Recommit "Implement [[msvc::no_unique_address]] (#65675)" (#67199)

This implements the [[msvc::no_unique_address]] attribute.

There is not ABI compatibility in this patch because the attribute is
relatively new and there's still some uncertainty in the MSVC version.

The recommit changes the attribute definitions so that instead of making
two separate attributes for no_unique_address
and msvc::no_unique_address, it modifies the attributes tablegen emitter
to allow spellings to be target-specific.

This reverts commit 71f9e76.

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

@@ -4513,9 +4513,14 @@ 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

@@ -4466,6 +4466,14 @@ 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);