On Sat, May 3, 2025 at 4:45 PM Luc Grosheintz <luc.groshei...@gmail.com>
wrote:
> Topic: follow up question about operator() for layout_stride.
>
> On 4/30/25 7:13 AM, Tomasz Kaminski wrote:
> > Hi,
> >
> > As we will be landing patches for extends, this will become a separate
> > patch series.
> > I would prefer, if you could commit per layout, and start with
> layout_right
> > (default)
> > I try to provide prompt responses, so if that works better for you, you
> can
> > post a patch
> > only with this layout first, as most of the comments will apply to all of
> > them.
> >
> > For the general design we have constructors that allow conversion between
> > rank-0
> > and rank-1 layouts left and right. This is done because they essentially
> > represents
> > the same layout. I think we could benefit from that in code by having a
> > base classes
> > for rank0 and rank1 mapping:
> > template<typename _Extents>
> > _Rank0_mapping_base
> > {
> > static_assert(_Extents::rank() == 0);
> >
> > template<OtherExtents>
> > // explicit, requires goes here
> > _Rank0_mapping_base(_Rank0_mapping_base<OtherExtents>);
> >
> > // All members layout_type goes her
> > };
> >
> > template<typename _Extents>
> > _Rank1_mapping_base
> > {
> > static_assert(_Extents::rank() == 1);
> > // Static assert for product is much simpler here, as we need to
> check one
> >
> > template<OtherExtents>
> > // explicit, requires goes here
> > _Rank1_mapping_base(_Rank1_mapping_base<OtherExtents>);
> >
> > // Call operator can also be simplified
> > index_type operator()(index_type i) const // conversion happens at
> user
> > side
> >
> > // cosntructor from strided_layout of Rank1 goes here.
> >
> > // All members layout_type goes her
> > };
> > Then we will specialize layout_left/right/stride to use
> _Rank0_mapping_base
> > as a base for rank() == 0
> > and layout_left/right to use _Rank1_mapping as base for rank()1;
> > template<typename T, unsigned... Ids>
> > struct extents {};
> >
> > struct layout
> > {
> > template<typename Extends>
> > struct mapping
> > {
> > // static assert that Extents mmyst be specialization of _Extents goes
> here.
> > }
> > };
> >
> > template<typename _IndexType>
> > struct layout::mapping<extents<_IndexType>>
> > : _Rank0_mapping_base<extents<_IndexType>>
> > {
> > using layout_type = layout_left;
> > // Provides converting constructor.
> > using _Rank0_mapping_base<extents<_IndexType>>::_Rank0_mapping_base;
> > // This one is implicit;
> > mapping(_Rank0_mapping_base<extents<_IndexType>> const&);
> > };
> >
> > template<typename _IndexType, unsigned _Ext>
> > struct layout::mapping<extents<_IndexType, _Ext>>
> > : _Rank1_mapping_base<extents<_IndexType>>
> >
> > {
> > using layout_type = layout_left;
> > // Provides converting constructor.
> > using _Rank0_mapping_base<extents<_IndexType>>::_Rank0_mapping_base;
> > // This one is implicit, allows construction from layout_right
> > mapping(_Rank1_mapping_base<extents<_IndexType>> const&);
> > };
> > };
> >
> > template<typename _IndexType, unsigned... _Ext>
> > requires sizeof..(_Ext) > = 2
> > struct layout::mapping<extents<_IndexType, _Ext>>
> >
> > The last one is a generic implementation that you can use in yours.
> > Please also include a comment explaining that we are deviating from
> > standard text here.
> >
> >
> > On Tue, Apr 29, 2025 at 2:56 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++/ChangeLog:
> >>
> >> * include/std/mdspan (layout_left): New class.
> >>
> >> Signed-off-by: Luc Grosheintz <luc.groshei...@gmail.com>
> >> ---
> >> libstdc++-v3/include/std/mdspan | 179 ++++++++++++++++++++++++++++++++
> >> 1 file changed, 179 insertions(+)
> >>
> >> diff --git a/libstdc++-v3/include/std/mdspan
> >> b/libstdc++-v3/include/std/mdspan
> >> index 39ced1d6301..e05048a5b93 100644
> >> --- a/libstdc++-v3/include/std/mdspan
> >> +++ b/libstdc++-v3/include/std/mdspan
> >> @@ -286,6 +286,26 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >>
> >> namespace __mdspan
> >> {
> >> + template<typename _Extents>
> >> + constexpr typename _Extents::index_type
> >> + __fwd_prod(const _Extents& __exts, size_t __r) noexcept
> >> + {
> >> + typename _Extents::index_type __fwd = 1;
> >> + for(size_t __i = 0; __i < __r; ++__i)
> >> + __fwd *= __exts.extent(__i);
> >> + return __fwd;
> >> + }
> >>
> > As we are inside the standard library implementation, we can do some
> tricks
> > here,
> > and provide two functions:
> > // Returns the std::span(_ExtentsStorage::_Ext).substr(f, l);
> > // For extents forward to __static_exts
> > span<typename Extends::index_type> __static_exts(size_t f, size_t l);
> > // Returns the
> >
> std::span(_ExtentsStorage::_M_dynamic_extents).substr(_S_dynamic_index[f],
> > _S_dynamic_index[l);
> > span<typename Extends::index_type> __dynamic_exts(Extents const& c);
> > Then you can befriend this function both to extents and _ExtentsStorage.
