[PATCH v5 5/7] libstdc++: Implement submdspan_mapping for layout_left_padded. [PR110352]
Tomasz Kaminski
tkaminsk@redhat.com
Tue Dec 9 09:48:41 GMT 2025
On Mon, Dec 8, 2025 at 9:21 PM Luc Grosheintz <luc.grosheintz@gmail.com>
wrote:
> Implements submdspan for layout_left_padded as described in P3663.
>
> PR libstdc++/110352
>
> libstdc++-v3/ChangeLog:
>
> * include/std/mdspan (layout_left_padded::submdspan_mapping):
> New friend method.
> * testsuite/23_containers/mdspan/layout_traits.h
> (LayoutTraits::layout_same_padded): New template type alias.
> *
> testsuite/23_containers/mdspan/submdspan/selections/left_padded_1.cc:
> Instantiate tests for layout_left_padded.
> *
> testsuite/23_containers/mdspan/submdspan/selections/left_padded_8.cc:
> Ditto.
> *
> testsuite/23_containers/mdspan/submdspan/selections/left_padded_dyn.cc:
> Ditto.
>
I will see if I can merge this 3 files and still be without timeout, if so
I will push them merged.
> * testsuite/23_containers/mdspan/submdspan/submdspan_mapping.cc:
> Ditto.
>
> Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>
> ---
>
LGTM.
> libstdc++-v3/include/std/mdspan | 56 +++++-
> .../23_containers/mdspan/layout_traits.h | 4 +
> .../submdspan/selections/left_padded_1.cc | 9 +
> .../submdspan/selections/left_padded_8.cc | 9 +
> .../submdspan/selections/left_padded_dyn.cc | 9 +
> .../mdspan/submdspan/submdspan_mapping.cc | 179 ++++++++++++++++--
> 6 files changed, 241 insertions(+), 25 deletions(-)
> create mode 100644
> libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/left_padded_1.cc
> create mode 100644
> libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/left_padded_8.cc
> create mode 100644
> libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/left_padded_dyn.cc
>
> diff --git a/libstdc++-v3/include/std/mdspan
> b/libstdc++-v3/include/std/mdspan
> index 16dda4c8468..520cf322d8c 100644
> --- a/libstdc++-v3/include/std/mdspan
> +++ b/libstdc++-v3/include/std/mdspan
> @@ -812,6 +812,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> template<typename _Mapping>
> constexpr bool __is_right_padded_mapping = __padded_mapping_of<
> layout_right_padded, _Mapping>;
> +
> + template<typename _Mapping>
> + constexpr bool __is_padded_mapping =
> __is_left_padded_mapping<_Mapping>
> + || __is_right_padded_mapping<_Mapping>;
> #endif
>
> template<typename _PaddedMapping>
> @@ -1276,11 +1280,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> }
> }
>
> - template<_LayoutSide _Side>
> + template<_LayoutSide _Side, bool _Padded>
> struct _SubMdspanMapping;
>
> template<>
> - struct _SubMdspanMapping<_LayoutSide::__left>
> + struct _SubMdspanMapping<_LayoutSide::__left, false>
> {
> using _Layout = layout_left;
> template<size_t _Pad> using _PaddedLayout =
> layout_left_padded<_Pad>;
> @@ -1304,7 +1308,42 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> };
>
> template<>
> - struct _SubMdspanMapping<_LayoutSide::__right>
> + struct _SubMdspanMapping<_LayoutSide::__left, true>
> + {
> + using _Layout = layout_left;
> + template<size_t _Pad> using _PaddedLayout =
> layout_left_padded<_Pad>;
> +
> + template<typename _Mapping, size_t _Us>
> + static consteval size_t
> + _S_pad()
> + {
> + using _Extents = typename _Mapping::extents_type;
> + constexpr auto __sta_exts
> + = __mdspan::__static_extents<_Extents>(1, _Us);
> + 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[0] == _SliceKind::__unit_strided_slice
> + || __slice_kinds[0] == _SliceKind::__full;
> + else
> + return false;
> + }
> + };
> +
> + template<>
> + struct _SubMdspanMapping<_LayoutSide::__right, false>
> {
> using _Layout = layout_right;
> template<size_t _Pad> using _PaddedLayout =
> layout_right_padded<_Pad>;
> @@ -1350,7 +1389,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
> constexpr auto __side = __mdspan::__mapping_side<_Mapping>();
