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[clang] Implement __is_virtual_base_of() intrinsic #100393

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Merged
merged 8 commits into from
Jul 26, 2024
Merged

[clang] Implement __is_virtual_base_of() intrinsic #100393

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772b5d8
[clang] Impement `__is_virtual_base_of()` intrinsic
Endilll Jul 24, 2024
8b0b0c5
Address reviewer feedback
Endilll Jul 24, 2024
da449c9
Remove short-circuiting
Endilll Jul 25, 2024
8913119
Rename to `__builtin_is_virtual_base_of`
Endilll Jul 25, 2024
46f02b2
Apply clang-format suggestions
Endilll Jul 25, 2024
60ccd79
Merge remote-tracking branch 'upstream/main' into is-virtual-base-of
Endilll Jul 25, 2024
8806fdb
Merge remote-tracking branch 'upstream/main' into is-virtual-base-of
Endilll Jul 25, 2024
47e274d
Merge remote-tracking branch 'upstream/main' into is-virtual-base-of
Endilll Jul 25, 2024
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1 change: 1 addition & 0 deletions clang/docs/LanguageExtensions.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -1546,6 +1546,7 @@ The following type trait primitives are supported by Clang. Those traits marked
* ``__array_extent(type, dim)`` (Embarcadero):
The ``dim``'th array bound in the type ``type``, or ``0`` if
``dim >= __array_rank(type)``.
* ``__builtin_is_virtual_base_of`` (C++, GNU, Microsoft)
* ``__can_pass_in_regs`` (C++)
Returns whether a class can be passed in registers under the current
ABI. This type can only be applied to unqualified class types.
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3 changes: 3 additions & 0 deletions clang/docs/ReleaseNotes.rst
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Expand Up @@ -81,6 +81,9 @@ C++23 Feature Support
C++2c Feature Support
^^^^^^^^^^^^^^^^^^^^^

- Add ``__builtin_is_virtual_base_of`` intrinsic, which supports
`P2985R0 A type trait for detecting virtual base classes <https://wg21.link/p2985r0>`_

Resolutions to C++ Defect Reports
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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1 change: 1 addition & 0 deletions clang/include/clang/Basic/TokenKinds.def
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Expand Up @@ -501,6 +501,7 @@ TYPE_TRAIT_1(__has_trivial_move_assign, HasTrivialMoveAssign, KEYCXX)
TYPE_TRAIT_1(__has_trivial_move_constructor, HasTrivialMoveConstructor, KEYCXX)

// GNU and MS Type Traits
TYPE_TRAIT_2(__builtin_is_virtual_base_of, IsVirtualBaseOf, KEYCXX)
TYPE_TRAIT_1(__has_nothrow_assign, HasNothrowAssign, KEYCXX)
TYPE_TRAIT_1(__has_nothrow_copy, HasNothrowCopy, KEYCXX)
TYPE_TRAIT_1(__has_nothrow_constructor, HasNothrowConstructor, KEYCXX)
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26 changes: 26 additions & 0 deletions clang/lib/Sema/SemaExprCXX.cpp
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Expand Up @@ -6030,6 +6030,32 @@ static bool EvaluateBinaryTypeTrait(Sema &Self, TypeTrait BTT, const TypeSourceI
return cast<CXXRecordDecl>(rhsRecord->getDecl())
->isDerivedFrom(cast<CXXRecordDecl>(lhsRecord->getDecl()));
}
case BTT_IsVirtualBaseOf: {
const RecordType *BaseRecord = LhsT->getAs<RecordType>();
const RecordType *DerivedRecord = RhsT->getAs<RecordType>();

if (!BaseRecord || !DerivedRecord) {
DiagnoseVLAInCXXTypeTrait(Self, Lhs,
tok::kw___builtin_is_virtual_base_of);
DiagnoseVLAInCXXTypeTrait(Self, Rhs,
tok::kw___builtin_is_virtual_base_of);
return false;
}

if (BaseRecord->isUnionType() || DerivedRecord->isUnionType())
return false;

if (!BaseRecord->isStructureOrClassType() ||
!DerivedRecord->isStructureOrClassType())
return false;
Comment on lines +6048 to +6050
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Is this check necessary? If it’s not a union, it has to be a class type, right? Or am I missing something obvious?

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It could be an enum (enums can't have an inheritance relationship) - but this check is enough, we don't need the union check above.
I think this is a mismatch between the standard - which treats union as classes, and clang - which treats everything as record.

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It could be an enum

I thought a TagType was either a RecordType (struct/class/union) or an EnumType (enum). Can a RecordType really be an enum?

