On Tue, May 20, 2025 at 10:45 AM Luc Grosheintz <luc.groshei...@gmail.com>
wrote:
>
>
> On 5/20/25 10:24 AM, Tomasz Kaminski wrote:
> > On Sun, May 18, 2025 at 10:16 PM Luc Grosheintz <
> luc.groshei...@gmail.com>
> > wrote:
> >
> >> Implements the remaining parts of layout_left and layout_right; and all
> >> of layout_stride.
> >>
> >> libstdc++-v3/ChangeLog:
> >>
> >> * include/std/mdspan(layout_stride): New class.
> >>
> >> Signed-off-by: Luc Grosheintz <luc.groshei...@gmail.com>
> >> ---
> >> libstdc++-v3/include/std/mdspan | 219 +++++++++++++++++++++++++++++++-
> >> 1 file changed, 216 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/libstdc++-v3/include/std/mdspan
> >> b/libstdc++-v3/include/std/mdspan
> >> index b1984eb2a33..31a38c736c2 100644
> >> --- a/libstdc++-v3/include/std/mdspan
> >> +++ b/libstdc++-v3/include/std/mdspan
> >> @@ -366,6 +366,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >> class mapping;
> >> };
> >>
> >> + struct layout_stride
> >> + {
> >> + template<typename _Extents>
> >> + class mapping;
> >> + };
> >> +
> >> namespace __mdspan
> >> {
> >> template<typename _Tp>
> >> @@ -434,7 +440,8 @@ _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>;
> >>
> >> template<typename M>
> >> concept __mapping_like = requires
> >> @@ -503,6 +510,16 @@ _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(
> >> + layout_left::mapping<_OExtents>(__other.extents()) ==
> __other);
> >>
> > Could this be *this == other?
> >
> >> + }
> >> +
> >> constexpr mapping&
> >> operator=(const mapping&) noexcept = default;
> >>
> >> @@ -518,8 +535,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >> constexpr index_type
> >> operator()(_Indices... __indices) const noexcept
> >> {
> >> - return __mdspan::__linear_index_left(
> >> - this->extents(), static_cast<index_type>(__indices)...);
> >> + return __mdspan::__linear_index_left(_M_extents,
> >> + static_cast<index_type>(__indices)...);
> >> }
> >>
> >> static constexpr bool
> >> @@ -633,6 +650,16 @@ _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(
> >> + layout_right::mapping<_OExtents>(__other.extents()) ==
> >> __other);
> >>
> > Similary here.
> >
> >> + }
> >> +
> >> constexpr mapping&
> >> operator=(const mapping&) noexcept = default;
> >>
> >> @@ -695,6 +722,192 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >> [[no_unique_address]] _Extents _M_extents;
> >> };
> >>
> >> + namespace __mdspan
> >> + {
> >> + template<typename _Mapping, size_t... _Counts>
> >> + constexpr typename _Mapping::index_type
> >> + __offset_impl(const _Mapping& __m, index_sequence<_Counts...>)
> >> noexcept
> >> + { return __m(((void) _Counts, 0)...); }
> >> +
> >> + template<typename _Mapping>
> >> + constexpr typename _Mapping::index_type
> >> + __offset(const _Mapping& __m) noexcept
> >> + {
> >>
> > Again, I would define __impl as nested lambda here:
> > auto __impl = [&]<size_t... Countws>(index_seqeunce<_Counts>) noexcept
> > { return __m(((void) _Counts, 0)...); }
> >
> >> + return __offset_impl(__m,
> >> + make_index_sequence<_Mapping::extents_type::rank()>());
> >> + }
> >> +
> >> + template<typename _Mapping, typename... _Indices>
> >> + constexpr typename _Mapping::index_type
> >> + __linear_index_strides(const _Mapping& __m,
> >> + _Indices... __indices)
> >> + {
> >> + 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
> >> + {
> >> + static_assert(__mdspan::__layout_extent<_Extents>,
> >> + "The size of extents_type must be representable as index_type");
> >> +
> >> + 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;
> >> +
> >> + constexpr
> >> + mapping() noexcept
> >> + {
> >> + auto __stride = index_type(1);
> >> + for(size_t __i = extents_type::rank(); __i > 0; --__i)
> >> + {
> >> + _M_strides[__i - 1] = __stride;
> >> + __stride *= _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_like _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(extents_type(__other.extents()))
> >> + {
> >> + 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&
> >> + extents() const noexcept { return _M_extents; }
> >> +
> >> + constexpr array<index_type, extents_type::rank()>
> >> + strides() const noexcept { return _M_strides; }
> >>
> > Does this constructor work? I mean _M_strides in not std::array,
> > just array. I assumed you would need to create local array and copy them.
