On Tue, Nov 18, 2025 at 3:28 PM Luc Grosheintz <[email protected]> wrote:
> Implement submdspan_extents as described in P3663 and adds it to the std > module. > > There's one deviation from the standard. Doesn't (under all > circumstances) require: > > 0 <= begin[k] <= end[k] <= exts.extent(k) > > where the k-th slice range is [begin[k], end[k]). Instead, it requires > that the k-th slice ranges is contained in the k-th extent interval. If > the the slice range is empty, then that condition is always satisfied, > even if > > begin[k] == end[k] > exts.extent(k) > This was always intended to work, so please submit the LWG issue. > > The deviation is that we enforce the above inequality through > preconditions. This is analogous to what the standard requires if > begin[k] is a constant wrapper. > > PR libstdc++/110352 > > libstdc++-v3/ChangeLog: > > * include/std/mdspan (submdspan_extents): New function. > * src/c++23/std.cc.in: Add submdspan_extents. > * testsuite/23_containers/mdspan/int_like.h: Add StructuralInt. > * testsuite/23_containers/mdspan/submdspan/submdspan_extents.cc: > New test. > * > testsuite/23_containers/mdspan/submdspan/submdspan_extents_neg.cc: New test. > > Signed-off-by: Luc Grosheintz <[email protected]> > --- > libstdc++-v3/include/std/mdspan | 135 ++++++++++++++ > libstdc++-v3/src/c++23/std.cc.in | 5 +- > .../testsuite/23_containers/mdspan/int_like.h | 9 + > .../mdspan/submdspan/submdspan_extents.cc | 169 ++++++++++++++++++ > .../mdspan/submdspan/submdspan_extents_neg.cc | 48 +++++ > 5 files changed, 365 insertions(+), 1 deletion(-) > create mode 100644 > libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_extents.cc > create mode 100644 > libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_extents_neg.cc > > diff --git a/libstdc++-v3/include/std/mdspan > b/libstdc++-v3/include/std/mdspan > index 70656bcdd01..36e04f7e1b5 100644 > --- a/libstdc++-v3/include/std/mdspan > +++ b/libstdc++-v3/include/std/mdspan > @@ -1003,20 +1003,44 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION > __assert_valid_slices(const _Extents& __exts, const _Slices&... > __slices) > { > constexpr auto __rank = _Extents::rank(); > auto __impl = [&]<size_t... _Is>(index_sequence<_Is...>) > { > ((__assert_valid_slice(__extract_extent<_Is>(__exts), > __slices...[_Is])),...); > }; > __impl(make_index_sequence<__rank>()); > } > + > + 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; > + } > #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; > @@ -2678,20 +2702,131 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION > typename _MappingType::layout_type>; > > template<typename _MappingType, typename _AccessorType> > mdspan(const typename _AccessorType::data_handle_type&, const > _MappingType&, > const _AccessorType&) > -> mdspan<typename _AccessorType::element_type, > typename _MappingType::extents_type, > typename _MappingType::layout_type, _AccessorType>; > > #if __glibcxx_submdspan > + namespace __mdspan > + { > + template<typename _Slice> > + consteval bool > + __is_statically_empty_strided_slice() > + { > + if constexpr (__is_strided_slice<_Slice>) > + { > + using _ExtentType = typename _Slice::extent_type; > + return __detail::__integral_constant_like<_ExtentType> > We do not need __intergral_constant_like (and the expressions matching) for concepts, is_constant_wrapper should be sufficient. This is why the normalization is in place. > + && _ExtentType() == 0; > + } > + else > + return false; > + } > + > + template<typename _Slice> > + consteval bool > + __is_statically_sized_strided_slice() > + { > + if constexpr (__is_strided_slice<_Slice>) > + { > + using _ExtentType = typename _Slice::extent_type; > + using _StrideType = typename _Slice::stride_type; > + return __detail::__integral_constant_like<_ExtentType> > + && __detail::__integral_constant_like<_StrideType>; > Same here. > + } > + else > + return