trivial_abi

[Richard Smith via cfe-commits]

On 3 January 2018 at 15:24, John McCall via cfe-commits <
cfe-commits at lists.llvm.org> wrote:

> On Jan 3, 2018, at 5:53 PM, Richard Smith <richard at metafoo.co.uk> wrote:
> On 3 January 2018 at 14:29, John McCall via cfe-commits <
> cfe-commits at lists.llvm.org> wrote:
>
>>
>> On Jan 3, 2018, at 5:12 PM, Richard Smith <richard at metafoo.co.uk> wrote:
>>
>> On 2 January 2018 at 20:55, John McCall via cfe-commits <
>> cfe-commits at lists.llvm.org> wrote:
>>
>>> On Jan 2, 2018, at 10:43 PM, Richard Smith <richard at metafoo.co.uk>
>>> wrote:
>>>
>>> On 2 January 2018 at 19:02, John McCall via cfe-commits <
>>> cfe-commits at lists.llvm.org> wrote:
>>>
>>>>
>>>> On Jan 2, 2018, at 9:15 PM, Akira Hatanaka <ahatanaka at apple.com> wrote:
>>>>
>>>>
>>>>
>>>> On Jan 2, 2018, at 4:56 PM, Richard Smith via cfe-commits <
>>>> cfe-commits at lists.llvm.org> wrote:
>>>>
>>>> On 2 January 2018 at 15:33, John McCall via cfe-commits <
>>>> cfe-commits at lists.llvm.org> wrote:
>>>>
>>>>> Hey, Richard et al.  Akira and I were talking about the right ABI rule
>>>>> for deciding can-pass-in-registers-ness for structs in the presence of
>>>>> trivial_abi, and I think I like Akira's approach but wanted to get your
>>>>> input.
>>>>>
>>>>> The current definition in Itanium is:
>>>>>
>>>>>   *non-trivial for the purposes of calls*
>>>>>
>>>>> A type is considered non-trivial for the purposes of calls if:
>>>>>
>>>>>    - it has a non-trivial copy constructor, move constructor, or
>>>>>    destructor, or
>>>>>
>>>>> I'm assuming we're implicitly excluding deleted functions here. (I'd
>>>> prefer to make that explicit; this has been the source of a number of ABI
>>>> mismatches.)
>>>>
>>>>>
>>>>>    - all of its copy and move constructors are deleted.
>>>>>
>>>>>
>>>>> I'd suggest modifying this to:
>>>>>
>>>>> A type is considered non-trivial for the purposes of calls if:
>>>>> - if has a copy constructor, move constructor, or destructor which is
>>>>> non-trivial for the purposes of calls, or
>>>>> - all of its copy and move constructors are deleted and it does not
>>>>> have the trivial_abi attribute.
>>>>>
>>>>> A copy/move constructor is considered trivial for the purposes of
>>>>> calls if:
>>>>> - it is user-provided and
>>>>> - the class has the trivial_abi attribute and
>>>>> - a defaulted definition of the constructor would be trivial for the
>>>>> purposes of calls; or
>>>>>
>>>>
>>>> We'd need to say what happens if the function in question cannot
>>>> validly be defaulted for any of the reasons in [dcl.fct.def.default]. Do we
>>>> try to infer whether it's a copy or move constructor, and use the rules for
>>>> a defaulted copy or move constructor? Or do we just say that's never
>>>> trivial for the purposes of calls? Or something else? Eg:
>>>>
>>>> struct [[clang::trivial_abi]] A {
>>>>   A(A && = make());
>>>> };
>>>>
>>>> Here, A::A(A&&) cannot validly be defaulted. Is A trivial for the
>>>> purpose of calls? Likewise:
>>>>
>>>> struct [[clang::trivial_abi]] B {
>>>>   B(...);
>>>> };
>>>> struct C {
>>>>   volatile B b;
>>>> };
>>>>
>>>> Here, C's copy constructor calls B::B(...). Is C trivial for the
>>>> purpose of calls? (OK, Clang crashes on that example today. But still...)
