On Fri, Mar 06, 2020 at 05:49:07PM -0500, Jason Merrill wrote:
> On 3/5/20 2:40 PM, Marek Polacek wrote:
> > 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;
> >
> > But while TYPE_USER_ALIGN of TenuredCell was 0, TYPE_USER_ALIGN of its
> > CLASSTYPE_AS_BASE was 1.
>
> Surely the bug is that TYPE_USER_ALIGN isn't set for TenuredCell?
That's my understanding. I think it's clear that we're losing the alignas
specifier and so the TYPE_SIZE of BaseShape is wrong. Before the patch from
June patch we'd set it...
> 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;
> > }
...here, but we no longer do. Is there any other direction I could pursue?
Well, perhaps...
> > --- 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. */
> > + && !CLASSTYPE_USER_ALIGN (t)
> > && tree_int_cst_equal (TYPE_SIZE (t),
> > TYPE_SIZE (CLASSTYPE_AS_BASE (t))))
> > CLASSTYPE_AS_BASE (t) = t;
...we could copy the TYPE_USER_ALIGN flag to t if its old CLASSTYPE_AS_BASE
had it. But my fix seems to be safer.
Marek
