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

[Richard Smith via cfe-commits]

On Mon, Feb 8, 2016 at 1:40 PM, H.J. Lu <hjl.tools at gmail.com> wrote:
>
> On Mon, Feb 8, 2016 at 12:38 PM, Richard Smith <richard at metafoo.co.uk> wrote:
> > On Mon, Feb 8, 2016 at 12:05 PM, H.J. Lu <hjl.tools at gmail.com> wrote:
> >>
> >> On Mon, Feb 8, 2016 at 11:33 AM, Jonathan Wakely <jwakely.gcc at gmail.com>
> >> wrote:
> >> > On 8 February 2016 at 19:23, Richard Smith wrote:
> >> >> "POD for the purpose of layout" is defined in the Itanium C++ ABI here:
> >> >>
> >> >>   http://mentorembedded.github.io/cxx-abi/abi.html#definitions
> >> >
> >> > Thanks. So there's no problem using "POD for the purposes of layout",
> >> > and the change to "POD for the purpose of standard-layout" was
> >> > unnecessary and just confused things.
> >>
> >> Here is the revised proposal:
> >>
> >> 1. "class type".  A class type is a structure, union or C++ class.
> >> 2. "empty class type".  An empty class type is:
> >>    a. A class type without member.  Or
> >>    b. A class type with only members of empty class types.  Or
> >
> >
> > (a) is a special case of (b).
> >
> >>    c. An array of empty class types.
> >
> >
> > It seems confusing to call an array a class type. Instead, how about:
> >
> >   2. An empty type is either an array of empty types or a class type where
> > every member is of empty type.
> >
> >> 3. "empty record".  An empty record is Plain Old Data (POD) for the
> >>    purposes of layout and
> >>    a. A class type without member.  Or
> >>    b. A class type with only members of empty class types.
> >
> >
> > (a) is a special case of (b).
> >
> >> 4. No memory slot nor register should be used to pass or return an object
> >> of empty record.
> >
> >
> > Objects of array type are never passed or returned (but if through some
> > language extension they were, we'd want this rule to apply). So you don't
> > need rule 3 and this can be just:
> >
> >  3. No memory slot nor register should be used to pass or return an object
> > of empty type.
>
> Thanks very much for your inputs.  Here is the proposal:
>
> 1. "class type".  A class type is a structure, union or C++ class.
> 2. "empty type".  An empty type is either an array of empty types or a
> class type where every member is of empty type.
> 3. No memory slot nor register should be used to pass or return an object
> of empty type.

David Majnemer points out that we also need to say something about
base classes. We could handle that case like this:

 2. "empty type".  An empty type is a type where it and all of its
subobjects are of class or array type.

Following the C++ rules, this also means that a class that contains
only unnamed bitfields is empty, because unnamed bitfields are not
subobjects, but might be worth explicitly stating for the C case. That
also matches Clang's behavior.