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. _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
