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Implement [[msvc::no_unique_address]]
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This attribute should match the behavior of MSVC's [[msvc::no_unique_address]] attribute.

Bug: llvm#49358

Differential Revision: https://reviews.llvm.org/D157762
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@amykhuang
amykhuang committed Sep 13, 2023
1 parent f5592f3 commit 481337f
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Showing 7 changed files with 540 additions and 27 deletions.
8 changes: 5 additions & 3 deletions clang/include/clang/Basic/Attr.td
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Original file line number Diff line number Diff line change
Expand Up @@ -1798,11 +1798,13 @@ def ArmMveStrictPolymorphism : TypeAttr, TargetSpecificAttr<TargetARM> {
let Documentation = [ArmMveStrictPolymorphismDocs];
}

def NoUniqueAddress : InheritableAttr, TargetSpecificAttr<TargetItaniumCXXABI> {
let Spellings = [CXX11<"", "no_unique_address", 201803>];
def NoUniqueAddress : InheritableAttr {
let Spellings = [CXX11<"", "no_unique_address", 201803>,
CXX11<"msvc", "no_unique_address", 201803>];
let Accessors = [Accessor<"isDefault", [CXX11<"", "no_unique_address", 201803>]>,
Accessor<"isMSVC", [CXX11<"msvc", "no_unique_address", 201803>]>];
let Subjects = SubjectList<[NonBitField], ErrorDiag>;
let Documentation = [NoUniqueAddressDocs];
let SimpleHandler = 1;
}

def ReturnsTwice : InheritableAttr {
Expand Down
4 changes: 4 additions & 0 deletions clang/include/clang/Basic/AttrDocs.td
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Original file line number Diff line number Diff line change
Expand Up @@ -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.
}];
}

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8 changes: 8 additions & 0 deletions clang/lib/AST/Decl.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -4505,6 +4505,14 @@ bool FieldDecl::isZeroSize(const ASTContext &Ctx) const {
if (!CXXRD->isEmpty())
return false;

// MS ABI: nonzero if class type with class type fields
if (Ctx.getTargetInfo().getCXXABI().isMicrosoft() &&
llvm::any_of(CXXRD->fields(), [](const FieldDecl *Field) {
return Field->getType()->getAs<RecordType>();
})) {
return false;
}

// 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
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191 changes: 167 additions & 24 deletions clang/lib/AST/RecordLayoutBuilder.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -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;
Expand All @@ -2562,7 +2565,11 @@ struct MicrosoftRecordLayoutBuilder {
void layoutNonVirtualBase(const CXXRecordDecl *RD,
const CXXRecordDecl *BaseDecl,
const ASTRecordLayout &BaseLayout,
const ASTRecordLayout *&PreviousBaseLayout);
const ASTRecordLayout *&PreviousBaseLayout,
BaseSubobjectInfo *Base);
BaseSubobjectInfo *computeBaseSubobjectInfo(const CXXRecordDecl *RD,
bool IsVirtual,
BaseSubobjectInfo *Derived);
void injectVFPtr(const CXXRecordDecl *RD);
void injectVBPtr(const CXXRecordDecl *RD);
/// Lays out the fields of the record. Also rounds size up to
Expand Down Expand Up @@ -2595,6 +2602,12 @@ struct MicrosoftRecordLayoutBuilder {
llvm::SmallPtrSetImpl<const CXXRecordDecl *> &HasVtorDispSet,
const CXXRecordDecl *RD) const;
const ASTContext &Context;
EmptySubobjectMap *EmptySubobjects;
llvm::SpecificBumpPtrAllocator<BaseSubobjectInfo> BaseSubobjectInfoAllocator;
typedef llvm::DenseMap<const CXXRecordDecl *, BaseSubobjectInfo *>
BaseSubobjectInfoMapTy;
BaseSubobjectInfoMapTy VirtualBaseInfo;

