[PATCH v4 2/2] libstdc++: Implement aligned_accessor from mdspan [PR120994]
Jonathan Wakely
jwakely@redhat.com
Wed Aug 20 17:26:48 GMT 2025
On 04/08/25 12:59 +0200, Luc Grosheintz wrote:
>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.
>
>Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>
>---
> libstdc++-v3/include/bits/version.def | 10 ++++
> libstdc++-v3/include/bits/version.h | 10 ++++
> libstdc++-v3/include/std/mdspan | 54 +++++++++++++++++++
> libstdc++-v3/src/c++23/std.cc.in | 9 ++--
> .../mdspan/accessors/aligned_neg.cc | 33 ++++++++++++
> .../accessors/debug/aligned_access_neg.cc | 23 ++++++++
> .../accessors/debug/aligned_offset_neg.cc | 23 ++++++++
> .../23_containers/mdspan/accessors/generic.cc | 31 +++++++++++
> .../testsuite/23_containers/mdspan/version.cc | 8 +++
> 9 files changed, 198 insertions(+), 3 deletions(-)
> create mode 100644 libstdc++-v3/testsuite/23_containers/mdspan/accessors/aligned_neg.cc
> create mode 100644 libstdc++-v3/testsuite/23_containers/mdspan/accessors/debug/aligned_access_neg.cc
> create mode 100644 libstdc++-v3/testsuite/23_containers/mdspan/accessors/debug/aligned_offset_neg.cc
>
>diff --git a/libstdc++-v3/include/bits/version.def b/libstdc++-v3/include/bits/version.def
>index 56ad9ee9d4b..9d5122fed17 100644
>--- a/libstdc++-v3/include/bits/version.def
>+++ b/libstdc++-v3/include/bits/version.def
>@@ -1025,6 +1025,16 @@ ftms = {
> };
> };
>
>+ftms = {
>+ name = aligned_accessor;
>+ values = {
>+ v = 202411;
>+ cxxmin = 26;
>+ extra_cond = "__glibcxx_assume_aligned "
>+ "&& __glibcxx_is_sufficiently_aligned";
>+ };
>+};
>+
> ftms = {
> name = ssize;
> values = {
>diff --git a/libstdc++-v3/include/bits/version.h b/libstdc++-v3/include/bits/version.h
>index 51805d292f0..6feeaa4d15a 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 691d4836262..9ac283347ce 100644
>--- a/libstdc++-v3/include/std/mdspan
>+++ b/libstdc++-v3/include/std/mdspan
>@@ -39,7 +39,12 @@
> #include <limits>
> #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
>@@ -1055,6 +1060,55 @@ _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),
>+ "ByteAlignment is too small for ElementType");
Please rephrase this in terms of "must", otherwise it's sometimes
unclear whether "assertion failed: A is too small for E" means that
the assertions was "it's too small" and that failed, or that the
assertion was something else and it failed _because_ it's too small.
In this case it's probably not confusing, but I think it's good to
consistently use the "must" form in our assertions where possible.
So either remove the string literal entirely (because the C++
expression used as the condition is pretty clear already) or change it
to something like:
"ByteAlignment must not be less than alignof(ElementType)".
I have a slight preference for just removing the string literal in
this case.
>+
>+ 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 (is_convertible_v<_OElementType(*)[], element_type(*)[]>
>+ && _OByteAlignment >= byte_alignment)
>+ 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 24411476ba0..40d2fa03c5a 100644
>--- a/libstdc++-v3/src/c++23/std.cc.in
>+++ b/libstdc++-v3/src/c++23/std.cc.in
>@@ -1858,10 +1858,13 @@ 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 00000000000..46d80bf4083
>--- /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 "ByteAlignment is too small for ElementType" }
>+// { dg-prune-output "ElementType must not be an array type" }
>+// { dg-prune-output "ElementType must not be an abstract" }
>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 00000000000..3511cef1c3a
>--- /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 00000000000..319da5ffef3
>--- /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 66009ad89c0..31cb13ed431 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 752060262a0..18826005971 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
>--
>2.50.0
>
>
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