(const ASTContext &Context) : Context(Context) {}
+  MicrosoftRecordLayoutBuilder(const ASTContext &Context,
+                               EmptySubobjectMap *EmptySubobjects)
+      : Context(Context), EmptySubobjects(EmptySubobjects) {}
+
 private:
   MicrosoftRecordLayoutBuilder(const MicrosoftRecordLayoutBuilder &) = delete;
   void operator=(const MicrosoftRecordLayoutBuilder &) = delete;
@@ -2595,6 +2598,8 @@ struct MicrosoftRecordLayoutBuilder {
       llvm::SmallPtrSetImpl<const CXXRecordDecl *> &HasVtorDispSet,
       const CXXRecordDecl *RD) const;
   const ASTContext &Context;
+  EmptySubobjectMap *EmptySubobjects;
+
   /// The size of the record being laid out.
   CharUnits Size;
   /// The non-virtual size of the record layout.
@@ -2908,8 +2913,7 @@ static bool recordUsesEBO(const RecordDecl *RD) {
 }
 
 void MicrosoftRecordLayoutBuilder::layoutNonVirtualBase(
-    const CXXRecordDecl *RD,
-    const CXXRecordDecl *BaseDecl,
+    const CXXRecordDecl *RD, const CXXRecordDecl *BaseDecl,
     const ASTRecordLayout &BaseLayout,
     const ASTRecordLayout *&PreviousBaseLayout) {
   // Insert padding between two bases if the left first one is zero sized or
@@ -2942,6 +2946,7 @@ void MicrosoftRecordLayoutBuilder::layoutNonVirtualBase(
   }
   Bases.insert(std::make_pair(BaseDecl, BaseOffset));
   Size += BaseLayout.getNonVirtualSize();
+  DataSize = Size;
   PreviousBaseLayout = &BaseLayout;
 }
 
@@ -2959,15 +2964,43 @@ void MicrosoftRecordLayoutBuilder::layoutField(const FieldDecl *FD) {
   LastFieldIsNonZeroWidthBitfield = false;
   ElementInfo Info = getAdjustedElementInfo(FD);
   Alignment = std::max(Alignment, Info.Alignment);
-  CharUnits FieldOffset;
-  if (UseExternalLayout)
+
+  const CXXRecordDecl *FieldClass = FD->getType()->getAsCXXRecordDecl();
+  bool IsOverlappingEmptyField = FD->isPotentiallyOverlapping() &&
+                                 FieldClass->isEmpty() &&
+                                 FieldClass->fields().empty();
+  CharUnits FieldOffset = CharUnits::Zero();
+
+  if (UseExternalLayout) {
     FieldOffset =
         Context.toCharUnitsFromBits(External.getExternalFieldOffset(FD));
-  else if (IsUnion)
+  } else if (IsUnion) {
     FieldOffset = CharUnits::Zero();
-  else
+  } else if (EmptySubobjects) {
+    if (!IsOverlappingEmptyField)
+      FieldOffset = DataSize.alignTo(Info.Alignment);
+
+    while (!EmptySubobjects->CanPlaceFieldAtOffset(FD, FieldOffset)) {
+      const CXXRecordDecl *ParentClass = cast<CXXRecordDecl>(FD->getParent());
+      bool HasBases = ParentClass && (!ParentClass->bases().empty() ||
+                                      !ParentClass->vbases().empty());
+      if (FieldOffset == CharUnits::Zero() && DataSize != CharUnits::Zero() &&
+          HasBases) {
+        // MSVC appears to only do this when there are base classes;
+        // otherwise it overlaps no_unique_address fields in non-zero offsets.
+        FieldOffset = DataSize.alignTo(Info.Alignment);
+      } else {
+        FieldOffset += Info.Alignment;
+      }
+    }
+  } else {
     FieldOffset = Size.alignTo(Info.Alignment);
+  }
   placeFieldAtOffset(FieldOffset);
+
+  if (!IsOverlappingEmptyField)
+    DataSize = std::max(DataSize, FieldOffset + Info.Size);
+
   Size = std::max(Size, FieldOffset + Info.Size);
 }
 
