[PATCH v5 4/7] libstdc++: Implement submdspan_mapping for layout_stride. [PR110352]

[Jonathan Wakely]

On Mon, 8 Dec 2025 at 22:27, Luc Grosheintz <luc.grosheintz@gmail.com> wrote:
>
> Add submdspan_mapping for layout_stride as in P3663.

OK


>
>         PR libstdc++/110352
>
> libstdc++-v3/ChangeLog:
>
>         * include/std/mdspan (layout_stride::mapping::submdspan_mapping): New
>         friend function.
>         * testsuite/23_containers/mdspan/submdspan/selections/stride.cc:
>         Instantiate tests for layout_stride.
>         * testsuite/23_containers/mdspan/submdspan/submdspan_neg.cc:
>         Ditto.
>         * testsuite/23_containers/mdspan/submdspan/submdspan_mapping.cc:
>         Add tests for layout_stride.
>
> Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>
> ---
>  libstdc++-v3/include/std/mdspan               | 20 +++++++++++++++++++
>  .../mdspan/submdspan/selections/stride.cc     |  9 +++++++++
>  .../mdspan/submdspan/submdspan_mapping.cc     | 18 +++++++++++++++++
>  .../mdspan/submdspan/submdspan_neg.cc         |  9 +++++++++
>  4 files changed, 56 insertions(+)
>  create mode 100644 libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/stride.cc
>
> diff --git a/libstdc++-v3/include/std/mdspan b/libstdc++-v3/include/std/mdspan
> index c962d0e0cd3..16dda4c8468 100644
> --- a/libstdc++-v3/include/std/mdspan
> +++ b/libstdc++-v3/include/std/mdspan
> @@ -1928,6 +1928,26 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>         }
>
>      private:
> +#if __glibcxx_submdspan
> +      template<typename... _Slices>
> +       requires (extents_type::rank() == sizeof...(_Slices))
> +       friend constexpr auto
> +       submdspan_mapping(const mapping& __mapping, _Slices... __slices)
> +       {
> +         if constexpr (sizeof...(_Slices) == 0)
> +           return submdspan_mapping_result{__mapping, 0};
> +         else
> +           {
> +             auto __offset = __mdspan::__suboffset(__mapping, __slices...);
> +             auto __sub_exts = __mdspan::__subextents(__mapping.extents(), __slices...);
> +             auto __sub_strides
> +               = __mdspan::__substrides<decltype(__sub_exts)>(__mapping, __slices...);
> +             return submdspan_mapping_result{
> +               layout_stride::mapping(__sub_exts, __sub_strides), __offset};
> +           }
> +       }
> +#endif
> +
>        using _Strides = typename __array_traits<index_type,
>                                                extents_type::rank()>::_Type;
>        [[no_unique_address]] extents_type _M_extents;
> diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/stride.cc b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/stride.cc
> new file mode 100644
> index 00000000000..19451d7f954
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/stride.cc
> @@ -0,0 +1,9 @@
> +// { dg-do run { target c++26 } }
> +#include "testcases.h"
> +
> +int
> +main()
> +{
> +  test_all<std::layout_stride>();
> +  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 cc832cdb415..cf6167dc3b0 100644
> --- a/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_mapping.cc
> +++ b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_mapping.cc
> @@ -123,6 +123,21 @@ template<typename Layout>
>      return true;
>    }
>
> +constexpr bool
> +test_layout_stride_return_types()
> +{
> +  auto exts = std::extents(3, 5);
> +  auto m = std::layout_stride::mapping(exts, std::array{2, 12});
> +
> +  using index_type = decltype(exts)::index_type;
> +  auto s1 = std::strided_slice{index_type(2), index_type(2),
> +                              std::cw<index_type(2)>};
> +  auto result = submdspan_mapping(m, index_type(1), s1);
> +  using layout_type = decltype(result.mapping)::layout_type;
> +  static_assert(std::same_as<layout_type, std::layout_stride>);
> +  return true;
> +}
> +
>  int
>  main()
>  {
> @@ -132,6 +147,9 @@ main()
>    test_layout_unpadded_return_types<std::layout_right>();
>    static_assert(test_layout_unpadded_return_types<std::layout_right>());
>
> +  test_layout_stride_return_types();
