On Mon, Mar 09, 2020 at 03:37:56PM -0400, Jason Merrill wrote:
> On 3/9/20 9:40 AM, Marek Polacek wrote:
> > On Mon, Mar 09, 2020 at 09:19:30AM -0400, Jason Merrill wrote:
> > > On 3/9/20 8:58 AM, Jakub Jelinek wrote:
> > > > On Fri, Mar 06, 2020 at 07:43:43PM -0500, Jason Merrill wrote:
> > > > > On 3/6/20 6:54 PM, Marek Polacek wrote:
> > > > > > I got a report that building Chromium fails with the "modifying a
> > > > > > const
> > > > > > object" error. After some poking I realized it's a bug in GCC, not
> > > > > > in
> > > > > > their codebase.
> > > > > >
> > > > > > Much like with ARRAY_REFs, which can be const even though the array
> > > > > > itself isn't, COMPONENT_REFs can be const although neither the
> > > > > > object
> > > > > > nor the field were declared const. So let's dial down the checking.
> > > > > > Here the COMPONENT_REF was const because of the "const_cast<const U
> > > > > > &>(m)"
> > > > > > thing -- cxx_eval_component_reference then builds a COMPONENT_REF
> > > > > > with
> > > > > > TREE_TYPE (t).
> > > > >
> > > > > What is folding the const into the COMPONENT_REF?
> > > >
> > > > cxx_eval_component_reference when it is called on
> > > > ((const struct array *) this)->elems
> > > > with /*lval=*/true and lval is true because we are evaluating
> > > > <retval> = (const int &) &((const struct array *)
> > > > this)->elems[VIEW_CONVERT_EXPR<size_t>(n)];
> > >
> > > Ah, sure. We're pretty loose with cv-quals in the constexpr code in
> > > general, so it's probably not worth trying to change that here. Getting
> > > back to the patch:
> >
> > Yes, here the additional const was caused by a const_cast adding a const.
> >
> > But this could also happen with wrapper functions like this one from
> > __array_traits in std::array:
> >
> > static constexpr _Tp&
> > _S_ref(const _Type& __t, std::size_t __n) noexcept
> > { return const_cast<_Tp&>(__t[__n]); }
> >
> > where the ref-to-const parameter added the const.
> >
> > > > + if (TREE_CODE (obj) == COMPONENT_REF)
> > > > + {
> > > > + tree op1 = TREE_OPERAND (obj, 1);
> > > > + if (CP_TYPE_CONST_P (TREE_TYPE (op1)))
> > > > + return true;
> > > > + else
> > > > + {
> > > > + tree op0 = TREE_OPERAND (obj, 0);
> > > > + /* The LHS of . or -> might itself be a COMPONENT_REF. */
> > > > + if (TREE_CODE (op0) == COMPONENT_REF)
> > > > + op0 = TREE_OPERAND (op0, 1);
> > > > + return CP_TYPE_CONST_P (TREE_TYPE (op0));
> > > > + }
> > > > + }
> > >
> > > Shouldn't this be a loop?
> >
> > I don't think so, though my earlier patch had a call to
> >
> > +static bool
> > +cref_has_const_field (tree ref)
> > +{
> > + while (TREE_CODE (ref) == COMPONENT_REF)
> > + {
> > + if (CP_TYPE_CONST_P (TREE_TYPE (TREE_OPERAND (ref, 1))))
> > + return true;
> > + ref = TREE_OPERAND (ref, 0);
> > + }
> > + return false;
> > +}
>
> > here. A problem arised when I checked even the outermost expression (which
> > is not a
> > field_decl), then I saw another problematical error.
> >
> > The more outer fields are expected to be checked in subsequent calls to
> > modifying_const_object_p in next iterations of the
> >
> > 4459 for (tree probe = target; object == NULL_TREE; )
> >
> > loop in cxx_eval_store_expression.
>
> OK, but then why do you want to check two levels here rather than just one?
It's a hack to keep constexpr-tracking-const7.C working. There we have
b.a.c.d.n
wherein 'd' is const struct D, but 'n' isn't const. Without the hack
const_object_being_modified would be 'b.a.c.d', but due to the problem I
desribed in the original mail[1] the constructor for D wouldn't have
TREE_READONLY set. With the hack const_object_being_modified will be
'b.a.c.d.n', which is of non-class type so we error:
4710 if (!CLASS_TYPE_P (const_objtype))
4711 fail = true;
I could remove the hack and maybe XFAIL constexpr-tracking-const7.C if you
want. Unfortunately I wasn't aware of [1] when I added that feature and
checking if the whole COMPONENT_REF is const seemed to be enough.
It's probably not a good idea to make this checking more strict at this
stage.
[1] "While looking into this I noticed that we don't detect modifying a const
object in certain cases like in
<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94074#c2>. That's because
we never evaluate an X::X() CALL_EXPR -- there's none. So there's no
CONSTRUCTOR to set TREE_READONLY on. No idea how to fix this, but it's
likely something for GCC 11 anyway."
Marek
