On Tue, Nov 7, 2017 at 2:40 PM, Richard Biener <rguent...@suse.de> wrote: > On November 7, 2017 7:21:17 PM GMT+01:00, Jason Merrill <ja...@redhat.com> > wrote: >>On Tue, Nov 7, 2017 at 3:14 AM, Richard Biener <rguent...@suse.de> >>wrote: >>> On Mon, 6 Nov 2017, Jason Merrill wrote: >>> >>>> On Mon, Nov 6, 2017 at 10:37 AM, Marek Polacek <pola...@redhat.com> >>wrote: >>>> > On Fri, Nov 03, 2017 at 12:19:05PM -0400, Jason Merrill wrote: >>>> >> On Fri, Nov 3, 2017 at 9:55 AM, Marek Polacek >><pola...@redhat.com> wrote: >>>> >> > + TYPE_EMPTY_P (t) = targetm.calls.empty_record_p (t); >>>> >> >>>> >> I think we want to set this in finalize_type_size; since the >>point of >>>> >> all this is becoming compliant with the psABI (and compatible >>with the >>>> >> C front end), I wouldn't think it should be specific to the C++ >>front >>>> >> end. >>>> > >>>> > Sure, that works. I'd wanted to do the TYPE_EMPTY_P setting in >>layout_type, >>>> > but that wasn't called for the classes in my testcases. I've >>moved the >>>> > setting of TYPE_EMPTY_P to finalize_type_size now. >>>> > >>>> >> > + TYPE_WARN_EMPTY_P (t) = warn_abi && abi_version_crosses >>(12); >>>> >> >>>> >> Can this flag go on the TRANSLATION_UNIT_DECL rather than the >>type? >>>> > >>>> > Yeah, that works, too. To avoid needing a lang hook, I'm setting >>the >>>> > flag in cxx_init_decl_processing, hope that's ok. >>>> >>>> Sure. >>>> >>>> >> > + if (TREE_CODE (field) == FIELD_DECL >>>> >> > + && (DECL_NAME (field) >>>> >> > + || RECORD_OR_UNION_TYPE_P (TREE_TYPE (field))) >>>> >> > + && !default_is_empty_type (TREE_TYPE (field))) >>>> >> > + return false; >>>> >> > + return true; >>>> >> >>>> >> Hmm, this assumes that any unnamed field can be ignored; I'm >>concerned >>>> >> that some front end might clear DECL_NAME for a reason that >>doesn't >>>> >> imply that the field is just padding. >>>> > >>>> > In that case I guess we need a new lang hook, right? Because in >>>> > default_is_empty_type we can't check FE-specific flags such as >>>> > DECL_C_BIT_FIELD. For C++, should that lang hook be just >>>> > is_really_empty_class? Or is there anything else I can do? >>>> >>>> Hmm, maybe leave it as it is and just document this assumption about >>>> FIELD_DECL with null DECL_NAME, both here and in tree.def. >>> >>> But are you sure you are not changing the ABI for a language other >>> than C++ then? >> >>No, that is the concern, we'd need to check that. >> >>> I don't think DECL_NAME has any special meaning - why >>> not check DECL_ARTIFICIAL or another flag that has appropriate >>> semantic - _what_ semantic are you looking for after all? >> >>That a struct consisting only of padding is considered empty. >> >>Perhaps we could use decl_flag_3 on FIELD_DECL for DECL_PADDING_P to >>indicate that a field isn't really data. Or perhaps we should remove >>such things from TYPE_FIELDS after layout is done. > > Hmm, I thought we had custom layout routines if you need padding that isn't > indicated by the real fields alignment? Why does C++ need explicit > FIELD_DECLs here?
The C front end builds FIELD_DECLs for unnamed bit-fields; I think the C++ front end inherited that decision. Jason
