[PATCH v1 4/6] libstdc++: Implement submdspan and submdspan_mapping for layout_left. [PR110352]
Luc Grosheintz
luc.grosheintz@gmail.com
Sat Nov 15 11:52:22 GMT 2025
On 11/14/25 15:09, Luc Grosheintz wrote:
> Implements `submdspan` and `submdspan_mapping` for layout_left as
> described in P3663 (Future proofing mdspan).
>
> When computing the offset of the submdspan, one must check that the
> lower bound of the slice range isn't out-of-range. There's a few
> cases when the lower bound is never out-of-range:
>
> - full_extent and exts.extent(k) != 0,
> - collapsing slice types.
>
> If those conditions are known to hold, no checks are generated.
>
> Similarly, if all slices are full_extent, there's no need to call
> mapping(0,...,0) for standardized mappings.
>
> The implementation prepares to use the symmetry between layout_left and
> layout_right and introduces concepts like a "layout side", i.e. left,
> right or unknown/strided.
>
> The tests use an iterator to replace nested for-loops. Which also makes
> it easier to write the core test logic in a rank-independent manner.
>
> PR libstdc++/110352
>
> libstdc++-v3/ChangeLog:
>
> * include/std/mdspan (layout_left::mapping::submdspan_mapping):
> New friend function.
> (submdspan): New function.
> * src/c++23/std.cc.in: Add submdspan.
> * testsuite/23_containers/mdspan/submdspan/submdspan.cc: New test.
> * testsuite/23_containers/mdspan/submdspan/submdspan_mapping.cc: New test.
> * testsuite/23_containers/mdspan/submdspan/submdspan_neg.cc: New test.
>
> Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>
> ---
> libstdc++-v3/include/std/mdspan | 420 +++++++++++++++++-
> libstdc++-v3/src/c++23/std.cc.in | 2 +-
> .../mdspan/submdspan/submdspan.cc | 369 +++++++++++++++
> .../mdspan/submdspan/submdspan_mapping.cc | 136 ++++++
> .../mdspan/submdspan/submdspan_neg.cc | 102 +++++
> 5 files changed, 1004 insertions(+), 25 deletions(-)
> create mode 100644 libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan.cc
> create mode 100644 libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_mapping.cc
> create mode 100644 libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_neg.cc
>
> diff --git a/libstdc++-v3/include/std/mdspan b/libstdc++-v3/include/std/mdspan
> index 1d506f0338e..1f6cbfd0967 100644
> --- a/libstdc++-v3/include/std/mdspan
> +++ b/libstdc++-v3/include/std/mdspan
> @@ -578,20 +578,30 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> return __r == 0 ? 1 : __exts.extent(0);
> else if constexpr (__all_dynamic(std::span(__sta_exts).first(__rank-1)))
> return __extents_prod(__exts, 1, 0, __r);
> else
> {
> size_t __sta_prod = __fwd_partial_prods<__sta_exts>[__r];
> return __extents_prod(__exts, __sta_prod, 0, __r);
> }
> }
>
> + template<typename _IndexType, size_t _Nm>
> + constexpr _IndexType
> + __fwd_prod(span<const _IndexType, _Nm> __values)
> + {
> + _IndexType __ret = 1;
> + for(auto __value : __values)
> + __ret *= __value;
> + return __ret;
> + }
> +
> // Preconditions: _r < _Extents::rank()
> template<typename _Extents>
> constexpr typename _Extents::index_type
> __rev_prod(const _Extents& __exts, size_t __r) noexcept
> {
> constexpr size_t __rank = _Extents::rank();
> constexpr auto& __sta_exts = __static_extents<_Extents>();
> if constexpr (__rank == 1)
> return 1;
> else if constexpr (__rank == 2)
> @@ -1003,20 +1013,383 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> __assert_valid_slices(const _Extents& __exts, const _Slices&... __slices)
> {
> constexpr auto __rank = _Extents::rank();
> if constexpr (__rank > 0)
> {
> constexpr auto [...__k] = _IotaArray<__rank>;
This is not supported by Clang:
<source>:6:5: error: decomposition declaration cannot be declared 'constexpr'
6 | constexpr auto [...i] = std::_IotaArray<5>;
| ^~~~~~~~~
https://godbolt.org/z/z5oqczahb
Do we use it anyway? (There's numerous other opportunities to
use this pattern, I wanted to ask first before making the patch
consistent.)
> ((__assert_valid_slice(__extract_extent<__k>(__exts),
> __slices...[__k])),...);
> }
> }
> +
> + template<typename _IndexType, typename _Slice>
> + constexpr _IndexType
> + __slice_begin(_Slice __slice)
> + {
> + if constexpr (convertible_to<_Slice, _IndexType>)
> + return __slice;
> + else if constexpr (__is_strided_slice<_Slice>)
> + return __slice.offset;
> + else
> + return 0; // full_extent
> + }
> +
> + template<typename _Mapping, typename... _Slices>
> + constexpr size_t
> + __suboffset(const _Mapping& __mapping, const _Slices&... __slices)
> + {
> + using _IndexType = typename _Mapping::index_type;
> + auto __any_past_the_end = [&]<size_t... _Is>(index_sequence<_Is...>)
> + {
> + auto __is_past_the_end = [](const auto& __slice, const auto& __ext)
> + {
> + using _Slice = remove_cvref_t<decltype(__slice)>;
> + if constexpr (is_convertible_v<_Slice, _IndexType>)
> + return false;
> + else if (same_as<_Slice, full_extent_t>
> + && __ext.static_extent(0) > 0
> + && __ext.static_extent(0) != dynamic_extent)
> + return false;
> + else
> + return __slice_begin<_IndexType>(__slice) == __ext.extent(0);
> + };
> +
> + const auto& __exts = __mapping.extents();
> + return ((__is_past_the_end(__slices...[_Is],
> + __extract_extent<_Is>(__exts))) || ...);
> + };
> +
> + if constexpr ((same_as<_Slices, full_extent_t> && ...))
