On Mon, Jul 13, 2026 at 12:51 PM Jonathan Wakely <[email protected]> wrote:
> 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? > I mean checking if permutation described in https://eel.is/c++draft/mdspan.layout#stride.cons-4.3 exists. Not sure how to comment that. > > > + // 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 > > > >
