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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 2:46 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).

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


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