> + return __offset(__mapping);
> +
> + if constexpr (!((convertible_to<_Slices, _IndexType>) && ...))
> + if (__any_past_the_end(make_index_sequence<sizeof...(__slices)>()))
> + return __mapping.required_span_size();
> + return __mapping(__slice_begin<_IndexType>(__slices)...);
> + }
> +
> + template<typename _IndexType, typename... _Slices>
> + consteval auto
> + __subrank()
> + {
> + return (static_cast<size_t>(!convertible_to<_Slices, _IndexType>)
> + + ... + 0);
> + }
> +
> + template<typename _IndexType, typename... _Slices>
> + consteval auto
> + __inv_map_rank()
> + {
> + constexpr auto __rank = sizeof...(_Slices);
> + constexpr auto __sub_rank = __subrank<_IndexType, _Slices...>();
> + auto __map = std::array<size_t, __sub_rank>{};
> + auto __is_int_like = std::array{convertible_to<_Slices, _IndexType>...};
> +
> + size_t __i = 0;
> + for (size_t __k = 0; __k < __rank; ++__k)
> + if (!__is_int_like[__k])
> + __map[__i++] = __k;
> + return __map;
> + }
> +
> + enum class _LayoutSide
> + {
> + __left,
> + __right,
> + __unknown
> + };
> +
> + template<typename _Mapping>
> + constexpr _LayoutSide
> + __deduce_mapping_side()
> + {
> + if constexpr (__is_left_padded_mapping<_Mapping>
> + || __mapping_of<layout_left, _Mapping>)
> + return _LayoutSide::__left;
> + if constexpr (__is_left_padded_mapping<_Mapping>
> + || __mapping_of<layout_right, _Mapping>)
> + return _LayoutSide::__right;
> + else
> + return _LayoutSide::__unknown;
> + }
> +
> + template<_LayoutSide _Side, size_t _Rank>
> + struct _StridesTrait
> + {
> + static constexpr const _LayoutSide _S_side = _Side;
> +
> + static constexpr size_t
> + _S_idx(size_t __k) noexcept
> + {
> + if constexpr (_Side == _LayoutSide::__left)
> + return __k;
> + else
> + return _Rank - 1 - __k;
> + }
> +
> + template<typename _Extents>
> + static constexpr typename _Extents::index_type
> + _S_extent(const _Extents& __exts, size_t __k)
> + { return __exts.extent(_S_idx(__k)); }
> +
> + template<typename _IndexType, typename... _Slices>
> + static consteval auto
> + _S_inv_map()
> + {
> + static_assert(_Side != _LayoutSide::__unknown);
> + constexpr auto [...__i] = _IotaArray<_Rank>;
> + return __inv_map_rank<_IndexType, _Slices...[_S_idx(__i)]...>();
> + }
> + };
> +
> + template<typename _SubExtents, typename _Mapping, typename... _Slices>
> + constexpr auto
> + __substrides_generic(const _Mapping& __mapping,
> + const _Slices&... __slices)
> + {
> + using _IndexType = typename _Mapping::index_type;
> + if constexpr (_SubExtents::rank() == 0)
> + return array<_IndexType, _SubExtents::rank()>{};
> + else
> + {
> + auto __stride = [&__mapping](size_t __k, auto __slice) -> _IndexType
> + {
> + if constexpr (__is_strided_slice<decltype(__slice)>)
> + if (__slice.stride < __slice.extent)
> + return __mapping.stride(__k) * __slice.stride;
> + return __mapping.stride(__k);
> + };
> +
> + constexpr auto [...__i] = _IotaArray<_SubExtents::rank()>;
> + constexpr auto __inv_map = __inv_map_rank<_IndexType, _Slices...>();
> + return array<_IndexType, _SubExtents::rank()>{
> + __stride(__inv_map[__i], __slices...[__inv_map[__i]])...};
> + }
> + };
> +
> + template<typename _SubExtents, typename _Mapping, typename... _Slices>
> + constexpr auto
> + __substrides_standarized(const _Mapping& __mapping,
> + const _Slices&... __slices)
> + {
The motivation is that __substrides_generic is quadratic in rank (it
computes stride(k) rank-times, and stride(k) is (in general) k
multiplications.
> + using _IndexType = typename _Mapping::index_type;
> + using _Trait = _StridesTrait<__deduce_mapping_side<_Mapping>(),
> + _Mapping::extents_type::rank()>;
> + using _SubTrait = _StridesTrait<__deduce_mapping_side<_Mapping>(),
> + _SubExtents::rank()>;
> +
> + constexpr size_t __sub_rank = _SubExtents::rank();
> +
> + array<_IndexType, __sub_rank> __ret;
> + if constexpr (__sub_rank > 0)
> + {
> + constexpr auto __inv_map
> + = _Trait::template _S_inv_map<_IndexType, _Slices...>();
> + auto __loop = [&]<size_t... _Ks>(index_sequence<_Ks...>)
> + {
> + size_t __i0 = 0;
> + size_t __stride = 1;
> + auto __body = [&](size_t __k, auto __slice)
> + {
> + for (size_t __i = __i0; __i < __inv_map[__k]; ++__i)
> + __stride *= _Trait::_S_extent(__mapping.extents(), __i);
> +
> + size_t __krev = _SubTrait::_S_idx(__k);
> + if constexpr (__is_strided_slice<decltype(__slice)>)
> + __ret[__krev] = __stride * __slice.stride;
> + else
> + __ret[__krev] = __stride;
> +
> + __i0 = __inv_map[__k];
> + };
> +
> + ((__body(_Ks, __slices...[_Trait::_S_idx(__inv_map[_Ks])])),...);
> + };
> + __loop(make_index_sequence<__sub_rank>());
> + }
> + return __ret;
> + }
> +
> + template<typename _SubExtents, typename _Mapping, typename... _Slices>
> + constexpr auto
> + __substrides(const _Mapping& __mapping, const _Slices&... __slices)
> + {
> + if constexpr (__deduce_mapping_side<_Mapping>() != _LayoutSide::__unknown)
> + return __substrides_standarized<_SubExtents>(__mapping, __slices...);
> + else
> + return __substrides_generic<_SubExtents>(__mapping, __slices...);
> + }
> +
> + template<typename _Slice, typename _IndexType>
> + concept __is_unit_stride_slice = (__is_strided_slice<_Slice>
> + && __detail::__integral_constant_like<typename _Slice::stride_type>
> + && _Slice::stride_type::value == 1)
> + || same_as<_Slice, full_extent_t>;
> +
> + // _BlockSize - 1
> + // [full, ..., full, unit_slice , ...]