> constexpr auto __rank = sizeof...(_Slices);
> - using _Trait = _SubMdspanMapping<__side>;
> + using _Trait = _SubMdspanMapping<__side,
> __is_padded_mapping<_Mapping>>;
> using _SliceView = span<const _SliceKind, __rank>;
>
> constexpr auto __slice_kinds =
> __mdspan::__make_slice_kind_array<_Slices...>();
> @@ -2558,6 +2597,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> operator==(const mapping& __self, const _LeftpadMapping& __other)
> noexcept
> { return __self._M_storage._M_equal(__other); }
> +
> + private:
> +#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
> };
>
> template<size_t _PaddingValue>
> diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/layout_traits.h
> b/libstdc++-v3/testsuite/23_containers/mdspan/layout_traits.h
> index 619cab53b9e..f0aeac320de 100644
> --- a/libstdc++-v3/testsuite/23_containers/mdspan/layout_traits.h
> +++ b/libstdc++-v3/testsuite/23_containers/mdspan/layout_traits.h
> @@ -87,6 +87,8 @@ template<>
> {
> using layout_same = std::layout_left;
> using layout_other = std::layout_right;
> + template<size_t PaddingValue>
> + using layout_same_padded = std::layout_left_padded<PaddingValue>;
>
> template<typename Extents>
> using extents_type = Extents;
> @@ -126,6 +128,8 @@ template<>
> struct LayoutTraits<PaddingSide::Right>
> {
> using layout_same = std::layout_right;
> + template<size_t PaddingValue>
> + using layout_same_padded = std::layout_right_padded<PaddingValue>;
> using layout_other = std::layout_left;
>
> template<typename IndexType, size_t... Extents>
> diff --git
> a/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/left_padded_1.cc
> b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/left_padded_1.cc
> new file mode 100644
> index 00000000000..b21de0cb59a
> --- /dev/null
> +++
> b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/left_padded_1.cc
> @@ -0,0 +1,9 @@
> +// { dg-do run { target c++26 } }
> +#include "testcases.h"
> +
> +int
> +main()
> +{
> + test_all<std::layout_left_padded<1>>();
> + return 0;
> +}
> diff --git
> a/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/left_padded_8.cc
> b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/left_padded_8.cc
> new file mode 100644
> index 00000000000..f2402d833f3
> --- /dev/null
> +++
> b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/left_padded_8.cc
> @@ -0,0 +1,9 @@
> +// { dg-do run { target c++26 } }
> +#include "testcases.h"
> +
> +int
> +main()
> +{
> + test_all<std::layout_left_padded<8>>();
> + return 0;
> +}
> diff --git
> a/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/left_padded_dyn.cc
> b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/left_padded_dyn.cc
> new file mode 100644
> index 00000000000..0f6e4e35550
> --- /dev/null
> +++
> b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/left_padded_dyn.cc
> @@ -0,0 +1,9 @@
> +// { dg-do run { target c++26 } }
> +#include "testcases.h"
> +
> +int
> +main()
> +{
> + test_all<std::layout_left_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 cf6167dc3b0..50836968a06 100644
> ---
> a/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_mapping.cc
> +++
> b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_mapping.cc
> @@ -6,6 +6,7 @@
> #include <testsuite_hooks.h>
>
> constexpr size_t dyn = std::dynamic_extent;
> +constexpr auto all = std::full_extent;
>
> template<typename Mapping, typename... Slices>
> constexpr auto
> @@ -18,10 +19,11 @@ template<typename Mapping, typename... Slices>
>
> template<typename Layout>
> constexpr bool
> - test_layout_unpadded_return_types()
> + test_layout_common_return_types()
> {
> constexpr auto padding_side =
> DeducePaddingSide::from_typename<Layout>();
> using Traits = LayoutTraits<padding_side>;
> + using layout_unpadded = typename Traits::layout_same;
>
> {
> auto m0 = typename Layout::mapping(std::extents());
