On Fri, 10 Jul 2026 at 16:39, Tomasz Kamiński <[email protected]> wrote:
>
> This add an assertions checking if the combination of strides
> and extents passed to layout_stride::mapping is unique.
>
> To avoid excessive performance impact, in non-debug mode only
> checks that are are O(rank) (i.e. similar cost as operator())
> are performed. To achieve that, for the ranks greater or equal,
> we perform two separate checks:
> * required_span_size() > mdspan::__size(_M_extents) - this is
> required by not sufficient condition, that is O(rank) and
"a necessary but not sufficient condition"
> is always performed.
> * building an permutation using insertion-sort based algorithm,
"a permutation"
> this is O(rank^2) and is performed only in debug modes.
>
> The check for non-zero stride values (performed for all non-zero
> ranks) is done only if the size of the multidimensional index space
> is not empty. This follows the resolution of LWG4603, and provides
> consistent behavior for constructing layout_stride mapping from
> layout_left/right directly or from strides extracted from it.
>
> libstdc++-v3/ChangeLog:
>
> * include/std/mdspan (layout_stride::mapping::_M_check_unique):
> Define.
> (layout_stride::mapping(const extents_type&, span<....>):
> Add __glibcxx_assert on _M_check_unique.
> * testsuite/23_containers/mdspan/layouts/stride_neg.cc:
> Test for the new added assertions.
> ---
> v3 removes the separate stride_unique.cc test, and uses dg-bogus
> instead (Jakub's no_specializations patch inspired me to do that).
>
> libstdc++-v3/include/std/mdspan | 75 ++++++++++++++++++-
> .../mdspan/layouts/stride_neg.cc | 72 +++++++++++++++++-
> 2 files changed, 142 insertions(+), 5 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/mdspan b/libstdc++-v3/include/std/mdspan
> index 23304e71b9d..30a079ca1ba 100644
> --- a/libstdc++-v3/include/std/mdspan
> +++ b/libstdc++-v3/include/std/mdspan
> @@ -1893,6 +1893,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> for (size_t __i = 0; __i < extents_type::rank(); ++__i)
> _M_strides[__i] =
>
> __mdspan::__index_type_cast<index_type>(as_const(__strides[__i]));
> +
> + __glibcxx_assert(_M_check_unique());
> }
>
> template<typename _OIndexType>
> @@ -2027,6 +2029,75 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> }
>
> private:
> + using _Strides = typename __array_traits<index_type,
> + extents_type::rank()>::_Type;
> +
> + constexpr bool
> + _M_check_unique() const
> + {
> + constexpr size_t __rank = extents_type::rank();
> + if constexpr (__rank == 0)
> + return true;
> + // _GLIBCXX_RESOLVE_LIB_DEFECTS
> + // 4603. `layout_stride` should accept zero strides for empty
> `extents`
> + else if (__mdspan::__empty(_M_extents))
> + return true;
> + else if constexpr (__rank == 1)
> + return _M_strides[0] > 0;
> + else if (__mdspan::__contains_zero<const index_type,
> __rank>(_M_strides))
> + return false;
> + else if constexpr (__rank == 2)
> + return (_M_strides[1] >= _M_strides[0] * _M_extents.extent(0))
> + || (_M_strides[0] >= _M_strides[1] * _M_extents.extent(1));
> + else
> + {
> + // This is necessary, but not sufficient condition for uniquness.
"uniqueness"
> + // It requires computation in range of call of operator(), so
> check
> + // if first, also in non-debug mode.
> + if (required_span_size() < __mdspan::__size(_M_extents))
> + return false;
> +
> + // Checking if pertmuation of strides exists is O(rank^2) so
> enabled
"permutation"
but I'm not sure what this comment is saying, are we checking for a
non-unique permutation of strides? An invalid permutation of strides?
> + // only in debug mode.
> +#ifdef _GLIBCXX_DEBUG
> + size_t __perm[__rank];
> + for (size_t __i = 0; __i < __rank; ++__i)
> + __perm[__i] = __i;
> +
> + auto __popmin = [&](size_t __from)
> + {
> + size_t __min = __perm[__rank - 1];
> + for (size_t __i = __rank - 1; __i > __from; --__i)
> + {
> + const size_t __cur = __perm[__i - 1];
> + const auto __res = _M_strides[__cur] <=> _M_strides[__min];
> + // Equal strides are allowed only for extent one, so apply
> it first.
> + if (__res < 0 || (__res == 0 && _M_extents.extent(__cur) ==
> 1))
> + {
> + __perm[__i] = __min;
> + __min = __cur;
> + }
> + else
> + __perm[__i] = __cur;
> + }
> + __perm[__from] = __min;
> + return __min;
> + };
> +
> + // We use eager insertion-sort based algorithm to permute the
> __perm.
