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 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
{
};


-- 
H.J.