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 10:46 AM, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
> On 8 February 2016 at 18:31, H.J. Lu <hjl.tools@gmail.com> wrote:
>> On Mon, Feb 8, 2016 at 10:30 AM, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
>>> On 8 February 2016 at 18:26, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
>>>> On 8 February 2016 at 17:58, H.J. Lu wrote:
>>>>> On Mon, Feb 8, 2016 at 7:59 AM, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
>>>>>>>> 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.
>>>>>>
>>>>>
>>>>> Apparently, not all standard-layout types are POD types.  GCC has
>>>>>
>>>>> /* Nonzero means that this class type is not POD for the purpose of layout
>>>>>    (as defined in the ABI).  This is different from the language's POD.  */
>>>>> CLASSTYPE_NON_LAYOUT_POD_P
>>>>>
>>>>> and
>>>>>
>>>>> /* Nonzero means that this class type is a non-standard-layout class.  */
>>>>> #define CLASSTYPE_NON_STD_LAYOUT
>>>>>
>>>>> They aren't the same.
>>>>>
>>>>> struct A { };
>>>>> struct B { };
>>>>> struct C : A, B { };
>>>>>
>>>>> C is a standard-layout type, but not a standard-layout POD type.
>>>>
>>>> As the comment says, "POD for the purposes of layout" is different
>>>> from the language's POD. All standard-layout types are POD types
>>>> according to the language.
>>>>
>>>> So when you previously had "POD for the purposes of layout" that was
>>>> at least partially clear that you meant something other than what the
>>>> language means. But as pointed out, using a GCC-specific term is not
>>>> ideal.
>>>>
>>>> When you changed it to "POD for the purpose of standard-layout" that
>>>> became a completely meaningless term. Where is that defined?
>>>>
>>>> Your next suggestion was "standard-layout Plain Old Data (POD)" which
>>>> is even worse, now you're using two terms defined by the C++ language,
>>>> but you mean something different.
>>>>
>>>> When you mean something that is the same as the language (like "class
>>>> type") it makes sense to use the same term.
>>>>
>>>> When you mean something that is not the same as the language (like
>>>> "POD") it makes sense to use a different term, or clearly define how
>>>> you are using it.
>>>
>>> To be clear: it's really confusing to take two terms defined by the
>>> language, "POD" and "standard-layout", and smash them together to mean
>>> something new.
>>>
>>> According to your proposal, struct C is a POD type, and  a
>>> standard-layout type, but not a "standard-layout POD type". That's
>>> just crazy.
>>
>> Can you suggest a better wording?
>
> I don't know what the definition of "POD for the purposes of layout"
> is, but if I was trying to define a better name for it I would start
> by trying to understand how it is specified, instead of just throwing
> existing terms together.
>
>
>> Another issue, if I define
>>
>> 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
>>    c. An array of empty class types.
>> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>
>> Will it confuse people?
>
> Yes :-)
>
> But that was already confusing when you called it an "empty
> collection" because an array isn't a collection.
>
> If I understand correctly, your proposal says that given:
>
> struct A { };  // empty class type
> typedef A A2[2];  // array of empty class types
>
> struct B { A a[2]; };  // empty record?
>
> struct B is an empty record ... is that right?

Yes, struct B is an empty record.   And also in

struct A { };
struct B { };
struct C : A, B { };

C isn't an empty record.

-- 
H.J.