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

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.

Bug: #49358

Also see https://reviews.llvm.org/D157762.

clang/include/clang/Basic/Attr.td

@@ -1798,11 +1798,24 @@ 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>;
+  // No spellings because instances of this attribute are created by
+  // MSNoUniqueAddress and ItaniumNoUniqueAddress
+  let Spellings = [];
+  let Documentation = [NoUniqueAddressDocs];
+}
+
+def MSNoUniqueAddress : InheritableAttr, TargetSpecificAttr<TargetMicrosoftCXXABI> {
+  let Subjects = SubjectList<[NonBitField], ErrorDiag>;
+  let Spellings = [CXX11<"msvc", "no_unique_address", 201803>];
+  let Documentation = [NoUniqueAddressDocs];
+}
+
+def ItaniumNoUniqueAddress : InheritableAttr, TargetSpecificAttr<TargetItaniumCXXABI> {
+  let Subjects = SubjectList<[NonBitField], ErrorDiag>;
+  let Spellings = [CXX11<"", "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.
   }];
 }
 

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

@@ -4374,7 +4374,8 @@ static bool IsBuiltInOrStandardCXX11Attribute(IdentifierInfo *AttrName,
   case ParsedAttr::AT_Deprecated:
   case ParsedAttr::AT_FallThrough:
   case ParsedAttr::AT_CXX11NoReturn:
-  case ParsedAttr::AT_NoUniqueAddress:
+  case ParsedAttr::AT_MSNoUniqueAddress:
+  case ParsedAttr::AT_ItaniumNoUniqueAddress:
   case ParsedAttr::AT_Likely:
   case ParsedAttr::AT_Unlikely:
     return true;

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 +9281,12 @@ ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D, const ParsedAttr &AL,
   case ParsedAttr::AT_NoMerge:
     handleNoMergeAttr(S, D, AL);
     break;
+  case ParsedAttr::AT_MSNoUniqueAddress:
+    handleNoUniqueAddressAttr(S, D, AL);
+    break;
+  case ParsedAttr::AT_ItaniumNoUniqueAddress:
+    handleNoUniqueAddressAttr(S, D, AL);
+    break;
 
   case ParsedAttr::AT_AvailableOnlyInDefaultEvalMethod:
     handleAvailableOnlyInDefaultEvalMethod(S, D, AL);