false; > + } > + > + template<typename _IndexType, size_t _Extent, typename _Slice> > + consteval size_t > + __deduce_static_slice_extent() > + { > + if constexpr (same_as<_Slice, full_extent_t>) > + return _Extent; > + else if constexpr (__is_statically_empty_strided_slice<_Slice>()) > + return 0; > + else if constexpr (__is_statically_sized_strided_slice<_Slice>()) > + return 1 + ((typename _Slice::extent_type{}) - 1) > + / (typename _Slice::stride_type{}); > + else > + return dynamic_extent; > + } > + > + template<size_t _K, typename _Extents, typename _Slice> > + constexpr typename _Extents::index_type > + __deduce_dynamic_slice_extent(const _Extents& __exts, _Slice > __slice) > + { > + using _IndexType = typename _Extents::index_type; > + 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, 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...>) > + { > + using _SubExtents = extents<_IndexType, > + (__mdspan::__deduce_static_slice_extent<_IndexType, > + _Extents...[__inv_map[_Indices]], > + _Slices...[__inv_map[_Indices]]>())...>; > + if constexpr (_SubExtents::rank_dynamic() == 0) > + return _SubExtents{}; > + else > + { > + using _StaticSubExtents = __mdspan::_StaticExtents< > + __mdspan::__static_extents<_SubExtents>()>; > + auto __create = [&]<size_t... _Is>(index_sequence<_Is...>) > + { > + constexpr auto __slice_idx = [__inv_map](size_t __i) > consteval > + { > + return > __inv_map[_StaticSubExtents::_S_dynamic_index_inv(__i)]; > + }; > + > + return _SubExtents{ > + > (__mdspan::__deduce_dynamic_slice_extent<__slice_idx(_Is)>( > + __exts, __slices...[__slice_idx(_Is)]))...}; > + }; > + constexpr auto __dyn_subrank = _SubExtents::rank_dynamic(); > + return __create(make_index_sequence<__dyn_subrank>()); > + } > + }; > + > + return __impl(make_index_sequence<__inv_map.size()>()); > + } > + } > + > + template<typename _IndexType, size_t... _Extents, typename... _Slices> > + requires (sizeof...(_Slices) == sizeof...(_Extents)) > + constexpr auto > + submdspan_extents(const extents<_IndexType, _Extents...>& __exts, > + _Slices... __raw_slices) > + { > + auto [...__slices] = submdspan_canonicalize_slices(__exts, > __raw_slices...); > I would much more prefer if we would not instantiate tuple here, as this is compile heavy, and completely unnecessary do: return __mdspan::__subextents(__exts, slice_cast(__slices)...); And insert __mdspan::__assert_valid_slices(__exts, __slices...) into __subextents. This will apply to any other calls to canonicalize_slices in later patches. > > + } > + > template<typename _IndexType, size_t... _Extents, typename... _Slices> > 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...); > diff --git a/libstdc++-v3/src/c++23/std.cc.in b/libstdc++-v3/src/c++23/ > std.cc.in > index 63b3d183bf5..c2a9293b05a 100644 > --- a/libstdc++-v3/src/c++23/std.cc.in > +++ b/libstdc++-v3/src/c++23/std.cc.in > @@ -1870,27 +1870,30 @@ export namespace std > using std::layout_right; > using std::layout_stride; > using std::default_accessor; > #if __glibcxx_aligned_accessor > using std::aligned_accessor; > #endif > using std::mdspan; > #if __glibcxx_padded_layouts > 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; > #endif > - // FIXME submdspan_extents, mdsubspan > + // 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/int_like.h > b/libstdc++-v3/testsuite/23_containers/mdspan/int_like.h > index 2f79b9dd3b5..b839be73ae9 100644 > --- a/libstdc++-v3/testsuite/23_containers/mdspan/int_like.h > +++ b/libstdc++-v3/testsuite/23_containers/mdspan/int_like.h > @@ -62,11 +62,20 @@ template<CustomIndexKind Kind, bool Copyable = false> > }; > > using IntLike = CustomIndexType<CustomIndexKind::Const>; > using ThrowingInt = CustomIndexType<CustomIndexKind::Throwing>; > using MutatingInt = CustomIndexType<CustomIndexKind::Mutating>; > using RValueInt = CustomIndexType<CustomIndexKind::RValue>; > > struct NotIntLike > { }; > > +struct StructuralInt > +{ > + constexpr > + operator int() const noexcept > + { return value; } > + > + int value; > +}; > + > #endif // TEST_MDSPAN_INT_LIKE_H > diff --git > a/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_extents.cc > b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_extents.cc > new file mode 100644 > index 00000000000..841910a77c8 > --- /dev/null > +++ > b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_extents.cc > @@ -0,0 +1,169 @@ > +// { dg-do run { target c++26 } } > +#include <mdspan> > + > +#include "../int_like.h" > +#include <testsuite_hooks.h> > + > +constexpr size_t dyn = std::dynamic_extent; > +constexpr auto all = std::full_extent; > + > +constexpr bool > +test_from_full_extent() > +{ > + auto exts = std::extents<int, 3, dyn, 7>{}; > + auto sub_exts = submdspan_extents(exts, 1, all, all); > + VERIFY(sub_exts.rank() == 2); > + VERIFY(sub_exts.static_extent(0) == dyn); > + VERIFY(sub_exts.extent(0) == exts.extent(1)); > + VERIFY(std::cmp_equal(sub_exts.static_extent(1), exts.extent(2))); > + return true; > +} > + > +template<template<typename, typename> typename Pair, template<int> > typename Cw> > + constexpr bool > + test_from_tuple() > + { > + auto exts = std::extents<int, 3, 5, 7>{}; > + auto s0 = Cw<1>{}; > + auto s1 = Pair{Cw<1>{}, Cw<2>{}}; > + auto s2 = Pair{Cw<1>{}, 4}; > + auto sub_exts = submdspan_extents(exts, s0, s1, s2); > + VERIFY(sub_exts.rank() == 2); > + VERIFY(sub_exts.static_extent(0) == size_t(get<1>(s1) - get<0>(s1))); > + VERIFY(sub_exts.static_extent(1) == dyn); > + VERIFY(std::cmp_equal(sub_exts.extent(1), get<1>(s2) - get<0>(s2))); > + return true; > + } > + > +template<template<int> typename Cw> > + constexpr bool > + test_from_tuple_all() > + { > + test_from_tuple<std::tuple, Cw>(); > + test_from_tuple<std::pair, Cw>(); > + return true; > + } > + > + > +template<typename Int> > + void > + test_from_int_like_as_scalar() > + { > + auto exts = std::extents<int, 3, 5>{}; > + auto sub_exts = submdspan_extents(exts, Int(1), std::tuple{1, 3}); > + VERIFY(sub_exts.rank() == 1); > + VERIFY(sub_exts.static_extent(0) == dyn); > + VERIFY(sub_exts.extent(0) == 2); > + } > + > +template<template<int> typename Cw> > + constexpr bool > + test_from_const_int() > + { > + auto exts = std::extents<int, 3, 5>{}; > + auto sub_exts = submdspan_extents(exts, Cw<1>{}, std::tuple{1, 3}); > + VERIFY(sub_exts.rank() == 1); > + VERIFY(sub_exts.static_extent(0) == dyn); > + VERIFY(sub_exts.extent(0) == 2); > + return true; > + } > + > +template<typename Int> > + constexpr bool > + test_from_int_like_in_tuple() > + { > + auto exts = std::extents<int, 3, 5>{}; > + auto sub_exts = submdspan_extents(exts, Int(1), std::tuple{Int(1), > Int(3)}); > + VERIFY(sub_exts.rank() == 1); > + VERIFY(sub_exts.static_extent(0) == dyn); > + VERIFY(sub_exts.extent(0) == 2); > + return true; > + } > + > +template<template<int> typename Cw> > + constexpr bool > + test_from_strided_slice() > + { > + auto exts = std::extents<int, 5, 7, 11>{}; > + { > + auto s0 = 1; > + auto s1 = std::strided_slice{0, 0, 0}; > + auto s2 = std::strided_slice{1, Cw<0>{}, 0}; > + auto sub_exts = submdspan_extents(exts, s0, s1, s2); > + VERIFY(sub_exts.rank() == 2); > + VERIFY(sub_exts.static_extent(0) == dyn); > + VERIFY(sub_exts.extent(0) == 0); > + VERIFY(sub_exts.static_extent(1) == 0); > + } > + > + { > + auto s0 = 1; > + auto s1 = std::strided_slice{0, 2, Cw<1>{}}; > + auto s2 = std::strided_slice{1, Cw<2>{}, 1}; > + auto sub_exts = submdspan_extents(exts, s0, s1, s2); > + VERIFY(sub_exts.rank() == 2); > + VERIFY(sub_exts.static_extent(0) == dyn); > + VERIFY(sub_exts.extent(0) == 2); > + VERIFY(sub_exts.static_extent(1) == dyn); > + VERIFY(sub_exts.extent(1) == 2); > + } > + > + { > + // selected = 1 x [1, 3] x [1, 4, 7, 10] > + auto s0 = 1; > + auto s1 = std::strided_slice{1, Cw<4>{}, 2}; > + auto s2 = std::strided_slice{1, Cw<10>{}, Cw<3>{}}; > + auto