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? >> 3. No memory slot nor register should be used to pass or return an object >> of empty type. >> >> >> -- >> H.J. -- H.J. _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
