[PATCH v4 3/4] libstdc++: Implement std::extents [PR107761].
Luc Grosheintz
luc.grosheintz@gmail.com
Fri Apr 18 17:47:54 GMT 2025
On 4/18/25 2:00 PM, Tomasz Kaminski wrote:
>
>
>
> On Fri, Apr 18, 2025 at 1:43 PM Luc Grosheintz <luc.grosheintz@gmail.com
> <mailto:luc.grosheintz@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.grosheintz@gmail.com
> <mailto:luc.grosheintz@gmail.com>>
> ---
>
> LGTM.
> Below, I shared one idea that I found interesting, and you could look into,
> but not necessary.
>
> 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..f7a47552485 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
> + _S_is_dyn(size_t __ext) noexcept
> + { return __ext == dynamic_extent; }
> +
> + template<typename _OIndexType>
> + static constexpr _IndexType
> + _S_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 += _S_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 (_S_is_dyn(_Extents[__i]))
> + __ret[__r++] = __i;
> + return __ret;
> + }();
> +
> + static constexpr size_t
> + _S_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] =
> _S_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](size_t __i)
> + { return __other._M_extent(__i); });
> + }
> +
> + template<typename _OIndexType, size_t _Nm>
> + constexpr
> + _ExtentsStorage(span<const _OIndexType, _Nm> __exts) noexcept
> + {
> + _M_init_dynamic_extents<_Nm>(
> + [&__exts](size_t __i) -> const _OIndexType&
> + { 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::_S_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 consteval bool
> + _S_is_less_dynamic(size_t __ext, size_t __oext)
> + { return (__ext != dynamic_extent) && (__oext ==
> dynamic_extent); }
> +
> + template<typename _OIndexType, size_t... _OExtents>
> + static consteval bool
> + _S_ctor_explicit()
> + {
> + return (_S_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) && ...)
>
> Let's imagine that we have a _S_is_compatible_function(e1, e2) that
> encapsulates this condition.
> return e1 == dynamic_extent || e2 == dynamic_extent || e1 == e2.
> Then ...
>
> + constexpr explicit(_S_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::_S_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(span<const _OIndexType, _Nm>(__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<const _OIndexType, _Nm>(__exts))
> + { }
> +
> + 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;
>
> .. in operator== we could add check:
> if constexpr (!(_S_is_compatible(_OExtents, _Extends) && ...))
> return false;
> This would avoid doing any runtime checks in some cases.
Very nice observation. I was hoping that in the cases where
everything could be determined at compile time, e.g.
std::extents<int, 1, 2>{} == std::extents<short, 1, 2>{}
the compiler would be able to reduce the expression to `true`.
I'd like to check the code the compiler generates in some of
these cases. Taken literally, I think the proposal doesn't go
far enough, because there's also cases in which we know at
compile time that the answer is `true` (not just `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>
> + consteval size_t
> + __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 5e18ad73908..e42f54df20e 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>
> @@ -1831,7 +1831,11 @@ export namespace std
> }
> }
>
> -// FIXME <mdspan>
> +// <mdspan>
> +{
> + using std::extents;
> + // FIXME layout_*, default_accessor and mdspan
> +}
>
> // 20.2 <memory>
> export namespace std
> --
> 2.49.0
>
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