@@ -3013,6 +3046,7 @@ void MicrosoftRecordLayoutBuilder::layoutBitField(const FieldDecl *FD) {
     Alignment = std::max(Alignment, Info.Alignment);
     RemainingBitsInField = Context.toBits(Info.Size) - Width;
   }
+  DataSize = Size;
 }
 
 void
@@ -3038,6 +3072,7 @@ MicrosoftRecordLayoutBuilder::layoutZeroWidthBitField(const FieldDecl *FD) {
     Size = FieldOffset;
     Alignment = std::max(Alignment, Info.Alignment);
   }
+  DataSize = Size;
 }
 
 void MicrosoftRecordLayoutBuilder::injectVBPtr(const CXXRecordDecl *RD) {
@@ -3304,8 +3339,9 @@ ASTContext::getASTRecordLayout(const RecordDecl *D) const {
   const ASTRecordLayout *NewEntry = nullptr;
 
   if (isMsLayout(*this)) {
-    MicrosoftRecordLayoutBuilder Builder(*this);
     if (const auto *RD = dyn_cast<CXXRecordDecl>(D)) {
+      EmptySubobjectMap EmptySubobjects(*this, RD);
+      MicrosoftRecordLayoutBuilder Builder(*this, &EmptySubobjects);
       Builder.cxxLayout(RD);
       NewEntry = new (*this) ASTRecordLayout(
           *this, Builder.Size, Builder.Alignment, Builder.Alignment,
@@ -3317,6 +3353,7 @@ ASTContext::getASTRecordLayout(const RecordDecl *D) const {
           Builder.EndsWithZeroSizedObject, Builder.LeadsWithZeroSizedBase,
           Builder.Bases, Builder.VBases);
     } else {
+      MicrosoftRecordLayoutBuilder Builder(*this, /*EmptySubobjects=*/nullptr);
       Builder.layout(D);
       NewEntry = new (*this) ASTRecordLayout(
           *this, Builder.Size, Builder.Alignment, Builder.Alignment,

clang/lib/Parse/ParseDeclCXX.cpp

@@ -4464,6 +4464,14 @@ bool Parser::ParseCXX11AttributeArgs(
   if (!Attrs.empty() &&
       IsBuiltInOrStandardCXX11Attribute(AttrName, ScopeName)) {
     ParsedAttr &Attr = Attrs.back();
+
+    // Ignore attributes that don't exist for the target.
+    if (!Attr.existsInTarget(getTargetInfo())) {
+      Diag(LParenLoc, diag::warn_unknown_attribute_ignored) << AttrName;
+      Attr.setInvalid(true);
+      return true;
+    }
+
     // If the attribute is a standard or built-in attribute and we are
     // parsing an argument list, we need to determine whether this attribute
     // was allowed to have an argument list (such as [[deprecated]]), and how

clang/lib/Sema/ParsedAttr.cpp

@@ -193,7 +193,9 @@ bool ParsedAttr::isTypeAttr() const { return getInfo().IsType; }
 bool ParsedAttr::isStmtAttr() const { return getInfo().IsStmt; }
 
 bool ParsedAttr::existsInTarget(const TargetInfo &Target) const {
-  return getInfo().existsInTarget(Target);
+  return getInfo().existsInTarget(Target) &&
+         getInfo().spellingExistsInTarget(Target,
+                                          getAttributeSpellingListIndex());
 }
 
 bool ParsedAttr::isKnownToGCC() const { return getInfo().IsKnownToGCC; }

clang/lib/Sema/SemaDeclAttr.cpp

@@ -8372,6 +8372,10 @@ static void handleNoMergeAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
   D->addAttr(NoMergeAttr::Create(S.Context, AL));
 }
 
+static void handleNoUniqueAddressAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
+  D->addAttr(NoUniqueAddressAttr::Create(S.Context, AL));
+}
+
 static void handleSYCLKernelAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
   // The 'sycl_kernel' attribute applies only to function templates.
   const auto *FD = cast<FunctionDecl>(D);
@@ -9277,6 +9281,9 @@ ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D, const ParsedAttr &AL,
   case ParsedAttr::AT_NoMerge:
     handleNoMergeAttr(S, D, AL);
     break;
+  case ParsedAttr::AT_NoUniqueAddress:
+    handleNoUniqueAddressAttr(S, D, AL);
+    break;
 
   case ParsedAttr::AT_AvailableOnlyInDefaultEvalMethod:
     handleAvailableOnlyInDefaultEvalMethod(S, D, AL);