libstdc++: Implement aligned_accessor from mdspan [PR120994]

This commit completes the implementation of P2897R7 by implementing and
testing the template class aligned_accessor.

	PR libstdc++/120994

libstdc++-v3/ChangeLog:

	* include/bits/version.def (aligned_accessor): Add.
	* include/bits/version.h: Regenerate.
	* include/std/mdspan (aligned_accessor): New class.
	* src/c++23/std.cc.in (aligned_accessor): Add.
	* testsuite/23_containers/mdspan/accessors/generic.cc: Add tests
	for aligned_accessor.
	* testsuite/23_containers/mdspan/accessors/aligned_neg.cc: New test.
	* testsuite/23_containers/mdspan/version.cc: Add test for
	__cpp_lib_aligned_accessor.

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

9 files changed, 197 insertions, 3 deletions

diff --git a/libstdc++-v3/include/bits/version.def b/libstdc++-v3/include/bits/version.def
index 56ad9ee..9d5122f 100644
--- a/libstdc++-v3/include/bits/version.def
+++ b/libstdc++-v3/include/bits/version.def
@@ -1026,6 +1026,16 @@ ftms = {
 };
 
 ftms = {
+  name = aligned_accessor;
+  values = {
+    v = 202411;
+    cxxmin = 26;
+    extra_cond = "__glibcxx_assume_aligned "
+    "&& __glibcxx_is_sufficiently_aligned";
+  };
+};
+
+ftms = {
   name = ssize;
   values = {
     v = 201902;
diff --git a/libstdc++-v3/include/bits/version.h b/libstdc++-v3/include/bits/version.h
index 51805d2..6feeaa4 100644
--- a/libstdc++-v3/include/bits/version.h
+++ b/libstdc++-v3/include/bits/version.h
@@ -1149,6 +1149,16 @@
 #endif /* !defined(__cpp_lib_mdspan) && defined(__glibcxx_want_mdspan) */
 #undef __glibcxx_want_mdspan
 
+#if !defined(__cpp_lib_aligned_accessor)
+# if (__cplusplus >  202302L) && (__glibcxx_assume_aligned && __glibcxx_is_sufficiently_aligned)
+#  define __glibcxx_aligned_accessor 202411L
+#  if defined(__glibcxx_want_all) || defined(__glibcxx_want_aligned_accessor)
+#   define __cpp_lib_aligned_accessor 202411L
+#  endif
+# endif
+#endif /* !defined(__cpp_lib_aligned_accessor) && defined(__glibcxx_want_aligned_accessor) */
+#undef __glibcxx_want_aligned_accessor
+
 #if !defined(__cpp_lib_ssize)
 # if (__cplusplus >= 202002L)
 #  define __glibcxx_ssize 201902L
diff --git a/libstdc++-v3/include/std/mdspan b/libstdc++-v3/include/std/mdspan
index b7b4375..6c7469c 100644
--- a/libstdc++-v3/include/std/mdspan
+++ b/libstdc++-v3/include/std/mdspan
@@ -38,7 +38,12 @@
 #include <type_traits>
 #include <utility>
 
+#if __cplusplus > 202302L
+#include <bits/align.h>
+#endif
+
 #define __glibcxx_want_mdspan
+#define __glibcxx_want_aligned_accessor
 #include <bits/version.h>
 
 #ifdef __glibcxx_mdspan
@@ -1184,6 +1189,54 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       { return __p + __i; }
     };
 
