* is_always_exhaustive should also check for rank 0
libstdc++-v3/include/std/mdspan | 229 ++++++++++++++++++++++++++++++-
libstdc++-v3/src/c++23/std.cc.in | 3 +-
2 files changed, 230 insertions(+), 2 deletions(-)
diff --git a/libstdc++-v3/include/std/mdspan
b/libstdc++-v3/include/std/mdspan
index 7daa0713716..d5f613a19fd 100644
--- a/libstdc++-v3/include/std/mdspan
+++ b/libstdc++-v3/include/std/mdspan
@@ -403,6 +403,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
class mapping;
};
+ struct layout_stride
+ {
+ template<typename _Extents>
+ class mapping;
+ };
+
namespace __mdspan
{
template<typename _Tp>
@@ -496,7 +502,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
template<typename _Mapping>
concept __standardized_mapping = __mapping_of<layout_left, _Mapping>
- || __mapping_of<layout_right,
_Mapping>;
+ || __mapping_of<layout_right,
_Mapping>
+ || __mapping_of<layout_stride,
_Mapping>;
template<typename M>
concept __mapping_like = requires
@@ -554,6 +561,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
: mapping(__other.extents(), __mdspan::__internal_ctor{})
{ }
+ template<typename _OExtents>
+ requires (is_constructible_v<extents_type, _OExtents>)
+ constexpr explicit(extents_type::rank() > 0)
+ mapping(const layout_stride::mapping<_OExtents>& __other)
+ : mapping(__other.extents(), __mdspan::__internal_ctor{})
+ { __glibcxx_assert(*this == __other); }
+
constexpr mapping&
operator=(const mapping&) noexcept = default;
@@ -684,6 +698,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
: mapping(__other.extents(), __mdspan::__internal_ctor{})
{ }
+ template<class _OExtents>
+ requires (is_constructible_v<extents_type, _OExtents>)
+ constexpr explicit(extents_type::rank() > 0)
+ mapping(const layout_stride::mapping<_OExtents>& __other) noexcept
+ : mapping(__other.extents(), __mdspan::__internal_ctor{})
+ { __glibcxx_assert(*this == __other); }
+
constexpr mapping&
operator=(const mapping&) noexcept = default;
@@ -757,6 +778,212 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
[[no_unique_address]] _Extents _M_extents{};
};
+ namespace __mdspan
+ {
+ template<typename _Mapping>
+ constexpr typename _Mapping::index_type
+ __offset(const _Mapping& __m) noexcept
+ {
+ using _IndexType = typename _Mapping::index_type;
+ constexpr auto __rank = _Mapping::extents_type::rank();
+
+ if constexpr (__standardized_mapping<_Mapping>)
+ return 0;
+ else
+ {
If the size of multidimensional index space is zero we cannot call mapping
at all,
and standard recognize it here:
https://eel.is/c++draft/mdspan.layout.stride#expo-1.2
otherwise 0, if the size of the multidimensional index space e is 0,
So we need two additional if in between:
else if constexpr (contains_zero(_static_exts<Mapping::extents_type>()))
return 0;
else if (contais_zero(_dyn_exts(_m.extents))
return 0;
else
+ auto __impl = [&__m]<size_t...
_Counts>(index_sequence<_Counts...>)
+ { return __m(((void) _Counts, _IndexType(0))...); };
+ return __impl(make_index_sequence<__rank>());
+ }
+ }
+
+ template<typename _Mapping, typename... _Indices>
+ constexpr typename _Mapping::index_type
+ __linear_index_strides(const _Mapping& __m,
+ _Indices... __indices)
Maka this noexcept, because it is called from noexcept function.