> > Also add index_type members to _ExtentsStorage.
> >
> > Then instead of having fwd-prod and rev-prod I would have:
> > template<typename _Extents>
> > consteval size_t __static_ext_prod(size_t f, size_t l)
> > {
> > // multiply E != dynamic_ext from __static_exts
> > }
> > constexpr size __ext_prod(const _Extents& __exts, size_t f, size_t l)
> > {
> > // multiply __static_ext_prod<_Extents>(f, l) and each elements of
> > __dynamic_exts(__exts, f, l);
> > }
> >
> > Then fwd-prod(e, n) would be __ext_prod(e, 0, n), and rev_prod(e, n)
> would
> > be __ext_prod(e, __ext.rank() -n, n, __ext.rank())
> >
> >
> >> +
> >> + template<typename _Extents>
> >> + constexpr typename _Extents::index_type
> >> + __rev_prod(const _Extents& __exts, size_t __r) noexcept
> >> + {
> >> + typename _Extents::index_type __rev = 1;
> >> + for(size_t __i = __r + 1; __i < __exts.rank(); ++__i)
> >> + __rev *= __exts.extent(__i);
> >> + return __rev;
> >> + }
> >> +
> >> template<typename _IndexType, size_t... _Counts>
> >> auto __build_dextents_type(integer_sequence<size_t, _Counts...>)
> >> -> extents<_IndexType, ((void) _Counts, dynamic_extent)...>;
> >> @@ -304,6 +324,165 @@ _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<size_t _Count>
> >> + struct _LinearIndexLeft
> >> + {
> >> + template<typename _Extents, typename... _Indices>
> >> + static constexpr typename _Extents::index_type
> >> + _S_value(const _Extents& __exts, typename _Extents::index_type
> >> __idx,
> >> + _Indices... __indices) noexcept
> >> + {
> >> + return __idx + __exts.extent(_Count)
> >> + * _LinearIndexLeft<_Count + 1>::_S_value(__exts,
> __indices...);
> >> + }
> >> +
> >> + template<typename _Extents>
> >> + static constexpr typename _Extents::index_type
> >> + _S_value(const _Extents&) noexcept
> >> + { return 0; }
> >> + };
> >> +
> >> + template<typename _Extents, typename... _Indices>
> >> + constexpr typename _Extents::index_type
> >> + __linear_index_left(const _Extents& __exts, _Indices...
> __indices)
> >> + {
> >> + return _LinearIndexLeft<0>::_S_value(__exts, __indices...);
> >> + }
> >>
> > This can be eliminated by fold expressions, see below.
> >
> >> +
> >> + template<typename _IndexType, typename _Tp, size_t _Nm>
> >> + consteval bool
> >> + __is_representable_product(array<_Tp, _Nm> __factors)
> >> + {
> >> + size_t __rest = numeric_limits<_IndexType>::max();
> >> + for(size_t __i = 0; __i < _Nm; ++__i)
> >> + {
> >> + if (__factors[__i] == 0)
> >> + return true;
> >> + __rest /= _IndexType(__factors[__i]);
> >> + }
> >> + return __rest > 0;
> >> + }
> >>
> > I would replace that with
> > template<IndexType>
> > consteval size_t __div_reminder(span<const size_t, _Nm> __factors, size_t
> > __val)
> > {
> > size_t __rest = val;
> > for(size_t __i = 0; __i < _Nm; ++__i)
> > {
> > if (__factors[__i] == dynamic_extent)
> > continue;
> > if (__factors[__i] != 0)
> > return val;
> > __rest /= _IndexType(__factors[__i]);
> > if (__res == 0)
> > return 0;
> > }
> > return __rest;
> > }
> >
> > We can express the is presentable check as
> > static constexpr __dyn_reminder =
> __div_reminder(__static_exts<Extents>(0,
> > rank()), std::numeric_limits<Index>::max());
> > static_assert(__dyn_reminder > 0);
> > However, with __dyn_reminder value, the precondition
> > https://eel.is/c++draft/mdspan.layout#left.cons-1,
> > can be checked by doing equivalent of __div_remainder for __dyn_extents
> > with __val being __dyn_reminder.
