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 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?

>> 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.