+ {
+ using _IndexType = typename _Mapping::index_type;
+ _IndexType __res = 0;
+ if constexpr (sizeof...(__indices) > 0)
+ {
+ auto __update = [&, __pos = 0u](_IndexType __idx) mutable
+ {
+ __res += __idx * __m.stride(__pos++);
+ };
+ (__update(__indices), ...);
+ }
+ return __res;
+ }
+ }
+
+ template<typename _Extents>
+ class layout_stride::mapping
+ {
+ public:
+ using extents_type = _Extents;
+ using index_type = typename extents_type::index_type;
+ using size_type = typename extents_type::size_type;
+ using rank_type = typename extents_type::rank_type;
+ using layout_type = layout_stride;
+
+ static_assert(__mdspan::__representable_size<_Extents, index_type>,
+ "The size of extents_type must be representable as index_type");
+
+ constexpr
+ mapping() noexcept
+ {
The standard has a precondition here
https://eel.is/c++draft/mdspan.layout.stride#cons-1:
*Preconditions*: layout_right::mapping<extents_type>().required_span_size(
) is representable as a value of type index_type ([basic.fundamental]
<https://eel.is/c++draft/basic.fundamental>).
<https://eel.is/c++draft/mdspan.layout.stride#cons-1.sentence-1>
However, for all static_extents this is already checked by static_assert
above.And if at least one extent is dynamic, required span size is zero,
so it is trivially true. Could you add a comment here.
+ size_t __stride = 1;
+ for (size_t __i = extents_type::rank(); __i > 0; --__i)
+ {
+ _M_strides[__i - 1] = index_type(__stride);
+ __stride *= size_t(_M_extents.extent(__i - 1));
+ }
+ }
+
+ constexpr
+ mapping(const mapping&) noexcept = default;
+
+ template<__mdspan::__valid_index_type<index_type> _OIndexType>
+ constexpr
+ mapping(const extents_type& __exts,
+ span<_OIndexType, extents_type::rank()> __strides) noexcept
+ : _M_extents(__exts)
+ {
We are not checking the preconditions here, could you add this information
to the commit
message, similarly as we have done for extents.
+ for (size_t __i = 0; __i < extents_type::rank(); ++__i)
+ _M_strides[__i] = index_type(as_const(__strides[__i]));
+ }
+
+ template<__mdspan::__valid_index_type<index_type> _OIndexType>
+ constexpr
+ mapping(const extents_type& __exts,
+ const array<_OIndexType, extents_type::rank()>& __strides)
+ noexcept
+ : mapping(__exts,
+ span<const _OIndexType, extents_type::rank()>(__strides))
+ { }
+
+ template<__mdspan::__mapping_like _StridedMapping>
+ requires (is_constructible_v<extents_type,
+ typename
_StridedMapping::extents_type>
+ && _StridedMapping::is_always_unique()
+ && _StridedMapping::is_always_strided())
+ constexpr explicit(!(
+ is_convertible_v<typename _StridedMapping::extents_type,
extents_type>
+ && __mdspan::__standardized_mapping<_StridedMapping>))
+ mapping(const _StridedMapping& __other) noexcept
+ : _M_extents(__other.extents())
+ {
+ if constexpr (cmp_greater(
+ numeric_limits<typename
_StridedMapping::index_type>::max(),
+ numeric_limits<index_type>::max()))
+ __glibcxx_assert(!cmp_less(numeric_limits<index_type>::max(),
+ __other.required_span_size()));
As we made __offset efficient for standard layouts, I will add:
__glibcxx_assert(__mdspan::offset(_other) == 0);
+ if constexpr (extents_type::rank() > 0)
+ for (size_t __i = 0; __i < extents_type::rank(); ++__i)
+ _M_strides[__i] = index_type(__other.stride(__i));
+ }
+
+ constexpr mapping&
+ operator=(const mapping&) noexcept = default;
+
+ constexpr const _Extents&
+ extents() const noexcept { return _M_extents; }
+