/// The size of the record being laid out.
CharUnits Size;
/// The non-virtual size of the record layout.
Expand Down Expand Up @@ -2818,6 +2831,8 @@ MicrosoftRecordLayoutBuilder::layoutNonVirtualBases(const CXXRecordDecl *RD) {
// Iterate through the bases and lay out the non-virtual ones.
for (const CXXBaseSpecifier &Base : RD->bases()) {
const CXXRecordDecl *BaseDecl = Base.getType()->getAsCXXRecordDecl();
BaseSubobjectInfo *BaseInfo =
computeBaseSubobjectInfo(BaseDecl, Base.isVirtual(), nullptr);
HasPolymorphicBaseClass |= BaseDecl->isPolymorphic();
const ASTRecordLayout &BaseLayout = Context.getASTRecordLayout(BaseDecl);
// Mark and skip virtual bases.
Expand All @@ -2839,7 +2854,8 @@ MicrosoftRecordLayoutBuilder::layoutNonVirtualBases(const CXXRecordDecl *RD) {
LeadsWithZeroSizedBase = BaseLayout.leadsWithZeroSizedBase();
}
// Lay out the base.
layoutNonVirtualBase(RD, BaseDecl, BaseLayout, PreviousBaseLayout);
layoutNonVirtualBase(RD, BaseDecl, BaseLayout, PreviousBaseLayout,
BaseInfo);
}
// Figure out if we need a fresh VFPtr for this class.
if (RD->isPolymorphic()) {
Expand All @@ -2864,10 +2880,21 @@ MicrosoftRecordLayoutBuilder::layoutNonVirtualBases(const CXXRecordDecl *RD) {
bool CheckLeadingLayout = !PrimaryBase;
// Iterate through the bases and lay out the non-virtual ones.
for (const CXXBaseSpecifier &Base : RD->bases()) {
if (Base.isVirtual())
continue;
const CXXRecordDecl *BaseDecl = Base.getType()->getAsCXXRecordDecl();
const ASTRecordLayout &BaseLayout = Context.getASTRecordLayout(BaseDecl);
BaseSubobjectInfo *BaseInfo =
computeBaseSubobjectInfo(BaseDecl, Base.isVirtual(), nullptr);

if (Base.isVirtual()) {
// Mark offset for virtual base.
CharUnits Offset = CharUnits::Zero();
while (!EmptySubobjects->CanPlaceBaseAtOffset(BaseInfo, Offset)) {
ElementInfo Info = getAdjustedElementInfo(BaseLayout);
Offset += Info.Alignment;
}
continue;
}

// Only lay out bases without extendable VFPtrs on the second pass.
if (BaseLayout.hasExtendableVFPtr()) {
VBPtrOffset = Bases[BaseDecl] + BaseLayout.getNonVirtualSize();
Expand All @@ -2880,7 +2907,8 @@ MicrosoftRecordLayoutBuilder::layoutNonVirtualBases(const CXXRecordDecl *RD) {
LeadsWithZeroSizedBase = BaseLayout.leadsWithZeroSizedBase();
}
// Lay out the base.
layoutNonVirtualBase(RD, BaseDecl, BaseLayout, PreviousBaseLayout);
layoutNonVirtualBase(RD, BaseDecl, BaseLayout, PreviousBaseLayout,
BaseInfo);
VBPtrOffset = Bases[BaseDecl] + BaseLayout.getNonVirtualSize();
}
// Set our VBPtroffset if we know it at this point.
Expand Down Expand Up @@ -2908,10 +2936,9 @@ 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) {
const ASTRecordLayout *&PreviousBaseLayout, BaseSubobjectInfo *Base) {
// Insert padding between two bases if the left first one is zero sized or
// contains a zero sized subobject and the right is zero sized or one leads
// with a zero sized base.
Expand All @@ -2927,7 +2954,7 @@ void MicrosoftRecordLayoutBuilder::layoutNonVirtualBase(
if (UseExternalLayout) {
FoundBase = External.getExternalNVBaseOffset(BaseDecl, BaseOffset);
if (BaseOffset > Size) {
Size = BaseOffset;
DataSize = Size = BaseOffset;
}
}

Expand All @@ -2937,14 +2964,97 @@ void MicrosoftRecordLayoutBuilder::layoutNonVirtualBase(
BaseOffset = CharUnits::Zero();
} else {
// Otherwise, lay the base out at the end of the MDC.
BaseOffset = Size = Size.alignTo(Info.Alignment);
BaseOffset = DataSize = Size = Size.alignTo(Info.Alignment);
}