+#ifdef __glibcxx_aligned_accessor
+  template<typename _ElementType, size_t _ByteAlignment>
+    struct aligned_accessor
+    {
+      static_assert(has_single_bit(_ByteAlignment),
+	"ByteAlignment must be a power of two");
+      static_assert(_ByteAlignment >= alignof(_ElementType));
+
+      using offset_policy = default_accessor<_ElementType>;
+      using element_type = _ElementType;
+      using reference = element_type&;
+      using data_handle_type = element_type*;
+
+      static constexpr size_t byte_alignment = _ByteAlignment;
+
+      constexpr
+      aligned_accessor() noexcept = default;
+
+      template<typename _OElementType, size_t _OByteAlignment>
+	requires (_OByteAlignment >= byte_alignment) 
+	    && is_convertible_v<_OElementType(*)[], element_type(*)[]>
+	constexpr
+	aligned_accessor(aligned_accessor<_OElementType, _OByteAlignment>)
+	noexcept
+	{ }
+
+      template<typename _OElementType>
+	requires is_convertible_v<_OElementType(*)[], element_type(*)[]>
+	constexpr explicit
+	aligned_accessor(default_accessor<_OElementType>) noexcept
+	{ }
+
+      template<typename _OElementType>
+	requires is_convertible_v<element_type(*)[], _OElementType(*)[]>
+	constexpr
+	operator default_accessor<_OElementType>() const noexcept
+	{ return {}; }
+
+      constexpr reference
+      access(data_handle_type __p, size_t __i) const noexcept
+      { return std::assume_aligned<byte_alignment>(__p)[__i]; }
+
+      constexpr typename offset_policy::data_handle_type
+      offset(data_handle_type __p, size_t __i) const noexcept
+      { return std::assume_aligned<byte_alignment>(__p) + __i; }
+    };
+#endif
+
   namespace __mdspan
   {
     template<typename _Extents, typename _IndexType, size_t _Nm>
diff --git a/libstdc++-v3/src/c++23/std.cc.in b/libstdc++-v3/src/c++23/std.cc.in
index 1596459..4888b8b 100644
--- a/libstdc++-v3/src/c++23/std.cc.in
+++ b/libstdc++-v3/src/c++23/std.cc.in
@@ -1865,10 +1865,13 @@ export namespace std
   using std::layout_right;
   using std::layout_stride;
   using std::default_accessor;
+#if __glibcxx_aligned_accessor
+  using std::aligned_accessor;
+#endif
   using std::mdspan;
-  // FIXME layout_left_padded, layout_right_padded, aligned_accessor,
-  // strided_slice, submdspan_mapping_result, full_extent_t, full_extent,
-  // submdspan_extents, mdsubspan
+  // FIXME layout_left_padded, layout_right_padded, strided_slice,
+  // submdspan_mapping_result, full_extent_t, full_extent, submdspan_extents,
+  // mdsubspan
 }
 #endif
 
diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/accessors/aligned_neg.cc b/libstdc++-v3/testsuite/23_containers/mdspan/accessors/aligned_neg.cc
new file mode 100644
index 0000000..f36ebb1
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/mdspan/accessors/aligned_neg.cc
@@ -0,0 +1,33 @@
+// { dg-do compile { target c++26 } }
+#include<mdspan>
+
+#include <cstdint>
+
+std::aligned_accessor<uint32_t, 0> a;          // { dg-error "required from here" }
+std::aligned_accessor<uint32_t, 7> b;          // { dg-error "required from here" }
+std::aligned_accessor<uint32_t, size_t(-1)> c; // { dg-error "required from here" }
+
+std::aligned_accessor<uint32_t, 2> d;          // { dg-error "required from here" }
+
+std::aligned_accessor<int[2], 32> e;           // { dg-error "required from here" }
+
+class Abstract
+{
+  virtual void
+  foo() const = 0;
+};
+
+class Derived : public Abstract
+{
+  void
+  foo() const override
+  { }
+};
+
+std::aligned_accessor<Derived, alignof(int)> f_ok;
+std::aligned_accessor<Abstract, alignof(int)> f_err; // { dg-error "required from here" }
+
+// { dg-prune-output "ByteAlignment must be a power of two" }
+// { dg-prune-output "ElementType must not be an array type" }
+// { dg-prune-output "ElementType must not be an abstract" }
+// { dg-prune-output "static assertion failed" } // no message for alignment being too small
diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/accessors/debug/aligned_access_neg.cc b/libstdc++-v3/testsuite/23_containers/mdspan/accessors/debug/aligned_access_neg.cc
new file mode 100644
index 0000000..3511cef
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/mdspan/accessors/debug/aligned_access_neg.cc
@@ -0,0 +1,23 @@
+// { dg-do run { target c++26 xfail *-*-* } }
+// { dg-require-debug-mode "" }
+
+#include <mdspan>
+#include <array>
+
+void
+test_unaligned_access()
+{
+  constexpr size_t N = 4;
+  alignas(N) std::array<char, 128> buffer{};
+  auto* unaligned = buffer.data() + 1;
+  auto a = std::aligned_accessor<char, N>{};
+
+  [[maybe_unused]] char x = a.access(unaligned, 0);
+}
+
+int
+main()
+{
+  test_unaligned_access();
+  return 0;
+};
diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/accessors/debug/aligned_offset_neg.cc b/libstdc++-v3/testsuite/23_containers/mdspan/accessors/debug/aligned_offset_neg.cc
new file mode 100644
index 0000000..319da5f
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/mdspan/accessors/debug/aligned_offset_neg.cc
@@ -0,0 +1,23 @@
+// { dg-do run { target c++26 xfail *-*-* } }
+// { dg-require-debug-mode "" }
+
+#include <mdspan>
+#include <array>
+
+void
+test_unaligned_offset()
+{
+  constexpr size_t N = 4;
+  alignas(N) std::array<char, 128> buffer{};
+  auto* unaligned = buffer.data() + 1;
+  auto a = std::aligned_accessor<char, N>{};
+
+  [[maybe_unused]] char* x = a.offset(unaligned, 0);
+}
+
+int
+main()
+{
+  test_unaligned_offset();
+  return 0;
+};
diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/accessors/generic.cc b/libstdc++-v3/testsuite/23_containers/mdspan/accessors/generic.cc
index 66009ad..31cb13e 100644
--- a/libstdc++-v3/testsuite/23_containers/mdspan/accessors/generic.cc
+++ b/libstdc++-v3/testsuite/23_containers/mdspan/accessors/generic.cc
@@ -97,10 +97,36 @@ template<template<typename T> typename Accessor>
 
