On Mon, May 26, 2025 at 9:15 PM Luc Grosheintz <luc.groshei...@gmail.com>
wrote:
>
>
> On 5/26/25 18:17, Tomasz Kaminski wrote:
> > On Mon, May 26, 2025 at 4:15 PM Luc Grosheintz <luc.groshei...@gmail.com
> >
> > wrote:
> >
> >> Implements the parts of layout_left that don't depend on any of the
> >> other layouts.
> >>
> >> libstdc++-v3/ChangeLog:
> >>
> >> * include/std/mdspan (layout_left): New class.
> >> * src/c++23/std.cc.in: Add layout_left.
> >>
> >> Signed-off-by: Luc Grosheintz <luc.groshei...@gmail.com>
> >> ---
> >> libstdc++-v3/include/std/mdspan | 304 ++++++++++++++++++++++++++++++-
> >> libstdc++-v3/src/c++23/std.cc.in | 1 +
> >> 2 files changed, 304 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/libstdc++-v3/include/std/mdspan
> >> b/libstdc++-v3/include/std/mdspan
> >> index 0f49b0e09a0..d81072596b4 100644
> >> --- a/libstdc++-v3/include/std/mdspan
> >> +++ b/libstdc++-v3/include/std/mdspan
> >> @@ -144,6 +144,20 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >> { return __exts[__i]; });
> >> }
> >>
> >> + static constexpr span<const size_t>
> >> + _S_static_extents(size_t __begin, size_t __end) noexcept
> >> + {
> >> + return {_Extents.data() + __begin, _Extents.data() + __end};
> >> + }
> >>
> > Oh, I think I was very unclear, regarding removing the dependency on
> > index_type.
> > What I was thinking of, is changing this function to:
> > + static consteval array<size_t, rank()>&
> > + _S_static_extents() noexcept
> > + {
> > + return _Extents;
> > + }
>
> Sorry, this was clear. I implemented it once, then we found the
> issues with overflow and while cleaning out the mess I'd made
> to compute things with an __unsigned_prod, I reverted this back
> to the original version.
>
> If you look at __static_extents_prod, it already doesn't depend
> on the index_type. Therefore, it a) felt like a change related
> to something we'd agreed to postpone; and b) I preferred the
> symmetry between __static_extents and __dynamic_extents; I'll
> change it to the proposed version.
>
I think we are good keeping current versions. and then return back to it
later, to see if we can make static_extent being folded at compile time,
and look at some runtime instruction/binary size tradeoffs later.
>
> >
> >> +
> >> + constexpr span<const _IndexType>
> >> + _M_dynamic_extents(size_t __begin, size_t __end) const noexcept
> >> + requires (_Extents.size() > 0)
> >> + {
> >> + return {_M_dyn_exts + _S_dynamic_index[__begin],
> >> + _M_dyn_exts + _S_dynamic_index[__end]};
> >> + }
> >> +
> >> private:
> >> using _S_storage = __array_traits<_IndexType,
> >> _S_rank_dynamic>::_Type;
> >> [[no_unique_address]] _S_storage _M_dyn_exts{};
> >> @@ -160,6 +174,22 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >> || _Extent <= numeric_limits<_IndexType>::max();
> >> }
> >>
> >> + namespace __mdspan
> >> + {
> >> + template<typename _Extents>
> >> + constexpr span<const size_t>
> >> + __static_extents(size_t __begin = 0, size_t __end =
> >> _Extents::rank())
> >> + { return _Extents::_S_storage::_S_static_extents(__begin,
> __end); }
> >
> > Also adjusting this one to have:
> > template<typename _Extents>
> > constexpr const std::array<size_t, N>&
> > __static_extents()
> > + { return _Extents::_S_storage::_S_static_extents; }
> >
> >> +
> >> + template<typename _Extents>
> >> + constexpr span<const typename _Extents::index_type>
> >> + __dynamic_extents(const _Extents& __exts, size_t __begin = 0,
> >> + size_t __end = _Extents::rank())
> >> + {
> >> + return __exts._M_exts._M_dynamic_extents(__begin, __end);
> >> + }
> >> + }
> >> +
> >> template<typename _IndexType, size_t... _Extents>
> >> class extents
> >> {
> >> @@ -251,7 +281,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >> : _M_exts(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())
> >> @@ -276,6 +305,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >> }
