[PATCH v5 3/7] libstdc++: Implement submdspan_mapping for layout_right. [PR110352]
Tomasz Kaminski
tkaminsk@redhat.com
Tue Dec 9 11:37:32 GMT 2025
On Tue, Dec 9, 2025 at 12:29 PM Jonathan Wakely <jwakely@redhat.com> wrote:
> On Mon, 8 Dec 2025 at 22:26, Luc Grosheintz <luc.grosheintz@gmail.com>
> wrote:
> >
> > Adds submdspan_mapping for layout_right as described in P3663.
> >
> > PR libstdc++/110352
> >
> > libstdc++-v3/ChangeLog:
> >
> > * include/std/mdspan (layout_right::mapping::submdspan_mapping):
> New
> > friend function.
>
> The new _SubMdspanMapping<__right> specialization isn't mentioned in
> the changelog.
>
Yes, I have updated changelog for this and next patches. And will push them
with all __mdspan changes mentioned.
>
> OK with that added.
>
> > * testsuite/23_containers/mdspan/submdspan/selections/right.cc:
> > Instantiate tests for layout_right.
> > * testsuite/23_containers/mdspan/submdspan/submdspan_mapping.cc:
> > Ditto.
> > * testsuite/23_containers/mdspan/submdspan/submdspan_neg.cc:
> > Ditto.
> >
> > Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>
> > ---
> > libstdc++-v3/include/std/mdspan | 40 +++++++++++++++++++
> > .../mdspan/submdspan/selections/right.cc | 9 +++++
> > .../mdspan/submdspan/submdspan_mapping.cc | 6 +++
> > .../mdspan/submdspan/submdspan_neg.cc | 9 +++++
> > 4 files changed, 64 insertions(+)
> > create mode 100644
> libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/right.cc
> >
> > diff --git a/libstdc++-v3/include/std/mdspan
> b/libstdc++-v3/include/std/mdspan
> > index 4c004335289..c962d0e0cd3 100644
> > --- a/libstdc++-v3/include/std/mdspan
> > +++ b/libstdc++-v3/include/std/mdspan
> > @@ -1303,6 +1303,38 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > { return __mdspan::__is_block(__slice_kinds, __sub_rank); }
> > };
> >
> > + template<>
> > + struct _SubMdspanMapping<_LayoutSide::__right>
> > + {
> > + 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);
> > + if constexpr (!__mdspan::__all_static(__sta_exts))
> > + return dynamic_extent;
> > + else
> > + return __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)
> > + {
> > + auto __rev_slice_kinds = array<_SliceKind, _Nm>{};
> > + for(size_t __i = 0; __i < _Nm; ++__i)
> > + __rev_slice_kinds[__i] = __slice_kinds[_Nm - 1 - __i];
> > + return __mdspan::__is_block(span(__rev_slice_kinds),
> __sub_rank);
> > + }
> > + };
> > +
> > template<typename _Mapping>
> > constexpr auto
> > __submdspan_mapping_impl(const _Mapping& __mapping)
> > @@ -1678,6 +1710,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >
> __glibcxx_assert(__mdspan::__is_representable_extents(_M_extents));
> > }
> >
> > +#if __glibcxx_submdspan
> > + 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
> > +
> > [[no_unique_address]] extents_type _M_extents{};
> > };
> >
> > diff --git
> a/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/right.cc
> b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/right.cc
> > new file mode 100644
> > index 00000000000..25570721072
> > --- /dev/null
> > +++
> b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/right.cc
> > @@ -0,0 +1,9 @@
> > +// { dg-do run { target c++26 } }
> > +#include "testcases.h"
> > +
> > +int
> > +main()
> > +{
> > + test_all<std::layout_right>();
> > + 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 a37d3cd588f..cc832cdb415 100644
> > ---
> a/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_mapping.cc
> > +++
> b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_mapping.cc
> > @@ -129,8 +129,14 @@ main()
> > test_layout_unpadded_return_types<std::layout_left>();
> > static_assert(test_layout_unpadded_return_types<std::layout_left>());
> >
> > + test_layout_unpadded_return_types<std::layout_right>();
> > + static_assert(test_layout_unpadded_return_types<std::layout_right>());
> > +
> > test_layout_unpadded_padding_value<std::layout_left>();
> > static_assert(test_layout_unpadded_padding_value<std::layout_left>());
> > +
> > + test_layout_unpadded_padding_value<std::layout_right>();
> > +
> static_assert(test_layout_unpadded_padding_value<std::layout_right>());
> > return 0;
> > }
> >
> > 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 1fc10a832eb..15b9b2e71ec 100644
> > ---
> a/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_neg.cc
> > +++
> b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_neg.cc
> > @@ -24,6 +24,7 @@ template<typename Layout>
> > return true;
> > }
> > static_assert(test_int_under<std::layout_left>()); // { dg-error
> "expansion of" }
> > +static_assert(test_int_under<std::layout_right>()); // { dg-error
> "expansion of" }
> >
> > template<typename Layout>
> > constexpr bool
> > @@ -33,6 +34,7 @@ template<typename Layout>
> > return true;
> > }
> > static_assert(test_int_over<std::layout_left>()); // { dg-error
> "expansion of" }
> > +static_assert(test_int_over<std::layout_right>()); // { dg-error
> "expansion of" }
> >
> > template<typename Layout>
> > constexpr bool
> > @@ -42,6 +44,7 @@ template<typename Layout>
> > return true;
> > }
> > static_assert(test_tuple_under<std::layout_left>()); // { dg-error
> "expansion of" }
> > +static_assert(test_tuple_under<std::layout_right>()); // { dg-error
> "expansion of" }
> >
> > template<typename Layout>
> > constexpr bool
> > @@ -51,6 +54,7 @@ template<typename Layout>
> > return true;
> > }
> > static_assert(test_tuple_reversed<std::layout_left>()); // { dg-error
> "expansion of" }
> > +static_assert(test_tuple_reversed<std::layout_right>()); // { dg-error
> "expansion of" }
> >
> > template<typename Layout>
> > constexpr bool
> > @@ -60,6 +64,7 @@ template<typename Layout>
> > return true;
> > }
> > static_assert(test_tuple_over<std::layout_left>()); // { dg-error
> "expansion of" }
> > +static_assert(test_tuple_over<std::layout_right>()); // { dg-error
> "expansion of" }
> >
> > template<typename Layout>
> > constexpr bool
> > @@ -69,6 +74,7 @@ template<typename Layout>
> > return true;
> > }
> > 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" }
> >
> > template<typename Layout>
> > constexpr bool
> > @@ -78,6 +84,7 @@ template<typename Layout>
> > return true;
> > }
> > 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" }
> >
> > template<typename Layout>
> > constexpr bool
> > @@ -87,6 +94,7 @@ template<typename Layout>
> > return true;
> > }
> > 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" }
> >
> > template<typename Layout>
> > constexpr bool
> > @@ -96,6 +104,7 @@ template<typename Layout>
> > return true;
> > }
> > 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" }
> >
> > // { dg-prune-output "static assertion failed" }
> > // { dg-prune-output "__glibcxx_assert_fail" }
> > --
> > 2.52.0
> >
>
>
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