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.
+ 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
>
>