>>>>
>>>> I'd be uncomfortable making the rules in [dcl.fct.def.default] part of
>>>> the ABI; they seem to be changing relatively frequently. Perhaps we could
>>>> say "if the function is a copy constructor ([class.copy.ctor]/1), then
>>>> consider what an implicitly-declared defaulted copy constructor would do;
>>>> if it's a move constructor ([class.copy.ctor]/2), then consider what an
>>>> implicitly-declared defaulted move constructor would do; otherwise, it's
>>>> not trivial for the purpose of calls". That'd mean A is trivial for the
>>>> purpose of calls and C is not, which I think is probably the right answer.
>>>>
>>>> - it is not user-provided and
>>>>> - the class has no virtual functions and no virtual base classes, and
>>>>> - the constructor used to copy/move each direct base class subobject
>>>>> is trivial for the purposes of calls, and
>>>>> - for each non-static data member that is of class type (or array
>>>>> thereof), the constructor selected to copy/move that member is trivial for
>>>>> the purposes of calls.
>>>>>
>>>>> A destructor is considered trivial for the purposes of calls if:
>>>>> - it is not user-provided or the class has the trivial_abi attribute,
>>>>> and
>>>>> - the destructor is not virtual, and
>>>>> - all of the direct base classes of its class have destructors that
>>>>> are trivial for the purposes of calls, and
>>>>> - for all of the non-static data members of its class that are of
>>>>> class type (or array thereof), each such class is trivial for the purposes
>>>>> of calls.
>>>>>
>>>>> These definitions are intended to follow [class.copy.ctor]p11 and
>>>>> [class.dtor]p6 except for the special rules applicable to trivial_abi
>>>>> classes.
>>>>>
>>>>
>>>> If I could rephrase: a *tor is considered trivial for for the purposes
>>>> of calls if it is either defaulted or the class has the trivial_abi
>>>> attribute, and the defaulted definition would satisfy the language rule for
>>>> being trivial but with the word "trivial" replaced by "trivial for the
>>>> purposes of calls". So only effect of the trivial_abi attribute is to
>>>> "undo" the non-triviality implied by a user-provided *tor when computing
>>>> triviality for the purpose of calls.
>>>>
>>>> I think that's a reasonable rule, if we have a satisfactory notion of
>>>> "defaulted definition".
>>>>
>>>> I'm not sure about the "defaulted definition" rule for copy/move
>>>>> constructors in trivial_abi classes.  The intent is to allow class
>>>>> temploids with trivial_abi that are instantiated to contain non-trivial
>>>>> classes to just silently become non-trivial.  I was thinking at first that
>>>>> it would be nice to have a general rule that trivial_abi classes only
>>>>> contain trivial_abi subobjects, but unfortunately that's not consistent
>>>>> with the standard triviality rule in some silly corner cases: a
>>>>> trivially-copyable class can have a non-trivially-copyable subobject if it
>>>>> happens to copy that subobject with a trivial copy constructor.  I couldn't
>>>>> think of a better way of capturing this than the "defaulted definition"
>>>>> rule.  I considered using the actual initializers used by the constructor,
>>>>> but that would introduce a lot of new complexity: suddenly we'd be asking
>>>>> about triviality for an arbitrary constructor, and copy/move elision make
>>>>> the question somewhat ambiguous anyway.
>>>>>
>>>>
>>>> Per the above examples, I don't think you can escape asking about
>>>> triviality for an arbitrary constructor if you take this path.
