On Thu, May 22, 2025 at 2:37 PM Tomasz Kaminski <tkami...@redhat.com> wrote:
>
>
> On Wed, May 21, 2025 at 12:04 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 | 216 +++++++++++++++++++++++++++++++-
>> 1 file changed, 213 insertions(+), 3 deletions(-)
>>
>> diff --git a/libstdc++-v3/include/std/mdspan
>> b/libstdc++-v3/include/std/mdspan
>> index 43676c3463c..732fc4eb1c2 100644
>> --- a/libstdc++-v3/include/std/mdspan
>> +++ b/libstdc++-v3/include/std/mdspan
>> @@ -399,6 +399,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>> class mapping;
>> };
>>
>> + struct layout_stride
>> + {
>> + template<typename _Extents>
>> + class mapping;
>> + };
>> +
>> namespace __mdspan
>> {
>> template<typename _Tp>
>> @@ -499,7 +505,8 @@ _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
>> @@ -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)
>>
> I think I would make it noexcept, as implementations can add noexcept to
> what is specified in standard.
> And just add appropriate comment.
>
>> + : mapping(__other.extents(), __mdspan::__internal_ctor{})
>> + { __glibcxx_assert(*this == __other); }
>> +
>> constexpr mapping&
>> operator=(const mapping&) noexcept = default;
>>
>> @@ -572,8 +586,8 @@ _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)...);
>>
> Could you move this change to layout_left commit.
>
>> }
>>
>> static constexpr bool
>> @@ -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;
>>
>> @@ -760,6 +781,195 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>> [[no_unique_address]] _Extents _M_extents;
>> };
>>
>> + namespace __mdspan
>> + {
>> + template<typename _Mapping>
>> + constexpr typename _Mapping::index_type
>> + __offset(const _Mapping& __m) noexcept
>> + {
>>
> As layout_stride::operator== working on generic mapping will be used with
other layouts, I would add here:
if constexpr (__standardized_mapping<_Mapping>)
return 0;
else
{
// current impl.
}
> + using _IndexType = typename _Mapping::index_type;
>> +
>> + auto __impl = [&__m]<size_t...
>> _Counts>(index_sequence<_Counts...>)
>> + { return __m(((void) _Counts, _IndexType(0))...); };
>> + return
>> __impl(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
>> + {
>> + 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, index_type>,
>> + "The size of extents_type must be representable as index_type");
>> +
>> + 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()))
>>
> I think this would also work: _M_extents(__other.extents())
>
>> + {
>>
> We could guard runtime check, with compile time for size)
> if constexpr(std::numeric_limits<typename
> _StridedMapping::index_type>::max()
> >
>
>
>> + if constexpr (extents_type::rank() > 0)
>> + for (size_t __i = 0; __i < extents_type::rank(); ++__i)
>> + _M_strides[__i] = index_type(__other.stride(__i));
>>
> I would add assert if other.required_span_size() <= numeric_limits::max.
>
> + }
>> +
>> + 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 {
>> + 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
>> + {
>> + 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];
>>
> If the index type is sized, this can overflow and cause UB, before we find
> zero.
> We need to check first if any of the extents is zero, before attempting
> multiplication.
>
> + }
>> + 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
>> + {
>>
> The brace in else should be indented.
>
>> + // 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.
>>
> Could re rephrase this:
> // We deviate from the standard, that does not thread all zero
> extents as exhaustive.
>
>> + 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
>>
>>