> + template<typename _IndexType, size_t _BlockSize, typename... _Slices>
> + consteval bool
> + __is_block()
> + {
> + if constexpr (_BlockSize == 0 || _BlockSize > sizeof...(_Slices))
> + return false;
> + else if constexpr (_BlockSize == 1)
> + return __is_unit_stride_slice<_Slices...[0], _IndexType>;
> + else if constexpr (same_as<_Slices...[0], full_extent_t>)
> + {
> + auto __recurse = []<size_t... _Is>(index_sequence<_Is...>)
> + {
> + return __is_block<_IndexType, _BlockSize - 1,
> + _Slices...[_Is + 1]...>();
> + };
> + return __recurse(make_index_sequence<sizeof...(_Slices) - 1>());
> + }
> + else
> + return false;
> + }
> +
> + // __u __u + _BlockSize - 1
> + // [*, full, ..., full, unit_slice, *]
> + template<typename _IndexType, size_t _Start, size_t _BlockSize,
> + typename... _Slices>
> + consteval size_t
> + __find_block()
> + {
> + static_assert(_BlockSize != dynamic_extent,
> + "The implementation can't handle submdspans with rank == size_t(-1)");
> +
> + if constexpr (sizeof...(_Slices) == 0)
> + return dynamic_extent;
> + else if constexpr (__is_block<_IndexType, _BlockSize, _Slices...>())
> + return _Start;
> + else
> + {
> + auto __recurse = []<size_t... _Is>(index_sequence<_Is...>)
> + {
> + return __find_block<_IndexType, _Start + 1, _BlockSize,
> + _Slices...[_Is + 1]...>();
> + };
> + return __recurse(make_index_sequence<sizeof...(_Slices) - 1>());
> + }
> + }
> +
> + template<typename _IndexType, size_t _SubRank, typename... _Slices>
> + static consteval bool
> + __is_compact_block()
> + {
> + if constexpr (_SubRank == 0)
> + return false;
> + else
> + return __find_block<_IndexType, 0, _SubRank, _Slices...>() == 0;
> + }
> +
> + // __u
> + // [unit_slice, i, ..., k, full, ..., full, unit_slice, *]
> + template<typename _IndexType, size_t _SubRank, typename _Slice,
> + typename... _Slices>
> + static consteval size_t
> + __padded_block_begin()
> + {
> + if constexpr (!__mdspan::__is_unit_stride_slice<_Slice, _IndexType>)
> + return dynamic_extent;
> + else if constexpr (sizeof...(_Slices) == 0)
> + return dynamic_extent;
> + else
> + {
> + constexpr auto __u = __find_block<_IndexType, 0, _SubRank - 1,
> + _Slices...>();
> + if constexpr (__u != dynamic_extent)
> + return __u + 1;
> + else
> + return dynamic_extent;
> + }
> + }
> +
> + template<_LayoutSide _Side>
> + struct _SubMdspanMapping;
> +
> + template<>
> + struct _SubMdspanMapping<_LayoutSide::__left>
> + {
> + using _Layout = layout_left;
> + template<size_t _Pad> using _PaddedLayout = layout_left_padded<_Pad>;
> +
> + template<typename _Extents, size_t _Us>
> + static constexpr size_t
> + _S_pad()
> + {
> + constexpr auto __sta_exts = __static_extents<_Extents>(0, _Us);
> + if constexpr (!__all_static(__sta_exts))
> + return dynamic_extent;
> + else
> + return __fwd_prod(__sta_exts);
> + }
> +
> + template<typename _IndexType, size_t _SubRank, typename... _Slices>
> + static consteval bool
> + _S_is_compact_block()
> + { return __is_compact_block<_IndexType, _SubRank, _Slices...>(); }
> +
> + template<typename _IndexType, size_t _SubRank, typename... _Slices>
> + static consteval size_t
> + _S_padded_block_begin()
> + { return __padded_block_begin<_IndexType, _SubRank, _Slices...>(); }
> + };
> +
All of __submdspan_mapping_impl is underindented.
