Thank you for another excellent review!
On 4/17/25 1:44 PM, Tomasz Kaminski wrote:
On Thu, Apr 17, 2025 at 1:18 PM Luc Grosheintz
<luc.groshei...@gmail.com> wrote:
This implements std::extents from <mdspan> according to N4950 and
contains partial progress towards PR107761.
If an extent changes its type, there's a precondition in the standard,
that the value is representable in the target integer type. This
precondition is not checked at runtime.
The precondition for 'extents::{static_,}extent' is that '__r <
rank()'.
For extents<T> this precondition is always violated and results in
calling __builtin_trap. For all other specializations it's checked via
__glibcxx_assert.
PR libstdc++/107761
libstdc++-v3/ChangeLog:
* include/std/mdspan (extents): New class.
* src/c++23/std.cc.in <http://std.cc.in>: Add 'using
std::extents'.
Signed-off-by: Luc Grosheintz <luc.groshei...@gmail.com>
Looks really solid. Few very minor suggestions regarding naming, and
use of
consteval where function is not needed at runtime.
---
libstdc++-v3/include/std/mdspan | 249
+++++++++++++++++++++++++++++++
libstdc++-v3/src/c++23/std.cc.in <http://std.cc.in> | 6 +-
2 files changed, 254 insertions(+), 1 deletion(-)
diff --git a/libstdc++-v3/include/std/mdspan
b/libstdc++-v3/include/std/mdspan
index 4094a416d1e..755c077f89c 100644
--- a/libstdc++-v3/include/std/mdspan
+++ b/libstdc++-v3/include/std/mdspan
@@ -33,6 +33,11 @@
#pragma GCC system_header
#endif
+#include <span>
+#include <array>
+#include <type_traits>
+#include <limits>
+
#define __glibcxx_want_mdspan
#include <bits/version.h>
@@ -41,6 +46,250 @@
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
+ namespace __mdspan
+ {
+ template<typename _IndexType, array _Extents>
+ class _ExtentsStorage
+ {
+ public:
+ static constexpr bool
+ _M_is_dyn(size_t __ext) noexcept
We use `_S_` prefix for static member functions, and `_M_` for
instance member functions.
Makes sense! The documentation states:
"""
Member data and function names: _M_.*
Examples: _M_num_elements _M_initialize ()
Static data members, constants, and enumerations: _S_.*
Examples: _S_max_elements _S_default_value
"""
https://gcc.gnu.org/onlinedocs/libstdc++/manual/source_code_style.html
Would you like me to make it more obvious in the docs?
+ { return __ext == dynamic_extent; }
+
+ template<typename _OIndexType>
+ static constexpr _IndexType
+ _M_int_cast(const _OIndexType& __other) noexcept
+ { return _IndexType(__other); }
+
+ static constexpr size_t _S_rank = _Extents.size();
+
+ // For __r in [0, _S_rank], _S_dynamic_index[__r] is the
number
+ // of dynamic extents up to (and not including) __r.
+ //
+ // If __r is the index of a dynamic extent, then
+ // _S_dynamic_index[__r] is the index of that extent in
+ // _M_dynamic_extents.
+ static constexpr auto _S_dynamic_index = [] consteval
+ {
+ array<size_t, _S_rank+1> __ret;
+ size_t __dyn = 0;
+ for(size_t __i = 0; __i < _S_rank; ++__i)
+ {
+ __ret[__i] = __dyn;
+ __dyn += _M_is_dyn(_Extents[__i]);
+ }
+ __ret[_S_rank] = __dyn;
+ return __ret;
+ }();
+
+ static constexpr size_t _S_rank_dynamic =
_S_dynamic_index[_S_rank];
+
+ // For __r in [0, _S_rank_dynamic),
_S_dynamic_index_inv[__r] is the
+ // index of the __r-th dynamic extent in _Extents.
