I have reviewed both patches, and posted v5 for layout_left_padded. I am
running a full test suite on them
will post them when they are finished.
One suggestion, I would consider in future. Instead of befriending
__dynamic_extents, __get_first_stride, __static_extents,
I think we could have a internal __storage function:
template<typename _ExtentsOrMapping>
const _ExtentsStorage& __storage(const _ExtentsOrMapping& __eom)
{
if constexpr (requires { __eom._M_extents})
return __mdspan::__storage(__eom._M_extents);
else
return __eom._M_storage;
}
This way __static_extents can just call __strorage(x)._M_static_extents,
and so on.
Similarly, when calling _M_equal in padded layouts operator== we could just
call
_M_equal(__mdspan::__storage(__other)).
And __get_static_stride will become __mdspan::__storage(x)._M_padstride();
We may consider having for layout_stride having an internal private class:
struct _Storage { _M_extents, __strides };
_Storage _M_storage
WDYT?
On Mon, Sep 29, 2025 at 8:03 AM Luc Grosheintz <[email protected]>
wrote:
> Changes since v3:
> - Improve comments in version.def
> - Avoid check statically asserted preconditions:
> + layout_left::mapping(LeftPaddedMapping)
> + layout_right::mapping(RightPaddedMapping)
> + _PaddedStorage(const _Extents&)
> + _PaddedStorage(const _LayoutSame&)
> + _PaddedStorage(const layout_stride&)
> + _PaddedStorage(const SamePaddedMapping&)
>
> - Use `constexpr static const size_t` in traits.
> - Reduce constraints in _PaddedStorage.
> - Use tag-types to completely eliminate constraints in _PaddedStorage;
> and
> simplify the traits.
> - Rename and inline:
> + _S_static_padextent
> + _M_padextent
> + _M_padstride
>
> - Introduce __index_type_cast that checks the representability
> and non-negativity conditions, and casts to index_type.
> - Simplify reverse.
> - Lift operator== to _PaddedStorage (as _M_equal).
> - Restructure __valid_padded_size to use _S_static_padextent.
> - Move is_padded_layout from ctors.cc to padded_traits.h
> - Eliminate _S_static_stride in the userfacing mappings.
> - Also make the multi arg ctor explicit.
> - Flatten else { if ... }.
> - Use __cplusplus > 202302L in tests.
> - Restructure _M_required_span_size
> - Add error message to a static_assert.
> - Move checking if static padding values are representable up.
> - Remove unneeded member initilizer in _PaddedStorage.
> - Fix formatting and whitespace.
>
> Not strictly requested changes:
>
> - When checking __valid_padded_size, use the fact that we
> can't handle index_type >= size_t. Therefore, only check if the padded
> size fits into index_type.
>
> - Include the bugzilla issue number in the commit messages.
> - Remove stray #include <iostream>
> - Rename make_expected to make_array.
>
> Luc Grosheintz (2):
> libstdc++: Implement std::layout_left_padded [PR110352].
> libstdc++: Implement layout_right_padded [PR110352].
>
> libstdc++-v3/include/bits/version.def | 10 +
> libstdc++-v3/include/bits/version.h | 9 +
> libstdc++-v3/include/std/mdspan | 903 +++++++++++++++++-
> libstdc++-v3/src/c++23/std.cc.in | 9 +-
> .../mdspan/layouts/class_mandate_neg.cc | 1 +
> .../23_containers/mdspan/layouts/ctors.cc | 60 +-
> .../23_containers/mdspan/layouts/empty.cc | 21 +-
> .../23_containers/mdspan/layouts/mapping.cc | 109 ++-
> .../23_containers/mdspan/layouts/padded.cc | 677 +++++++++++++
> .../mdspan/layouts/padded_neg.cc | 352 +++++++
> .../mdspan/layouts/padded_traits.h | 149 +++
> 11 files changed, 2283 insertions(+), 17 deletions(-)
> create mode 100644
> libstdc++-v3/testsuite/23_containers/mdspan/layouts/padded.cc
> create mode 100644
> libstdc++-v3/testsuite/23_containers/mdspan/layouts/padded_neg.cc
> create mode 100644
> libstdc++-v3/testsuite/23_containers/mdspan/layouts/padded_traits.h
>
> --
> 2.50.1
>
>
