Re: [PATCH 1/2] libstdc++: Convert _RangeAdaptorClosure into a CRTP class [PR108827]

[Jonathan Wakely <jwakely.gcc@gmail.com>]

On Wed, 16 Aug 2023 at 17:06, Patrick Palka via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> On Sun, Apr 16, 2023 at 11:24 PM Patrick Palka <ppalka@redhat.com> wrote:
> >
> > On Fri, 14 Apr 2023, Patrick Palka wrote:
> >
> > > Using the CRTP idiom for this base class avoids bloating the size of a
> > > pipeline when adding distinct empty range adaptor closure objects to it,
> > > as detailed in section 4.1 of P2387R3.
> > >
> > > But it means we can no longer define its operator| overloads as hidden
> > > friends, since each instantiation of _RangeAdaptorClosure would then
> > > introduce its own logically different hidden friends.  So for example
> > > during overload resolution for the outer pipe operator in
> > >
> > >  :x | (views::reverse | views::join)
> > >
> > > we'd have to consider 6 different hidden operator| friends:
> > >
> > >   2 from _RangeAdaptorClosure<_Reverse>
> > >   2 from _RangeAdaptorClosure<_Join>
> > >   2 from _RangeAdaptorClosure<_Pipe<_Reverse, _Join>>
> > >
> > > which is wasteful and can even cause hard errors in some cases.  So we
> > > instead define the operator| overloads at namespace scope in an isolated
> > > namespace.
> >
> > On second thought, since this doesn't fix a bug or add new functionality
> > it seems more like GCC 14 material.  The size reduction is nice but it's
> > probably not a big deal in practice since adaptor pipelines are usually
> > very transient objects that don't get passed around as function
> > arguments etc.
>
> Ping, does this look OK for trunk?

OK for trunk, thanks.


>
> >
> > But perhaps the second patch implementing range_adaptor_closure would be
> > desirable for GCC 13?  I'll post an updated standalone version of that
> > patch for separate consideration.
> >
> > >
> > >       PR libstdc++/108827
> > >
> > > libstdc++-v3/ChangeLog:
> > >
> > >       * include/std/ranges (__adaptor::_RangeAdaptorClosure): Move ...
> > >       (__adaptor::__closure::_RangeAdaptorClosure): ... here and turn
> > >       it into a CRTP class template.  Move hidden operator| friends
> > >       into namespace scope and adjust their constraints.  Add a
> > >       using-declaration for this at __adaptor::_RangeAdaptorClosure.
> > >       (__closure::__is_range_adaptor_closure_fn): Define.
> > >       (__closure::__is_range_adaptor_closure): Define.
> > >       (__adaptor::_Partial): Adjust use of _RangeAdaptorClosure.
> > >       (__adaptor::_Pipe): Likewise.
> > >       (views::_All): Likewise.
> > >       (views::_Join): Likewise.
> > >       (views::_Common): Likewise.
> > >       (views::_Reverse): Likewise.
> > >       (views::_Elements): Likewise.
> > >       (views::_Adjacent): Likewise.
> > >       (views::_AsRvalue): Likewise.
> > >       (views::_Enumerate): Likewise.
> > >       (views::_AsConst): Likewise.
> > >       * testsuite/std/ranges/adaptors/all.cc: Reintroduce
> > >       static_assert expecting that adding empty range adaptor
> > >       closure objects to a pipeline doesn't increase the size of a
> > >       pipeline.
> > > ---
> > >  libstdc++-v3/include/std/ranges               | 69 +++++++++++--------
> > >  .../testsuite/std/ranges/adaptors/all.cc      |  7 --
> > >  2 files changed, 42 insertions(+), 34 deletions(-)
> > >
> > > diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
> > > index 283d757faa4..531ec6f68b3 100644
> > > --- a/libstdc++-v3/include/std/ranges
> > > +++ b/libstdc++-v3/include/std/ranges
> > > @@ -872,30 +872,45 @@ namespace views::__adaptor
> > >    template<typename _Lhs, typename _Rhs>
> > >      struct _Pipe;
> > >
> > > -  // The base class of every range adaptor closure.