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IIRC ObjC interfaces are also RecordTypes, and I don't think we want to bother with them.

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ObjC types don't inherit from RecordType, so I don't think this check is needed.

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(and I don't think you can have a virtual objective c interface so there is nothing of ObjC to support here)

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isStructureOrClassType is defined as follows:

llvm-project/clang/lib/AST/Type.cpp

Lines 657 to 663 in 0ee32c4

bool Type::isStructureOrClassType() const {
if (const auto *RT = getAs<RecordType>()) {
RecordDecl *RD = RT->getDecl();
return RD->isStruct() || RD->isClass() || RD->isInterface();
}
return false;
}

So (1) ObjC doesn't have to inherit from RecordType to get involved, and (2) the function has a misleading name.

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Hmm, isInterface() is defined on TagDecl, so it seems that a RecordType can still not be an ObjcInterface from what I can tell. Looks like RD->isInterface() check in here is just pointless.

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It’s also maybe worth noting that the line containing the check is from 2014, so things may have been different back then


if (Self.RequireCompleteType(Rhs->getTypeLoc().getBeginLoc(), RhsT,
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What does it mean for there to be a complete derived class type but an incomplete base class type? Shouldn't this require a complete type in both cases to give reasonable diagnostic behavior?

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The proposal only mentions that the second type has to be complete. My guess is that the reasoning behind this is that in e.g. __is_virtual_base_of(A, B), if B is complete but A isn’t, then A can’t be a base of B—since a complete class can’t have an incomplete base—and therefore, it can’t be a virtual base either.

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I hadn't realized that is_base_of already has the same requirement, so I don't insist on a change here. But that said, this presumes users don't have typos they'd like to catch. e.g.,

struct Bee { };
struct Be;

struct Derived : virtual Bee {};

static_assert(!__is_virtual_base_of(Be, Derived)); // Oops, I can't spell very well!

I think it would be much kinder to tell the user that Be is incomplete than let them be surprised.

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What does it mean for there to be a complete derived class type but an incomplete base class type? Shouldn't this require a complete type in both cases to give reasonable diagnostic behavior?

There is no incomplete base type. We are asking whether some type is a base, and if it isn't, we don't care that it is complete. Type traits don't mandate completeness if they don't need to.

diag::err_incomplete_type))
return false;

return cast<CXXRecordDecl>(DerivedRecord->getDecl())
->isVirtuallyDerivedFrom(cast<CXXRecordDecl>(BaseRecord->getDecl()));
}
case BTT_IsSame:
return Self.Context.hasSameType(LhsT, RhsT);
case BTT_TypeCompatible: {
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74 changes: 72 additions & 2 deletions clang/test/SemaCXX/type-traits.cpp
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Expand Up @@ -2402,11 +2402,11 @@ template<typename T> struct DerivedB : BaseA<T> { };
template<typename T> struct CrazyDerived : T { };


class class_forward; // expected-note 2 {{forward declaration of 'class_forward'}}
class class_forward; // expected-note 4 {{forward declaration of 'class_forward'}}

template <class T> class DerivedTemp : Base {};
template <class T> class NonderivedTemp {};
template <class T> class UndefinedTemp; // expected-note {{declared here}}
template <class T> class UndefinedTemp; // expected-note 2 {{declared here}}

void is_base_of() {
static_assert(__is_base_of(Base, Derived));
Expand Down Expand Up @@ -2457,6 +2457,76 @@ void is_base_of() {
static_assert(!__is_base_of(DerivedB<int>, BaseA<int>));
}

struct DerivedTransitiveViaNonVirtual : Derived3 {};
struct DerivedTransitiveViaVirtual : virtual Derived3 {};

template <typename T>
struct CrazyDerivedVirtual : virtual T {};

struct DerivedPrivate : private virtual Base {};
struct DerivedProtected : protected virtual Base {};
struct DerivedPrivatePrivate : private DerivedPrivate {};
struct DerivedPrivateProtected : private DerivedProtected {};
struct DerivedProtectedPrivate : protected DerivedProtected {};
struct DerivedProtectedProtected : protected DerivedProtected {};

