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/submdspan.cc:
        Restructed and new tests for layout_left_padded.
        * testsuite/23_containers/mdspan/submdspan/submdspan_mapping.cc:
        Ditto.

Signed-off-by: Luc Grosheintz <[email protected]>
---
 libstdc++-v3/include/std/mdspan               |  52 ++++-
 .../23_containers/mdspan/layout_traits.h      |   4 +
 .../mdspan/submdspan/submdspan.cc             |   3 +
 .../mdspan/submdspan/submdspan_mapping.cc     | 179 ++++++++++++++++--
 4 files changed, 213 insertions(+), 25 deletions(-)

diff --git a/libstdc++-v3/include/std/mdspan b/libstdc++-v3/include/std/mdspan
index e291bad04f1..154213e02a9 100644
--- a/libstdc++-v3/include/std/mdspan
+++ b/libstdc++-v3/include/std/mdspan
@@ -791,20 +791,24 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        _Layout<_Mapping::padding_value>, _Mapping>;
 
 #ifdef __glibcxx_padded_layouts
     template<typename _Mapping>
       constexpr bool __is_left_padded_mapping = __padded_mapping_of<
        layout_left_padded, _Mapping>;
 
     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>
       consteval size_t
       __get_static_stride()
       { return _PaddedMapping::_PaddedStorage::_S_static_stride; }
 
     template<typename _Mapping>
       concept __standardized_mapping = __mapping_of<layout_left, _Mapping>
                                       || __mapping_of<layout_right, _Mapping>
@@ -1333,25 +1337,25 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
                _IndexType, _SubRank, _Slices...[__rank - 1 - _Is]...>();
              if (__u == dynamic_extent)
                return dynamic_extent;
              else
                return __rank - 1 - __u;
            };
            return __impl(make_index_sequence<__rank>());
          }
       }
 
-    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>;
 
        template<typename _Mapping, size_t _Us>
          static consteval size_t
          _S_pad()
          {
            using _Extents = typename _Mapping::extents_type;
            constexpr auto __sta_exts = __static_extents<_Extents>(0, _Us);
@@ -1361,21 +1365,52 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
              return __fwd_prod(__sta_exts);
          }
 
        template<typename _IndexType, size_t _SubRank, typename... _Slices>
          static consteval bool
          _S_is_unpadded_submdspan()
          { return __is_compact_block<_IndexType, _SubRank, _Slices...>(); }
       };
 
     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 = __static_extents<_Extents>(1, _Us);
+           constexpr auto __sta_padstride = __get_static_stride<_Mapping>();
+           if constexpr (__sta_padstride == dynamic_extent
+                         || !__all_static(__sta_exts))
+             return dynamic_extent;
+           else
+             return __sta_padstride * __fwd_prod(__sta_exts);
+         }
+
+       template<typename _IndexType, size_t _SubRank, typename... _Slices>
+         static consteval bool
+         _S_is_unpadded_submdspan()
+         {
+           if constexpr (_SubRank == 1)
+             return __is_unit_stride_slice<_Slices...[0], _IndexType>;
+           else
+             return false;
+         }
+      };
+
+    template<>
+      struct _SubMdspanMapping<_LayoutSide::__right, false>
       {
        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();
@@ -1406,21 +1441,21 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       __submdspan_mapping_impl(const _Mapping& __mapping)
       { return submdspan_mapping_result{__mapping, 0}; }
 
     template<typename _Mapping, typename... _Slices>
       requires (sizeof...(_Slices) > 0)
       constexpr auto
       __submdspan_mapping_impl(const _Mapping& __mapping, _Slices... __slices)
       {
        using _IndexType= typename _Mapping::index_type;
        constexpr auto __side = __deduce_mapping_side<_Mapping>();
-       using _Trait = _SubMdspanMapping<__side>;
+       using _Trait = _SubMdspanMapping<__side, __is_padded_mapping<_Mapping>>;
 
        auto __offset = __suboffset(__mapping, __slices...);
        auto __sub_exts = submdspan_extents(__mapping.extents(), __slices...);
        using _SubExtents = decltype(__sub_exts);
        constexpr auto __sub_rank = _SubExtents::rank();
        if constexpr (_SubExtents::rank() == 0)
          return submdspan_mapping_result{
            typename _Trait::_Layout::mapping(__sub_exts), __offset};
        else if constexpr (
            _Trait::template _S_is_unpadded_submdspan<_IndexType, __sub_rank,
@@ -2613,20 +2648,29 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
              static_cast<size_t>(__mdspan::__fwd_prod(extents(), 1, __r)));
        }
 
