[PATCH v5 6/8] libstdc++: Implement layout_stride from mdspan.
Luc Grosheintz
luc.grosheintz@gmail.com
Wed Jun 4 11:40:17 GMT 2025
On 6/3/25 14:49, Tomasz Kaminski wrote:
> On Fri, May 30, 2025 at 6:47 PM Luc Grosheintz <luc.grosheintz@gmail.com>
> 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 <luc.grosheintz@gmail.com>
>> ---
>>
> LGTM. Only two comments for formatting and unused variable.
>
>> libstdc++-v3/include/std/mdspan | 245 ++++++++++++++++++++++++++++++-
>> libstdc++-v3/src/c++23/std.cc.in | 3 +-
>> 2 files changed, 246 insertions(+), 2 deletions(-)
>>
>> diff --git a/libstdc++-v3/include/std/mdspan
>> b/libstdc++-v3/include/std/mdspan
>> index 8e406731568..1250adefc14 100644
>> --- a/libstdc++-v3/include/std/mdspan
>> +++ b/libstdc++-v3/include/std/mdspan
>> @@ -418,6 +418,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>> class mapping;
>> };
>>
>> + struct layout_stride
>> + {
>> + template<typename _Extents>
>> + class mapping;
>> + };
>> +
>> namespace __mdspan
>> {
>> template<typename _Tp>
>> @@ -511,7 +517,8 @@ _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
>> @@ -557,6 +564,13 @@ _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;
>>
>> @@ -687,6 +701,13 @@ _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;
>>
>> @@ -759,6 +780,228 @@ _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
>
> I would write this as
> else if (__empty(__m.extents())
> return 0;
> else
> {
> /// ....
> }
>
>> + {
>> + if (__empty(__m.extents()))
>> + return 0;
>> +
>> + 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())
>> + {
>> + __glibcxx_assert(__mdspan::__offset(__other) == 0);
>> + if constexpr (cmp_greater(
>> + numeric_limits<typename
>> _StridedMapping::index_type>::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();
>>
> The rank variable is unused now.
It's used in the very next line; and if I inline it the next line is
81 characters wide, so I end up with two lines anyway.
>
>> + 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
>>
>>
>
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