> + template<typename _Mapping>
> + constexpr auto
> + __submdspan_mapping_impl(const _Mapping& __mapping)
> + { return submdspan_mapping_result{__mapping, 0}; }
> +
> + template<typename _Mapping, typename... _Slices>
> + requires (sizeof...(_Slices) > 0)
> + constexpr auto
> + __submdspan_mapping_impl(const _Mapping& __mapping,
> + _Slices... __slices)
> + {
> + using _Extents = typename _Mapping::extents_type;
> + using _IndexType= typename _Mapping::index_type;
> + using _Trait = _SubMdspanMapping<__deduce_mapping_side<_Mapping>()>;
> +
> + auto __offset = __suboffset(__mapping, __slices...);
> + auto __sub_exts = submdspan_extents(__mapping.extents(), __slices...);
> + using _SubExtents = decltype(__sub_exts);
> + constexpr auto __sub_rank = _SubExtents::rank();
> + if constexpr (_SubExtents::rank() == 0)
> + return submdspan_mapping_result{
> + typename _Trait::_Layout::mapping(__sub_exts), __offset};
> + else if constexpr (_Trait::template _S_is_compact_block<_IndexType,
> + __sub_rank,
> + _Slices...>())
> + return submdspan_mapping_result{
> + typename _Trait::_Layout::mapping(__sub_exts), __offset};
> + else if constexpr (
> + constexpr auto __u
> + = _Trait::template _S_padded_block_begin<_IndexType, __sub_rank,
> + _Slices...>();
> + __u != dynamic_extent)
> + {
> + constexpr auto __pad = _Trait::template _S_pad<_Extents, __u>();
> + using _Layout = typename _Trait::template _PaddedLayout<__pad>;
> + return submdspan_mapping_result{
> + typename _Layout::mapping(__sub_exts, __mapping.stride(__u)),
> + __offset};
> + }
> + else
> + {
> + auto __sub_strides
> + = __substrides<_SubExtents>(__mapping, __slices...);
> + return submdspan_mapping_result{
> + layout_stride::mapping(__sub_exts, __sub_strides), __offset};
> + }
> + }
> #endif // __glibcxx_submdspan
> }
>
> template<typename _Extents>
> class layout_left::mapping
> {
> public:
> using extents_type = _Extents;
> using index_type = typename extents_type::index_type;
> using size_type = typename extents_type::size_type;
> @@ -1144,20 +1517,28 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> template<typename _OExtents>
> constexpr explicit
> mapping(const _OExtents& __oexts, __mdspan::__internal_ctor) noexcept
> : _M_extents(__oexts)
> {
> static_assert(__mdspan::__representable_size<_OExtents, index_type>,
> "The size of OtherExtents must be representable as index_type");
> __glibcxx_assert(__mdspan::__is_representable_extents(_M_extents));
> }
>
> +#if __glibcxx_submdspan
> + template<__mdspan::__valid_canonical_slice_type<index_type>... _Slices>
> + requires (extents_type::rank() == sizeof...(_Slices))
> + friend constexpr auto
> + submdspan_mapping(const mapping& __mapping, _Slices... __slices)
> + { return __mdspan::__submdspan_mapping_impl(__mapping, __slices...); }
> +#endif // __glibcxx_submdspan
> +
> [[no_unique_address]] extents_type _M_extents{};
> };
>
> namespace __mdspan
> {
> template<typename _Extents, typename... _Indices>
> constexpr typename _Extents::index_type
> __linear_index_right(const _Extents& __exts, _Indices... __indices)
> noexcept
> {
> @@ -2739,44 +3120,20 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> if constexpr (__is_strided_slice<_Slice>)
> return __slice.extent == 0 ? 0 :
> 1 + (__slice.extent - 1) / __slice.stride;
> else if constexpr (convertible_to<_Slice, _IndexType>)
> return 1;
> else
> return __exts.extent(_K);
>
> }
>
> - template<typename _IndexType, typename... _Slices>
> - consteval auto
> - __subrank()
> - {
> - return (static_cast<size_t>(!convertible_to<_Slices, _IndexType>)
> - + ... + 0);
> - }
> -
> - template<typename _IndexType, typename... _Slices>
> - consteval auto
> - __inv_map_rank()
> - {
> - constexpr auto __rank = sizeof...(_Slices);
> - constexpr auto __sub_rank = __subrank<_IndexType, _Slices...>();
> - auto __map = std::array<size_t, __sub_rank>{};
> - auto __is_int_like = std::array{convertible_to<_Slices, _IndexType>...};
> -
> - size_t __i = 0;
> - for (size_t __k = 0; __k < __rank; ++__k)
> - if (!__is_int_like[__k])
> - __map[__i++] = __k;
> - return __map;
> - }
> -
> template<typename _IndexType, size_t... _Extents, typename... _Slices>
> requires (sizeof...(_Slices) == sizeof...(_Extents))
> constexpr auto
> __subextents(const extents<_IndexType, _Extents...>& __exts,
> _Slices... __slices)
> {
> constexpr auto __inv_map = __mdspan::__inv_map_rank<_IndexType,
> _Slices...>();
> auto __impl = [&]<size_t... _Indices>(index_sequence<_Indices...>)
> {
> @@ -2824,16 +3181,31 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> requires (sizeof...(_Extents) == sizeof...(_Slices))
> constexpr auto
> submdspan_canonicalize_slices(const extents<_IndexType, _Extents...>& __exts,
> _Slices... __raw_slices)
> {
> auto [...__slices]
> = make_tuple(__mdspan::__slice_cast<_IndexType>(__raw_slices)...);
> __mdspan::__assert_valid_slices(__exts, __slices...);
> return make_tuple(__slices...);
> }
> +
> + template<typename _ElementType, typename _Extents, typename _Layout,
> + typename _Accessor, typename... _Slices>
> + requires (sizeof...(_Slices) == _Extents::rank())
> + constexpr auto
> + submdspan(
> + const mdspan<_ElementType, _Extents, _Layout, _Accessor>& __md,
> + _Slices... __raw_slices)
> + {
> + auto [...__slices] = submdspan_canonicalize_slices(__md.extents(),
> + __raw_slices...);
> + auto __result = submdspan_mapping(__md.mapping(), __slices...);
> + return mdspan(__md.accessor().offset(__md.data_handle(), __result.offset),
> + __result.mapping, typename _Accessor::offset_policy(__md.accessor()));
> + }
> #endif // __glibcxx_submdspan
>
> _GLIBCXX_END_NAMESPACE_VERSION
> }
> #endif
> #endif
> diff --git a/libstdc++-v3/src/c++23/std.cc.in b/libstdc++-v3/src/c++23/std.cc.in
> index 2dfdf13f0d1..6306aad0498 100644
> --- a/libstdc++-v3/src/c++23/std.cc.in
> +++ b/libstdc++-v3/src/c++23/std.cc.in