> >>
> >> private:
> >> + friend span<const size_t>
> >> + __mdspan::__static_extents<extents>(size_t, size_t);
> >> +
> >> + friend span<const index_type>
> >> + __mdspan::__dynamic_extents<extents>(const extents&, size_t,
> >> size_t);
> >> +
> >> using _S_storage = __mdspan::_ExtentsStorage<
> >> _IndexType, array<size_t, sizeof...(_Extents)>{_Extents...}>;
> >> [[no_unique_address]] _S_storage _M_exts;
> >> @@ -286,6 +321,58 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >>
> >> namespace __mdspan
> >> {
> >> + template<typename _Tp, size_t _Nm>
> >> + constexpr bool
> >> + __contains_zero(span<_Tp, _Nm> __exts)
> >> + {
> >> + for (size_t __i = 0; __i < __exts.size(); ++__i)
> >> + if (__exts[__i] == 0)
> >> + return true;
> >> + return false;
> >> + }
> >> +
> >> + constexpr size_t
> >> + __static_extents_prod(const auto& __sta_exts)
> >>
> > Then this could be implemented as:
> > template<auto& _Exts>
> > __static_extents_prod(size_t __begin, size_t __end);
> > // We provide the array as template parameter.
> > {
> > }
> >
> >> + {
> >> + size_t __ret = 1;
> >> + for (auto __factor : span(_Ext).subspan(__begin, __end))
> >> + if (__factor != dynamic_extent)
> >> + __ret *= __factor;
> >> + return __ret;
> >> + }
> >> +
> >> + template<typename _Extents>
> >> + constexpr typename _Extents::index_type
> >> + __exts_prod(const _Extents& __exts, size_t __begin, size_t __end)
> >> noexcept
> >> + {
> >> + using _IndexType = typename _Extents::index_type;
> >> +
> >> + auto __sta_exts = __static_extents<_Extents>(__begin, __end);
> >> + size_t __sta_prod = __static_extents_prod(__sta_exts);
> >>
> > And then it will be called as follows:
> > constexpr auto& __sta_exts = __static_extents<_Extents>();
> > size_t __sta_prod = __static_extents_prod<__sta_exts>(__begin,
> __end);
> > This way __static_extents_prod will not depend on index type.
>
> The current implementation of __static_extents_prod doesn't depend
> on index_type either. The static extents are returned as a
> span<const size_t>.
>
> > We seem to also compute the product twice.
> >
> >> +
> >> + size_t __ret = 1;
> >> + if constexpr (_Extents::rank_dynamic() != _Extents::rank())
> >> + __ret = __static_extents_prod(__sta_exts);
> >> +
> >> + if (__ret == 0)
> >> + return 0;
> >> +
> >> + if constexpr (_Extents::rank_dynamic() > 0)
> >> + for (auto __factor : __dynamic_extents(__exts, __begin,
> __end))
> >> + __ret *= size_t(__factor);
> >> + return _IndexType(__ret);
> >> + }
> >> +
> >> + template<typename _Extents>
> >> + constexpr typename _Extents::index_type
> >> + __fwd_prod(const _Extents& __exts, size_t __r) noexcept
> >> + { return __exts_prod(__exts, 0, __r); }
> >> +
> >> + template<typename _Extents>
> >> + constexpr typename _Extents::index_type
> >> + __rev_prod(const _Extents& __exts, size_t __r) noexcept
> >> + { return __exts_prod(__exts, __r + 1, __exts.rank()); }
> >> +
> >> template<typename _IndexType, size_t... _Counts>
> >> auto __build_dextents_type(integer_sequence<size_t, _Counts...>)
> >> -> extents<_IndexType, ((void) _Counts, dynamic_extent)...>;
> >> @@ -304,6 +391,221 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >> explicit extents(_Integrals...) ->
> >> extents<size_t, __mdspan::__dynamic_extent<_Integrals>()...>;
> >>
> >> + struct layout_left
> >> + {
> >> + template<typename _Extents>
> >> + class mapping;
> >> + };
> >> +
> >> + namespace __mdspan
> >> + {
> >> + template<typename _Tp>
> >> + constexpr bool __is_extents = false;
> >> +
> >> + template<typename _IndexType, size_t... _Extents>
> >> + constexpr bool __is_extents<extents<_IndexType, _Extents...>> =
> >> true;
> >> +
> >> + template<typename _Extents, typename... _Indices>
> >> + constexpr typename _Extents::index_type
> >> + __linear_index_left(const _Extents& __exts, _Indices...
