C++ PATCH to implement P1064R0, Virtual Function Calls in Constant Expressions (v4)

Jason Merrill jason@redhat.com
Tue Sep 18 18:36:00 GMT 2018


On Tue, Sep 18, 2018 at 11:25 AM, Marek Polacek <polacek@redhat.com> wrote:
> On Mon, Sep 17, 2018 at 11:28:06PM -0400, Jason Merrill wrote:
>> On Mon, Sep 17, 2018 at 5:39 PM, Marek Polacek <polacek@redhat.com> wrote:
>> > On Fri, Sep 14, 2018 at 04:45:22PM -0400, Marek Polacek wrote:
>> >> On Fri, Sep 14, 2018 at 04:30:46PM -0400, Jason Merrill wrote:
>> >> > On Fri, Sep 14, 2018 at 1:19 PM, Marek Polacek <polacek@redhat.com> wrote:
>> >> > > This patch implements another bit of C++20, virtual calls in constant
>> >> > > expression:
>> >> > > <http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1064r0.html>
>> >> > > The basic idea is that since in a constant expression we know the dynamic
>> >> > > type (to detect invalid code etc.), the restriction that prohibits virtual
>> >> > > calls is unnecessary.
>> >> > >
>> >> > > Handling virtual function calls turned out to be fairly easy (as anticipated);
>> >> > > I simply let the constexpr machinery figure out the dynamic type and then
>> >> > > OBJ_TYPE_REF_TOKEN gives us the index into the virtual function table.  That
>> >> > > way we get the function decl we're interested in, and cxx_eval_call_expression
>> >> > > takes it from there.
>> >> > >
>> >> > > But handling pointer-to-virtual-member-functions doesn't work like that.
>> >> > > get_member_function_from_ptrfunc creates a COND_EXPR which looks like
>> >> > > if (pf.__pfn & 1) // is it a virtual function?
>> >> > >   // yes, find the pointer in the vtable
>> >> > > else
>> >> > >   // no, just return the pointer
>> >> > > so ideally we want to evaluate the then-branch.  Eventually it'll evaluate it
>> >> > > to something like _ZTV2X2[2], but the vtable isn't constexpr so we'd end up
>> >> > > with "not a constant expression" error.
>> >> >
>> >> > Then let's mark the vtable as constexpr, there's no reason for it not to be.
>> >
>> > Done.  But then I had to add indexes to the vtable's ctor (because find_array_ctor_elt
>> > expects it), which broke an assert in gimple_get_virt_method_for_vtable.  But I
>> > don't need the array_ref hack anymore!
>>
>> > Also, I had to set DECL_DECLARED_CONSTEXPR_P after maybe_commonize_var,
>> > otherwise we run into the sorry in that function with -fno-weak...
>>
>> Hmm, we shouldn't give that sorry for DECL_ARTIFICIAL variables.
>>
>> Looking more closely, it seems that the call to maybe_commonize_var
>> from initialize_artificial_var did nothing before this change, since
>> the vtable is DECL_ARTIFICIAL, so it didn't pass the condition at the
>> top.  I suppose we should extend the !DECL_ARTIFICIAL check in
>> maybe_commonize_var to the inline variable case as well.
>
> Done.  And then I could move setting DECL_DECLARED_CONSTEXPR_P to
> initialize_artificial_var, which is where I think it belongs.
>
>> >> Ok, unfortunately it wasn't as easy as merely marking it DECL_DECLARED_CONSTEXPR_P
>> >> in initialize_artificial_var because then I saw "used in its own initializer"
>> >> error.  Which I don't know why, but now that I know you agree with this direction
>> >> I can dig deeper.
>> >>
>> >> > > Since the vtable initializer is
>> >> > > a compile-time constant, I thought we could make it work by a hack as the one
