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 1:10 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Tue, Feb 16, 2016 at 1:02 PM, Richard Smith <richard@metafoo.co.uk> wrote:
>> 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?
>
> How about this?
>
> Empty type is defined as a trivially-copyable aggregate occupying zero bytes
> (excluding any padding or contributing zero bytes to the size of derived
> classes in C++).
>
> This will cover
>
> struct dummy0
> {
>   void bar (void);
> };
> struct dummy1
> {
>   void foo (void);
> };
> struct dummy : dummy0, dummy1 { };
>
> But not
>
> struct dummy0
> {
> };
> struct dummy1
> {
>   unsigned : 15;
> };
> struct dummy : dummy0, dummy1
> {
> };

Why not? That looks empty to me. The ABI will classify the
corresponding eightbyte as NO_CLASS, as it has no members, so it
should not be passed.