> __indices)
> >> + {
> >> + using _IndexType = typename _Extents::index_type;
> >> + _IndexType __res = 0;
> >> + if constexpr (sizeof...(__indices) > 0)
> >> + {
> >> + _IndexType __mult = 1;
> >> + auto __update = [&, __pos = 0u](_IndexType __idx) mutable
> >> + {
> >> + __res += __idx * __mult;
> >> + __mult *= __exts.extent(__pos);
> >> + ++__pos;
> >> + };
> >> + (__update(__indices), ...);
> >> + }
> >> + return __res;
> >> + }
> >> +
> >> + template<typename _Extents,
> >> + typename _IndexType = typename _Extents::index_type>
> >> + consteval _IndexType
> >> + __static_quotient(_IndexType __nom =
> >> numeric_limits<_IndexType>::max())
> >> + {
> >> + auto __sta_exts = __static_extents<_Extents>();
> >> + for (auto __factor : __sta_exts)
> >> + {
> >> + if (__factor != dynamic_extent)
> >> + __nom /= _IndexType(__factor);
> >> + if (__nom == 0)
> >> + break;
> >> + }
> >> + return __nom;
> >> + }
> >> +
> >> + template<typename _Extents>
> >> + constexpr bool
> >> + __is_representable_extents(const _Extents& __exts)
> >> + {
> >> + using _IndexType = _Extents::index_type;
> >> +
> >> + if constexpr (__contains_zero(__static_extents<_Extents>()))
> >> + return true;
> >> + else
> >> + {
> >> + constexpr auto __sta_quo = __static_quotient<_Extents>();
> >>
> > + if constexpr (_Extents::rank_dynamic() == 0)
> >> + return __sta_quo != 0;
> >> + else
> >> + {
> >> + auto __dyn_exts = __dynamic_extents(__exts);
> >> + if (__contains_zero(__dyn_exts))
> >> + return true;
> >> +
> >> + if constexpr (__sta_quo == 0)
> >> + return false;
> >> + else
> >> + {
> >> + constexpr auto __dyn_rank =
> _Extents::rank_dynamic();
> >>
> > These seem to be unused.
> >
> >> + auto __dyn_quo = _IndexType(__sta_quo);
> >> + for (auto __factor : __dyn_exts)
> >> + {
> >> + __dyn_quo /= __factor;
> >> + if (__dyn_quo == 0)
> >> + return false;
> >> + }
> >> + return true;
> >> + }
> >> + }
> >> + }
> >> + }
> >> +
> >> + template<typename _Extents, typename _IndexType>
> >> + concept __representable_size = _Extents::rank_dynamic() != 0
> >> + || __contains_zero(__static_extents<_Extents>())
> >> + || (__static_quotient<_Extents, _IndexType>() != 0);
> >> +
> >> + template<typename _Layout, typename _Mapping>
> >> + concept __mapping_of =
> >> + is_same_v<typename _Layout::mapping<typename
> >> _Mapping::extents_type>,
> >> + _Mapping>;
> >> +
> >> + template<typename _Mapping>
> >> + concept __standardized_mapping = __mapping_of<layout_left,
> >> _Mapping>;
> >> +
> >> + template<typename M>
> >> + concept __mapping_like = requires
> >> + {
> >> + requires __is_extents<typename M::extents_type>;
> >> + { M::is_always_strided() } -> same_as<bool>;
> >> + { M::is_always_exhaustive() } -> same_as<bool>;
> >> + { M::is_always_unique() } -> same_as<bool>;
> >> + bool_constant<M::is_always_strided()>::value;
> >> + bool_constant<M::is_always_exhaustive()>::value;
> >> + bool_constant<M::is_always_unique()>::value;
> >> + };
> >> +
> >> + // A tag type to create internal ctors.