> > > -  //
> > > -  // The derived class should define the optional static data member
> > > -  // _S_has_simple_call_op to true if the behavior of this adaptor is
> > > -  // independent of the constness/value category of the adaptor object.
> > > -  struct _RangeAdaptorClosure
> > > +  namespace __closure
> > >    {
> > > +    // The base class of every range adaptor closure.
> > > +    //
> > > +    // The derived class should define the optional static data member
> > > +    // _S_has_simple_call_op to true if the behavior of this adaptor is
> > > +    // independent of the constness/value category of the adaptor object.
> > > +    template<typename _Derived>
> > > +      struct _RangeAdaptorClosure
> > > +      { };
> > > +
> > > +    template<typename _Tp, typename _Up>
> > > +      requires (!same_as<_Tp, _RangeAdaptorClosure<_Up>>)
> > > +      void __is_range_adaptor_closure_fn
> > > +     (const _Tp&, const _RangeAdaptorClosure<_Up>&); // not defined
> > > +
> > > +    template<typename _Tp>
> > > +      concept __is_range_adaptor_closure
> > > +     = requires (_Tp __t) { __closure::__is_range_adaptor_closure_fn(__t, __t); };
> > > +
> > >      // range | adaptor is equivalent to adaptor(range).
> > >      template<typename _Self, typename _Range>
> > > -      requires derived_from<remove_cvref_t<_Self>, _RangeAdaptorClosure>
> > > +      requires __is_range_adaptor_closure<_Self>
> > >       && __adaptor_invocable<_Self, _Range>
> > > -      friend constexpr auto
> > > +      constexpr auto
> > >        operator|(_Range&& __r, _Self&& __self)
> > >        { return std::forward<_Self>(__self)(std::forward<_Range>(__r)); }
> > >
> > >      // Compose the adaptors __lhs and __rhs into a pipeline, returning
> > >      // another range adaptor closure object.
> > >      template<typename _Lhs, typename _Rhs>
> > > -      requires derived_from<_Lhs, _RangeAdaptorClosure>
> > > -     && derived_from<_Rhs, _RangeAdaptorClosure>
> > > -      friend constexpr auto
> > > +      requires __is_range_adaptor_closure<_Lhs>
> > > +     && __is_range_adaptor_closure<_Rhs>
> > > +      constexpr auto
> > >        operator|(_Lhs __lhs, _Rhs __rhs)
> > >        { return _Pipe<_Lhs, _Rhs>{std::move(__lhs), std::move(__rhs)}; }
> > > -  };
> > > +  } // namespace __closure
> > > +
> > > +  using __closure::_RangeAdaptorClosure;
> > >
> > >    // The base class of every range adaptor non-closure.
> > >    //
> > > @@ -937,7 +952,7 @@ namespace views::__adaptor
> > >    // A range adaptor closure that represents partial application of
> > >    // the range adaptor _Adaptor with arguments _Args.
> > >    template<typename _Adaptor, typename... _Args>
> > > -    struct _Partial : _RangeAdaptorClosure
> > > +    struct _Partial : _RangeAdaptorClosure<_Partial<_Adaptor, _Args...>>
> > >      {
> > >        tuple<_Args...> _M_args;
> > >
> > > @@ -978,7 +993,7 @@ namespace views::__adaptor
> > >    // A lightweight specialization of the above primary template for
> > >    // the common case where _Adaptor accepts a single extra argument.
> > >    template<typename _Adaptor, typename _Arg>
> > > -    struct _Partial<_Adaptor, _Arg> : _RangeAdaptorClosure
> > > +    struct _Partial<_Adaptor, _Arg> : _RangeAdaptorClosure<_Partial<_Adaptor, _Arg>>
> > >      {
> > >        _Arg _M_arg;
> > >
> > > @@ -1011,7 +1026,7 @@ namespace views::__adaptor
> > >    template<typename _Adaptor, typename... _Args>
> > >      requires __adaptor_has_simple_extra_args<_Adaptor, _Args...>
> > >        && (is_trivially_copyable_v<_Args> && ...)