>>>>
>>>> Another option, similar to your general rule, would be to say that a
>>>> type is considered trivial for the purpose of calls if either: (1) it is
>>>> trivial for the purpose of calls under the current Itanium ABI rule, or (2)
>>>> it has the trivial_abi attribute and all members and base classes have
>>>> types that are trivial for the purpose of calls. That would sidestep the
>>>> "defaulted definition" complexity entirely, and while it differs from the
>>>> way that the language computes triviality normally, it doesn't seem
>>>> fundamentally unreasonable: when we're thinking about triviality for the
>>>> purpose of calls, there's notionally a call to the trivial copy/move ctor
>>>> being elided, not a call to an arbitrary ctor selected by overload
>>>> resolution, and we'd just be pushing that effect from the class itself to
>>>> its subobjects with this attribute.
>>>>
>>>>
>>>>
>>>> It sounds like a class containing a member that has a type annotated
>>>> with “trivial_abi” would not necessarily be considered trivial for the
>>>> purpose of calls according to rule (2)? For example, S1 would not be
>>>> trivial for the purpose of calls because it isn’t annotated with
>>>> “trivial_abi” in the code below:
>>>>
>>>> struct [[clang::trivial_abi]] S0 {
>>>>   // user-provided special functions declared here.
>>>> };
>>>>
>>>> struct S1 {
>>>>   S0 f0;
>>>> };
>>>>
>>>> I thought we wanted containing classes (S1 in this case) to be trivial
>>>> for the purpose of calls too?
>>>>
>>>>
>>>> I would like that, yeah.
>>>>
>>>
>>> OK, I think that's fair. Then we probably need the more complex rule.
>>> Which I think means we're at something equivalent to:
>>>
>>> A type is considered non-trivial for the purposes of calls if:
>>> - if has a copy constructor, move constructor, or destructor *that is
>>> not deleted and* is non-trivial for the purposes of calls, or
>>> - all of its copy and move constructors are deleted and it does not have
>>> the trivial_abi attribute.
>>>
>>> Hold on... this final "and it does not have the trivial_abi attribute"
>> looks wrong to me; it seems to break the "do what I mean"ness of the
>> attribute. Consider:
>>
>> template<typename T, typename U> struct [[clang::trivial_abi]] pair { ...
>> };
>>
>> std::pair<ContainsPointerToSelf, int> f(); // returned indirect
>> std::pair<ContainsPointerToSelf, NonCopyable> g(); // returned in
>> registers because all copy/move ctors deleted
>>
>> That seems like a bug. Can we just strike that addition, or does one of
>> your intended use cases need it?
>>
>>
>> It was a last-minute addition that seemed like a good idea, but I was
>> just thinking about all the copy/move ctors being explicitly deleted on the
>> class, not any of the inheritance cases.  I agree with striking it.
>>
>> The only use cases we really have in mind are
>>   - simple resource-owning classes like smart pointers, which would adopt
>> the attribute, and
>>   - classes with defaulted copy/destruction semantics, which should
>> propagate triviality if possible.
>>
>> I just think we need to be prepared to make the rule more general than
>> that.
>>
>> A copy/move constructor is considered trivial for the purposes of calls
>>> if:
>>> - it is user-provided and
>>> - the class has the trivial_abi attribute and
>>> - *a defaulted definition of a constructor with the signature of the
>>> implicit copy/move constructor for the class would be trivial for the
>>> purposes of calls*; or
>>>
>>> One other concern here: what if the defaulted definition would be
>> deleted? I think in that case the constructor we're considering should also
>> be treated as if it were deleted. And that applies recursively: if the
>> implicit copy/move constructor would itself have been deleted, we want to
>> treat the original member of the type we're checking as being deleted. And
>> likewise, if a defaulted copy/move constructor invokes a copy/move
>> constructor of a trivial_abi class, and a defaulted copy/move constructor
>> for that class would have been deleted, we want to act as if the original
>> defaulted copy/move constructor was deleted. That seems awkward to specify
>> in the fashion we've been using until now, since the result of a triviality
>> calculation is now "deleted", "non-trivial", or "trivial", and deletedness
>> can change in either direction as a result of the attribute.
>>
>>
>> Ugh.  I feel like this problem is mostly a further indictment of the idea
>> of basing this on what a defaulted definition would look like.