sub_exts = submdspan_extents(exts, s0, s1, s2); > + VERIFY(sub_exts.rank() == 2); > + VERIFY(sub_exts.static_extent(0) == dyn); > + VERIFY(sub_exts.extent(0) == 2); > + VERIFY(sub_exts.static_extent(1) == 4); > + } > + > + { > + // selected = [0, 2] x [1, 3] x [0, 3, 6] > + auto s0 = std::strided_slice(0, 3, 2); > + auto s1 = std::strided_slice(1, 4, 2); > + auto s2 = std::strided_slice(0, 7, 3); > + auto sub_exts = submdspan_extents(exts, s0, s1, s2); > + VERIFY(sub_exts.rank() == 3); > + VERIFY(sub_exts.extent(0) == 2); > + VERIFY(sub_exts.extent(1) == 2); > + VERIFY(sub_exts.extent(2) == 3); > + } > + return true; > + } > + > +template<int Value> > + using CW = std::constant_wrapper<Value, int>; > + > +template<int Value> > + using IC = std::integral_constant<int, Value>; > + > +constexpr bool > +test_all() > +{ > + test_from_full_extent(); > + test_from_tuple_all<CW>(); > + test_from_tuple_all<IC>(); > + test_from_const_int<CW>(); > + test_from_const_int<IC>(); > + test_from_strided_slice<CW>(); > + test_from_strided_slice<IC>(); > + test_from_int_like_in_tuple<StructuralInt>(); > + return true; > +} > + > +int > +main() > +{ > + test_all(); > + static_assert(test_all()); > + > + test_from_int_like_as_scalar<CustomIndexType<CustomIndexKind::Const, > true>>(); > + test_from_int_like_as_scalar<CustomIndexType<CustomIndexKind::Throwing, > true>>(); > + test_from_int_like_as_scalar<CustomIndexType<CustomIndexKind::Mutating, > true>>(); > + test_from_int_like_as_scalar<CustomIndexType<CustomIndexKind::RValue, > true>>(); > + test_from_int_like_as_scalar<StructuralInt>(); > + return 0; > +} > diff --git > a/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_extents_neg.cc > b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_extents_neg.cc > new file mode 100644 > index 00000000000..cf27c0c7e4d > --- /dev/null > +++ > b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_extents_neg.cc > @@ -0,0 +1,48 @@ > +// { dg-do compile { target c++26 } } > +#include <mdspan> > + > +#include <cstdint> > + > +struct NotASlice > +{ }; > + > +constexpr bool > +test_unrelated_stride_type() > +{ > + auto exts = std::extents(3, 5, 7); > + auto sub_exts = submdspan_extents(exts, 1, NotASlice{}, 2); // { > dg-error "required from" } > + return true; > +} > +static_assert(test_unrelated_stride_type()); > + > +constexpr bool > +test_invalid_stride_zero() > +{ > + auto exts = std::extents(3, 5, 7); > + auto s = std::strided_slice{0, 1, 0}; > + auto sub_exts = submdspan_extents(exts, 1, s, 2); // { dg-error > "expansion of" } > + return true; > +} > +static_assert(test_invalid_stride_zero()); > + > +template<typename Slice> > +constexpr bool > +test_out_of_bounds(const Slice& slice) > +{ > + auto exts = std::extents<uint16_t, 3, 5, 7>{}; > + auto sub_exts = submdspan_extents(exts, 1, slice, 2); // { dg-error > "expansion of" } > + return true; > +} > +static_assert(test_out_of_bounds(std::strided_slice{0, 6, 1})); // { > dg-error "expansion of" } > +static_assert(test_out_of_bounds(std::strided_slice{0, 7, 2})); // { > dg-error "expansion of" } > +static_assert(test_out_of_bounds(std::strided_slice{1, 6, 1})); // { > dg-error "expansion of" } > +static_assert(test_out_of_bounds(std::strided_slice{1, 6, 2})); // { > dg-error "expansion of" } > +static_assert(test_out_of_bounds(std::tuple{1, 6})); // { > dg-error "expansion of" } > +static_assert(test_out_of_bounds(std::tuple{std::cw<1>, std::cw<6>})); // > { dg-error "expansion of" } > +static_assert(test_out_of_bounds(std::strided_slice{-1, 2, 1})); // { > dg-error "expansion of" } > +static_assert(test_out_of_bounds(std::tuple{-1, 2})); // { > dg-error "expansion of" } > + > +// { dg-prune-output "cannot decompose class type 'NotASlice'" } > +// { dg-prune-output "static assertion failed" } > +// { dg-prune-output "__glibcxx_assert_fail" } > +// { dg-prune-output "non-constant condition" } > -- > 2.51.2 > >
