Re: RFC: Update Intel386, x86-64 and IA MCU psABIs for passing/returning empty struct

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Mon, Feb 8, 2016 at 3:28 PM, Richard Smith <richard@metafoo.co.uk> wrote:
> On Mon, Feb 8, 2016 at 3:01 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>> On Mon, Feb 8, 2016 at 2:58 PM, Richard Smith <richard@metafoo.co.uk> wrote:
>>> On Mon, Feb 8, 2016 at 2:54 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>>> On Mon, Feb 8, 2016 at 2:51 PM, Richard Smith <richard@metafoo.co.uk> wrote:
>>>>> On Mon, Feb 8, 2016 at 2:46 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>>>>> On Mon, Feb 8, 2016 at 2:35 PM, Richard Smith <richard@metafoo.co.uk> wrote:
>>>>>>> On Mon, Feb 8, 2016 at 1:40 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>>>>>>>
>>>>>>>> On Mon, Feb 8, 2016 at 12:38 PM, Richard Smith <richard@metafoo.co.uk> wrote:
>>>>>>>> > On Mon, Feb 8, 2016 at 12:05 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>>>>>>> >>
>>>>>>>> >> On Mon, Feb 8, 2016 at 11:33 AM, Jonathan Wakely <jwakely.gcc@gmail.com>
>>>>>>>> >> wrote:
>>>>>>>> >> > On 8 February 2016 at 19:23, Richard Smith wrote:
>>>>>>>> >> >> "POD for the purpose of layout" is defined in the Itanium C++ ABI here:
>>>>>>>> >> >>
>>>>>>>> >> >>   http://mentorembedded.github.io/cxx-abi/abi.html#definitions
>>>>>>>> >> >
>>>>>>>> >> > Thanks. So there's no problem using "POD for the purposes of layout",
>>>>>>>> >> > and the change to "POD for the purpose of standard-layout" was
>>>>>>>> >> > unnecessary and just confused things.
>>>>>>>> >>
>>>>>>>> >> Here is the revised proposal:
>>>>>>>> >>
>>>>>>>> >> 1. "class type".  A class type is a structure, union or C++ class.
>>>>>>>> >> 2. "empty class type".  An empty class type is:
>>>>>>>> >>    a. A class type without member.  Or
>>>>>>>> >>    b. A class type with only members of empty class types.  Or
>>>>>>>> >
>>>>>>>> >
>>>>>>>> > (a) is a special case of (b).
>>>>>>>> >
>>>>>>>> >>    c. An array of empty class types.
>>>>>>>> >
>>>>>>>> >
>>>>>>>> > It seems confusing to call an array a class type. Instead, how about:
>>>>>>>> >
>>>>>>>> >   2. An empty type is either an array of empty types or a class type where
>>>>>>>> > every member is of empty type.
>>>>>>>> >
>>>>>>>> >> 3. "empty record".  An empty record is Plain Old Data (POD) for the
>>>>>>>> >>    purposes of layout and
>>>>>>>> >>    a. A class type without member.  Or
>>>>>>>> >>    b. A class type with only members of empty class types.
>>>>>>>> >
>>>>>>>> >
>>>>>>>> > (a) is a special case of (b).
>>>>>>>> >
>>>>>>>> >> 4. No memory slot nor register should be used to pass or return an object
>>>>>>>> >> of empty record.
>>>>>>>> >
>>>>>>>> >
>>>>>>>> > Objects of array type are never passed or returned (but if through some
>>>>>>>> > language extension they were, we'd want this rule to apply). So you don't
>>>>>>>> > need rule 3 and this can be just:
>>>>>>>> >
>>>>>>>> >  3. No memory slot nor register should be used to pass or return an object
>>>>>>>> > of empty type.
>>>>>>>>
>>>>>>>> Thanks very much for your inputs.  Here is the proposal:
>>>>>>>>
>>>>>>>> 1. "class type".  A class type is a structure, union or C++ class.
>>>>>>>> 2. "empty type".  An empty type is either an array of empty types or a
>>>>>>>> class type where every member is of empty type.
>>>>>>>> 3. No memory slot nor register should be used to pass or return an object
>>>>>>>> of empty type.
>>>>>>>
>>>>>>> David Majnemer points out that we also need to say something about
>>>>>>> base classes. We could handle that case like this:
>>>>>>>
>>>>>>>  2. "empty type".  An empty type is a type where it and all of its
>>>>>>> subobjects are of class or array type.
>>>>>>>
>>>>>>> Following the C++ rules, this also means that a class that contains
>>>>>>> only unnamed bitfields is empty, because unnamed bitfields are not
>>>>>>> subobjects, but might be worth explicitly stating for the C case. That
>>>>>>> also matches Clang's behavior.
>>>>>>
>>>>>> Like this?
>>>>>>
>>>>>> 1. "class type".  A class type is a structure, union or C++ class.
>>>>>> 2. "empty type".  An empty type is
>>>>>>    a. A type where it and all of its subobjects are of class or array
>>>>>>    type. And
>>>>>>    b. Either an array of empty types or a class type where every member
>>>>>>    is of empty type.
>>>>>
>>>>> You don't need (b). It's implied by (a).
>>>>
>>>> Does (a) cover empty type?
>>>
>>> Yes, (a) is a complete definition of "empty type" by itself: the
>>> definition is, essentially, that a complete recursive walk of the type
>>> and all its subobjects never sees anything that would require any
>>> storage (that is, it only sees class types and array types).
>>
>> Is there a definition of subobject?
>
> Yes, the C++ standard defines the term (as an object that is contained
> within another via member, base class, and array element
> relationships). The C standard uses it in 6.7.9 to mean the same thing
> (recursing into class and array elements) but doesn't provide a formal
> definition.

Here is the new one:

1. "class type".  A class type is a structure, union or C++ class.
2. "empty type".  An empty type is Plain Old Data (POD) for the
   purposes of layout, and.a type where it and all of its subobjects
   are of class or array type.

I put back " Plain Old Data (POD) for the purposes of layout" to
explicitly exclude array here.

-- 
H.J.