https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94050
--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
commit r10-7089-g8475f2902a2e2ca5f7ace8bc5265bd1a815dda20
Author: Marek Polacek <pola...@redhat.com>
Date: Thu Mar 5 14:07:25 2020 -0500
c++: Fix ABI issue with alignas on armv7hl [PR94050]
The static_assert in the following test was failing on armv7hl because
we were disregarding the alignas specifier on Cell. BaseShape's data
takes up 20B on 32-bit architectures, but we failed to round up its
TYPE_SIZE. This happens since the
<https://gcc.gnu.org/ml/gcc-patches/2019-06/msg01189.html>
patch: here, in layout_class_type for TenuredCell, we see that the size
of TenuredCell and its CLASSTYPE_AS_BASE match, so we set
CLASSTYPE_AS_BASE (t) = t;
While TYPE_USER_ALIGN of TenuredCell was 0, because finalize_type_size
called from finish_record_layout reset it, TYPE_USER_ALIGN of its
CLASSTYPE_AS_BASE still remained 1. After we replace it, it's no longer
1. Then we perform layout_empty_base_or_field for TenuredCell and since
TYPE_USER_ALIGN of its CLASSTYPE_AS_BASE is now 0, we don't do this
adjustment:
if (CLASSTYPE_USER_ALIGN (type))
{
rli->record_align = MAX (rli->record_align, CLASSTYPE_ALIGN (type));
if (warn_packed)
rli->unpacked_align = MAX (rli->unpacked_align, CLASSTYPE_ALIGN
(type));
TYPE_USER_ALIGN (rli->t) = 1;
}
where rli->t is BaseShape. Then finalize_record_size won't use the
correct rli->record_align and therefore
/* Round the size up to be a multiple of the required alignment. */
TYPE_SIZE (rli->t) = round_up (unpadded_size, TYPE_ALIGN (rli->t));
after this we end up with the wrong size.
Since the original fix was to avoid creating extra copies for LTO
purposes, I think the following fix should be acceptable.
PR c++/94050 - ABI issue with alignas on armv7hl.
* class.c (layout_class_type): Don't replace a class's
CLASSTYPE_AS_BASE if their TYPE_USER_ALIGN don't match.
* g++.dg/abi/align3.C: New test.
