[PATCH v4 0/2] libstdc++: Implement padded layouts.

[Tomasz Kaminski]

On Tue, Sep 30, 2025 at 1:12 PM Tomasz Kaminski <tkaminsk@redhat.com> wrote:

> I have tested both patches on x86_64-linux
> and powerpc64le-unknown-linux-gnu locally,
> and everything passed.
> OK for trunk?
>
I have also included a follow up patch in the series:
https://gcc.gnu.org/pipermail/libstdc++/2025-October/063749.html


>
> On Mon, Sep 29, 2025 at 8:03 AM Luc Grosheintz <luc.grosheintz@gmail.com>
> 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
>>
>>
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