+ static constexpr auto _S_dynamic_index_inv = [] consteval
+ {
+ array<size_t, _S_rank_dynamic> __ret;
+ for (size_t __i = 0, __r = 0; __i < _S_rank; ++__i)
+ if (_M_is_dyn(_Extents[__i]))
+ __ret[__r++] = __i;
+ return __ret;
+ }();
+
+ static constexpr size_t
+ _M_static_extent(size_t __r) noexcept
+ { return _Extents[__r]; }
+
+ constexpr _IndexType
+ _M_extent(size_t __r) const noexcept
+ {
+ auto __se = _Extents[__r];
+ if (__se == dynamic_extent)
+ return _M_dynamic_extents[_S_dynamic_index[__r]];
+ else
+ return __se;
+ }
+
+ template<size_t _OtherRank, typename _GetOtherExtent>
+ constexpr void
+ _M_init_dynamic_extents(_GetOtherExtent __get_extent)
noexcept
+ {
+ for(size_t __i = 0; __i < _S_rank_dynamic; ++__i)
+ {
+ size_t __di = __i;
+ if constexpr (_OtherRank != _S_rank_dynamic)
+ __di = _S_dynamic_index_inv[__i];
+ _M_dynamic_extents[__i] =
_M_int_cast(__get_extent(__di));
+ }
+ }
+
+ constexpr
+ _ExtentsStorage() noexcept = default;
+
+ template<typename _OIndexType, array _OExtents>
+ constexpr
+ _ExtentsStorage(const _ExtentsStorage<_OIndexType,
_OExtents>&
+ __other) noexcept
+ {
+ _M_init_dynamic_extents<_S_rank>([&__other](auto __i)
There is no need to be generic here, just use size_t as lambda
parameter, instead of auto.
+ { return __other._M_extent(__i); });
+ }
+
+ template<typename _OIndexType, size_t _Nm>
+ constexpr
+ _ExtentsStorage(const span<_OIndexType, _Nm>& __exts)
noexcept
+ {
+ _M_init_dynamic_extents<_Nm>(
+ [&__exts](auto __i) -> const _OIndexType&
Same as above.
+ { return __exts[__i]; });
+ }
+
+ private:
+ using _S_storage = __array_traits<_IndexType,
_S_rank_dynamic>::_Type;
+ [[no_unique_address]] _S_storage _M_dynamic_extents;
+ };
+
+ template<typename _OIndexType, typename _SIndexType>
+ concept __valid_index_type =
+ is_convertible_v<_OIndexType, _SIndexType> &&
+ is_nothrow_constructible_v<_SIndexType, _OIndexType>;
+
+ template<size_t _Extent, typename _IndexType>
+ concept
+ __valid_static_extent = _Extent == dynamic_extent
+ || _Extent <= numeric_limits<_IndexType>::max();
+ }
+
+ template<typename _IndexType, size_t... _Extents>
+ class extents
+ {
+ static_assert(is_integral_v<_IndexType>, "_IndexType must
be integral.");
+ static_assert(
+ (__mdspan::__valid_static_extent<_Extents, _IndexType>
&& ...),
+ "Extents must either be dynamic or representable as
_IndexType");
+
+ public:
+ using index_type = _IndexType;
+ using size_type = make_unsigned_t<index_type>;
+ using rank_type = size_t;
+
+ static constexpr rank_type
+ rank() noexcept { return _S_storage::_S_rank; }
+
+ static constexpr rank_type
+ rank_dynamic() noexcept { return _S_storage::_S_rank_dynamic; }
+
+ static constexpr size_t
+ static_extent(rank_type __r) noexcept
+ {
+ __glibcxx_assert(__r < rank());
+ if constexpr (rank() == 0)
+ __builtin_trap();
+
+ return _S_storage::_M_static_extent(__r);
+ }
+
+ constexpr index_type
+ extent(rank_type __r) const noexcept
+ {
+ __glibcxx_assert(__r < rank());
+ if constexpr (rank() == 0)
+ __builtin_trap();
+
+ return _M_dynamic_extents._M_extent(__r);
+ }
+
+ constexpr
+ extents() noexcept = default;
+
+ private:
+ static constexpr bool
I think this can be made consteval, we do not need symbols emitted to
binary.
+ _M_is_less_dynamic(size_t __ext, size_t __oext)
+ { return (__ext != dynamic_extent) && (__oext ==
dynamic_extent); }
+
+ template<typename _OIndexType, size_t... _OExtents>
+ static constexpr bool
I think this can be made consteval, we do not need symbols emitted to
binary.
+ _M_ctor_explicit()
+ {
+ return (_M_is_less_dynamic(_Extents, _OExtents) || ...)