> > > -    struct _Partial<_Adaptor, _Args...> : _RangeAdaptorClosure
> > > +    struct _Partial<_Adaptor, _Args...> : _RangeAdaptorClosure<_Partial<_Adaptor, _Args...>>
> > >      {
> > >        tuple<_Args...> _M_args;
> > >
> > > @@ -1041,7 +1056,7 @@ namespace views::__adaptor
> > >    template<typename _Adaptor, typename _Arg>
> > >      requires __adaptor_has_simple_extra_args<_Adaptor, _Arg>
> > >        && is_trivially_copyable_v<_Arg>
> > > -    struct _Partial<_Adaptor, _Arg> : _RangeAdaptorClosure
> > > +    struct _Partial<_Adaptor, _Arg> : _RangeAdaptorClosure<_Partial<_Adaptor, _Arg>>
> > >      {
> > >        _Arg _M_arg;
> > >
> > > @@ -1066,7 +1081,7 @@ namespace views::__adaptor
> > >    // A range adaptor closure that represents composition of the range
> > >    // adaptor closures _Lhs and _Rhs.
> > >    template<typename _Lhs, typename _Rhs>
> > > -    struct _Pipe : _RangeAdaptorClosure
> > > +    struct _Pipe : _RangeAdaptorClosure<_Pipe<_Lhs, _Rhs>>
> > >      {
> > >        [[no_unique_address]] _Lhs _M_lhs;
> > >        [[no_unique_address]] _Rhs _M_rhs;
> > > @@ -1102,7 +1117,7 @@ namespace views::__adaptor
> > >    template<typename _Lhs, typename _Rhs>
> > >      requires __closure_has_simple_call_op<_Lhs>
> > >        && __closure_has_simple_call_op<_Rhs>
> > > -    struct _Pipe<_Lhs, _Rhs> : _RangeAdaptorClosure
> > > +    struct _Pipe<_Lhs, _Rhs> : _RangeAdaptorClosure<_Pipe<_Lhs, _Rhs>>
> > >      {
> > >        [[no_unique_address]] _Lhs _M_lhs;
> > >        [[no_unique_address]] _Rhs _M_rhs;
> > > @@ -1264,7 +1279,7 @@ namespace views::__adaptor
> > >       concept __can_owning_view = requires { owning_view{std::declval<_Range>()}; };
> > >      } // namespace __detail
> > >
> > > -    struct _All : __adaptor::_RangeAdaptorClosure
> > > +    struct _All : __adaptor::_RangeAdaptorClosure<_All>
> > >      {
> > >        template<typename _Range>
> > >       static constexpr bool
> > > @@ -3048,7 +3063,7 @@ namespace views::__adaptor
> > >         = requires { join_view<all_t<_Range>>{std::declval<_Range>()}; };
> > >      } // namespace __detail
> > >
> > > -    struct _Join : __adaptor::_RangeAdaptorClosure
> > > +    struct _Join : __adaptor::_RangeAdaptorClosure<_Join>
> > >      {
> > >        template<viewable_range _Range>
> > >       requires __detail::__can_join_view<_Range>
> > > @@ -3842,7 +3857,7 @@ namespace views::__adaptor
> > >         = requires { common_view{std::declval<_Range>()}; };
> > >      } // namespace __detail
> > >
> > > -    struct _Common : __adaptor::_RangeAdaptorClosure
> > > +    struct _Common : __adaptor::_RangeAdaptorClosure<_Common>
> > >      {
> > >        template<viewable_range _Range>
> > >       requires __detail::__already_common<_Range>
> > > @@ -3963,7 +3978,7 @@ namespace views::__adaptor
> > >         = requires { reverse_view{std::declval<_Range>()}; };
> > >      } // namespace __detail
> > >
> > > -    struct _Reverse : __adaptor::_RangeAdaptorClosure
> > > +    struct _Reverse : __adaptor::_RangeAdaptorClosure<_Reverse>
> > >      {
> > >        template<viewable_range _Range>
> > >       requires __detail::__is_reverse_view<remove_cvref_t<_Range>>
> > > @@ -4363,7 +4378,7 @@ namespace views::__adaptor
> > >      } // namespace __detail
> > >
> > >      template<size_t _Nm>
