On Wed, 4 Jun 2025 at 16:16, Luc Grosheintz <[email protected]> wrote:
>
> Implements the remaining parts of layout_left and layout_right; and all
> of layout_stride.
>
> The implementation of layout_stride::mapping::is_exhaustive applies
> the following change to the standard:
>
> 4266. layout_stride::mapping should treat empty mappings as exhaustive
>
> https://cplusplus.github.io/LWG/issue4266
>
> The preconditions for layout_stride(extents, strides) are not checked.
>
> libstdc++-v3/ChangeLog:
>
> * include/std/mdspan (layout_stride): New class.
> * src/c++23/std.cc.in: Add layout_stride.
>
> Signed-off-by: Luc Grosheintz <[email protected]>
OK for trunk
> ---
> libstdc++-v3/include/std/mdspan | 249 ++++++++++++++++++++++++++++++-
> libstdc++-v3/src/c++23/std.cc.in | 3 +-
> 2 files changed, 250 insertions(+), 2 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/mdspan b/libstdc++-v3/include/std/mdspan
> index 8ab7a1a52fb..fe182c35a55 100644
> --- a/libstdc++-v3/include/std/mdspan
> +++ b/libstdc++-v3/include/std/mdspan
> @@ -415,6 +415,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> class mapping;
> };
>
> + struct layout_stride
> + {
> + template<typename _Extents>
> + class mapping;
> + };
> +
> namespace __mdspan
> {
> template<typename _Tp>
> @@ -508,7 +514,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
> template<typename _Mapping>
> concept __standardized_mapping = __mapping_of<layout_left, _Mapping>
> - || __mapping_of<layout_right,
> _Mapping>;
> + || __mapping_of<layout_right, _Mapping>
> + || __mapping_of<layout_stride,
> _Mapping>;
>
> // A tag type to create internal ctors.
> class __internal_ctor
> @@ -554,6 +561,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> : mapping(__other.extents(), __mdspan::__internal_ctor{})
> { }
>
> + template<typename _OExtents>
> + requires is_constructible_v<extents_type, _OExtents>
> + constexpr explicit(extents_type::rank() > 0)
> + mapping(const layout_stride::mapping<_OExtents>& __other)
> + : mapping(__other.extents(), __mdspan::__internal_ctor{})
> + { __glibcxx_assert(*this == __other); }
> +
> constexpr mapping&
> operator=(const mapping&) noexcept = default;
>
> @@ -684,6 +698,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> : mapping(__other.extents(), __mdspan::__internal_ctor{})
> { }
>
> + template<class _OExtents>
> + requires is_constructible_v<extents_type, _OExtents>
> + constexpr explicit(extents_type::rank() > 0)
> + mapping(const layout_stride::mapping<_OExtents>& __other) noexcept
> + : mapping(__other.extents(), __mdspan::__internal_ctor{})
> + { __glibcxx_assert(*this == __other); }
> +
> constexpr mapping&
> operator=(const mapping&) noexcept = default;
>
> @@ -756,6 +777,232 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> [[no_unique_address]] extents_type _M_extents{};
> };
>
> + namespace __mdspan
> + {
> + template<typename M>
> + concept __mapping_alike = requires
> + {
> + requires __is_extents<typename M::extents_type>;
> + { M::is_always_strided() } -> same_as<bool>;
> + { M::is_always_exhaustive() } -> same_as<bool>;
> + { M::is_always_unique() } -> same_as<bool>;
> + bool_constant<M::is_always_strided()>::value;
> + bool_constant<M::is_always_exhaustive()>::value;
> + bool_constant<M::is_always_unique()>::value;
> + };
> +
> + template<typename _Mapping>
> + constexpr typename _Mapping::index_type
> + __offset(const _Mapping& __m) noexcept
> + {
> + using _IndexType = typename _Mapping::index_type;
> + constexpr auto __rank = _Mapping::extents_type::rank();
> +
> + if constexpr (__standardized_mapping<_Mapping>)
> + return 0;
> + else if (__empty(__m.extents()))
> + return 0;
> + else
> + {
> + auto __impl = [&__m]<size_t...
