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.
Marek
