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

[H.J. Lu via cfe-commits]

On Sun, Feb 7, 2016 at 11:31 AM, Florian Weimer <fw at deneb.enyo.de> wrote:
> * H. J. Lu:
>
>> I am proposing to update Intel386, x86-64 and IA MCU psABIs to specify
>> how to pass/return empty struct:
>>
>> 1. "collection".  A collection is a structure, union or C++ class.
>> 2. "empty collection".  An empty collection is:
>>    a. A collection without member.  Or
>>    b. A collection with only empty collections.  Or
>>    c. An array of empty collections.
>> 3. "empty record".  An empty record is Plain Old Data (POD) for the purpose
>>    of layout and
>>    a. A collection without member.  Or
>>    b. A collection with only empty collections.
>> 4. No memory slot nor register should be used to pass or return an object of
>> empty collection.
>
> “Aggregate” may be the more standard term instead of collection.

Aggregate also include array, which is excluded here.

> I think you mean “empty record” under 4.

Yes. I will fix it.

> Any syntactical array argument (at the C level) is should be passed as
> a pointer.  The language appears to change that.

I didn't use aggregate so that array is excluded here.

> For 2., static members and non-data members do not count.

They do count here.  That is why I used "POD for the purpose of
layout.

> Does the definition of POD vary between C++ standards?  Then the
> calling convention would vary as well, which is probably not what we
> want.

I believe that POD for the purpose of layout doesn't change between
C++ standards.

> How do existing C++ compilers implement empty array members (an
> extension)?  Does the type of such members affect whether a class is a
> standard-layout class?
>
> Florian

Are they "POD for the purpose of layout"? If yes, they are covered here.


-- 
H.J.