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

[H.J. Lu via cfe-commits]

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). 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.