[gcc r16-5991] libstdc++: Implement submdspan_mapping for layout_stride. [PR110352]

[Tomasz Kaminski]

https://gcc.gnu.org/g:57d53be024b561c767d7d3e47e11edf571003965

commit r16-5991-g57d53be024b561c767d7d3e47e11edf571003965
Author: Luc Grosheintz <luc.grosheintz@gmail.com>
Date:   Mon Dec 8 21:23:43 2025 +0100

    libstdc++: Implement submdspan_mapping for layout_stride. [PR110352]
    
    Add submdspan_mapping for layout_stride as in P3663.
    
            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.
    
    Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
    Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>

Diff:
---
 libstdc++-v3/include/std/mdspan                      | 20 ++++++++++++++++++++
 .../mdspan/submdspan/selections/stride.cc            |  9 +++++++++
 .../mdspan/submdspan/submdspan_mapping.cc            | 18 ++++++++++++++++++
 .../23_containers/mdspan/submdspan/submdspan_neg.cc  |  9 +++++++++
 4 files changed, 56 insertions(+)

diff --git a/libstdc++-v3/include/std/mdspan b/libstdc++-v3/include/std/mdspan
index 5ee29e135875..6386a26e392d 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 000000000000..19451d7f954e
--- /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 cc832cdb415a..cf6167dc3b0f 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 15b9b2e71ec1..39eb18cb03df 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" }