// Place in EmptySubobjects map but don't check the position? MSVC seems to
// not allow fields to overlap at the end of a virtual base, but they can
// overlap with other bass.
EmptySubobjects->CanPlaceBaseAtOffset(Base, BaseOffset);
}

Bases.insert(std::make_pair(BaseDecl, BaseOffset));
Size += BaseLayout.getNonVirtualSize();
DataSize = Size;
PreviousBaseLayout = &BaseLayout;
}

BaseSubobjectInfo *MicrosoftRecordLayoutBuilder::computeBaseSubobjectInfo(
const CXXRecordDecl *RD, bool IsVirtual, BaseSubobjectInfo *Derived) {
// This is copied directly from ItaniumRecordLayoutBuilder::ComputeBaseSubobjectInfo.
BaseSubobjectInfo *Info;

if (IsVirtual) {
// Check if we already have info about this virtual base.
BaseSubobjectInfo *&InfoSlot = VirtualBaseInfo[RD];
if (InfoSlot) {
assert(InfoSlot->Class == RD && "Wrong class for virtual base info!");
return InfoSlot;
}

// We don't, create it.
InfoSlot = new (BaseSubobjectInfoAllocator.Allocate()) BaseSubobjectInfo;
Info = InfoSlot;
} else {
Info = new (BaseSubobjectInfoAllocator.Allocate()) BaseSubobjectInfo;
}

Info->Class = RD;
Info->IsVirtual = IsVirtual;
Info->Derived = nullptr;
Info->PrimaryVirtualBaseInfo = nullptr;

const CXXRecordDecl *PrimaryVirtualBase = nullptr;
BaseSubobjectInfo *PrimaryVirtualBaseInfo = nullptr;

// Check if this base has a primary virtual base.
if (RD->getNumVBases()) {
const ASTRecordLayout &Layout = Context.getASTRecordLayout(RD);
if (Layout.isPrimaryBaseVirtual()) {
// This base does have a primary virtual base.
PrimaryVirtualBase = Layout.getPrimaryBase();
assert(PrimaryVirtualBase && "Didn't have a primary virtual base!");

// Now check if we have base subobject info about this primary base.
PrimaryVirtualBaseInfo = VirtualBaseInfo.lookup(PrimaryVirtualBase);

if (PrimaryVirtualBaseInfo) {
if (PrimaryVirtualBaseInfo->Derived) {
// We did have info about this primary base, and it turns out that it
// has already been claimed as a primary virtual base for another
// base.
PrimaryVirtualBase = nullptr;
} else {
// We can claim this base as our primary base.
Info->PrimaryVirtualBaseInfo = PrimaryVirtualBaseInfo;
PrimaryVirtualBaseInfo->Derived = Info;
}
}
}
}

// Now go through all direct bases.
for (const auto &I : RD->bases()) {
bool IsVirtual = I.isVirtual();

const CXXRecordDecl *BaseDecl = I.getType()->getAsCXXRecordDecl();

Info->Bases.push_back(computeBaseSubobjectInfo(BaseDecl, IsVirtual, Info));
}

if (PrimaryVirtualBase && !PrimaryVirtualBaseInfo) {
// Traversing the bases must have created the base info for our primary
// virtual base.
PrimaryVirtualBaseInfo = VirtualBaseInfo.lookup(PrimaryVirtualBase);
assert(PrimaryVirtualBaseInfo && "Did not create a primary virtual base!");

// Claim the primary virtual base as our primary virtual base.
Info->PrimaryVirtualBaseInfo = PrimaryVirtualBaseInfo;
PrimaryVirtualBaseInfo->Derived = Info;
}
return Info;
}

void MicrosoftRecordLayoutBuilder::layoutFields(const RecordDecl *RD) {
LastFieldIsNonZeroWidthBitfield = false;
for (const FieldDecl *Field : RD->fields())
Expand All @@ -2956,18 +3066,48 @@ void MicrosoftRecordLayoutBuilder::layoutField(const FieldDecl *FD) {
layoutBitField(FD);
return;
}

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();
const CXXRecordDecl *ParentClass = cast<CXXRecordDecl>(FD->getParent());
bool HasBases =
!ParentClass->bases().empty() || !ParentClass->vbases().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)) {
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);
}

