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

[Richard Smith <richard at metafoo dot co dot uk>]

On Tue, Feb 16, 2016 at 12:25 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Tue, Feb 16, 2016 at 12:22 PM, Richard Smith <richard@metafoo.co.uk> wrote:
>> On Tue, Feb 16, 2016 at 10:24 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>>
>>> On Fri, Feb 12, 2016 at 11:39 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>> > On Fri, Feb 12, 2016 at 6:58 AM, Matthijs van Duin
>>> > <matthijsvanduin@gmail.com> wrote:
>>> >> On 11 February 2016 at 16:31, H.J. Lu <hjl.tools@gmail.com> wrote:
>>> >>> struct A {
>>> >>> static void foo (void) ();
>>> >>> static int xxx;
>>> >>> };
>>> >>
>>> >> What about it? It's an empty struct.  (And it declares a function and
>>> >> a variable in the namespace of A, which however do not have any
>>> >> relevant impact here.)
>>> >>
>>> >
>>> > Thanks for all the feedbacks.  Here is the new proposal:
>>> >
>>> > 1. "empty type".  An empty type is a trivially-copyable aggregate
>>> > occupying zero bytes (excluding any padding).
>>> > 2. No memory slot nor register should be used to pass or return an object
>>> > of empty type.
>>> >
>>> > Footnote: Array of empty type can only passed by reference in C/C++.
>>> >
>>>
>>> I updated intel386, x86-64 and IA MCU psABIs:
>>>
>>> https://github.com/hjl-tools/x86-psABI/wiki/X86-psABI
>>>
>>> to specify:
>>>
>>> Empty type is defined as a trivially-copyable aggregate occupying zero bytes
>>> (excluding any padding).
>>
>> I think this is now extremely unclear. Does an empty struct in C++
>> occupy zero bytes? sizeof applied to it will produce at least 1.
>
> Can it be considered as padding?

Perhaps, but I would contend that that's unclear in at least some
cases (when the class is used as the type of a member, ...). What
about this case:

  struct X { unsigned : 15; };

Is that empty or not? Do we have a formal definition somewhere of what
does, and does not, count as padding?

>>> No memory slot nor register should be used to pass or
>>> return an object object of empty type.
>>>
>>> with footnote: Array of empty type can only passed by reference in C and C++.
>>>
>>> Any comments?
>>>
>>> Thanks.
>>>
>>>
>>> --
>>> H.J.
>
>
>
> --
> H.J.