> > > -      struct _Elements : __adaptor::_RangeAdaptorClosure
> > > +      struct _Elements : __adaptor::_RangeAdaptorClosure<_Elements<_Nm>>
> > >        {
> > >       template<viewable_range _Range>
> > >         requires __detail::__can_elements_view<_Nm, _Range>
> > > @@ -5484,7 +5499,7 @@ namespace views::__adaptor
> > >      }
> > >
> > >      template<size_t _Nm>
> > > -      struct _Adjacent : __adaptor::_RangeAdaptorClosure
> > > +      struct _Adjacent : __adaptor::_RangeAdaptorClosure<_Adjacent<_Nm>>
> > >        {
> > >       template<viewable_range _Range>
> > >         requires (_Nm == 0) || __detail::__can_adjacent_view<_Nm, _Range>
> > > @@ -8609,7 +8624,7 @@ namespace views::__adaptor
> > >       concept __can_as_rvalue_view = requires { as_rvalue_view(std::declval<_Tp>()); };
> > >      }
> > >
> > > -    struct _AsRvalue : __adaptor::_RangeAdaptorClosure
> > > +    struct _AsRvalue : __adaptor::_RangeAdaptorClosure<_AsRvalue>
> > >      {
> > >        template<viewable_range _Range>
> > >       requires __detail::__can_as_rvalue_view<_Range>
> > > @@ -8918,7 +8933,7 @@ namespace views::__adaptor
> > >         = requires { enumerate_view<all_t<_Tp>>(std::declval<_Tp>()); };
> > >      }
> > >
> > > -    struct _Enumerate : __adaptor::_RangeAdaptorClosure
> > > +    struct _Enumerate : __adaptor::_RangeAdaptorClosure<_Enumerate>
> > >      {
> > >        template<viewable_range _Range>
> > >       requires __detail::__can_enumerate_view<_Range>
> > > @@ -9004,7 +9019,7 @@ namespace views::__adaptor
> > >       concept __can_as_const_view = requires { as_const_view(std::declval<_Range>()); };
> > >      }
> > >
> > > -    struct _AsConst : __adaptor::_RangeAdaptorClosure
> > > +    struct _AsConst : __adaptor::_RangeAdaptorClosure<_AsConst>
> > >      {
> > >        template<viewable_range _Range>
> > >        constexpr auto
> > > diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/all.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/all.cc
> > > index 57cb542174b..6bbaf73a20a 100644
> > > --- a/libstdc++-v3/testsuite/std/ranges/adaptors/all.cc
> > > +++ b/libstdc++-v3/testsuite/std/ranges/adaptors/all.cc
> > > @@ -105,12 +105,6 @@ static_assert(std::is_empty_v<decltype(views::common
> > >                                      | views::common
> > >                                      | views::keys
> > >                                      | views::reverse)>);
> > > -#if 0
> > > -// Adding empty range adaptor closure objects to a pipeline used to not
> > > -// increase the size of the pipeline, but now that our range adaptor closure
> > > -// objects derive from a common empty base class, [[no_unique_address]] can no
> > > -// longer make two empty adjacent range adaptor closure objects occupy the same
> > > -// data member address.
> > >  static_assert(sizeof(decltype(views::take(5) | views::drop(5)))
> > >             == sizeof(decltype(views::take(5)
> > >                                | views::join
> > > @@ -119,7 +113,6 @@ static_assert(sizeof(decltype(views::take(5) | views::drop(5)))
> > >                                | views::keys
> > >                                | views::drop(5)
> > >                                | views::reverse)));
> > > -#endif
> > >
> > >  template<auto all = views::all>
> > >  void
> > > --
> > > 2.40.0.335.g9857273be0
> > >
> > >
>