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

[Patrick Palka <ppalka@redhat.com>]

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.

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
> 
>