> + size_t __prev = __popmin(0);
> + for (size_t __i = 1; __i < __rank; ++__i)
> + {
> + const size_t __next = __popmin(__i);
> + if (_M_strides[__next] < _M_strides[__prev] *
> _M_extents.extent(__prev))
> + return false;
> + __prev = __next;
> + }
> +#endif
> + return true;
> + }
> + }
> +
> #if __glibcxx_submdspan
> template<typename... _Slices>
> requires (extents_type::rank() == sizeof...(_Slices))
> @@ -2047,8 +2118,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> }
> #endif
>
> - using _Strides = typename __array_traits<index_type,
> - extents_type::rank()>::_Type;
> [[no_unique_address]] extents_type _M_extents;
> [[no_unique_address]] _Strides _M_strides;
> };
> @@ -3433,7 +3502,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> __mdspan::__check_inrange_index(__ext, __slice.offset);
> else if constexpr (__is_constant_wrapper_v<typename
> _Slice::extent_type>)
> {
> - __mdspan::__check_inrange_index(__ext, __slice.offset);
> + __mdspan::__check_inrange_index(__ext, __slice.offset);
> if constexpr (__is_constant_wrapper_v<typename
> _Slice::stride_type>)
> static_assert(_Slice::stride_type::value > 0);
> else
> diff --git
> a/libstdc++-v3/testsuite/23_containers/mdspan/layouts/stride_neg.cc
> b/libstdc++-v3/testsuite/23_containers/mdspan/layouts/stride_neg.cc
> index 153560bc82f..692caeac812 100644
> --- a/libstdc++-v3/testsuite/23_containers/mdspan/layouts/stride_neg.cc
> +++ b/libstdc++-v3/testsuite/23_containers/mdspan/layouts/stride_neg.cc
> @@ -1,7 +1,5 @@
> // { dg-do compile { target c++23 } }
> #include <mdspan>
> -
> -#include "../layout_traits.h"
> #include <cstdint>
>
> constexpr size_t dyn = std::dynamic_extent;
> @@ -27,5 +25,75 @@ test_stride_negative()
> }
> static_assert(test_stride_negative()); // { dg-error "expansion of" }
>
> +
> +using std::array;
> +template<size_t Rank>
> + using mapping = std::layout_stride::mapping<std::dextents<size_t, Rank>>;
> +
> +constexpr auto m0 = mapping<0>(array<int, 0>{}, array<int, 0>{}); // {
> dg-bogus "expansion of" }
> +
> +// Zero strides allowed for empty mappings
> +constexpr auto m00 = mapping<1>(array{0}, array{0}); // { dg-bogus
> "expansion of" }
> +constexpr auto m01 = mapping<2>(array{0, 1}, array{3, 0}); // { dg-bogus
> "expansion of" }
> +constexpr auto m02 = mapping<2>(array{0, 1}, array{0, 0}); // { dg-bogus
> "expansion of" }
> +constexpr auto m03 = mapping<3>(array{0, 1, 2}, array{3, 0, 0}); // {
> dg-bogus "expansion of" }
> +constexpr auto m04 = mapping<3>(array{0, 1, 2}, array{3, 3, 0}); // {
> dg-bogus "expansion of" }
> +constexpr auto m05 = mapping<4>(array{0, 1, 2, 3}, array{5, 3, 0, 2}); // {
> dg-bogus "expansion of" }
> +
> +// Otherwise leads to non-unique mappings.
> +constexpr auto m06 = mapping<1>(array{1}, array{0}); // {
> dg-error "expansion of" }
> +constexpr auto m07 = mapping<2>(array{1, 2}, array{3, 0}); // {
> dg-error "expansion of" }
> +constexpr auto m08 = mapping<3>(array{1, 2, 3}, array{3, 3, 0}); // {
> dg-error "expansion of" }
> +constexpr auto m09 = mapping<4>(array{1, 2, 3, 5}, array{5, 3, 0, 2}); // {
> dg-error "expansion of" }
> +
> +// required_span_size is smaller size of multidimensional index space
> +constexpr auto m10 = mapping<2>(array{3, 4}, array{2, 3}); // { dg-error
> "expansion of" }
> +constexpr auto m11 = mapping<2>(array{3, 4}, array{1, 2}); // { dg-error
> "expansion of" }
> +constexpr auto m12 = mapping<3>(array{3, 4, 5}, array{1, 3, 11}); // {
> dg-error "expansion of" }
> +constexpr auto m13 = mapping<3>(array{3, 4, 5}, array{1, 2, 12}); // {
> dg-error "expansion of" }
> +constexpr auto m14 = mapping<4>(array{3, 4, 5, 6}, array{1, 3, 12, 50}); //
> { dg-error "expansion of" }
> +constexpr auto m15 = mapping<4>(array{3, 4, 5, 6}, array{1, 3, 11, 60}); //
> { dg-error "expansion of" }
> +constexpr auto m16 = mapping<4>(array{3, 4, 5, 6}, array{1, 2, 12, 60}); //
> { dg-error "expansion of" }