> @@ -32,21 +34,13 @@ template<typename Layout>
>
> auto exts = Traits::make_extents(std::dims<5, int>(3, 5, 7, 11, 13));
> auto m = typename Layout::mapping(exts);
> - auto all = std::full_extent;
> auto s251 = std::strided_slice{2, 5, std::cw<1>};
>
> {
> auto slices = std::tuple{0, 0, 0, 0, 0};
> auto result = call_submdspan_mapping(m, Traits::make_tuple(slices));
> using layout_type = typename decltype(result.mapping)::layout_type;
> - static_assert(std::same_as<layout_type, Layout>);
> - }
> -
> - {
> - auto slices = std::tuple{all, all, all, s251, 0};
> - auto result = call_submdspan_mapping(m, Traits::make_tuple(slices));
> - using layout_type = typename decltype(result.mapping)::layout_type;
> - static_assert(std::same_as<layout_type, Layout>);
> + static_assert(std::same_as<layout_type, layout_unpadded>);
> }
>
> {
> @@ -98,6 +92,72 @@ template<typename Layout>
> return true;
> }
>
> +template<typename Layout>
> + constexpr bool
> + test_layout_unpadded_return_types()
> + {
> + constexpr auto padding_side =
> DeducePaddingSide::from_typename<Layout>();
> + using Traits = LayoutTraits<padding_side>;
> +
> + auto exts = Traits::make_extents(std::dims<5, int>(3, 5, 7, 11, 13));
> + auto m = typename Layout::mapping(exts);
> + auto s251 = std::strided_slice{2, 5, std::cw<1>};
> +
> + {
> + auto slices = std::tuple{all, all, all, s251, 0};
> + auto result = call_submdspan_mapping(m, Traits::make_tuple(slices));
> + using layout_type = typename decltype(result.mapping)::layout_type;
> + static_assert(std::same_as<layout_type, Layout>);
> + }
> + return true;
> + }
> +
> +template<typename Layout>
> + constexpr bool
> + test_layout_padded_return_types()
> + {
> + constexpr auto padding_side =
> DeducePaddingSide::from_typename<Layout>();
> + using Traits = LayoutTraits<padding_side>;
> +
> + auto exts = Traits::make_extents(std::dims<5, int>(3, 5, 7, 11, 13));
> + auto m = typename Layout::mapping(exts);
> + auto s251 = std::strided_slice{2, 5, std::cw<1>};
> +
> + {
> + auto slices = std::tuple{all, all, all, s251, 0};
> + auto result = call_submdspan_mapping(m, Traits::make_tuple(slices));
> + using layout_type = typename decltype(result.mapping)::layout_type;
> + using layout_expected = typename Traits::layout_same_padded<dyn>;
> + static_assert(std::same_as<layout_type, layout_expected>);
> + }
> +
> + {
> + auto slices = std::tuple{all, 0, 0, 0, 0};
> + auto result = call_submdspan_mapping(m, Traits::make_tuple(slices));
> + using layout_type = typename decltype(result.mapping)::layout_type;
> + using layout_expected = typename Traits::layout_same;
> + static_assert(std::same_as<layout_type, layout_expected>);
> + }
> +
> + {
> + auto s121 = std::strided_slice{1, 2, std::cw<1>};
> + auto slices = std::tuple{s121, 0, 0, 0, 0};
> + auto result = call_submdspan_mapping(m, Traits::make_tuple(slices));
> + using layout_type = typename decltype(result.mapping)::layout_type;
> + using layout_expected = typename Traits::layout_same;
> + static_assert(std::same_as<layout_type, layout_expected>);
> + }
> +
> + {
> + auto s121 = std::strided_slice{1, 2, std::cw<1>};
> + auto slices = std::tuple{0, s121, 0, 0, 0};
> + auto result = call_submdspan_mapping(m, Traits::make_tuple(slices));
> + using layout_type = typename decltype(result.mapping)::layout_type;
> + static_assert(std::same_as<layout_type, std::layout_stride>);
> + }
> + return true;
> + }
> +
> template<typename Layout>
> constexpr bool
> test_layout_unpadded_padding_value()
> @@ -105,7 +165,6 @@ template<typename Layout>
> using Traits =
> LayoutTraits<DeducePaddingSide::from_typename<Layout>()>;
> auto s0 = std::strided_slice{size_t(1), size_t(2),
> std::cw<size_t(1)>};
> auto s3 = std::strided_slice{size_t(2), size_t(5),
> std::cw<size_t(1)>};
> - auto all = std::full_extent;
>
> auto check = [&](auto exts, size_t expected)
