[v2,0/5] libstdc++: Implement std::layout_left_padded.

  Changes since v1:

  - Use cmp_* when comparing two integers of differnt types.
  - Remove the FTM submdspan and replace it with an unofficial
  purely internal FTM called padded_layouts.
  - Make __get_static_stride consteval.
  - Make __is_*_padded_layout only available inside
    __glibcxx_padded_layout regions.
  - Use size_t for __least_multiple_at_least.
  - Implement checks (mandate and prerequisite) for representable padded
    static stride.
  - Implement checks for representable padded "size", i.e. the product
    of padded extents. This product might be larger than the
    required_span_size, but the standard requires the product to be
    representable (not just required_span_size).
  - New overload for __static_extents(__begin, __end).
  - Remove the double checked prerequisite.
  - Skip some calculations for static first stride (2x).
  - Replace array<..., ...> with _Stride.
  - s/_S_stride_type/_Stride
  - Test additional cases for default ctor.
  - Rename ConversionRule::Conventional to Regular.
  - Remove empty file testsuite/mdspan/layouts/debug/padded_neg.cc.
  - Ensure all tests are run.
  - Test remaining prerequisite in mapping(layout_stride).

I also noticed that some of the changes can be split into separate
commits.

Luc Grosheintz (5):
  libstdc++: Refactor layout mapping tests to use a concept.
  libstdc++: Fix bug in layout mapping tests.
  libstdc++: Prepare mapping layout tests for left padded.
  libstdc++: Refactor __mdspan::__static_quotient.
  libstdc++: Implement std::layout_left_padded.

 libstdc++-v3/include/bits/version.def         |  10 +
 libstdc++-v3/include/bits/version.h           |   9 +
 libstdc++-v3/include/std/mdspan               | 541 +++++++++++++++-
 libstdc++-v3/src/c++23/std.cc.in              |   8 +-
 .../mdspan/layouts/class_mandate_neg.cc       |   1 +
 .../23_containers/mdspan/layouts/ctors.cc     |  61 +-
 .../mdspan/layouts/debug/padded_neg.cc        |  22 +
 .../23_containers/mdspan/layouts/empty.cc     |  12 +-
 .../23_containers/mdspan/layouts/mapping.cc   | 204 ++++--
 .../23_containers/mdspan/layouts/padded.cc    | 611 ++++++++++++++++++
 .../mdspan/layouts/padded_neg.cc              | 280 ++++++++
 11 files changed, 1692 insertions(+), 67 deletions(-)
 create mode 100644 libstdc++-v3/testsuite/23_containers/mdspan/layouts/debug/padded_neg.cc
 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
  

On Thu, Sep 11, 2025 at 1:50 PM Luc Grosheintz <luc.grosheintz@gmail.com>
wrote:

> Changes since v1:
>
>   - Use cmp_* when comparing two integers of differnt types.
>   - Remove the FTM submdspan and replace it with an unofficial
>   purely internal FTM called padded_layouts.
>   - Make __get_static_stride consteval.
>   - Make __is_*_padded_layout only available inside
>     __glibcxx_padded_layout regions.
>   - Use size_t for __least_multiple_at_least.
>   - Implement checks (mandate and prerequisite) for representable padded
>     static stride.
>   - Implement checks for representable padded "size", i.e. the product
>     of padded extents. This product might be larger than the
>     required_span_size, but the standard requires the product to be
>     representable (not just required_span_size).
>   - New overload for __static_extents(__begin, __end).
>   - Remove the double checked prerequisite.
>   - Skip some calculations for static first stride (2x).
>   - Replace array<..., ...> with _Stride.
>   - s/_S_stride_type/_Stride
>   - Test additional cases for default ctor.
>   - Rename ConversionRule::Conventional to Regular.
>   - Remove empty file testsuite/mdspan/layouts/debug/padded_neg.cc.
>   - Ensure all tests are run.
>   - Test remaining prerequisite in mapping(layout_stride).
>
> I also noticed that some of the changes can be split into separate
> commits.
>
Hi,
The first four patches LGTM, however I will put them in one patch,
that just least all made changes, they are really small.
I do not think I will be able to review layout_left_padded today fully,
and I will continue review in two weeks. I will post a partial notes
marking them as so.