>> >> > > in cxx_eval_array_reference.  We'll then let cxx_eval_call_expression do its
>> >> > > job and everything is hunky-dory.
>> >> > >
>> >> > > Except when it isn't: I noticed that the presence of _vptr doesn't make the
>> >> > > class non-empty, and when cxx_eval_constant_expression saw a decl with an empty
>> >> > > class type, it just evaluated it to { }.  But such a class still had gotten an
>> >> > > initializer that looks like {.D.2082 = {._vptr.X2 = &_ZTV2X2 + 16}}.  So
>> >> > > replacing it with { } will lose the proper initializer whereupon we fail.
>> >> > > The check I've added to cxx_eval_constant_expression won't win any beauty
>> >> > > contests but unfortunately EMPTY_CONSTRUCTOR_P doesn't work there.
>> >> >
>> >> > Perhaps we should check !TYPE_POLYMORPHIC_P as well as
>> >> > is_really_empty_class.  Perhaps there should be a predicate for that,
>> >> > say, is_really_nearly_empty_class...
>> >
>> > For now I've only added the !TYPE_POLYMORPHIC_P check, which works just fine.
>> >
>> > Bootstrapped/regtested on x86_64-linux, ok for trunk?
>> >
>> > 2018-09-17  Marek Polacek  <polacek@redhat.com>
>> >
>> >         P1064R0 - Allowing Virtual Function Calls in Constant Expressions
>> >         * call.c (build_over_call): Add FIXME.
>> >         * class.c (initialize_vtable): Mark the vtable as constexpr.
>> >         (build_vtbl_initializer): Build vtable's constructor with indexes.
>> >         * constexpr.c (cxx_eval_constant_expression): Don't ignore _vptr's
>> >         initializer.  Handle OBJ_TYPE_REF.
>> >         (potential_constant_expression_1): Handle OBJ_TYPE_REF.
>> >         * decl.c (grokdeclarator): Change error to pedwarn.  Only warn when
>> >         pedantic and not C++2a.
>> >
>> >         * gimple-fold.c (gimple_get_virt_method_for_vtable): Remove assert.
>> >
>> >         * g++.dg/cpp0x/constexpr-virtual5.C: Adjust dg-error.
>> >         * g++.dg/cpp2a/constexpr-virtual1.C: New test.
>> >         * g++.dg/cpp2a/constexpr-virtual2.C: New test.
>> >         * g++.dg/cpp2a/constexpr-virtual3.C: New test.
>> >         * g++.dg/cpp2a/constexpr-virtual4.C: New test.
>> >         * g++.dg/cpp2a/constexpr-virtual5.C: New test.
>> >         * g++.dg/cpp2a/constexpr-virtual6.C: New test.
>> >         * g++.dg/cpp2a/constexpr-virtual7.C: New test.
>> >         * g++.dg/cpp2a/constexpr-virtual8.C: New test.
>> >         * g++.dg/cpp2a/constexpr-virtual9.C: New test.
>> >         * g++.dg/diagnostic/virtual-constexpr.C: Skip for C++2a.  Use
>> >         -pedantic-errors.  Adjust dg-error.
>> >
>> > diff --git gcc/cp/call.c gcc/cp/call.c
>> > index 69503ca7920..6c70874af40 100644
>> > --- gcc/cp/call.c
>> > +++ gcc/cp/call.c
>> > @@ -8401,7 +8401,8 @@ build_over_call (struct z_candidate *cand, int flags, tsubst_flags_t complain)
>> >
>> >    if (DECL_VINDEX (fn) && (flags & LOOKUP_NONVIRTUAL) == 0
>> >        /* Don't mess with virtual lookup in instantiate_non_dependent_expr;
>> > -        virtual functions can't be constexpr.  */
>> > +        virtual functions can't be constexpr.  FIXME Actually, no longer
>> > +        true in C++2a.  */
>> >        && !in_template_function ())
>>
>> I notice that removing the in_template_function check doesn't break
>> template/virtual4.C nowadays.  Does it break anything else?
>
> It doesn't seem to!  So, removed.