+ || (numeric_limits<index_type>::max()
+ < numeric_limits<_OIndexType>::max());
+ }
+
+ public:
+ template<typename _OIndexType, size_t... _OExtents>
+ requires (sizeof...(_OExtents) == rank())
+ && ((_OExtents == dynamic_extent || _Extents ==
dynamic_extent
+ || _OExtents == _Extents) && ...)
+ constexpr explicit(_M_ctor_explicit<_OIndexType,
_OExtents...>())
+ extents(const extents<_OIndexType, _OExtents...>& __other)
noexcept
+ : _M_dynamic_extents(__other._M_dynamic_extents)
+ { }
+
+ template<__mdspan::__valid_index_type<index_type>... _OIndexTypes>
+ requires (sizeof...(_OIndexTypes) == rank()
+ || sizeof...(_OIndexTypes) == rank_dynamic())
+ constexpr explicit extents(_OIndexTypes... __exts) noexcept
+ : _M_dynamic_extents(span<const _IndexType,
sizeof...(_OIndexTypes)>(
+ initializer_list{_S_storage::_M_int_cast(__exts)...}))
+ { }
+
+ template<__mdspan::__valid_index_type<index_type> _OIndexType,
size_t _Nm>
+ requires (_Nm == rank() || _Nm == rank_dynamic())
+ constexpr explicit(_Nm != rank_dynamic())
+ extents(span<_OIndexType, _Nm> __exts) noexcept
+ : _M_dynamic_extents(__exts)
+ { }
+
+
+ template<__mdspan::__valid_index_type<index_type> _OIndexType,
size_t _Nm>
+ requires (_Nm == rank() || _Nm == rank_dynamic())
+ constexpr explicit(_Nm != rank_dynamic())
+ extents(const array<_OIndexType, _Nm>& __exts) noexcept
+ : _M_dynamic_extents(span<add_const_t<_OIndexType>,
_Nm>(__exts))
We can say const _OIndexType here.
We certainly can and I was convinced I had.
+ { }
+
+ template<typename _OIndexType, size_t... _OExtents>
+ friend constexpr bool
+ operator==(const extents& __self,
+ const extents<_OIndexType, _OExtents...>&
__other) noexcept
+ {
+ if constexpr (__self.rank() != __other.rank())
+ return false;
+
+ for (size_t __i = 0; __i < __self.rank(); ++__i)
+ if (__self.extent(__i) != __other.extent(__i))
+ return false;
+ return true;
+ }
+
+ private:
+ using _S_storage = __mdspan::_ExtentsStorage<
+ _IndexType, array<size_t, sizeof...(_Extents)>{_Extents...}>;
+ [[no_unique_address]] _S_storage _M_dynamic_extents;
+
+ template<typename _OIndexType, size_t... _OExtents>
+ friend class extents;
+ };
+
+ namespace __mdspan
+ {
+ template<typename _IndexType, size_t... _Counts>
+ auto __build_dextents_type(integer_sequence<size_t,
_Counts...>)
+ -> extents<_IndexType, (_Counts, dynamic_extent)...>;
+
+ template<typename _Tp>
+ constexpr size_t
Make it consteval.
+ __dynamic_extent() { return dynamic_extent; }
+ }
+
+ template<typename _IndexType, size_t _Rank>
+ using dextents =
decltype(__mdspan::__build_dextents_type<_IndexType>(
+ make_index_sequence<_Rank>()));
+
+ template<typename... _Integrals>
+ requires (is_convertible_v<_Integrals, size_t> && ...)
+ explicit extents(_Integrals...) ->
+ extents<size_t, __mdspan::__dynamic_extent<_Integrals>()...>;
_GLIBCXX_END_NAMESPACE_VERSION
}
diff --git a/libstdc++-v3/src/c++23/std.cc.in <http://std.cc.in>
b/libstdc++-v3/src/c++23/std.cc.in <http://std.cc.in>
index 12253b95c5a..481e39b2821 100644
--- a/libstdc++-v3/src/c++23/std.cc.in <http://std.cc.in>
+++ b/libstdc++-v3/src/c++23/std.cc.in <http://std.cc.in>
@@ -1825,7 +1825,11 @@ export namespace std
}
}
-// FIXME <mdspan>
+// <mdspan>
+{
+ using std::extents;
+ // FIXME layout_*, default_accessor and mdspan
+}
// 20.2 <memory>
export namespace std
--
2.49.0