> @@ -1876,22 +1876,22 @@ export namespace std
> using std::layout_left_padded;
> using std::layout_right_padded;
> #endif
> #if __glibcxx_submdspan
> using std::strided_slice;
> using std::full_extent_t;
> using std::full_extent;
> using std::submdspan_mapping_result;
> using std::submdspan_canonicalize_slices;
> using std::submdspan_extents;
> + using std::submdspan;
> #endif
> - // FIXME mdsubspan
> }
> #endif
>
> // 20.2 <memory>
> export namespace std
> {
> using std::align;
> using std::allocator;
> using std::allocator_arg;
> using std::allocator_arg_t;
> diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan.cc b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan.cc
> new file mode 100644
> index 00000000000..53e91407a9c
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan.cc
> @@ -0,0 +1,369 @@
> +// { dg-do run { target c++26 } }
> +#include <mdspan>
> +
> +#include <iostream> // TODO remove
> +#include <vector>
> +#include <numeric>
> +#include "../layout_traits.h"
> +#include <testsuite_hooks.h>
> +
> +constexpr size_t dyn = std::dynamic_extent;
> +constexpr auto all = std::full_extent;
> +
> +template<typename T>
> + constexpr bool is_strided_slice = false;
> +
> +template<typename O, typename E, typename S>
> + constexpr bool is_strided_slice<std::strided_slice<O, E, S>> = true;
> +
> +template<typename MDSpan>
> + constexpr void
> + fill(const MDSpan& md)
> + {
> + using IndexType = typename MDSpan::index_type;
> + auto exts = md.extents();
> + if constexpr (exts.rank() == 3)
> + for(IndexType i = 0; i < exts.extent(0); ++i)
> + for(IndexType j = 0; j < exts.extent(1); ++j)
> + for(IndexType k = 0; k < exts.extent(2); ++k)
> + md[i, j, k] = 100 * i + 10 * j + k;
> + }
> +
> +template<typename Int, size_t Rank>
> + class multi_index_generator
> + {
> + class EndIt
> + { };
> +
> + class BeginIt
> + {
> + public:
> + constexpr
> + BeginIt(const std::array<Int, Rank>& shape)
> + : M_shape(shape)
> + { }
> +
> + constexpr BeginIt&
> + operator++()
> + {
> + if constexpr (Rank > 0)
> + {
> + ++M_indices[Rank-1];
> + for(size_t i = Rank; i > 1; --i)
> + if (M_indices[i-1] == M_shape[i-1])
> + {
> + M_indices[i-1] = 0;
> + ++M_indices[i-2];
> + }
> + }
> + return *this;
> + }
> +
> + constexpr auto
> + operator*()
> + { return M_indices; }
> +
> + constexpr bool
> + operator==(EndIt)
> + {
> + if constexpr (Rank > 0)
> + return M_indices[0] == M_shape[0];
> + else
> + return true;
> + }
> +
> + private:
> + std::array<Int, Rank> M_indices{};
> + std::array<Int, Rank> M_shape;
> + };
> +
> + public:
> + constexpr
> + multi_index_generator(std::array<Int, Rank> shape)
> + : M_shape(shape)
> + { }
> +
> + constexpr BeginIt
> + begin() const
> + { return BeginIt(M_shape); }
> +
> + constexpr EndIt
> + end() const
> + { return EndIt{}; }
> +
> + private:
> + std::array<Int, Rank> M_shape;
> + };
> +
> +constexpr bool
> +test_multi_index()
> +{
> + auto shape = std::array{3, 5, 7, 1};
> +
> + std::vector<std::array<int, 4>> expected;
> + for (int i = 0; i < shape[0]; ++i)
> + for (int j = 0; j < shape[1]; ++j)
> + for (int k = 0; k <shape[2]; ++k)
> + for (int l = 0; l <shape[3]; ++l)
> + expected.push_back(std::array{i, j, k, l});
> +
> + size_t i = 0;
> + for (auto actual : multi_index_generator(shape))
> + VERIFY(expected[i++] == actual);
> + return true;
> +}
> +
> +static_assert(test_multi_index());
> +
> +struct
> +collapse
> +{ };
> +
> +template<typename... Slices>
> + consteval auto
> + inv_collapsed_index_map()
> + {
> + constexpr size_t rank = sizeof...(Slices);
> + auto is_collapsing = std::array{std::same_as<Slices, collapse>...};
> + constexpr auto collapsed_rank = ((!std::same_as<Slices, collapse>) + ... + 0);
> +
> + std::array<size_t, collapsed_rank> ret;
> + if constexpr (collapsed_rank > 0)
> + for(size_t k = 0, i = 0; i < rank; ++i)
> + if (!is_collapsing[i])
> + ret[k++] = i;
> + return ret;
> + }
> +
> +static_assert(inv_collapsed_index_map<collapse, collapse, collapse>()
> + == std::array<size_t, 0>{});
> +
> +static_assert(inv_collapsed_index_map<collapse, decltype(all), collapse>()
> + == std::array<size_t, 1>{1});
> +
> +template<typename IndexType, typename Slice>
> + constexpr std::vector<IndexType>
> + make_selection(IndexType extent, const Slice& slice)
> + {
> + if constexpr (std::convertible_to<Slice, IndexType>)
> + return {static_cast<IndexType>(slice)};
> + else if constexpr (std::same_as<Slice, std::full_extent_t>)
> + {
> + auto ret = std::vector<IndexType>(static_cast<size_t>(extent));
> + std::ranges::iota(ret, 0);
> + return ret;
> + }
> + else if constexpr (is_strided_slice<Slice>)
> + {
> + auto ret = std::vector<IndexType>{};
> + size_t n = static_cast<size_t>(slice.extent);
> + for(size_t i = 0; i < n; i += slice.stride)
> + ret.push_back(slice.offset + i);
> + return ret;
> + }
> + else
> + {
> + auto [begin, end] = slice;
> + auto ret = std::vector<IndexType>(static_cast<size_t>(end - begin));
> + std::ranges::iota(ret, begin);
> + return ret;
> + }
> + }
> +
> +template<typename Layout, size_t... I, typename... Slices>
> + constexpr bool
> + check_selection(std::index_sequence<I...>, auto md, Slices... slices)
> + {
> + auto exts = md.extents();
> + auto outer_shape = std::array{exts.extent(0), exts.extent(1), exts.extent(2)};
> +
> + constexpr auto full_index = inv_collapsed_index_map<Slices...>();
> + auto make_slice = [](size_t i, auto slice)
> + {
> + if constexpr (std::same_as<decltype(slice), collapse>)
> + return i;
> + else
> + return slice;
> + };
> +
> + auto loop_body = [&]<size_t... J>(std::index_sequence<J...>, auto ijk,
> + auto... slices)
> + {
> + auto submd = submdspan(md, slices...[I]...);
> + auto selection = std::tuple{make_selection(exts.extent(I), slices...[I])...};
> + auto inner_shape = std::array<size_t, full_index.size()>{
> + std::get<full_index[J]>(selection).size()...