>>
>> We could just base it on the overall trivial-for-calls-ness of the
>> subobject types.  It's a very different rule from the standard triviality
>> rule, but it's okay to differ here because this *only* affects special
>> members of classes with the attribute.
>>
>
> I like this idea a lot. Here's a concrete suggestion:
>
> """
> A type has a *triviality override* if it has the trivial_abi attribute,
> and it has no virtual functions nor virtual base classes, and every
> subobject is trivial for the purposes of calls. The attribute is ill-formed
> if applied to a non-template class that does not meet these criteria; the
> attribute is ill-formed, no diagnostic required, if applied to a templated
> class and no instantiation of that class can meet these criteria.
>
>
> David B. and I were talking about whether this should be a required
> diagnostic even in the template case, and I think we settled on "no"
> because it could interfere with portability.  Imagine that std::unique_ptr
> were made trivial_abi in some STL; classes containing a std::unique_ptr
> could only be trivial_abi on that target.  On the other hand, I get that
> it's nice to have a static guarantee that the attribute meant something.
>
> Maybe we could make it ill-formed, no diagnostic required, if the
> attribute is present but the class can never have a triviality override
> (for any instantiation, if a template).  That would give us wide leeway to
> complain about putting it on a class with a direct virtual base, or when
> there's a non-trivial subobject whose type is defined in the "same
> library", or if specifically requested to.
>
> Besides, we're not actually promising to pass it "directly".  It's a
> totally legal implementation (right now) to just ignore the attribute.
> That wouldn't be ABI-compatible with compilers that implement it, of
> course, but not everyone cares about that.
>

I'd be OK with, say, downgrading this from an error to a warning, or making
it an error-with-a-warning-flag. But even in the case of a unique_ptr
member, I think a user would typically want to be told that the attribute
didn't have the effect they're looking for: that's exactly the case where I
would expect people to try to "override" the triviality of a subobject
using the attribute, so I think the diagnostic should be enabled by default.

I suppose the "ill-formed, no diagnostic required" formulation at least
gives other implementers of the attribute a hint that they should also
consider producing a diagnostic.

> A type is trivial for the purposes of calls if:
>   - it has a triviality override, or
>   - it is trivial for the purposes of calls as specified in the Itanium
> C++ ABI, or would be so if all direct or indirect construction and
> destruction of types with a triviality override were ignored when computing
> the triviality (but not deletedness) of functions
> """
>
>
> I like this wording, since we don't have to actually repeat anything from
> the standard.
>
> So we would still compute both a "trivial" and a "trivial for the purposes
> of calls" flag for defaulted copy constructors, move constructors, and
> destructors, but we'd only do the overload resolution and deletedness
> analysis once; trivial would always imply trivial for the purposes of
> calls, and the converse only fails when there is a subobject whose type has
> a triviality override.
>
>
> Right.
>
> John.
>
> Put another way, we'd have four levels of triviality for special members:
> deleted, non-trivial, trivial for purposes of calls, and trivial. The
> triviality of a deleted member is "deleted". The triviality of any
> trivial_abi member is "trivial for purposes of calls". The triviality of
> any other user-provided member is "non-trivial". And the triviality of a
> non-user-provided non-deleted member is "deleted" if any subobject call is
> ill-formed, otherwise "non-trivial" for the special cases involving virtual
> bases and virtual functions, otherwise the mimimum of that value over all
> subobject calls. And a type is trivial for the purposes of calls unless any
> copy ctor, move ctor or dtor is "non-trivial" or all copy and move
> constructors are "deleted".
>
>> Here's a terse summary of the rule I'm considering:
>>
>> """
>> For the determination of triviality for the purposes of calls, a modified
>> form of the program is considered. In this modified form, each copy or move
>> constructor or destructor of a class with the trivial_abi attribute is
>> replaced by a defaulted copy or move constructor or destructor (with the
>> signature of an implicit such declaration), and calls to the former are
>> transformed into calls to the latter within the implicit definitions of
>> defaulted special member functions. A function is deleted for the purposes
>> of calls in the original program if the corresponding function is deleted
>> in the modified program, and is otherwise trivial for the purposes of calls
>> in the original program if the corresponding function is trivial in the
>> modified program.
