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


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.


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