void is_virtual_base_of(int n) {
static_assert(!__builtin_is_virtual_base_of(Base, Derived));
static_assert(!__builtin_is_virtual_base_of(const Base, Derived));
static_assert(!__builtin_is_virtual_base_of(Derived, Base));
static_assert(!__builtin_is_virtual_base_of(Derived, int));
static_assert(!__builtin_is_virtual_base_of(Base, Base));
static_assert(!__builtin_is_virtual_base_of(Base, Derived3));
static_assert(!__builtin_is_virtual_base_of(Derived, Derived3));
static_assert(__builtin_is_virtual_base_of(Derived2b, Derived3));
static_assert(__builtin_is_virtual_base_of(Derived2a, Derived3));
static_assert(!__builtin_is_virtual_base_of(BaseA<int>, DerivedB<int>));
static_assert(!__builtin_is_virtual_base_of(DerivedB<int>, BaseA<int>));
static_assert(!__builtin_is_virtual_base_of(Union, Union));
static_assert(!__builtin_is_virtual_base_of(Empty, Empty));
static_assert(!__builtin_is_virtual_base_of(class_forward, class_forward)); // expected-error {{incomplete type 'class_forward' where a complete type is required}}
static_assert(!__builtin_is_virtual_base_of(Empty, class_forward)); // expected-error {{incomplete type 'class_forward' where a complete type is required}}
static_assert(!__builtin_is_virtual_base_of(class_forward, Empty));
static_assert(!__builtin_is_virtual_base_of(Base&, Derived&));
static_assert(!__builtin_is_virtual_base_of(Base[10], Derived[10]));
static_assert(!__builtin_is_virtual_base_of(Base[n], Derived[n])); // expected-error 2 {{variable length arrays are not supported in '__builtin_is_virtual_base_of'}}
static_assert(!__builtin_is_virtual_base_of(int, int));
static_assert(!__builtin_is_virtual_base_of(int[], int[]));
static_assert(!__builtin_is_virtual_base_of(long, int));
static_assert(!__builtin_is_virtual_base_of(Base, DerivedTemp<int>));
static_assert(!__builtin_is_virtual_base_of(Base, NonderivedTemp<int>));
static_assert(!__builtin_is_virtual_base_of(Base, UndefinedTemp<int>)); // expected-error {{implicit instantiation of undefined template 'UndefinedTemp<int>'}}
static_assert(__builtin_is_virtual_base_of(Base, DerivedPrivate));
static_assert(__builtin_is_virtual_base_of(Base, DerivedProtected));
static_assert(__builtin_is_virtual_base_of(Base, DerivedPrivatePrivate));
static_assert(__builtin_is_virtual_base_of(Base, DerivedPrivateProtected));
static_assert(__builtin_is_virtual_base_of(Base, DerivedProtectedPrivate));
static_assert(__builtin_is_virtual_base_of(Base, DerivedProtectedProtected));
static_assert(__builtin_is_virtual_base_of(Derived2a, DerivedTransitiveViaNonVirtual));
static_assert(__builtin_is_virtual_base_of(Derived2b, DerivedTransitiveViaNonVirtual));
static_assert(__builtin_is_virtual_base_of(Derived2a, DerivedTransitiveViaVirtual));
static_assert(__builtin_is_virtual_base_of(Derived2b, DerivedTransitiveViaVirtual));
static_assert(!__builtin_is_virtual_base_of(Base, CrazyDerived<Base>));
static_assert(!__builtin_is_virtual_base_of(CrazyDerived<Base>, Base));
static_assert(__builtin_is_virtual_base_of(Base, CrazyDerivedVirtual<Base>));
static_assert(!__builtin_is_virtual_base_of(CrazyDerivedVirtual<Base>, Base));

static_assert(!__builtin_is_virtual_base_of(IncompleteUnion, IncompleteUnion));
static_assert(!__builtin_is_virtual_base_of(Union, IncompleteUnion));
static_assert(!__builtin_is_virtual_base_of(IncompleteUnion, Union));
static_assert(!__builtin_is_virtual_base_of(IncompleteStruct, IncompleteUnion));
static_assert(!__builtin_is_virtual_base_of(IncompleteUnion, IncompleteStruct));
static_assert(!__builtin_is_virtual_base_of(Empty, IncompleteUnion));
static_assert(!__builtin_is_virtual_base_of(IncompleteUnion, Empty));
static_assert(!__builtin_is_virtual_base_of(int, IncompleteUnion));
static_assert(!__builtin_is_virtual_base_of(IncompleteUnion, int));
static_assert(!__builtin_is_virtual_base_of(Empty, Union));
static_assert(!__builtin_is_virtual_base_of(Union, Empty));
static_assert(!__builtin_is_virtual_base_of(int, Empty));
static_assert(!__builtin_is_virtual_base_of(Union, int));
static_assert(!__builtin_is_virtual_base_of(IncompleteStruct, IncompleteStruct[n])); // expected-error {{variable length arrays are not supported in '__builtin_is_virtual_base_of'}}
}

template<class T, class U>
class TemplateClass {};

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