 static_assert(test_properties<std::default_accessor>());
 
+#ifdef __glibcxx_aligned_accessor
+template<size_t Mult>
+struct OverAlignedAccessorTrait
+{
+  template<typename T>
+    using type = std::aligned_accessor<T, Mult*alignof(T)>;
+};
+
+static_assert(test_properties<OverAlignedAccessorTrait<1>::type>());
+static_assert(test_properties<OverAlignedAccessorTrait<2>::type>());
+static_assert(test_ctor<OverAlignedAccessorTrait<2>::type,
+			std::default_accessor, false>());
+static_assert(test_ctor<OverAlignedAccessorTrait<2>::type,
+			OverAlignedAccessorTrait<4>::type>());
+static_assert(test_ctor<std::default_accessor,
+			OverAlignedAccessorTrait<2>::type>());
+static_assert(!std::is_constructible_v<std::aligned_accessor<char, 4>,
+				       std::aligned_accessor<char, 2>>);
+#endif
+
 template<typename A>
   constexpr size_t
   accessor_alignment = alignof(typename A::element_type);
 
+#ifdef __glibcxx_aligned_accessor
+template<typename T, size_t N>
+  constexpr size_t
+  accessor_alignment<std::aligned_accessor<T, N>> = N;
+#endif
+
 template<typename Accessor>
   constexpr void
   test_access(Accessor accessor)
@@ -136,5 +162,10 @@ main()
 {
   test_all<std::default_accessor<double>>();
   static_assert(test_all<std::default_accessor<double>>());
+
+#ifdef __glibcxx_aligned_accessor
+  test_all<typename OverAlignedAccessorTrait<4>::type<double>>();
+  static_assert(test_all<typename OverAlignedAccessorTrait<4>::type<double>>());
+#endif
   return 0;
 }
diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/version.cc b/libstdc++-v3/testsuite/23_containers/mdspan/version.cc
index 7520602..1882600 100644
--- a/libstdc++-v3/testsuite/23_containers/mdspan/version.cc
+++ b/libstdc++-v3/testsuite/23_containers/mdspan/version.cc
@@ -10,3 +10,11 @@
 #elif __cplusplus > 202302L && __cpp_lib_mdspan != 202406L
 #error "Feature test macro __cpp_lib_mdspan has the wrong value for C++26"
 #endif
+
+#if __cplusplus > 202302L
+#ifndef __cpp_lib_aligned_accessor
+#error "Feature test macro __cpp_lib_aligned_accessor is missing for <mdspan>"
+#elif __cpp_lib_aligned_accessor != 202411L
+#error "Feature test macro __cpp_lib_aligned_accessor has the wrong value"
+#endif
+#endif