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.

1 files changed, 4 insertions, 0 deletions

diff --git a/gcc/cp/class.c b/gcc/cp/class.c
index b3787f7..5340799 100644
--- a/gcc/cp/class.c
+++ b/gcc/cp/class.c
@@ -6705,6 +6705,10 @@ layout_class_type (tree t, tree *virtuals_p)
 
   /* If we didn't end up needing an as-base type, don't use it.  */
   if (CLASSTYPE_AS_BASE (t) != t
+      /* If T's CLASSTYPE_AS_BASE is TYPE_USER_ALIGN, but T is not,
+	 replacing the as-base type would change CLASSTYPE_USER_ALIGN,
+	 causing us to lose the user-specified alignment as in PR94050.  */
+      && TYPE_USER_ALIGN (t) == TYPE_USER_ALIGN (CLASSTYPE_AS_BASE (t))
       && tree_int_cst_equal (TYPE_SIZE (t),
 			     TYPE_SIZE (CLASSTYPE_AS_BASE (t))))
     CLASSTYPE_AS_BASE (t) = t;