> + };
> +
> + for (auto ij : multi_index_generator(inner_shape))
> + {
> + ((ijk[full_index[J]] = get<full_index[J]>(selection)[ij[J]]),...);
> + VERIFY(submd[ij] == md[ijk]);
> + }
> + };
> +
> + for (auto ijk : multi_index_generator(outer_shape))
> + loop_body(std::make_index_sequence<full_index.size()>(), ijk,
> + make_slice(ijk[I], slices...[I])...);
> + return true;
> + }
> +
> +template<typename Layout, typename...MD, typename... Slices>
> + constexpr bool
> + check_selection(std::mdspan<MD...> md, Slices... slices)
> + {
> + auto indices = std::make_index_sequence<sizeof...(slices)>();
> + return check_selection<Layout>(indices, md, slices...);
> + }
> +
> +template<typename Layout, typename IndexType, size_t... Extents,
> + typename... Slices>
> + constexpr bool
> + check_selection(std::extents<IndexType, Extents...>exts, Slices... slices)
> + {
> + auto run = [&](auto m)
> + {
> + auto storage = std::vector<double>(m.required_span_size());
> + auto md = std::mdspan(storage.data(), m);
> + fill(md);
> + return check_selection<Layout>(md, slices...);
> + };
> +
> + if constexpr (std::same_as<Layout, std::layout_stride>)
> + {
> + auto m = typename Layout::mapping(exts, std::array{15, 2, 50});
> + return run(m);
> + }
> + else
> + {
> + auto m = typename Layout::mapping(exts);
> + return run(m);
> + }
> + }
> +
> +template<typename Layout>
> + constexpr bool
> + test_scalar_selection(auto exts)
> + {
> + check_selection<Layout>(exts, collapse{}, collapse{}, collapse{});
> + return true;
> + }
> +
> +template<typename Layout>
> + constexpr bool
> + test_full_lines(auto exts)
> + {
> + check_selection<Layout>(exts, all, collapse{}, collapse{});
> + check_selection<Layout>(exts, collapse{}, all, collapse{});
> + check_selection<Layout>(exts, collapse{}, collapse{}, all);
> + return true;
> + }
> +
> +template<typename Layout>
> + constexpr bool
> + test_full_blocks(auto exts)
> + {
> + check_selection<Layout>(exts, all, all, collapse{});
> + check_selection<Layout>(exts, all, collapse{}, all);
> + check_selection<Layout>(exts, collapse{}, all, all);
> + return true;
> + }
> +
> +template<typename Layout>
> + constexpr bool
> + test_cubes(auto exts)
> + {
> + auto s0 = std::pair{0, 2};
> + auto s1 = std::pair{1, 4};
> + auto s2 = std::pair{3, 7};
> +
> + check_selection<Layout>(exts, all, all, all);
> + check_selection<Layout>(exts, all, all, s2);
> + check_selection<Layout>(exts, s0, all, all);
> + check_selection<Layout>(exts, s0, all, s2);
> + check_selection<Layout>(exts, s0, s1, s2);
> + return true;
> + }
> +
> +template<typename Layout>
> + constexpr bool
> + test_strided_line_selection(auto exts)
> + {
> + auto check = [&](auto s)
> + {
> + check_selection<Layout>(exts, collapse{}, s, collapse{});
> + };
> +
> + check(std::strided_slice(0, 2, 2));
> + check(std::strided_slice(0, 3, 2));
> + check(std::strided_slice(1, 3, 2));
> + check(std::strided_slice(1, std::cw<3>, std::cw<2>));
> + return true;
> + }
> +
> +template<typename Layout>
> + constexpr bool
> + test_strided_box_selection(auto exts)
> + {
> + auto s0 = std::strided_slice(0, 3, 2);
> + auto s1 = std::strided_slice(1, 4, 2);
> + auto s2 = std::strided_slice(0, 7, 3);
> +
> + check_selection<Layout>(exts, s0, s1, s2);
> + return true;
> + }
> +
> +template<typename Layout>
> + constexpr bool
> + test_all_cheap()
> + {
> + constexpr auto dyn_exts = std::extents(3, 5, 7);
> + constexpr auto sta_exts = std::extents<int, 3, 5, 7>{};
> +
> + test_scalar_selection<Layout>(dyn_exts);
> + test_scalar_selection<Layout>(sta_exts);
> + static_assert(test_scalar_selection<Layout>(dyn_exts));
> + static_assert(test_scalar_selection<Layout>(sta_exts));
> +
> + test_full_lines<Layout>(dyn_exts);
> + test_full_lines<Layout>(sta_exts);
> + static_assert(test_full_lines<Layout>(dyn_exts));
> + static_assert(test_full_lines<Layout>(sta_exts));
> +
> + test_strided_box_selection<Layout>(dyn_exts);
> + test_strided_box_selection<Layout>(sta_exts);
> + static_assert(test_strided_box_selection<Layout>(dyn_exts));
> + static_assert(test_strided_box_selection<Layout>(sta_exts));
> + return true;
> + }
> +
> +template<typename Layout>
> + constexpr bool
> + test_all_expensive()
> + {
> + auto run = [](auto exts)
> + {
> + test_full_blocks<Layout>(exts);
> + test_cubes<Layout>(exts);
> + };
> +
> + run(std::extents(3, 5, 7));
> + run(std::extents<int, 3, 5, 7>{});
> + return true;
> + }
> +
> +template<typename Layout>
> + constexpr bool
> + test_all()
> + {
> + test_all_cheap<Layout>();