+ constexpr array<index_type, extents_type::rank()>
+ strides() const noexcept {
+ array<index_type, extents_type::rank()> __ret;
+ for (size_t __i = 0; __i < extents_type::rank(); ++__i)
+ __ret[__i] = _M_strides[__i];
+ return __ret;
+ }
+
+ constexpr index_type
+ required_span_size() const noexcept
+ {
We can use the fact that we have __static_exts and __dyn_exts and optimize
this to:
if constexpr (contains_zero(_static_exts<Mapping::extents_type>()))
return 0;
else if (contais_zero(_dyn_exts(_m.extents))
return 0;
else
// Given this is 3rd time we are checking that (in __offset,
__respresetnable_size
+ for (size_t __i = 0; __i < extents_type::rank(); ++__i)
+ if (_M_extents.extent(__i) == 0)
+ return 0;
+
+ index_type __ret = 1;
+ for (size_t __i = 0; __i < extents_type::rank(); ++__i)
+ __ret += (_M_extents.extent(__i) - 1) * _M_strides[__i];
+ return __ret;
+ }
+
+ template<__mdspan::__valid_index_type<index_type>... _Indices>
+ requires (sizeof...(_Indices) == extents_type::rank())
+ constexpr index_type
+ operator()(_Indices... __indices) const noexcept
+ {
+ return __mdspan::__linear_index_strides(*this,
+ static_cast<index_type>(__indices)...);
+ }
+
+ static constexpr bool
+ is_always_unique() noexcept { return true; }
+
+ // _GLIBCXX_RESOLVE_LIB_DEFECTS
+ // 4266. layout_stride::mapping should treat empty mappings as
exhaustive
+ static constexpr bool
+ is_always_exhaustive() noexcept
+ {
In the issue I have suggested moving the rank check here:
*> Returns*: true if *rank_* is 0 or if there is a rank index r of extents()
such that extents_type::static_extent(r) is 0, otherwise false.
So we need to have rank() == 0 || contains_zeror.
+ return __mdspan::__contains_zero(
+ __mdspan::__static_extents<_Extents>());
+ }
+
+ static constexpr bool
+ is_always_strided() noexcept { return true; }
+
+ static constexpr bool
+ is_unique() noexcept { return true; }
+
+ // _GLIBCXX_RESOLVE_LIB_DEFECTS
+ // 4266. layout_stride::mapping should treat empty mappings as
exhaustive
+ constexpr bool
+ is_exhaustive() const noexcept
+ {
+ constexpr auto __rank = extents_type::rank();
+ if constexpr (__rank == 0)
And here we can say if constexpr (is_always_exhaustive()).
I would write this as:
if constexpr (!is_always_exhaustive())
{
auto __size = __mdspan::__fwd_prod(_M_extents, __rank);
if (__size > 0)
return __size == required_span_size()
}
return true;
+ return true;
+ else
+ {
+ // Here we deviate from the standard. The standard requires
that
+ // for some extents with size zero, is_exhaustive returns
false.
Remove this comment, as you mentioned that you are implementing the 4266
issue.
+ auto __size = __mdspan::__fwd_prod(_M_extents, __rank);
+ if (__size == 0)
+ return true;
+ return __size == required_span_size();
+ }
+ }
+
+ static constexpr bool
+ is_strided() noexcept { return true; }
+
+ constexpr index_type
+ stride(rank_type __r) const noexcept { return _M_strides[__r]; }
+
+ template<__mdspan::__mapping_like _OMapping>
+ requires ((extents_type::rank() == _OMapping::extents_type::rank())
+ && _OMapping::is_always_strided())
+ friend constexpr bool
+ operator==(const mapping& __self, const _OMapping& __other)
noexcept
+ {
+ if (__self.extents() != __other.extents())
+ return false;
+ if constexpr (extents_type::rank() > 0)
+ for (size_t __i = 0; __i < extents_type::rank(); ++__i)
+ if (__self.stride(__i) != __other.stride(__i))
I think we want to use cmp_equal here, to avoid warnings for
signed/unsigned comparison.
