> From: Jason Merrill via Gcc-patches <gcc-patches@gcc.gnu.org> > Date: Fri, 17 Dec 2021 04:26:55 +0100
> C++14 changed the definition of 'aggregate' to allow default member > initializers, but such classes still need to be considered "non-POD for the > purpose of layout" for ABI compatibility with C++11 code. It seems rare to > derive from such a class, as evidenced by how long this bug has > survived (since r216750 in 2014), but it's certainly worth fixing. > > We only warn when we were failing to allocate another field into the > tail padding of the newly aggregate class; this is the only ABI impact. > > This also changes end_of_class to consider all data members, not just empty > data members; that used to be an additional flag, removed in r9-5710, but I > don't see any reason not to always include them. This makes the result of > the function correspond to the ABI nvsize term and its nameless counterpart > that does include virtual bases. > > When looking closely at other users of end_of_class, I realized that we were > assuming that the latter corresponded to the ABI dsize term, but it doesn't > if the class ends with an empty virtual base (in the rare case that the > empty base can't be assigned offset 0), and this matters for layout of > [[no_unique_address]]. So I added another mode that returns the desired > value for that case. I'm not adding a warning for this ABI fix because it's > a C++20 feature. > > Tested x86_64-pc-linux-gnu, applying to trunk. > > PR c++/103681 > > gcc/ChangeLog: > > * common.opt (fabi-version): Add v17. > > gcc/cp/ChangeLog: > > * cp-tree.h (struct lang_type): Add non_pod_aggregate. > (CLASSTYPE_NON_POD_AGGREGATE): New. > * class.c (check_field_decls): Set it. > (check_bases_and_members): Check it. > (check_non_pod_aggregate): New. > (enum eoc_mode): New. > (end_of_class): Always include non-empty fields. > Add eoc_nv_or_dsize mode. > (include_empty_classes, layout_class_type): Adjust. Looks like this caused https://gcc.gnu.org/PR104139. brgds, H-P