> {
> @@ -123,6 +182,52 @@ template<typename Layout>
> return true;
> }
>
> +template<typename Layout>
> +constexpr size_t static_padding_value = 1;
> +
> +template<size_t PaddingValue>
> +constexpr size_t
> static_padding_value<std::layout_left_padded<PaddingValue>> = PaddingValue;
> +
> +template<size_t PaddingValue>
> +constexpr size_t
> static_padding_value<std::layout_right_padded<PaddingValue>> = PaddingValue;
> +
> +template<typename Layout>
> + constexpr bool
> + test_layout_padded_padding_value()
> + {
> + using Traits =
> LayoutTraits<DeducePaddingSide::from_typename<Layout>()>;
> + auto s0 = std::strided_slice{size_t(1), size_t(2),
> std::cw<size_t(1)>};
> + auto s3 = std::strided_slice{size_t(2), size_t(5),
> std::cw<size_t(1)>};
> +
> + auto check = [&](auto exts, size_t expected)
> + {
> + auto m = typename Layout::mapping(Traits::make_extents(exts));
> + auto slices = std::tuple{s0, size_t(0), all, s3, size_t(0)};
> + auto result = call_submdspan_mapping(m, Traits::make_tuple(slices));
> + auto padding_value = decltype(result.mapping)::padding_value;
> + VERIFY(padding_value == expected);
> + };
> +
> + auto pad = [](int n, int m) -> size_t
> + {
> + constexpr auto padding_value = static_padding_value<Layout>;
> + if constexpr (padding_value != dyn)
> + {
> + auto npad = ((n + padding_value - 1) / padding_value) *
> padding_value;
> + return npad * m;
> + }
> + else
> + return dyn;
> + };
> +
> + check(std::extents(std::cw<3>, std::cw<5>, std::cw<7>, 11, 13),
> pad(3, 5));
> + check(std::extents(std::cw<3>, std::cw<5>, 7, 11, 13), pad(3, 5));
> + check(std::extents(std::cw<3>, std::cw<6>, 7, 11, 13), pad(3, 6));
> + check(std::extents(std::cw<3>, 5, 7, 11, 13), dyn);
> + check(std::extents(3, 5, 7, 11, 13), dyn);
> + return true;
> + }
> +
> constexpr bool
> test_layout_stride_return_types()
> {
> @@ -138,23 +243,55 @@ test_layout_stride_return_types()
> return true;
> }
>
> +template<typename Layout>
> + constexpr bool
> + test_return_types_all()
> + {
> + return true;
> + }
> +
> +template<typename Layout>
> + constexpr bool
> + test_return_types_unpadded_all()
> + {
> + test_layout_common_return_types<Layout>();
> + static_assert(test_layout_common_return_types<Layout>());
> +
> + test_layout_unpadded_return_types<Layout>();
> + static_assert(test_layout_unpadded_return_types<Layout>());
> +
> + test_layout_unpadded_padding_value<Layout>();
> + static_assert(test_layout_unpadded_padding_value<Layout>());
> + return true;
> + }
> +
> +template<typename Layout>
> + constexpr bool
> + test_return_types_padded_all()
> + {
> + test_layout_common_return_types<Layout>();
> + static_assert(test_layout_common_return_types<Layout>());
> +
> + test_layout_padded_return_types<Layout>();
> + static_assert(test_layout_padded_return_types<Layout>());
> +
> + test_layout_padded_padding_value<Layout>();
> + static_assert(test_layout_padded_padding_value<Layout>());
> + return true;
> + }
> +
> int
> main()
> {
> - test_layout_unpadded_return_types<std::layout_left>();
> - static_assert(test_layout_unpadded_return_types<std::layout_left>());
> + test_return_types_unpadded_all<std::layout_left>();
> + test_return_types_unpadded_all<std::layout_right>();
>
> - test_layout_unpadded_return_types<std::layout_right>();
> - static_assert(test_layout_unpadded_return_types<std::layout_right>());
> + test_return_types_padded_all<std::layout_left_padded<1>>();
> + test_return_types_padded_all<std::layout_left_padded<2>>();
> + test_return_types_padded_all<std::layout_left_padded<dyn>>();
>
> 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>());
> -
> - test_layout_unpadded_padding_value<std::layout_right>();
> - static_assert(test_layout_unpadded_padding_value<std::layout_right>());
> return 0;
> }
>
> --
> 2.52.0
>
>
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