> +
> +// Passes required_span_size check (smaller and bigger stride), detected
> only in debug mode
> +constexpr auto m17 = mapping<3>(array{3, 4, 5}, array{1, 2, 25}); // {
> dg-error "expansion of" "" { target debug_mode } }
> +constexpr auto m18 = mapping<3>(array{3, 4, 5}, array{1, 5, 12}); // {
> dg-error "expansion of" "" { target debug_mode } }
> +constexpr auto m19 = mapping<4>(array{3, 4, 5, 6}, array{1, 2, 12, 100}); //
> { dg-error "expansion of" "" { target debug_mode } }
> +constexpr auto m20 = mapping<4>(array{3, 4, 5, 6}, array{1, 5, 12, 60}); //
> { dg-error "expansion of" "" { target debug_mode } }
> +
> +// Equal indicies allowed if extents are 1
> +constexpr auto m21 = mapping<2>(array{5, 1}, array{1, 1}); // { dg-bogus
> "expansion of" }
> +constexpr auto m22 = mapping<3>(array{1, 5, 1}, array{1, 1, 1}); // {
> dg-bogus "expansion of" }
> +constexpr auto m23 = mapping<4>(array{1, 1, 5, 1}, array{1, 1, 1, 1}); // {
> dg-bogus "expansion of" }
> +constexpr auto m24 = mapping<5>(array{3, 4, 1, 1, 5}, array{3, 10, 3, 3,
> 45}); // { dg-bogus "expansion of" }
> +constexpr auto m25 = mapping<5>(array{3, 4, 1, 1, 5}, array{3, 10, 3, 10,
> 45}); // { dg-bogus "expansion of" }
> +constexpr auto m26 = mapping<5>(array{3, 4, 1, 1, 5}, array{3, 10, 10, 3,
> 45}); // { dg-bogus "expansion of" }
> +constexpr auto m27 = mapping<5>(array{3, 4, 1, 1, 5}, array{3, 10, 10, 10,
> 45}); // { dg-bogus "expansion of" }
> +
> +// Only some permutations are valid, invalid permutations detected only in
> debug mode.
> +
> +// Unique if stride 3 corresponds to extent 3
> +constexpr auto m28 = mapping<2>(array{3, 4}, array{3, 10}); // { dg-bogus
> "expansion of" }
> +constexpr auto m29 = mapping<2>(array{3, 4}, array{10, 3}); // { dg-error
> "expansion of" "" { target debug_mode } }
> +constexpr auto m30 = mapping<2>(array{4, 3}, array{10, 3}); // { dg-bogus
> "expansion of" }
> +constexpr auto m31 = mapping<2>(array{4, 3}, array{3, 10}); // { dg-error
> "expansion of" "" { target debug_mode } }
> +
> +// Unique if stride 11 corresponds to extent 3, as 11 * 3 < 35, and 2 * 5 <
> 11
> +constexpr auto m32 = mapping<3>(array{3, 4, 5}, array{11, 2, 35}); // {
> dg-bogus "expansion of" }
> +constexpr auto m33 = mapping<3>(array{3, 4, 5}, array{11, 35, 2}); // {
> dg-bogus "expansion of" }
> +constexpr auto m34 = mapping<3>(array{3, 4, 5}, array{2, 11, 35}); // {
> dg-error "expansion of" "" { target debug_mode } }
> +constexpr auto m35 = mapping<3>(array{3, 4, 5}, array{35, 2, 11}); // {
> dg-error "expansion of" "" { target debug_mode } }
> +constexpr auto m36 = mapping<3>(array{5, 4, 3}, array{2, 35, 11}); // {
> dg-bogus "expansion of" }
> +constexpr auto m37 = mapping<3>(array{5, 4, 3}, array{2, 11, 35}); // {
> dg-error "expansion of" "" { target debug_mode } }
> +constexpr auto m38 = mapping<3>(array{5, 3, 4}, array{35, 11, 2}); // {
> dg-bogus "expansion of" }
> +constexpr auto m39 = mapping<3>(array{3, 5, 4}, array{35, 2, 11}); // {
> dg-error "expansion of" "" { target debug_mode } }
> +
> +// Unique for stride -> extent mapping: 3 * 5 < 16, 15 * 4 < 65, 65 * 3 < 200
> +constexpr auto m40 = mapping<4>(array{3, 4, 5, 6}, array{65, 16, 3, 200});
> // { dg-bogus "expansion of" }
> +constexpr auto m41 = mapping<4>(array{3, 4, 5, 6}, array{3, 16, 65, 200});
> // { dg-error "expansion of" "" { target debug_mode } }
> +constexpr auto m42 = mapping<4>(array{3, 4, 5, 6}, array{200, 65, 3, 16});
> // { dg-error "expansion of" "" { target debug_mode } }
> +constexpr auto m43 = mapping<4>(array{5, 3, 6, 4}, array{3, 65, 200, 16});
> // { dg-bogus "expansion of" }
> +constexpr auto m44 = mapping<4>(array{5, 3, 6, 4}, array{3, 16, 200, 65});
> // { dg-error "expansion of" "" { target debug_mode } }
> +
> // { dg-prune-output "non-constant condition for static assertion" }
> // { dg-prune-output "__glibcxx_assert_fail()" }
> --
> 2.54.0
>
