On Wed, Nov 27, 2019 at 10:47:58PM -0500, Jason Merrill wrote:
> On 11/27/19 6:35 PM, Marek Polacek wrote:
> > On Wed, Nov 27, 2019 at 04:47:01PM -0500, Jason Merrill wrote:
> > > On 11/27/19 2:36 PM, Marek Polacek wrote:
> > > > On Sun, Nov 24, 2019 at 12:24:48PM -0500, Jason Merrill wrote:
> > > > > On 11/16/19 5:23 PM, Marek Polacek wrote:
> > > > > > [ Working virtually on Baker Island. ]
> > > > > >
> > > > > > This patch implements C++20 P1331, allowing trivial default
> > > > > > initialization in
> > > > > > constexpr contexts.
> > > > > >
> > > > > > I used Jakub's patch from the PR which allowed uninitialized
> > > > > > variables in
> > > > > > constexpr contexts. But the hard part was handling
> > > > > > CONSTRUCTOR_NO_CLEARING
> > > > > > which is always cleared in cxx_eval_call_expression. We need to
> > > > > > set it in
> > > > > > the case a constexpr constructor doesn't initialize all the
> > > > > > members, so that
> > > > > > we can give proper diagnostic instead of value-initializing. A lot
> > > > > > of my
> > > > > > attempts flopped but then I came up with this approach, which
> > > > > > handles various
> > > > > > cases as tested in constexpr-init8.C, where S is initialized by a
> > > > > > non-default
> > > > > > constexpr constructor, and constexpr-init9.C, using delegating
> > > > > > constructors.
> > > > > > And the best part is that I didn't need any new
> > > > > > cx_check_missing_mem_inits
> > > > > > calls! Just save the information whether a constructor is missing
> > > > > > an init
> > > > > > into constexpr_fundef_table and retrieve it when needed.
> > > > >
> > > > > Is it necessary to clear the flag for constructors that do happen to
> > > > > initialize all the members? I would think that leaving that clearing
> > > > > to
> > > > > reduced_constant_expression_p would be enough.
> > > >
> > > > It seems so: if I tweak cxx_eval_call_expression to only call
> > > > clear_no_implicit_zero
> > > > when 'fun' isn't DECL_CONSTRUCTOR_P, then a lot breaks, e.g.
> > > > constexpr-base.C
> > > > where the constructor initializes all the members. By breaking I mean
> > > > spurious
> > > > errors coming from
> > > >
> > > > 5937 if (TREE_CODE (r) == CONSTRUCTOR && CONSTRUCTOR_NO_CLEARING (r))
> > > > 5938 {
> > > > 5939 if (!allow_non_constant)
> > > > 5940 error ("%qE is not a constant expression because it refers
> > > > to "
> > > > 5941 "an incompletely initialized variable", t);
> > > > 5942 TREE_CONSTANT (r) = false;
> > > > 5943 non_constant_p = true;
> > > > 5944 }
> > >
> > > Why didn't reduced_constant_expression_p unset CONSTRUCTOR_NO_CLEARING?
> >
> > We have a constructor that initializes a base class and members of a class:
> >
> > {.D.2364={.i=12}, .a={.i=24}, .j=36}
> >
> > Say we don't clear CONSTRUCTOR_NO_CLEARING in this ctor in
> > cxx_eval_call_expression.
> > Then soon in reduced_constant_expression_p we do
> > 2221 field = next_initializable_field (TYPE_FIELDS (TREE_TYPE
> > (t)));
> > and since "Implement P0017R1, C++17 aggregates with bases. / r241187" we
> > skip
> > base fields in C++17 so 'field' is set to 'a'.
>
> Hmm?
>
> > next_initializable_field (tree field)
> > {
> > while (field
> > && (TREE_CODE (field) != FIELD_DECL
> > || DECL_UNNAMED_BIT_FIELD (field)
> > || (DECL_ARTIFICIAL (field)
> > && !(cxx_dialect >= cxx17 && DECL_FIELD_IS_BASE (field)))))
> > field = DECL_CHAIN (field);
>
> This skips artificial fields except that in C++17 and up base fields are
> *not* skipped.
>
> How are you getting field starting with 'a'? Are you compiling in a lower
> standard mode? The code using next_initializable_field doesn't work for
> lower -std because of skipping base fields.
Duh, I'm sorry, you're right of course, I got it backwards. I didn't realize
I was debugging without -std=c++2a :/
> So perhaps we want to always clear_no_implicit_zero before c++20, and always
> for c++20 and up?
This doesn't work for constexpr-init8.C, where S is initialized by a non-default
constexpr constructor:
struct S {
constexpr S(int) {}
};
struct W {
constexpr W(int) : s(8), p() {}
S s;
int *p;
};
constexpr auto a = W(42);
When we perform register_constexpr_fundef the result of massage_constexpr_body
is
{.s=S::S (&((struct W *) this)->s, NON_LVALUE_EXPR <8>), .p=0B}
i.e. a ctor that initializes all the members. But later
reduced_constant_expression_p only ever sees {.p=0B} which seemingly doesn't
initialize all the members, so CONSTRUCTOR_NO_CLEARING is not cleared, and we
give an error.
That's why I opted for the approach in my original patch: in
register_constexpr_fundef
we still see all the initializers.
Marek
