trivial_abi

[Richard Smith via cfe-commits]

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@
> 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.

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
- 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*.

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.


> 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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