[PATCH v1 4/4] libstdc++: Implement submdspan_extents. [PR110352]
Luc Grosheintz
luc.grosheintz@gmail.com
Thu Oct 2 09:27:17 GMT 2025
Implement submdspan_extents as described in N5014 and adds it to the std
module.
The implementation contains a casting or canonicalization layer.
Similar, but different to the one described in P3663. It's purpose is
twofold:
- Reduce the number of template instantiations and isolate the
complexity of dealing with user-defined integer like types to this
layer.
- Handle special user-defined types that only convert to index_type
as rvalues, etc.
The rules for canonicalization are:
- collapsing slices, i.e. those that convert to index_type, are
canonicalized as follows:
+ integral-constant-like objects are canonicalized to the corresponding
integral_constant<index_type, ...> object,
+ everything else is canonicalized via
static_cast<index_type>(std::move(...)),
- for any strided_slice it's the analogous strided_slice with all
three members canonicalized,
- index-pair-like objects are converted to std::tuple with both
elements canonicalized,
- anything convertable to full_extent_t is canonicalized to
full_extent.
PR libstdc++/110352
libstdc++-v3/ChangeLog:
* include/std/mdspan (submdspan_extents): New function.
* 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 <luc.grosheintz@gmail.com>
---
libstdc++-v3/include/std/mdspan | 276 ++++++++++++++++++
.../testsuite/23_containers/mdspan/int_like.h | 9 +
.../mdspan/submdspan/submdspan_extents.cc | 159 ++++++++++
.../mdspan/submdspan/submdspan_extents_neg.cc | 66 +++++
4 files changed, 510 insertions(+)
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 cef6d625982..a88d4f8ff76 100644
--- a/libstdc++-v3/include/std/mdspan
+++ b/libstdc++-v3/include/std/mdspan
@@ -40,6 +40,7 @@
#if __cplusplus > 202302L
#include <bits/align.h>
+#include <tuple>
#endif
#define __glibcxx_want_mdspan
@@ -368,6 +369,123 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
[[no_unique_address]] _Mapping mapping = _Mapping();
size_t offset{};
};
+
+ namespace __mdspan
+ {
+ template<typename _Tp>
+ constexpr _Tp
+ __unwrap_constant(_Tp __value)
+ { return __value; }
+
+ template<__detail::__integral_constant_like _Tp>
+ constexpr auto
+ __unwrap_constant(_Tp)
+ { return _Tp::value; }
+
+ template<typename _Tp, typename _IndexType>
+ concept __index_pair_like = __pair_like<_Tp>
+ && convertible_to<tuple_element_t<0,_Tp>, _IndexType>
+ && convertible_to<tuple_element_t<1,_Tp>, _IndexType>;
+
+ template<typename _Tp>
+ inline constexpr bool __is_strided_slice = false;
+
+ template<typename _OffsetType, typename _ExtentType, typename _StrideType>
+ inline constexpr bool __is_strided_slice<strided_slice<_OffsetType,
+ _ExtentType, _StrideType>> = true;
+
+ template<typename _IndexType, typename... _Slices>
+ consteval auto __submdspan_rank()
+ {
+ 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 = __submdspan_rank<_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, typename _Slice>
+ constexpr auto
+ __slice_cast(_Slice&& __slice)
+ {
+ if constexpr (convertible_to<_Slice, _IndexType>
+ && __detail::__integral_constant_like<_Slice>)
+ return integral_constant<_IndexType, _IndexType{_Slice::value}>{};
+ else if constexpr (convertible_to<_Slice, _IndexType>)
+ return __index_type_cast<_IndexType>(std::move(__slice));
+ else if constexpr (__index_pair_like<_Slice, _IndexType>)
+ {
+ auto [__begin, __end] = std::move(__slice);
+ return std::tuple{__slice_cast<_IndexType>(std::move(__begin)),
+ __slice_cast<_IndexType>(std::move(__end))};
+ }
+ else if constexpr (__is_strided_slice<_Slice>)
+ return strided_slice{
+ __slice_cast<_IndexType>(std::move(__slice.offset)),
+ __slice_cast<_IndexType>(std::move(__slice.extent)),
+ __slice_cast<_IndexType>(std::move(__slice.stride))};
+ else
+ return full_extent;
+ }
+
+ template<typename _IndexType, typename _Slice>
+ constexpr _IndexType
+ __slice_begin(_Slice __slice)
+ {
+ if constexpr (convertible_to<_Slice, _IndexType>)
