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? Richard. >Jason
