On 3/6/20 8:12 PM, Marek Polacek wrote:
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.
So why is that? Where is setting TYPE_USER_ALIGN on as-base
TenuredCell, and why isn't it setting it on the main type as well? Or
is it getting cleared on the main type for some reason?
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;
}
I would expect it to be set here on TenuredCell prime, and then copied
to the as-base type with
TYPE_USER_ALIGN (base_t) = TYPE_USER_ALIGN (t);
...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.
The flag should be set properly regardless of whether we do this
optimization.
Jason
