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

[Jason Merrill via cfe-commits]

On 02/29/2016 10:13 AM, Michael Matz via cfe-commits wrote:
> Also this insistence that all of "trivially copyable" is
> already quite nicely specified in the C++ ABI is still not really relevant
> because C++ _is not the only language out there_.  I'm not sure how often
> I have to repeat this until people get it.

Other language ABIs can handle language specific calling conventions as 
appropriate for them.  The psABI can only talk about things that are in 
its domain.

Jason