> >
> >> +
> >> + constexpr index_type
> >> + required_span_size() const noexcept
> >> + {
> >> + index_type __ret = 1;
> >> + for(size_t __i = 0; __i < extents_type::rank(); ++__i)
> >> + {
> >> + index_type __ext = _M_extents.extent(__i);
> >> + if(__ext == 0)
> >> + return 0;
> >> + __ret += (__ext - 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; }
> >> +
> >> + static constexpr bool
> >> + is_always_exhaustive() noexcept { return false; }
> >> +
> >> + static constexpr bool
> >> + is_always_strided() noexcept { return true; }
> >> +
> >> + static constexpr bool
> >> + is_unique() noexcept { return true; }
> >> +
> >> + constexpr bool
> >> + is_exhaustive() const noexcept
> >> + {
> >> + if constexpr (extents_type::rank() == 0)
> >> + return true;
> >> + else
> >> + {
> >> + // Under the assumption that the mapping is unique and has
> >> positive
> >> + // size, the mapping is exhaustive, if and only if the largest
> >> value
> >> + // returned by m(i...) is within the required range.
> >> + // However, the standard requires implementing a condition
> >> that's not
> >> + // always true when the size of the mapping is zero.
> >>
> > With the precondition on the constructor
> > https://eel.is/c++draft/mdspan.layout.stride#cons-4.3,
> > I do not think you can produce a layout_stride where this would produce a
> > different result.
> > If you agree I would change the comment that "C++23
> > [mdspan.layout.stride.cons] p4.3"
> > implies that this produces the same result.
>
> If the size of the extent is zero, then almost any choice of strides
> will work as a counter example. For example:
>
> extents = [0, 1, 1]
> strides = [2, 4, 8]
>
> is exhaustive, because every element (of the empty set) is covered.
> However, [2, 4, 8] don't match the formula in the spec [1].
>
My point is that constructing layout_stride with extents and strides from
the
example violates the precondition on the constructor:
https://eel.is/c++draft/mdspan.layout.stride#cons-4.3.
So this function cannot observe the difference.
>
> If the size of extents isn't zero, then the two approaches are the
> same.
>
> [1]: https://eel.is/c++draft/mdspan.layout.stride#obs-5.2
>
> >>
> >> + auto __size = __mdspan::__fwd_prod(_M_extents,
> >> extents_type::rank());
> >> + if (__size == 0)
> >> + return true;
> >> + return __size == required_span_size();
> >> + }
> >> + }
> >> +
> >> + static constexpr bool
> >> + is_strided() noexcept { return true; }
> >> +
> >> + constexpr index_type
> >> + stride(rank_type __r) const noexcept { return _M_strides[__r]; }
> >> +
> >> + template<__mdspan::__mapping_like _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 (__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 _M_extents;
> >> + [[no_unique_address]] _S_strides_t _M_strides;
> >> + };
> >> +
> >> _GLIBCXX_END_NAMESPACE_VERSION
> >> }
> >> #endif
> >> --
> >> 2.49.0
> >>
> >>
> >
>
>
