[PATCH v5 6/7] libstdc++: Implement submdspan_mapping for layout_right_padded. [PR110352]

[Tomasz Kaminski]

On Mon, Dec 8, 2025 at 9:22 PM Luc Grosheintz <luc.grosheintz@gmail.com>
wrote:

> Implements submdspan for layout_right_padded as described in P3663.
>
>         PR libstdc++/110352
>
> libstdc++-v3/ChangeLog:
>
>         * include/std/mdspan (layout_right_padded::submdspan_mapping):
>         New friend method.
>         *
> testsuite/23_containers/mdspan/submdspan/selections/right_padded_1.cc:
>         Instantiate tests for layout_right_padded.
>         *
> testsuite/23_containers/mdspan/submdspan/selections/right_padded_8.cc:
>         Ditto.
>         *
> testsuite/23_containers/mdspan/submdspan/selections/right_padded_dyn.cc:
>         Ditto.
>
I will also try to merge above files.

>         * testsuite/23_containers/mdspan/submdspan/submdspan_mapping.cc:
>         Ditto.
>
> Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>
> ---
>
LGTM.

>  libstdc++-v3/include/std/mdspan               | 46 +++++++++++++++++++
>  .../submdspan/selections/right_padded_1.cc    |  9 ++++
>  .../submdspan/selections/right_padded_8.cc    |  9 ++++
>  .../submdspan/selections/right_padded_dyn.cc  |  9 ++++
>  .../mdspan/submdspan/submdspan_mapping.cc     |  4 ++
>  5 files changed, 77 insertions(+)
>  create mode 100644
> libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/right_padded_1.cc
>  create mode 100644
> libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/right_padded_8.cc
>  create mode 100644
> libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/right_padded_dyn.cc
>
> diff --git a/libstdc++-v3/include/std/mdspan
> b/libstdc++-v3/include/std/mdspan
> index 520cf322d8c..b9d993e47d6 100644
> --- a/libstdc++-v3/include/std/mdspan
> +++ b/libstdc++-v3/include/std/mdspan
> @@ -1374,6 +1374,43 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>           }
>        };
>
> +    template<>
> +      struct _SubMdspanMapping<_LayoutSide::__right, true>
> +      {
> +       using _Layout = layout_right;
> +       template<size_t _Pad> using _PaddedLayout =
> layout_right_padded<_Pad>;
> +
> +       template<typename _Mapping, size_t _Us>
> +         static consteval size_t
> +         _S_pad()
> +         {
> +           using _Extents = typename _Mapping::extents_type;
> +           constexpr auto __rank = _Extents::rank();
> +           constexpr auto __sta_exts
> +             = __mdspan::__static_extents<_Extents>(_Us + 1, __rank - 1);
> +           constexpr auto __sta_padstride
> +             = __mdspan::__get_static_stride<_Mapping>();
> +           if constexpr (__sta_padstride == dynamic_extent
> +                         || !__mdspan::__all_static(__sta_exts))
> +             return dynamic_extent;
> +           else
> +             return __sta_padstride * __mdspan::__fwd_prod(__sta_exts);
> +         }
> +
> +       template<size_t _Nm>
> +         static consteval bool
> +         _S_is_unpadded_submdspan(span<const _SliceKind, _Nm>
> __slice_kinds,
> +                                  size_t __sub_rank)
> +         {
> +           if (__sub_rank == 1)
> +             return __slice_kinds[_Nm - 1] ==
> _SliceKind::__unit_strided_slice
> +               || __slice_kinds[_Nm - 1] == _SliceKind::__full;
> +           else
> +             return false;
> +         }
> +      };
> +
> +
>      template<typename _Mapping>
>        constexpr auto
>        __submdspan_mapping_impl(const _Mapping& __mapping)
> @@ -2775,6 +2812,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>           operator==(const mapping& __self, const _RightPaddedMapping&
> __other)
>           noexcept
>           { return __self._M_storage._M_equal(__other); }
> +
> +#if __glibcxx_submdspan
> +      private:
> +       template<typename... _Slices>
> +         requires (extents_type::rank() == sizeof...(_Slices))
> +         friend constexpr auto
> +         submdspan_mapping(const mapping& __mapping, _Slices... __slices)
> +         { return __mdspan::__submdspan_mapping_impl(__mapping,
> __slices...); }
> +#endif // __glibcxx_submdspan
>        };
>  #endif // __glibcxx_padded_layouts
>
> diff --git
> a/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/right_padded_1.cc
> b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/right_padded_1.cc
> new file mode 100644
> index 00000000000..af18f6ba5be
> --- /dev/null
> +++
> b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/right_padded_1.cc
> @@ -0,0 +1,9 @@
> +// { dg-do run { target c++26 } }
> +#include "testcases.h"
> +
> +int
> +main()
> +{
> +  test_all<std::layout_right_padded<1>>();
> +  return 0;
> +}
> diff --git
> a/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/right_padded_8.cc
> b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/right_padded_8.cc
> new file mode 100644
> index 00000000000..cbe5a6cd645
> --- /dev/null
> +++
> b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/right_padded_8.cc
> @@ -0,0 +1,9 @@
> +// { dg-do run { target c++26 } }
> +#include "testcases.h"
> +
> +int
> +main()
> +{
> +  test_all<std::layout_right_padded<8>>();
> +  return 0;
> +}
> diff --git
> a/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/right_padded_dyn.cc
> b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/right_padded_dyn.cc
> new file mode 100644
> index 00000000000..fd65b48a185
> --- /dev/null
> +++
> b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/right_padded_dyn.cc
> @@ -0,0 +1,9 @@
> +// { dg-do run { target c++26 } }
> +#include "testcases.h"
> +
> +int
> +main()
> +{
> +  test_all<std::layout_right_padded<dyn>>();
> +  return 0;
> +}
> diff --git
> a/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_mapping.cc
> b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_mapping.cc
> index 50836968a06..efd71d10f9d 100644
> ---
> a/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_mapping.cc
> +++
> b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_mapping.cc
> @@ -290,6 +290,10 @@ main()
>    test_return_types_padded_all<std::layout_left_padded<2>>();
>    test_return_types_padded_all<std::layout_left_padded<dyn>>();
>
> +  test_return_types_padded_all<std::layout_right_padded<1>>();
> +  test_return_types_padded_all<std::layout_right_padded<2>>();
> +  test_return_types_padded_all<std::layout_right_padded<dyn>>();
> +
>    test_layout_stride_return_types();
>    static_assert(test_layout_stride_return_types());
>    return 0;
> --
> 2.52.0
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://gcc.gnu.org/pipermail/libstdc++/attachments/20251209/cc4fdcb4/attachment.htm>