> >
> >
> >> +
> >> + template<typename _Extents>
> >> + consteval array<typename _Extents::index_type, _Extents::rank()>
> >> + __static_extents_array()
> >> + {
> >> + array<typename _Extents::index_type, _Extents::rank()> __exts;
> >> + for(size_t __i = 0; __i < _Extents::rank(); ++__i)
> >> + __exts[__i] = _Extents::static_extent(__i);
> >> + return __exts;
> >> + }
> >>
> >
> > Replaced by __static_exts accessor, as described above.
> >
> >
> >> +
> >> + template<typename _Extents, typename _IndexType>
> >> + concept __representable_size = _Extents::rank_dynamic() != 0
> >> + || __is_representable_product<_IndexType>(
> >> + __static_extents_array<_Extents>());
> >> +
> >> + template<typename _Extents>
> >> + concept __layout_extent = __representable_size<
> >> + _Extents, typename _Extents::index_type>;
> >> + }
> >>
> > +
> >> + template<typename _Extents>
> >> + class layout_left::mapping
> >> + {
> >> + static_assert(__mdspan::__layout_extent<_Extents>,
> >> + "The size of extents_type is not representable as index_type.");
> >> + 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;
> >> +
> >> + constexpr
> >> + mapping() noexcept = default;
> >> +
> >> + constexpr
> >> + mapping(const mapping&) noexcept = default;
> >> +
> >> + constexpr
> >> + mapping(const extents_type& __extents) noexcept
> >> + : _M_extents(__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
> >> + : _M_extents(__other.extents())
> >> + {
> >
> > Here we could do checks at compile time:
> > if constexpr(_OExtents::rank_dynamic() == 0)
> > static_assert( __div_remainder(...) > 0);
> > }
> >
> >
> >> }
> >> +
> >> + constexpr mapping&
> >> + operator=(const mapping&) noexcept = default;
> >> +
> >> + constexpr const extents_type&
> >> + extents() const noexcept { return _M_extents; }
> >> +
> >> + constexpr index_type
> >> + required_span_size() const noexcept
> >> + { return __mdspan::__fwd_prod(_M_extents, _M_extents.rank()); }
> >> +
> >> + template<__mdspan::__valid_index_type<index_type>... _Indices>
> >>
> > // Because we extracted rank0 and rank1 specializations
> >
> >> + requires (sizeof...(_Indices) + 1 == extents_type::rank())
> >> + constexpr index_type
> >> + operator()(index_type __idx, _Indices... __indices) const
> noexcept
> >> + {
> >>
> > This could be implemented as, please synchronize the names.
> > if constexpr (!is_same_v<_Indices, index_type> || ...)
> > // Reduce the number of instantations.
> > return operator()(index_type _idx0,
> > static_cast<index_type>(std::move(__indices))....);
> > else
> > {
> > // This can be used for layout stride, if you start with __res =
> 0;
> > index_type __res = _idx0;
> > index_type __mult = _M_extents.extent(0);
> > auto __update = [&__res, &__mult, __pos = 1u](index_type __idx)
> > mutable
> > {
> > __res += __idx * __mult;
> > __mult *= _M_extents.extent(__pos);
> > ++__pos;
> > };
> > // Fold over invocation of lambda
> > (__update(_Indices), ....);
> > return __res;
> > }
> >
> > This could be even simpler and written as (use for layout stride):
> > size_t __pos = 0;
> > return (index_type(0) + ... + __indices * stride(__pos++));
> > Here, I prefer to avoid multiplying multiple times.
>
> I'm unsure about the expression above. If written out naively it
> would be:
>
> return (((0 + __i[0]*stride(__pos++)) + ...) +
> __i[n-1]*stride(__pos++);
>
> This looks a lot like UB. After reading:
> https://en.cppreference.com/w/cpp/language/eval_order
Indeed, the order of evaluation causes troubles here, however, we can use
the fact that comma introduces a sequence point:
index_type __res = 0;
size_t __ pos = 0;
(_res += __indices * stride(__pos++), ....);
return __res;
>
>
> I didn't spot anything that suggests that the compiler must evaluate
> the parenthesized sub-expressions left to right. IIUC, the compiler is
> free to first compute `__i[n-1]*stride(__pos++)` and then
> `__i[0]*stride(__pos++)`.
>
> Is there an additional rule for folds?
>
> >
> >
> > + 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:
> >> + [[no_unique_address]] extents_type _M_extents;
> >> + };
> >> +
> >> _GLIBCXX_END_NAMESPACE_VERSION
> >> }
> >> #endif
> >> --
> >> 2.49.0
> >>
> >>
> >
>
>