+ return __slice;
+ else if constexpr (__index_pair_like<_Slice, _IndexType>)
+ return get<0>(__slice);
+ else if constexpr (__is_strided_slice<_Slice>)
+ return __slice.offset;
+ else
+ return 0; // full_extent
+ }
+
+ template<size_t _K, typename _Extents, typename _Slice>
+ constexpr typename _Extents::index_type
+ __slice_end(_Extents __exts, _Slice __slice)
+ {
+ using _IndexType = typename _Extents::index_type;
+ if constexpr (convertible_to<_Slice, _IndexType>)
+ return __unwrap_constant(__slice) + 1;
+ else if constexpr (__index_pair_like<_Slice, _IndexType>)
+ return get<1>(__slice);
+ else if constexpr (__is_strided_slice<_Slice>)
+ return __unwrap_constant(__slice.offset)
+ + __unwrap_constant(__slice.extent);
+ else
+ return __exts.extent(_K); // full_extent
+ }
+
+ template<size_t _K, typename _Extents, typename _Slice>
+ constexpr typename _Extents::index_type
+ __slice_size(const _Extents& __exts, _Slice __slice)
+ {
+ using _IndexType = typename _Extents::index_type;
+ if constexpr (convertible_to<_Slice, _IndexType>)
+ return 1;
+ else if constexpr (__index_pair_like<_Slice, _IndexType>)
+ return __unwrap_constant(get<1>(__slice))
+ - __unwrap_constant(get<0>(__slice));
+ else if constexpr (__is_strided_slice<_Slice>)
+ return __unwrap_constant(__slice.extent);
+ else
+ return __exts.extent(_K);
+ }
+ }
#endif
template<typename _IndexType, size_t... _Extents>
@@ -2492,6 +2610,164 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
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>
+ && _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>;
+ }
+ else
+ return false;
+ }
+
+ template<typename _IndexType, typename _Slice>
+ consteval bool
+ __is_statically_sized_tuple()
+ {
+ if constexpr (__index_pair_like<_Slice, _IndexType>)
+ return __detail::__integral_constant_like<tuple_element_t<0, _Slice>>
+ && __detail::__integral_constant_like<tuple_element_t<1, _Slice>>;
+ else
+ return false;
+ }
+
+
+ template<typename _IndexType, size_t _Extent, typename _Slice>
+ consteval size_t
+ __deduce_static_slice_extent()
+ {
+ if constexpr (is_convertible_v<_Slice, full_extent_t>)
+ return _Extent;
+ else if constexpr (__is_statically_sized_tuple<_IndexType, _Slice>())
+ return __unwrap_constant(tuple_element_t<1, _Slice>())
+ - __unwrap_constant(tuple_element_t<0, _Slice>());
+ else if constexpr (__is_statically_empty_strided_slice<_Slice>())
+ return 0;
+ else if constexpr (__is_statically_sized_strided_slice<_Slice>())
+ return 1 + (__unwrap_constant(typename _Slice::extent_type()) - 1)
+ / __unwrap_constant(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)
+ {
+ if constexpr (__is_strided_slice<_Slice>)
+ return __slice.extent == 0 ? 0 :
+ 1 + (__unwrap_constant(__slice.extent) - 1)
+ / __unwrap_constant(__slice.stride);
+ else
+ return __slice_size<_K>(__exts, __slice);
+ }
+
+ template<typename _Slice, typename _IndexType>
+ concept __valid_slice_type = convertible_to<_Slice, _IndexType>
+ + __index_pair_like<_Slice, _IndexType>
+ + is_convertible_v<_Slice, full_extent_t>
+ + __is_strided_slice<_Slice> == 1;
+
+ template<size_t _K, typename _Extents, typename _Slice>
+ constexpr bool
+ __is_valid_slice(const _Extents& __exts, const _Slice& __slice)
+ {
+ using _IndexType = typename _Extents::index_type;
+ if constexpr (__is_strided_slice<_Slice>)
+ if (__slice.extent != 0 && __slice.stride == 0)
+ return false;
+
+ auto __begin = __slice_begin<_IndexType>(__slice);
+ auto __end = __slice_end<_K>(__exts, __slice);
+ return std::cmp_less_equal(__begin, __end)
+ && std::cmp_less_equal(__end, __exts.extent(_K));
+ }
+
+ template<typename _Extents, typename... _Slices>
+ constexpr bool
+ __all_valid_slices(const _Extents& __exts, const _Slices&... __slices)
+ {
+ auto __impl = [&]<size_t... _Is>(index_sequence<_Is...>){
+ return (__is_valid_slice<_Is>(__exts, __slices...[_Is]) && ...);
+ };
+ return __impl(make_index_sequence<sizeof...(_Slices)>());
+ }
+
+
+ template<typename _IndexType, size_t... _Extents, typename... _Slices>