> + test_all_expensive<Layout>();
> + return true;
> + }
> +
> +int
> +main()
> +{
> + test_all<std::layout_left>();
> + return 0;
> +}
> diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_mapping.cc b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_mapping.cc
> new file mode 100644
> index 00000000000..a37d3cd588f
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_mapping.cc
> @@ -0,0 +1,136 @@
> +// { dg-do run { target c++26 } }
> +#include <mdspan>
> +
> +#include <iostream> // TODO remove
> +#include "../layout_traits.h"
> +#include <testsuite_hooks.h>
> +
> +constexpr size_t dyn = std::dynamic_extent;
> +
> +template<typename Mapping, typename... Slices>
> + constexpr auto
> + call_submdspan_mapping(const Mapping& m, std::tuple<Slices...> slices)
> + {
> + auto impl = [&]<size_t... I>(std::index_sequence<I...>)
> + { return submdspan_mapping(m, get<I>(slices)...); };
> + return impl(std::make_index_sequence<sizeof...(Slices)>());
> + }
> +
> +template<typename Layout>
> + constexpr bool
> + test_layout_unpadded_return_types()
> + {
> + constexpr auto padding_side = DeducePaddingSide::from_typename<Layout>();
> + using Traits = LayoutTraits<padding_side>;
> +
> + {
> + auto m0 = typename Layout::mapping(std::extents());
> + auto result = submdspan_mapping(m0);
> + using layout_type = typename decltype(result.mapping)::layout_type;
> + static_assert(std::same_as<layout_type, Layout>);
> + }
> +
> + auto exts = Traits::make_extents(std::dims<5, int>(3, 5, 7, 11, 13));
> + auto m = typename Layout::mapping(exts);
> + auto all = std::full_extent;
> + auto s251 = std::strided_slice{2, 5, std::cw<1>};
> +
> + {
> + auto slices = std::tuple{0, 0, 0, 0, 0};
> + auto result = call_submdspan_mapping(m, Traits::make_tuple(slices));
> + using layout_type = typename decltype(result.mapping)::layout_type;
> + static_assert(std::same_as<layout_type, Layout>);
> + }
> +
> + {
> + auto slices = std::tuple{all, all, all, s251, 0};
> + auto result = call_submdspan_mapping(m, Traits::make_tuple(slices));
> + using layout_type = typename decltype(result.mapping)::layout_type;
> + static_assert(std::same_as<layout_type, Layout>);
> + }
> +
> + {
> + auto s0 = std::strided_slice{1, 1, std::cw<1>};
> + auto slices = std::tuple{s0, all, all, s251, 0};
> + auto result = call_submdspan_mapping(m, Traits::make_tuple(slices));
> + using layout_type = typename decltype(result.mapping)::layout_type;
> + static_assert(is_same_padded<padding_side, layout_type>);
> + }
> +
> + {
> + auto s0 = std::strided_slice{1, 2, std::cw<1>};
> + auto slices = std::tuple{s0, all, all, s251, 0};
> + auto result = call_submdspan_mapping(m, Traits::make_tuple(slices));
> + using layout_type = typename decltype(result.mapping)::layout_type;
> + static_assert(is_same_padded<padding_side, layout_type>);
> + }
> +
> + {
> + auto s0 = std::strided_slice{1, 2, std::cw<1>};
> + auto slices = std::tuple{s0, 0, all, s251, 0};
> + auto result = call_submdspan_mapping(m, Traits::make_tuple(slices));
> + using layout_type = typename decltype(result.mapping)::layout_type;
> + static_assert(is_same_padded<padding_side, layout_type>);
> + }
> +
> + {
> + auto s0 = std::strided_slice{1, 2, 1};
> + auto slices = std::tuple{s0, all, all, s251, 0};
> + auto result = call_submdspan_mapping(m, Traits::make_tuple(slices));
> + using layout_type = decltype(result.mapping)::layout_type;
> + static_assert(std::same_as<layout_type, std::layout_stride>);
> + }
> +
> + {
> + auto slices = std::tuple{1, all, all, s251, 0};
> + auto result = call_submdspan_mapping(m, Traits::make_tuple(slices));
> + using layout_type = decltype(result.mapping)::layout_type;
> + static_assert(std::same_as<layout_type, std::layout_stride>);
> + }
> +
> + {
> + auto s3 = std::strided_slice{2, std::cw<7>, std::cw<2>};
> + auto slices = std::tuple{all, all, all, s3, 0};
> + auto result = call_submdspan_mapping(m, Traits::make_tuple(slices));
> + using layout_type = decltype(result.mapping)::layout_type;
> + static_assert(std::same_as<layout_type, std::layout_stride>);
> + }
> + return true;
> + }
> +
> +template<typename Layout>
> + constexpr bool
> + test_layout_unpadded_padding_value()
> + {
> + using Traits = LayoutTraits<DeducePaddingSide::from_typename<Layout>()>;
> + auto s0 = std::strided_slice{size_t(1), size_t(2), std::cw<size_t(1)>};
> + auto s3 = std::strided_slice{size_t(2), size_t(5), std::cw<size_t(1)>};
> + auto all = std::full_extent;
> +
> + auto check = [&](auto exts, size_t expected)