>>
>> A type is considered non-trivial for the purposes of calls if:
>> - if has a copy constructor, move constructor, or destructor that is
>> non-deleted and non-trivial for the purposes of calls, or
>> - all of its copy and move constructors are deleted for purposes of calls.
>>
>> """
>>
>>
>> Yikes.  I feel like I would have no ability to explain this rule to a
>> user.
>>
>> - it is not user-provided and
>>> - the class has no virtual functions and no virtual base classes, and
>>> - the constructor used to copy/move each direct base class subobject is
>>> trivial for the purposes of calls, and
>>> - for each non-static data member that is of class type (or array
>>> thereof), the constructor selected to copy/move that member is trivial for
>>> the purposes of calls.
>>> *A constructor that is neither a copy constructor nor a move constructor
>>> is considered non-trivial for the purposes of calls*.
>>>
>>>
>>> This clause is there to handle constructors that are copy/move
>>> constructors only because of defaulted arguments?  I wonder if this is
>>> necessary; I think the allocator-like use cases would prefer that we just
>>> ignore the non-initial arguments, wouldn't they?
>>>
>>
>> This doesn't affect the default argument case: if a constructor has a
>> first parameter of type T / cv T& / cv T&&, and all further parameters (if
>> any) have default arguments, it is still a copy or move constructor.
>> Rather, we reach this clause in any case where "the constructor
>> used/selected to copy/move [...]" has some other first parameter type or is
>> X::X(...); such a constructor is only selected when there is no viable
>> copy/move constructor.
>>
>>
>> Oh, which can happen even for non-user-provided constructors because it's
>> just the ordinary overload rules, of course.
>>
>> A destructor is considered trivial for the purposes of calls if:
>>> - it is not user-provided or the class has the trivial_abi attribute, and
>>> - the destructor is not virtual, and
>>> - all of the direct base classes of its class have destructors that are
>>> trivial for the purposes of calls, and
>>> - for all of the non-static data members of its class that are of class
>>> type (or array thereof), each such class is trivial for the purposes of
>>> calls.
>>>
>>> Bolded phrases are changed from John's initial email.
>>>
>>>
>>> Thank you for the revision; this is much improved.
>>>
>>
>> I'm concerned about the level of complexity we've discovered to be
>> necessary here, and in particular the necessity of having a side-notion of
>> "trivial for the purpose of calls" for all copy/move ctors and dtors, even
>> in classes that do not directly use the trivial_abi attribute. But I
>> suppose that's fundamental if we want to pass struct S1 (above) directly.
>> I'd like a simpler rule, but I'm not convinced there is one.
>>
>>
>> Well, I think the adjustment I suggest above would cap the complexity a
>> bit; at least we would need these speculative investigation into defaulted
>> definitions that don't actually exist.  But we'd still need to track the
>> new kind of triviality for each ctor/dtor.
>>
>> John.
>>
>>
>>
>>> John.
>>>
>>>
>>>
>>>> John.
>>>>
>>>>
>>>> I'm also not sure about the right rules about virtual methods.  Should
>>>>> we allow polymorphic classes to be made trivial by application of the
>>>>> attribute?
>>>>>
>>>>
>>>> I think that it probably doesn't make much sense to pass dynamic
>>>> classes indirectly unless we can avoid passing the vptr; otherwise I'd
>>>> expect we'd use too many registers for it to be worthwhile. Perhaps as a
>>>> compromise, we could make the attribute ill-formed if used on a class
>>>> definition that introduces any virtual bases or explicitly declares any
>>>> member functions as 'virtual'. That gives us the room to make this decision
>>>> later if we find we want to.
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