>
>> > +  /* The C++ FE now produces indexed fields but we can index the array
>> > +     directly. */
>> >    if (access_index < CONSTRUCTOR_NELTS (init))
>> >      {
>> >        fn = CONSTRUCTOR_ELT (init, access_index)->value;
>> > -      gcc_checking_assert (!CONSTRUCTOR_ELT (init, access_index)->index);
>>
>> Rather than remove this assert, let's fix it to check that the ->index
>> matches access_index.
>
> Done.
>
> Bootstrapped/regtested on ppc64le-linux, additionally tested dg.exp with
> GXX_TESTSUITE_STDS=98,11,14,17,2a on x86_64-linux, ok for trunk?
>
> 2018-09-18  Marek Polacek  <polacek@redhat.com>
>
>         P1064R0 - Allowing Virtual Function Calls in Constant Expressions
>         * call.c (build_over_call): No longer check if we're outside a template
>         function.
>         * class.c (build_vtbl_initializer): Build vtable's constructor with
>         indexes.
>         * constexpr.c (cxx_eval_constant_expression): Don't ignore _vptr's
>         initializer.  Handle OBJ_TYPE_REF.
>         (potential_constant_expression_1): Handle OBJ_TYPE_REF.
>         * decl.c (maybe_commonize_var): Bail out for any DECL_ARTIFICIAL.
>         (initialize_artificial_var): Mark the variable as constexpr.
>         (grokdeclarator): Change error to pedwarn.  Only warn when
>         pedantic and not C++2a.
>
>         * gimple-fold.c (gimple_get_virt_method_for_vtable): Adjust assert.
>
>         * g++.dg/cpp0x/constexpr-virtual5.C: Adjust dg-error.
>         * g++.dg/cpp2a/constexpr-virtual1.C: New test.
>         * g++.dg/cpp2a/constexpr-virtual2.C: New test.
>         * g++.dg/cpp2a/constexpr-virtual3.C: New test.
>         * g++.dg/cpp2a/constexpr-virtual4.C: New test.
>         * g++.dg/cpp2a/constexpr-virtual5.C: New test.
>         * g++.dg/cpp2a/constexpr-virtual6.C: New test.
>         * g++.dg/cpp2a/constexpr-virtual7.C: New test.
>         * g++.dg/cpp2a/constexpr-virtual8.C: New test.
>         * g++.dg/cpp2a/constexpr-virtual9.C: New test.
>         * g++.dg/diagnostic/virtual-constexpr.C: Skip for C++2a.  Use
>         -pedantic-errors.  Adjust dg-error.
>
> diff --git gcc/cp/call.c gcc/cp/call.c
> index 69503ca7920..ddf0ed044a0 100644
> --- gcc/cp/call.c
> +++ gcc/cp/call.c
> @@ -8399,10 +8399,7 @@ build_over_call (struct z_candidate *cand, int flags, tsubst_flags_t complain)
>        && DECL_BUILT_IN_CLASS (fn) == BUILT_IN_NORMAL)
>      maybe_warn_class_memaccess (input_location, fn, args);
>
> -  if (DECL_VINDEX (fn) && (flags & LOOKUP_NONVIRTUAL) == 0
> -      /* Don't mess with virtual lookup in instantiate_non_dependent_expr;
> -        virtual functions can't be constexpr.  */
> -      && !in_template_function ())
> +  if (DECL_VINDEX (fn) && (flags & LOOKUP_NONVIRTUAL) == 0)
>      {
>        tree t;
>        tree binfo = lookup_base (TREE_TYPE (TREE_TYPE (argarray[0])),
> diff --git gcc/cp/class.c gcc/cp/class.c
> index e950a7423f7..b9cde037337 100644
> --- gcc/cp/class.c
> +++ gcc/cp/class.c
> @@ -9266,6 +9266,7 @@ build_vtbl_initializer (tree binfo,
>        tree vcall_index;
>        tree fn, fn_original;
>        tree init = NULL_TREE;
> +      tree idx = build_int_cst (size_type_node, jx++);

Let's use size_int here, like in process_init_constructor_array.

OK with that change.

Jason



More information about the Gcc-patches mailing list