[PATCH v5 6/8] libstdc++: Implement layout_stride from mdspan.
Tomasz Kaminski
tkaminsk@redhat.com
Wed Jun 4 11:46:23 GMT 2025
On Wed, Jun 4, 2025 at 1:40 PM Luc Grosheintz <luc.grosheintz@gmail.com>
wrote:
>
>
> 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.
>
Ah, indeed. I thought that we have a default argument for the __fwd_prod
being rank,
as this is used in required_span_size() and would be part of mdspan::size().
No change needed.
>
> >
> >> + 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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