> _Counts>(index_sequence<_Counts...>)
> + { return __m(((void) _Counts, _IndexType(0))...); };
> + return __impl(make_index_sequence<__rank>());
> + }
> + }
> +
> + template<typename _Mapping, typename... _Indices>
> + constexpr typename _Mapping::index_type
> + __linear_index_strides(const _Mapping& __m, _Indices... __indices)
> + noexcept
> + {
> + using _IndexType = typename _Mapping::index_type;
> + _IndexType __res = 0;
> + if constexpr (sizeof...(__indices) > 0)
> + {
> + auto __update = [&, __pos = 0u](_IndexType __idx) mutable
> + {
> + __res += __idx * __m.stride(__pos++);
> + };
> + (__update(__indices), ...);
> + }
> + return __res;
> + }
> + }
> +
> + template<typename _Extents>
> + class layout_stride::mapping
> + {
> + public:
> + using extents_type = _Extents;
> + using index_type = typename extents_type::index_type;
> + using size_type = typename extents_type::size_type;
> + using rank_type = typename extents_type::rank_type;
> + using layout_type = layout_stride;
> +
> + static_assert(__mdspan::__representable_size<extents_type, index_type>,
> + "The size of extents_type must be representable as index_type");
> +
> + constexpr
> + mapping() noexcept
> + {
> + // The precondition is either statically asserted, or automatically
> + // satisfied because dynamic extents are zero-initialzied.
> + size_t __stride = 1;
> + for (size_t __i = extents_type::rank(); __i > 0; --__i)
> + {
> + _M_strides[__i - 1] = index_type(__stride);
> + __stride *= size_t(_M_extents.extent(__i - 1));
> + }
> + }
> +
> + constexpr
> + mapping(const mapping&) noexcept = default;
> +
> + template<__mdspan::__valid_index_type<index_type> _OIndexType>
> + constexpr
> + mapping(const extents_type& __exts,
> + span<_OIndexType, extents_type::rank()> __strides) noexcept
> + : _M_extents(__exts)
> + {
> + for (size_t __i = 0; __i < extents_type::rank(); ++__i)
> + _M_strides[__i] = index_type(as_const(__strides[__i]));
> + }
> +
> + template<__mdspan::__valid_index_type<index_type> _OIndexType>
> + constexpr
> + mapping(const extents_type& __exts,
> + const array<_OIndexType, extents_type::rank()>& __strides)
> + noexcept
> + : mapping(__exts,
> + span<const _OIndexType, extents_type::rank()>(__strides))
> + { }
> +
> + template<__mdspan::__mapping_alike _StridedMapping>
> + requires (is_constructible_v<extents_type,
> + typename _StridedMapping::extents_type>
> + && _StridedMapping::is_always_unique()
> + && _StridedMapping::is_always_strided())
> + constexpr explicit(!(
> + is_convertible_v<typename _StridedMapping::extents_type,
> extents_type>
> + && __mdspan::__standardized_mapping<_StridedMapping>))
> + mapping(const _StridedMapping& __other) noexcept
> + : _M_extents(__other.extents())
> + {
> + using _OIndexType = _StridedMapping::index_type;
> + using _OExtents = _StridedMapping::extents_type;
> +
> + __glibcxx_assert(__mdspan::__offset(__other) == 0);
> + static_assert(__mdspan::__representable_size<_OExtents, index_type>,
> + "The size of StridedMapping::extents_type must be representable
> as"
> + " index_type");
> + if constexpr (cmp_greater(numeric_limits<_OIndexType>::max(),
> + numeric_limits<index_type>::max()))
> + __glibcxx_assert(!cmp_less(numeric_limits<index_type>::max(),
> + __other.required_span_size())
> + && "other.required_span_size() must be representable"
> + " as index_type");
> + if constexpr (extents_type::rank() > 0)
> + for (size_t __i = 0; __i < extents_type::rank(); ++__i)
> + _M_strides[__i] = index_type(__other.stride(__i));
> + }
> +
> + constexpr mapping&