        template<typename _LeftpadMapping>
          requires(__mdspan::__is_left_padded_mapping<_LeftpadMapping>
                   && _LeftpadMapping::extents_type::rank() == _S_rank)
          friend constexpr bool
          operator==(const mapping& __self, const _LeftpadMapping& __other)
          noexcept
          { return __self._M_storage._M_equal(__other); }
+
+      private:
+#if __glibcxx_submdspan
+       template<__mdspan::__valid_canonical_slice_type<index_type>... _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>
     template<typename _Extents>
       class layout_right_padded<_PaddingValue>::mapping {
       public:
        static constexpr size_t padding_value = _PaddingValue;
        using extents_type = _Extents;
        using index_type = typename extents_type::index_type;
        using size_type = typename extents_type::size_type;
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
@@ -80,20 +80,22 @@ struct DeducePaddingSide
 };
 
 template<PaddingSide Side>
   struct LayoutTraits;
 
 template<>
   struct LayoutTraits<PaddingSide::Left>
   {
     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;
 
     template<typename Extents>
       constexpr static extents_type<Extents>
       make_extents(const Extents& exts)
       { return exts; }
 
     template<typename T, size_t N>
@@ -119,20 +121,22 @@ template<>
     template<typename Extents>
       constexpr static auto
       padded_extent(const Extents& exts)
       { return exts.extent(0); }
   };
 
 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>
       constexpr static auto
       make_extents(const std::extents<IndexType, Extents...>& exts)
       {
        constexpr size_t rank = sizeof...(Extents);
        auto impl = [&]<size_t... I>(std::index_sequence<I...>)
        {
          auto dyn_exts = make_array(dynamic_extents_array(exts));
diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan.cc 
b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan.cc
index 645c4711294..927b5d06c6b 100644
--- a/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan.cc
+++ b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan.cc
@@ -358,14 +358,17 @@ template<typename Layout>
   {
     test_all_cheap<Layout>();
     test_all_expensive<Layout>();
     return true;
   }
 
 int
 main()
 {
   test_all<std::layout_left>();
+  test_all<std::layout_left_padded<1>>();
+  test_all<std::layout_left_padded<8>>();
+  test_all<std::layout_left_padded<dyn>>();
   test_all<std::layout_right>();
   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 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
@@ -1,59 +1,53 @@
 // { dg-do run { target c++26 } }
 #include <mdspan>
 
 #include <iostream> // TODO remove
 #include "../layout_traits.h"
 #include <testsuite_hooks.h>
 
 constexpr size_t dyn = std::dynamic_extent;
+constexpr auto all = std::full_extent;
 
 template<typename Mapping, typename... Slices>
   constexpr auto
   call_submdspan_mapping(const Mapping& m, std::tuple<Slices...> slices)
   {
     auto impl = [&]<size_t... I>(std::index_sequence<I...>)
     { return submdspan_mapping(m, get<I>(slices)...); };
     return impl(std::make_index_sequence<sizeof...(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());
       auto result = submdspan_mapping(m0);
       using layout_type = typename decltype(result.mapping)::layout_type;
       static_assert(std::same_as<layout_type, 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>);
     }
 
     {
       auto s0 = std::strided_slice{1, 1, std::cw<1>};
       auto slices = std::tuple{s0, 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(is_same_padded<padding_side, layout_type>);
     }
 
@@ -91,70 +85,213 @@ template<typename Layout>
     {
       auto s3 = std::strided_slice{2, std::cw<7>, std::cw<2>};
       auto slices = std::tuple{all, all, all, s3, 0};
       auto result = call_submdspan_mapping(m, Traits::make_tuple(slices));
       using layout_type = 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_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()
   {
     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)
     {
       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);
     };
 
     check(std::extents(std::cw<3>, std::cw<5>, std::cw<7>, 11, 13), 3*5);
     check(std::extents(std::cw<3>, std::cw<5>, 7, 11, 13), 3*5);
     check(std::extents(std::cw<3>, 5, 7, 11, 13), dyn);
     check(std::extents(3, 5, 7, 11, 13), dyn);
     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()
 {
   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;
 }
 
+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;
 }
 
-- 
2.51.2

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