[PATCH v1 0/3] Implement aligned_accessor [P2897R7].

[Luc Grosheintz]

How would you like to track this on bugzilla:

   1. As part of submdspan:
   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110352
   https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/p2630r4.html

   2. As a new issue, because it's a different paper:
   https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p2897r7.html

On 7/3/25 12:33, Luc Grosheintz wrote:
> This patch series implements the aligned_accessor paper P2897R7 in three
> parts:
> 
>    - Implement `is_sufficiently_aligned` which is part of <memory>.
>    - Prepare the accessor tests for reuse.
>    - Implement aligned_accessor.
> 
> A couple of remarks:
> 
>    - The paper P2897R7 and spec N5008 don't specify that the aligment
>    for is_sufficiently_aligned must be a power of two.
> 
>    - The reasoning for why is_sufficiently_aligned isn't constexpr is
>    nicely described in the paper.
> 
>    - Use of `class` in is_sufficiently_aligned is for consistency within
>    that file.
> 
>    - The tests create new unsupported tests and expected failures. The
>    testsuite doesn't have all that many of those; so there's likely a
>    strategy to avoid this. However, I don't know how.
> 
>    - These changes are independent of mdspan, but due to the precise
>    location of the code it might conflict with the mdspan patch series.
> 
>    - I skipped updating `cxxapi-data.csv` for is_sufficiently_aligned
>    due to consistency with the rest of the mdspan patches, i.e. there
>    will be a bulk update of the file later.
> 
>    - Each commit was tested with/without PCH and with/without
>    _GLIBCXX_DEBUG filtered by 20_util/is_sufficiently_aligned
>    and 23_containers/mdspan.
>    The last commit was tested fully with/without PCH. All tests on
>    x86_64-linux.
> 
> As always I'm happy to reorganize into different commits, if the
> grouping doesn't make sense.
> 
> Luc Grosheintz (3):
>    libstdc++: Implement is_sufficiently_aligned.
>    libstdc++: Prepare test code for default_accessor for reuse.
>    libstdc++: Implement aligned_accessor from mdspan.
> 
>   libstdc++-v3/include/bits/align.h             |  16 ++
>   libstdc++-v3/include/bits/version.def         |  18 ++
>   libstdc++-v3/include/bits/version.h           |  20 +++
>   libstdc++-v3/include/std/mdspan               |  72 ++++++++
>   libstdc++-v3/include/std/memory               |   1 +
>   libstdc++-v3/src/c++23/std.cc.in              |   4 +-
>   .../20_util/is_sufficiently_aligned/1.cc      |  31 ++++
>   .../20_util/is_sufficiently_aligned/2.cc      |   7 +
>   .../23_containers/mdspan/accessors/aligned.cc |  43 +++++
>   .../mdspan/accessors/aligned_ftm.cc           |   6 +
>   .../mdspan/accessors/aligned_neg.cc           |  33 ++++
>   .../accessors/debug/aligned_access_neg.cc     |  23 +++
>   .../accessors/debug/aligned_offset_neg.cc     |  23 +++
>   .../23_containers/mdspan/accessors/default.cc |  99 -----------
>   .../23_containers/mdspan/accessors/generic.cc | 168 ++++++++++++++++++
>   15 files changed, 464 insertions(+), 100 deletions(-)
>   create mode 100644 libstdc++-v3/testsuite/20_util/is_sufficiently_aligned/1.cc
>   create mode 100644 libstdc++-v3/testsuite/20_util/is_sufficiently_aligned/2.cc
>   create mode 100644 libstdc++-v3/testsuite/23_containers/mdspan/accessors/aligned.cc
>   create mode 100644 libstdc++-v3/testsuite/23_containers/mdspan/accessors/aligned_ftm.cc
>   create mode 100644 libstdc++-v3/testsuite/23_containers/mdspan/accessors/aligned_neg.cc
>   create mode 100644 libstdc++-v3/testsuite/23_containers/mdspan/accessors/debug/aligned_access_neg.cc
>   create mode 100644 libstdc++-v3/testsuite/23_containers/mdspan/accessors/debug/aligned_offset_neg.cc
>   delete mode 100644 libstdc++-v3/testsuite/23_containers/mdspan/accessors/default.cc
>   create mode 100644 libstdc++-v3/testsuite/23_containers/mdspan/accessors/generic.cc
>