From: Luc Grosheintz <luc.grosheintz@gmail.com>
Date: Mon, 8 Dec 2025 20:23:45 +0000 (+0100)
Subject: libstdc++: Implement submdspan_mapping for layout_right_padded. [PR110352]
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=f5d72af36d406b55f31e709f01b5fc184019ff26;p=thirdparty%2Fgcc.git

libstdc++: Implement submdspan_mapping for layout_right_padded. [PR110352]

Implements submdspan for layout_right_padded as described in P3663.

	PR libstdc++/110352

libstdc++-v3/ChangeLog:

	* include/std/mdspan
	(__mdspan::_SubMdspanMapping<_LayoutSide::__right, true>): Define.
	(layout_right_padded::submdspan_mapping): New friend function.
	* testsuite/23_containers/mdspan/submdspan/selections/right_padded.cc:
	Instantiate tests for layout_right_padded.
	* testsuite/23_containers/mdspan/submdspan/submdspan_mapping.cc:
	Ditto.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>
---

diff --git a/libstdc++-v3/include/std/mdspan b/libstdc++-v3/include/std/mdspan
index 12413a0662f..dc0aa4f9584 100644
--- a/libstdc++-v3/include/std/mdspan
+++ b/libstdc++-v3/include/std/mdspan
@@ -1374,6 +1374,43 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  }
       };
 
+    template<>
+      struct _SubMdspanMapping<_LayoutSide::__right, true>
+      {
+	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 - 1);
+	    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[_Nm - 1] == _SliceKind::__unit_strided_slice
+		|| __slice_kinds[_Nm - 1] == _SliceKind::__full;
+	    else
+	      return false;
+	  }
+      };
+
+
     template<typename _Mapping>
       constexpr auto
       __submdspan_mapping_impl(const _Mapping& __mapping)
@@ -2775,6 +2812,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  operator==(const mapping& __self, const _RightPaddedMapping& __other)
 	  noexcept
 	  { return __self._M_storage._M_equal(__other); }
+
+#if __glibcxx_submdspan
+      private:
+	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
       };
 #endif // __glibcxx_padded_layouts
 
diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/right_padded.cc b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/right_padded.cc
new file mode 100644
index 00000000000..24ed6cb7d62
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/right_padded.cc
@@ -0,0 +1,12 @@
+// { dg-do run { target c++26 } }
+// { dg-timeout-factor 2 }
+#include "testcases.h"
+
+int
+main()
+{
+  test_all<std::layout_right_padded<1>>();
+  test_all<std::layout_right_padded<8>>();
+  test_all<std::layout_right_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 50836968a06..efd71d10f9d 100644
--- a/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_mapping.cc
+++ b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_mapping.cc
@@ -290,6 +290,10 @@ main()
   test_return_types_padded_all<std::layout_left_padded<2>>();
   test_return_types_padded_all<std::layout_left_padded<dyn>>();
 
+  test_return_types_padded_all<std::layout_right_padded<1>>();
+  test_return_types_padded_all<std::layout_right_padded<2>>();
+  test_return_types_padded_all<std::layout_right_padded<dyn>>();
+
   test_layout_stride_return_types();
   static_assert(test_layout_stride_return_types());
   return 0;