+ requires (sizeof...(_Slices) == sizeof...(_Extents))
+ constexpr auto __submdspan_extents(
+ const extents<_IndexType, _Extents...>& __exts, _Slices... __slices)
+ {
+ __glibcxx_assert(__mdspan::__all_valid_slices(__exts, __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... __slices)
+ {
+ static_assert((__mdspan::__valid_slice_type<_Slices, _IndexType> && ...));
+ return __mdspan::__submdspan_extents(__exts,
+ __mdspan::__slice_cast<_IndexType>(std::move(__slices))...);
+ }
+#endif
+
_GLIBCXX_END_NAMESPACE_VERSION
}
#endif
diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/int_like.h b/libstdc++-v3/testsuite/23_containers/mdspan/int_like.h
index e9172c13455..67c759245f9 100644
--- a/libstdc++-v3/testsuite/23_containers/mdspan/int_like.h
+++ b/libstdc++-v3/testsuite/23_containers/mdspan/int_like.h
@@ -60,4 +60,13 @@ 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..a20509a7fc6
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_extents.cc
@@ -0,0 +1,159 @@
+// { dg-do run { target c++26 } }
+#include <mdspan>
+#include <tuple>
+
+#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<int> typename Cw>
+ constexpr bool
+ test_from_tuple()
+ {
+ auto exts = std::extents<int, 3, 5, 7>{};
+ auto s0 = Cw<1>{};
+ auto s1 = std::tuple{Cw<1>{}, Cw<2>{}};
+ auto s2 = std::tuple{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<typename Int>
+ bool
+ 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);
+ return true;
+ }
+
+template<typename Int>
+ bool
+ test_from_const_int()
+ {
+ 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);
+ 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<CW>();
+ test_from_tuple<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<IntLike>();
+ test_from_int_like_as_scalar<MutatingInt>();
+ test_from_int_like_as_scalar<ThrowingInt>();
+ test_from_int_like_as_scalar<RValueInt>();
+ 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..dca294c185e
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_extents_neg.cc
@@ -0,0 +1,66 @@
+// { dg-do compile { target c++26 } }
+#include <mdspan>
+
+#include <cstdint>
+
+struct NotASlice
+{ };
+
+struct AmbiguousSlice
+{
+ constexpr
+ operator int() const
+ { return 0; }
+
+ constexpr
+ operator std::full_extent_t() const
+ { return std::full_extent; }
+};
+
+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_ambiguous_stride_type()
+{
+ auto exts = std::extents(3, 5, 7);
+ auto sub_exts = submdspan_extents(exts, 1, AmbiguousSlice{}, 2); // { dg-error "required from" }
+ return true;
+}
+static_assert(test_ambiguous_stride_type());
+
+constexpr bool
+test_invalid_stride_zero()
+{
+ auto exts = std::extents(3, 5, 7);
+ auto sub_exts = submdspan_extents(exts, 1, std::strided_slice{0, 1, 0}, 2); // { dg-error "expansion of" }
+ return true;
+}
+static_assert(test_invalid_stride_zero());
+
+template<typename Slice>
+constexpr bool
+test_out_of_bounds_selection(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_selection(std::strided_slice{0, 6, 1})); // { dg-error "expansion of" }
+static_assert(test_out_of_bounds_selection(std::strided_slice{0, 7, 2})); // { dg-error "expansion of" }
+static_assert(test_out_of_bounds_selection(std::strided_slice{1, 6, 1})); // { dg-error "expansion of" }
+static_assert(test_out_of_bounds_selection(std::strided_slice{1, 6, 2})); // { dg-error "expansion of" }
+static_assert(test_out_of_bounds_selection(std::tuple{1, 6})); // { dg-error "expansion of" }
+static_assert(test_out_of_bounds_selection(std::tuple{std::cw<1>, std::cw<6>})); // { dg-error "expansion of" }
+static_assert(test_out_of_bounds_selection(std::strided_slice{-1, 2, 1})); // { dg-error "expansion of" }
+static_assert(test_out_of_bounds_selection(std::tuple{-1, 2})); // { dg-error "expansion of" }
+
+// { dg-prune-output "static assertion failed" }
+// { dg-prune-output "__glibcxx_assert_fail" }
+// { dg-prune-output "non-constant condition" }
--
2.50.1
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