On 9/30/25 10:29 AM, Tomasz Kaminski wrote:
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;
}
I like the idea of removing the numerous friends. I'm not
fully sold on the idea that sometimes it calls the storage
of the object (i.e. for padded layouts and extents); and
sometimes is returns the storage of _M_extents. It's likely
easily fixed by giving everything an _M_storage (as you
propose further down. I feels like it's made to make other
code convenient, but right now (and that often changes with
time) I don't have a good way of thinking of __storage without
remembering if-conditions.
Making layout_left and layout_right DRYer by using a similar
storage object as the padded layouts is also appealing.
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
