[gcc r17-2356] libstdc++: Assert that produced layout_stride::mapping is unique.
Tomasz Kaminski
tkaminsk@gcc.gnu.org
Mon Jul 13 13:26:14 GMT 2026
https://gcc.gnu.org/g:1a4d8dc0d2dc045bbb93654ae66fb80276dc6690
commit r17-2356-g1a4d8dc0d2dc045bbb93654ae66fb80276dc6690
Author: Tomasz Kamiński <tkaminsk@redhat.com>
Date: Tue Jul 7 10:27:33 2026 +0200
libstdc++: Assert that produced layout_stride::mapping is unique.
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
a necessary but not sufficient condition, that is O(rank) and
is always performed.
* building a permutation using insertion-sort based algorithm,
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.
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>
Diff:
---
libstdc++-v3/include/std/mdspan | 73 +++++++++++++++++++++-
.../23_containers/mdspan/layouts/stride_neg.cc | 72 ++++++++++++++++++++-
2 files changed, 141 insertions(+), 4 deletions(-)
diff --git a/libstdc++-v3/include/std/mdspan b/libstdc++-v3/include/std/mdspan
index 23304e71b9df..025ce4d75aba 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 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 for a valid permutation of strides is O(rank^2) so
+ // so enabled 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;
};
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 153560bc82ff..692caeac8123 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()" }
More information about the Libstdc++-cvs
mailing list