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 7:59 AM, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
> On 8 February 2016 at 15:42, H.J. Lu <hjl.tools@gmail.com> wrote:
>> On Mon, Feb 8, 2016 at 7:02 AM, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
>>> On 8 February 2016 at 13:54, H.J. Lu <hjl.tools@gmail.com> wrote:
>>>> On Sun, Feb 7, 2016 at 12:52 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>>>
>>>> The standard-layout POD is well defined:
>>>>
>>>> https://en.wikipedia.org/wiki/C%2B%2B11#Modification_to_the_definition_of_plain_old_data
>>>>
>>>> Here is the updated proposal for  Intel386, x86-64 and IA MCU psABIs:
>>>>
>>>> 1. "collection".  A collection is a structure, union or C++ class.
>>>
>>> These are all "class types". Why invent a new term?
>>
>> Because it applies to both C and C++.  There is no class in C.
>
> Then you could use the term "class type" in the ABI, defining it to
> mean structure or union in C, or class type in C++. No need for a new
> term.

I will do it.

>
>>>> 2. "empty collection".  An empty collection is:
>>>>    a. A collection without member.  Or
>>>
>>> What about base classes?
>>>
>>> What about bit-fields of length 0?
>>
>> Is a collection with them standard-layout POD type?
>
> (I'm not sure what the "bit-fields of length 0" part is for, but my
> point is it would be useful to examine similar concepts in the
> standard and align with them, not just make up entirely new
> classifications.)

I am replying on C++ compiler to tell if it is standard-layout
or not.

> For base classes, yes. A standard-layout class can have base classes
> of standard-layout type.
>
> struct A { };
> struct B { };
> struct C : A, B { };
>
> C is a standard-layout type. Is it an empty collection?

My understanding is

A type that is standard-layout means that it orders and packs its
members in a way that is compatible with C.

What is the corresponding compatible type in C?

>>>>    b. A collection with only empty collections.  Or
>>>
>>> What does "with" mean? Only members, or bases too?
>>
>> Is "A collection with only members of empty collections" better?
>
> Should it mention base classes?

It depends on the answer of my question above.

>
>>>>    c. An array of empty collections.
>>>> 3. "empty record".  An empty record is Plain Old Data (POD) for the purpose
>>>>    of standard-layout and
>>>
>>> "For the purposes of standard-layout" doesn't mean anything.
>>>
>>> A type is a standard-layout type, or it isn't.
>>
>> How about "An empty record is standard-layout Plain Old Data (POD)
>> type and ..."?
>
> That's redundant, all POD types are standard-layout types.
>

I will update it.

Thanks.


-- 
H.J.