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

[H.J. Lu via cfe-commits]

On Tue, Feb 16, 2016 at 1:15 PM, Richard Smith <richard at metafoo.co.uk> wrote:
> On Tue, Feb 16, 2016 at 1:10 PM, H.J. Lu <hjl.tools at gmail.com> wrote:
>> On Tue, Feb 16, 2016 at 1:02 PM, Richard Smith <richard at metafoo.co.uk> wrote:
>>> On Tue, Feb 16, 2016 at 12:25 PM, H.J. Lu <hjl.tools at gmail.com> wrote:
>>>> On Tue, Feb 16, 2016 at 12:22 PM, Richard Smith <richard at metafoo.co.uk> wrote:
>>>>> On Tue, Feb 16, 2016 at 10:24 AM, H.J. Lu <hjl.tools at gmail.com> wrote:
>>>>>>
>>>>>> On Fri, Feb 12, 2016 at 11:39 AM, H.J. Lu <hjl.tools at gmail.com> wrote:
>>>>>> > On Fri, Feb 12, 2016 at 6:58 AM, Matthijs van Duin
>>>>>> > <matthijsvanduin at gmail.com> wrote:
>>>>>> >> On 11 February 2016 at 16:31, H.J. Lu <hjl.tools at 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.

Do you have a wording to describe it?


-- 
H.J.