> +  static_assert(test_layout_stride_return_types());
> +
>    test_layout_unpadded_padding_value<std::layout_left>();
>    static_assert(test_layout_unpadded_padding_value<std::layout_left>());
>
> diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_neg.cc b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_neg.cc
> index 15b9b2e71ec..39eb18cb03d 100644
> --- a/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_neg.cc
> +++ b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_neg.cc
> @@ -25,6 +25,7 @@ template<typename Layout>
>    }
>  static_assert(test_int_under<std::layout_left>());   // { dg-error "expansion of" }
>  static_assert(test_int_under<std::layout_right>());  // { dg-error "expansion of" }
> +static_assert(test_int_under<std::layout_stride>()); // { dg-error "expansion of" }
>
>  template<typename Layout>
>    constexpr bool
> @@ -35,6 +36,7 @@ template<typename Layout>
>    }
>  static_assert(test_int_over<std::layout_left>());    // { dg-error "expansion of" }
>  static_assert(test_int_over<std::layout_right>());   // { dg-error "expansion of" }
> +static_assert(test_int_over<std::layout_stride>());  // { dg-error "expansion of" }
>
>  template<typename Layout>
>    constexpr bool
> @@ -45,6 +47,7 @@ template<typename Layout>
>    }
>  static_assert(test_tuple_under<std::layout_left>());     // { dg-error "expansion of" }
>  static_assert(test_tuple_under<std::layout_right>());    // { dg-error "expansion of" }
> +static_assert(test_tuple_under<std::layout_stride>());   // { dg-error "expansion of" }
>
>  template<typename Layout>
>    constexpr bool
> @@ -55,6 +58,7 @@ template<typename Layout>
>    }
>  static_assert(test_tuple_reversed<std::layout_left>());   // { dg-error "expansion of" }
>  static_assert(test_tuple_reversed<std::layout_right>());  // { dg-error "expansion of" }
> +static_assert(test_tuple_reversed<std::layout_stride>()); // { dg-error "expansion of" }
>
>  template<typename Layout>
>    constexpr bool
> @@ -65,6 +69,7 @@ template<typename Layout>
>    }
>  static_assert(test_tuple_over<std::layout_left>());   // { dg-error "expansion of" }
>  static_assert(test_tuple_over<std::layout_right>());  // { dg-error "expansion of" }
> +static_assert(test_tuple_over<std::layout_stride>()); // { dg-error "expansion of" }
>
>  template<typename Layout>
>    constexpr bool
> @@ -75,6 +80,7 @@ template<typename Layout>
>    }
>  static_assert(test_strided_slice_zero<std::layout_left>());   // { dg-error "expansion of" }
>  static_assert(test_strided_slice_zero<std::layout_right>());  // { dg-error "expansion of" }
> +static_assert(test_strided_slice_zero<std::layout_stride>()); // { dg-error "expansion of" }
>
>  template<typename Layout>
>    constexpr bool
> @@ -85,6 +91,7 @@ template<typename Layout>
>    }
>  static_assert(test_strided_slice_offset_under<std::layout_left>());   // { dg-error "expansion of" }
>  static_assert(test_strided_slice_offset_under<std::layout_right>());  // { dg-error "expansion of" }
> +static_assert(test_strided_slice_offset_under<std::layout_stride>()); // { dg-error "expansion of" }
>
>  template<typename Layout>
>    constexpr bool
> @@ -95,6 +102,7 @@ template<typename Layout>
>    }
>  static_assert(test_strided_slice_offset_over<std::layout_left>());   // { dg-error "expansion of" }
>  static_assert(test_strided_slice_offset_over<std::layout_right>());  // { dg-error "expansion of" }
> +static_assert(test_strided_slice_offset_over<std::layout_stride>()); // { dg-error "expansion of" }
>
>  template<typename Layout>
>    constexpr bool
> @@ -105,6 +113,7 @@ template<typename Layout>
>    }
>  static_assert(test_strided_slice_extent_over<std::layout_left>());   // { dg-error "expansion of" }
>  static_assert(test_strided_slice_extent_over<std::layout_right>());  // { dg-error "expansion of" }
> +static_assert(test_strided_slice_extent_over<std::layout_stride>()); // { dg-error "expansion of" }
>
>  // { dg-prune-output "static assertion failed" }
>  // { dg-prune-output "__glibcxx_assert_fail" }
> --
> 2.52.0
>