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

[Michael Matz via cfe-commits]

Hi,

On Thu, 18 Feb 2016, Richard Smith wrote:

> >> An empty type is a type where it and all of its subobjects 
> >> (recursively) are of class, structure, union, or array type.  No 
> >> memory slot nor register should be used to pass or return an object 
> >> of empty type.
> >
> > The trivially copyable is gone again.  Why is it not necessary?
> 
> The C++ ABI doesn't defer to the C psABI for types that aren't 
> trivially-copyable. See 
> http://mentorembedded.github.io/cxx-abi/abi.html#normal-call

Hmm, yes, but we don't want to define something for only C and C++, but 
language independend (so far as possible).  And given only the above 
language I think this type:

struct S {
  S() {something();}
};

would be an empty type, and that's not what we want.  "Trivially copyable" 
is a reasonably common abstraction (if in doubt we could even define it in 
the ABI), and captures the idea that we need well (namely that a bit-copy 
is enough).


Ciao,
Michael.