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

[Richard Smith via cfe-commits]

On 20 Feb 2016 10:01 p.m., "H.J. Lu" <hjl.tools at gmail.com> wrote:
>
> On Sat, Feb 20, 2016 at 9:47 PM, Richard Smith <richard at metafoo.co.uk>
wrote:
> > On 20 Feb 2016 6:54 p.m., "H.J. Lu" <hjl.tools at gmail.com> wrote:
> >>
> >> On Sat, Feb 20, 2016 at 4:57 PM, Matthijs van Duin
> >> <matthijsvanduin at gmail.com> wrote:
> >> > On 20 February 2016 at 23:35, H.J. Lu <hjl.tools at gmail.com> wrote:
> >> >> Can a compiler tell if a copy constructor or destructor is trivial
> >> >> from the class declaration without function body?
> >> >
> >> > Yes, the mere presence of the declaration suffices to render it
> >> > non-trivial (unless explicitly declared "= default" like I did with
> >> > the default constructor, in which case there's no function body).
> >>
> >> How about this?
> >>
> >> An empty type is a type where it and all of its subobjects
(recursively)
> >> are of class, structure, union, or array type.  An empty type may only
> >> have static member functions, default  constructor, default copy
> >> constructor, default copy assignment operator or default destructor.
> >
> > No, that's the wrong rule still. Please leave the C++ rule here to the
C++
> > ABI rather than trying to reinvent it. Whether a type is empty is
completely
> > orthogonal to whether it must be passed through memory for C++ ABI /
> > semantics reasons.
>
> What is the correct wording?  The last one:
>
> An empty type is a type where it and all of its subobjects (recursively)
> are of class, structure, union, or array type.
>
> doesn't cover "trivially-copyable".

That's correct. Whether a type is trivially copyable is unrelated to
whether it is empty.
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