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


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.
3. No memory slot nor register should be used to pass or return an object
of empty type.


-- 
H.J.


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