> + operator=(const mapping&) noexcept = default;
> +
> + constexpr const extents_type&
> + extents() const noexcept { return _M_extents; }
> +
> + constexpr array<index_type, extents_type::rank()>
> + strides() const noexcept
> + {
> + array<index_type, extents_type::rank()> __ret;
> + for (size_t __i = 0; __i < extents_type::rank(); ++__i)
> + __ret[__i] = _M_strides[__i];
> + return __ret;
> + }
> +
> + constexpr index_type
> + required_span_size() const noexcept
> + {
> + if (__mdspan::__empty(_M_extents))
> + return 0;
> +
> + index_type __ret = 1;
> + for (size_t __i = 0; __i < extents_type::rank(); ++__i)
> + __ret += (_M_extents.extent(__i) - 1) * _M_strides[__i];
> + return __ret;
> + }
> +
> + template<__mdspan::__valid_index_type<index_type>... _Indices>
> + requires (sizeof...(_Indices) == extents_type::rank())
> + constexpr index_type
> + operator()(_Indices... __indices) const noexcept
> + {
> + return __mdspan::__linear_index_strides(*this,
> + static_cast<index_type>(__indices)...);
> + }
> +
> + static constexpr bool
> + is_always_unique() noexcept { return true; }
> +
> + // _GLIBCXX_RESOLVE_LIB_DEFECTS
> + // 4266. layout_stride::mapping should treat empty mappings as
> exhaustive
> + static constexpr bool
> + is_always_exhaustive() noexcept
> + {
> + return (_Extents::rank() == 0) || __mdspan::__contains_zero(
> + __mdspan::__static_extents<extents_type>());
> + }
> +
> + static constexpr bool
> + is_always_strided() noexcept { return true; }
> +
> + static constexpr bool
> + is_unique() noexcept { return true; }
> +
> + // _GLIBCXX_RESOLVE_LIB_DEFECTS
> + // 4266. layout_stride::mapping should treat empty mappings as
> exhaustive
> + constexpr bool
> + is_exhaustive() const noexcept
> + {
> + if constexpr (!is_always_exhaustive())
> + {
> + constexpr auto __rank = extents_type::rank();
> + auto __size = __mdspan::__fwd_prod(_M_extents, __rank);
> + if(__size > 0)
> + return __size == required_span_size();
> + }
> + return true;
> + }
> +
> + static constexpr bool
> + is_strided() noexcept { return true; }
> +
> + constexpr index_type
> + stride(rank_type __r) const noexcept { return _M_strides[__r]; }
> +
> + template<__mdspan::__mapping_alike _OMapping>
> + requires ((extents_type::rank() == _OMapping::extents_type::rank())
> + && _OMapping::is_always_strided())
> + friend constexpr bool
> + operator==(const mapping& __self, const _OMapping& __other) noexcept
> + {
> + if (__self.extents() != __other.extents())
> + return false;
> + if constexpr (extents_type::rank() > 0)
> + for (size_t __i = 0; __i < extents_type::rank(); ++__i)
> + if (!cmp_equal(__self.stride(__i), __other.stride(__i)))
> + return false;
> + return __mdspan::__offset(__other) == 0;
> + }
> +
> + private:
> + using _S_strides_t = typename __array_traits<index_type,
> +
> extents_type::rank()>::_Type;
> + [[no_unique_address]] extents_type _M_extents;
> + [[no_unique_address]] _S_strides_t _M_strides;
> + };
> +
> _GLIBCXX_END_NAMESPACE_VERSION
> }
> #endif
> diff --git a/libstdc++-v3/src/c++23/std.cc.in
> b/libstdc++-v3/src/c++23/std.cc.in
> index fe1407d71da..9a52a7e7728 100644
> --- a/libstdc++-v3/src/c++23/std.cc.in
> +++ b/libstdc++-v3/src/c++23/std.cc.in
> @@ -1846,7 +1846,8 @@ export namespace std
> using std::extents;
> using std::layout_left;
> using std::layout_right;
> - // FIXME layout_*, default_accessor and mdspan
> + using std::layout_stride;
> + // FIXME layout_left_padded, layout_right_padded, default_accessor and
> mdspan
> }
> #endif
>
> --
> 2.49.0
>