> >> + class __internal_ctor
> >> + { };
> >> + }
> >> +
> >> + template<typename _Extents>
> >> + class layout_left::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_left;
> >> +
> >> + static_assert(__mdspan::__representable_size<_Extents,
> index_type>,
> >> + "The size of extents_type must be representable as index_type");
> >> +
> >> + constexpr
> >> + mapping() noexcept = default;
> >> +
> >> + constexpr
> >> + mapping(const mapping&) noexcept = default;
> >> +
> >> + constexpr
> >> + mapping(const _Extents& __extents) noexcept
> >> + : _M_extents(__extents)
> >> + {
> >> __glibcxx_assert(__mdspan::__is_representable_extents(_M_extents)); }
> >> +
> >> + template<typename _OExtents>
> >> + requires (is_constructible_v<extents_type, _OExtents>)
> >> + constexpr explicit(!is_convertible_v<_OExtents, extents_type>)
> >> + mapping(const mapping<_OExtents>& __other) noexcept
> >> + : mapping(__other.extents(), __mdspan::__internal_ctor{})
> >> + { }
> >> +
> >> + constexpr mapping&
> >> + operator=(const mapping&) noexcept = default;
> >> +
> >> + constexpr const _Extents&
> >> + extents() const noexcept { return _M_extents; }
> >> +
> >> + constexpr index_type
> >> + required_span_size() const noexcept
> >> + { return __mdspan::__fwd_prod(_M_extents, extents_type::rank());
> }
> >> +
> >> + 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_left(_M_extents,
> >> + static_cast<index_type>(__indices)...);
> >> + }
> >> +
> >> + static constexpr bool
> >> + is_always_unique() noexcept { return true; }
> >> +
> >> + static constexpr bool
> >> + is_always_exhaustive() noexcept { return true; }
> >> +
> >> + static constexpr bool
> >> + is_always_strided() noexcept { return true; }
> >> +
> >> + static constexpr bool
> >> + is_unique() noexcept { return true; }
> >> +
> >> + static constexpr bool
> >> + is_exhaustive() noexcept { return true; }
> >> +
> >> + static constexpr bool
> >> + is_strided() noexcept { return true; }
> >> +
> >> + constexpr index_type
> >> + stride(rank_type __i) const noexcept
> >> + requires (extents_type::rank() > 0)
> >> + {
> >> + __glibcxx_assert(__i < extents_type::rank());
> >> + return __mdspan::__fwd_prod(_M_extents, __i);
> >> + }
> >> +
> >> + template<typename _OExtents>
> >> + requires (extents_type::rank() == _OExtents::rank())
> >> + friend constexpr bool
> >> + operator==(const mapping& __self, const mapping<_OExtents>&
> >> __other)
> >> + noexcept
> >> + { return __self.extents() == __other.extents(); }
> >> +
> >> + private:
> >> + template<typename _OExtents>
> >> + constexpr explicit
> >> + mapping(const _OExtents& __oexts, __mdspan::__internal_ctor)
> >> noexcept
> >> + : _M_extents(__oexts)
> >> + {
> >> + static_assert(__mdspan::__representable_size<_OExtents,
> >> index_type>,
> >> + "The size of OtherExtents must be representable as
> >> index_type");
> >> +
> >> __glibcxx_assert(__mdspan::__is_representable_extents(_M_extents));
> >> + }
> >> +
> >> + public:
> >> + [[no_unique_address]] _Extents _M_extents{};
> >> + };
> >> +
> >> _GLIBCXX_END_NAMESPACE_VERSION
> >> }
> >> #endif
> >> diff --git a/libstdc++-v3/src/c++23/std.cc.in b/libstdc++-v3/src/c++23/
> >> std.cc.in
> >> index 417c8a1a562..b4b0bb24a8c 100644
> >> --- a/libstdc++-v3/src/c++23/std.cc.in
> >> +++ b/libstdc++-v3/src/c++23/std.cc.in
> >> @@ -1841,6 +1841,7 @@ export namespace std
> >> export namespace std
> >> {
> >> using std::extents;
> >> + using std::layout_left;
> >> // FIXME layout_*, default_accessor and mdspan
> >> }
> >> #endif
> >> --
> >> 2.49.0
> >>
> >>
> >
>
>