Expand Down Expand Up @@ -2999,17 +3139,17 @@ void MicrosoftRecordLayoutBuilder::layoutBitField(const FieldDecl *FD) {
auto NewSize = Context.toCharUnitsFromBits(
llvm::alignDown(FieldBitOffset, Context.toBits(Info.Alignment)) +
Context.toBits(Info.Size));
Size = std::max(Size, NewSize);
DataSize = Size = std::max(Size, NewSize);
Alignment = std::max(Alignment, Info.Alignment);
} else if (IsUnion) {
placeFieldAtOffset(CharUnits::Zero());
Size = std::max(Size, Info.Size);
DataSize = Size = std::max(Size, Info.Size);
// TODO: Add a Sema warning that MS ignores bitfield alignment in unions.
} else {
// Allocate a new block of memory and place the bitfield in it.
CharUnits FieldOffset = Size.alignTo(Info.Alignment);
placeFieldAtOffset(FieldOffset);
Size = FieldOffset + Info.Size;
DataSize = Size = FieldOffset + Info.Size;
Alignment = std::max(Alignment, Info.Alignment);
RemainingBitsInField = Context.toBits(Info.Size) - Width;
}
Expand All @@ -3029,13 +3169,13 @@ MicrosoftRecordLayoutBuilder::layoutZeroWidthBitField(const FieldDecl *FD) {
ElementInfo Info = getAdjustedElementInfo(FD);
if (IsUnion) {
placeFieldAtOffset(CharUnits::Zero());
Size = std::max(Size, Info.Size);
DataSize = Size = std::max(Size, Info.Size);
// TODO: Add a Sema warning that MS ignores bitfield alignment in unions.
} else {
// Round up the current record size to the field's alignment boundary.
CharUnits FieldOffset = Size.alignTo(Info.Alignment);
placeFieldAtOffset(FieldOffset);
Size = FieldOffset;
DataSize = Size = FieldOffset;
Alignment = std::max(Alignment, Info.Alignment);
}
}
Expand All @@ -3055,7 +3195,7 @@ void MicrosoftRecordLayoutBuilder::injectVBPtr(const CXXRecordDecl *RD) {
// It is possible that there were no fields or bases located after vbptr,
// so the size was not adjusted before.
if (Size < FieldStart)
Size = FieldStart;
DataSize = Size = FieldStart;
return;
}
// Make sure that the amount we push the fields back by is a multiple of the
Expand Down Expand Up @@ -3103,6 +3243,7 @@ void MicrosoftRecordLayoutBuilder::injectVFPtr(const CXXRecordDecl *RD) {
void MicrosoftRecordLayoutBuilder::layoutVirtualBases(const CXXRecordDecl *RD) {
if (!HasVBPtr)
return;

// Vtordisps are always 4 bytes (even in 64-bit mode)
CharUnits VtorDispSize = CharUnits::fromQuantity(4);
CharUnits VtorDispAlignment = VtorDispSize;
Expand Down Expand Up @@ -3136,7 +3277,7 @@ void MicrosoftRecordLayoutBuilder::layoutVirtualBases(const CXXRecordDecl *RD) {
if ((PreviousBaseLayout && PreviousBaseLayout->endsWithZeroSizedObject() &&
BaseLayout.leadsWithZeroSizedBase() && !recordUsesEBO(RD)) ||
HasVtordisp) {
Size = Size.alignTo(VtorDispAlignment) + VtorDispSize;
DataSize = Size = Size.alignTo(VtorDispAlignment) + VtorDispSize;
Alignment = std::max(VtorDispAlignment, Alignment);
}
// Insert the virtual base.
Expand All @@ -3154,7 +3295,7 @@ void MicrosoftRecordLayoutBuilder::layoutVirtualBases(const CXXRecordDecl *RD) {

VBases.insert(std::make_pair(BaseDecl,
ASTRecordLayout::VBaseInfo(BaseOffset, HasVtordisp)));
Size = BaseOffset + BaseLayout.getNonVirtualSize();
DataSize = Size = BaseOffset + BaseLayout.getNonVirtualSize();
PreviousBaseLayout = &BaseLayout;
}
}
Expand Down Expand Up @@ -3304,8 +3445,9 @@ 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,
Expand All @@ -3317,6 +3459,7 @@ 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,
Expand Down
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