> + {
> + auto m = typename Layout::mapping(Traits::make_extents(exts));
> + auto slices = std::tuple{s0, size_t(0), all, s3, size_t(0)};
> + auto result = call_submdspan_mapping(m, Traits::make_tuple(slices));
> + auto padding_value = decltype(result.mapping)::padding_value;
> + VERIFY(padding_value == expected);
> + };
> +
> + check(std::extents(std::cw<3>, std::cw<5>, std::cw<7>, 11, 13), 3*5);
> + check(std::extents(std::cw<3>, std::cw<5>, 7, 11, 13), 3*5);
> + check(std::extents(std::cw<3>, 5, 7, 11, 13), dyn);
> + check(std::extents(3, 5, 7, 11, 13), dyn);
> + return true;
> + }
> +
> +int
> +main()
> +{
> + test_layout_unpadded_return_types<std::layout_left>();
> + static_assert(test_layout_unpadded_return_types<std::layout_left>());
> +
> + test_layout_unpadded_padding_value<std::layout_left>();
> + static_assert(test_layout_unpadded_padding_value<std::layout_left>());
> + return 0;
> +}
> +
> diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_neg.cc b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_neg.cc
> new file mode 100644
> index 00000000000..4f9aad81cb7
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_neg.cc
> @@ -0,0 +1,102 @@
> +// { dg-do compile { target c++26 } }
> +#include <mdspan>
> +
> +#include <vector>
> +
> +template<typename Layout, typename... Slices>
> + constexpr bool
> + check_slice_range(Slices... slices)
> + {
> + auto m = typename Layout::mapping<std::extents<int, 3, 5, 7>>{};
> + auto storage = std::vector<double>(m.required_span_size());
> + auto md = std::mdspan(storage.data(), m);
> +
> + auto submd = submdspan(md, slices...); // { dg-error "expansion of" }
> + (void) submd;
> + return true;
> + }
> +
> +template<typename Layout>
> + constexpr bool
> + test_int_under()
> + {
> + check_slice_range<Layout>(1, -1, 2); // { dg-error "expansion of" }
> + return true;
> + }
> +static_assert(test_int_under<std::layout_left>()); // { dg-error "expansion of" }
> +
> +template<typename Layout>
> + constexpr bool
> + test_int_over()
> + {
> + check_slice_range<Layout>(1, 5, 2); // { dg-error "expansion of" }
> + return true;
> + }
> +static_assert(test_int_over<std::layout_left>()); // { dg-error "expansion of" }
> +
> +template<typename Layout>
> + constexpr bool
> + test_tuple_under()
> + {
> + check_slice_range<Layout>(1, std::tuple{-1, 2}, 2); // { dg-error "expansion of" }
> + return true;
> + }
> +static_assert(test_tuple_under<std::layout_left>()); // { dg-error "expansion of" }
> +
> +template<typename Layout>
> + constexpr bool
> + test_tuple_reversed()
> + {
> + check_slice_range<Layout>(1, std::tuple{3, 2}, 2); // { dg-error "expansion of" }
> + return true;
> + }
> +static_assert(test_tuple_reversed<std::layout_left>()); // { dg-error "expansion of" }
> +
> +template<typename Layout>
> + constexpr bool
> + test_tuple_over()
> + {
> + check_slice_range<Layout>(1, std::tuple{0, 6}, 2); // { dg-error "expansion of" }
> + return true;
> + }
> +static_assert(test_tuple_over<std::layout_left>()); // { dg-error "expansion of" }
> +
> +template<typename Layout>
> + constexpr bool
> + test_strided_slice_zero()
> + {
> + check_slice_range<Layout>(1, std::strided_slice{1, 1, 0}, 2); // { dg-error "expansion of" }
> + return true;
> + }
> +static_assert(test_strided_slice_zero<std::layout_left>()); // { dg-error "expansion of" }
> +
> +template<typename Layout>
> + constexpr bool
> + test_strided_slice_offset_under()
> + {
> + check_slice_range<Layout>(1, std::strided_slice{-1, 1, 1}, 2); // { dg-error "expansion of" }
> + return true;
> + }
> +static_assert(test_strided_slice_offset_under<std::layout_left>()); // { dg-error "expansion of" }
> +
> +template<typename Layout>
> + constexpr bool
> + test_strided_slice_offset_over()
> + {
> + check_slice_range<Layout>(1, std::strided_slice{6, 0, 1}, 2); // { dg-error "expansion of" }
> + return true;
> + }
> +static_assert(test_strided_slice_offset_over<std::layout_left>()); // { dg-error "expansion of" }
> +
> +template<typename Layout>
> + constexpr bool
> + test_strided_slice_extent_over()
> + {
> + check_slice_range<Layout>(1, std::strided_slice{1, 5, 1}, 2); // { dg-error "expansion of" }
> + return true;
> + }
> +static_assert(test_strided_slice_extent_over<std::layout_left>()); // { dg-error "expansion of" }
> +
> +// { dg-prune-output "static assertion failed" }
> +// { dg-prune-output "__glibcxx_assert_fail" }
> +// { dg-prune-output "non-constant condition" }
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