On Tue, Feb 9, 2016 at 6:45 AM, H.J. Lu <hjl.to...@gmail.com> wrote: > On Mon, Feb 8, 2016 at 3:28 PM, Richard Smith <rich...@metafoo.co.uk> wrote: >> On Mon, Feb 8, 2016 at 3:01 PM, H.J. Lu <hjl.to...@gmail.com> wrote: >>> On Mon, Feb 8, 2016 at 2:58 PM, Richard Smith <rich...@metafoo.co.uk> wrote: >>>> On Mon, Feb 8, 2016 at 2:54 PM, H.J. Lu <hjl.to...@gmail.com> wrote: >>>>> On Mon, Feb 8, 2016 at 2:51 PM, Richard Smith <rich...@metafoo.co.uk> >>>>> wrote: >>>>>> On Mon, Feb 8, 2016 at 2:46 PM, H.J. Lu <hjl.to...@gmail.com> wrote: >>>>>>> On Mon, Feb 8, 2016 at 2:35 PM, Richard Smith <rich...@metafoo.co.uk> >>>>>>> wrote: >>>>>>>> On Mon, Feb 8, 2016 at 1:40 PM, H.J. Lu <hjl.to...@gmail.com> wrote: >>>>>>>>> >>>>>>>>> On Mon, Feb 8, 2016 at 12:38 PM, Richard Smith >>>>>>>>> <rich...@metafoo.co.uk> wrote: >>>>>>>>> > On Mon, Feb 8, 2016 at 12:05 PM, H.J. Lu <hjl.to...@gmail.com> >>>>>>>>> > wrote: >>>>>>>>> >> >>>>>>>>> >> On Mon, Feb 8, 2016 at 11:33 AM, Jonathan Wakely >>>>>>>>> >> <jwakely....@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. >>>>>>> >>>>>>> Like this? >>>>>>> >>>>>>> 1. "class type". A class type is a structure, union or C++ class. >>>>>>> 2. "empty type". An empty type is >>>>>>> a. A type where it and all of its subobjects are of class or array >>>>>>> type. And >>>>>>> b. Either an array of empty types or a class type where every member >>>>>>> is of empty type. >>>>>> >>>>>> You don't need (b). It's implied by (a). >>>>> >>>>> Does (a) cover empty type? >>>> >>>> Yes, (a) is a complete definition of "empty type" by itself: the >>>> definition is, essentially, that a complete recursive walk of the type >>>> and all its subobjects never sees anything that would require any >>>> storage (that is, it only sees class types and array types). >>> >>> Is there a definition of subobject? >> >> Yes, the C++ standard defines the term (as an object that is contained >> within another via member, base class, and array element >> relationships). The C standard uses it in 6.7.9 to mean the same thing >> (recursing into class and array elements) but doesn't provide a formal >> definition. > > Here is the new one: > > 1. "class type". A class type is a structure, union or C++ class. > 2. "empty type". An empty type is Plain Old Data (POD) for the > purposes of layout, and.a type where it and all of its subobjects > are of class or array type. > > I put back " Plain Old Data (POD) for the purposes of layout" to > explicitly exclude array here.
I updated PDF files for passing/returning empty type: https://github.com/hjl-tools/x86-psABI/wiki/X86-psABI -- H.J.