>
> Luc Grosheintz (5):
>   libstdc++: Refactor layout mapping tests to use a concept.
>   libstdc++: Fix bug in layout mapping tests.
>   libstdc++: Prepare mapping layout tests for left padded.
>   libstdc++: Refactor __mdspan::__static_quotient.
>   libstdc++: Implement std::layout_left_padded.
>
>  libstdc++-v3/include/bits/version.def         |  10 +
>  libstdc++-v3/include/bits/version.h           |   9 +
>  libstdc++-v3/include/std/mdspan               | 541 +++++++++++++++-
>  libstdc++-v3/src/c++23/std.cc.in              |   8 +-
>  .../mdspan/layouts/class_mandate_neg.cc       |   1 +
>  .../23_containers/mdspan/layouts/ctors.cc     |  61 +-
>  .../mdspan/layouts/debug/padded_neg.cc        |  22 +
>  .../23_containers/mdspan/layouts/empty.cc     |  12 +-
>  .../23_containers/mdspan/layouts/mapping.cc   | 204 ++++--
>  .../23_containers/mdspan/layouts/padded.cc    | 611 ++++++++++++++++++
>  .../mdspan/layouts/padded_neg.cc              | 280 ++++++++
>  11 files changed, 1692 insertions(+), 67 deletions(-)
>  create mode 100644
> libstdc++-v3/testsuite/23_containers/mdspan/layouts/debug/padded_neg.cc
>  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
>
> --
> 2.50.1
>
>
  

On 9/11/25 14:41, Tomasz Kaminski wrote:
> On Thu, Sep 11, 2025 at 1:50 PM Luc Grosheintz <luc.grosheintz@gmail.com>
> wrote:
> 
>> Changes since v1:
>>
>>    - Use cmp_* when comparing two integers of differnt types.
>>    - Remove the FTM submdspan and replace it with an unofficial
>>    purely internal FTM called padded_layouts.
>>    - Make __get_static_stride consteval.
>>    - Make __is_*_padded_layout only available inside
>>      __glibcxx_padded_layout regions.
>>    - Use size_t for __least_multiple_at_least.
>>    - Implement checks (mandate and prerequisite) for representable padded
>>      static stride.
>>    - Implement checks for representable padded "size", i.e. the product
>>      of padded extents. This product might be larger than the
>>      required_span_size, but the standard requires the product to be
>>      representable (not just required_span_size).
>>    - New overload for __static_extents(__begin, __end).
>>    - Remove the double checked prerequisite.
>>    - Skip some calculations for static first stride (2x).
>>    - Replace array<..., ...> with _Stride.
>>    - s/_S_stride_type/_Stride
>>    - Test additional cases for default ctor.
>>    - Rename ConversionRule::Conventional to Regular.
>>    - Remove empty file testsuite/mdspan/layouts/debug/padded_neg.cc.
>>    - Ensure all tests are run.
>>    - Test remaining prerequisite in mapping(layout_stride).
>>
>> I also noticed that some of the changes can be split into separate
>> commits.
>>
> Hi,
> The first four patches LGTM, however I will put them in one patch,
> that just least all made changes, they are really small.
> I do not think I will be able to review layout_left_padded today fully,
> and I will continue review in two weeks. I will post a partial notes
> marking them as so.

Thank you for the reviews. Sorry for reacting slowly, but it might be
easiest to postpone the review. I'll likely be able to make progress
on layout_right_padded (now that many things are cleared up).

The right-padded layout will likely require that I generalize certain
concepts, e.g. a "padded stride" instead of a first/last stride.

> 
> 
>>
>> Luc Grosheintz (5):
>>    libstdc++: Refactor layout mapping tests to use a concept.
>>    libstdc++: Fix bug in layout mapping tests.
>>    libstdc++: Prepare mapping layout tests for left padded.
>>    libstdc++: Refactor __mdspan::__static_quotient.
>>    libstdc++: Implement std::layout_left_padded.
>>
>>   libstdc++-v3/include/bits/version.def         |  10 +
>>   libstdc++-v3/include/bits/version.h           |   9 +
>>   libstdc++-v3/include/std/mdspan               | 541 +++++++++++++++-
>>   libstdc++-v3/src/c++23/std.cc.in              |   8 +-
>>   .../mdspan/layouts/class_mandate_neg.cc       |   1 +
>>   .../23_containers/mdspan/layouts/ctors.cc     |  61 +-
>>   .../mdspan/layouts/debug/padded_neg.cc        |  22 +
>>   .../23_containers/mdspan/layouts/empty.cc     |  12 +-
>>   .../23_containers/mdspan/layouts/mapping.cc   | 204 ++++--
>>   .../23_containers/mdspan/layouts/padded.cc    | 611 ++++++++++++++++++
>>   .../mdspan/layouts/padded_neg.cc              | 280 ++++++++
>>   11 files changed, 1692 insertions(+), 67 deletions(-)
>>   create mode 100644
>> libstdc++-v3/testsuite/23_containers/mdspan/layouts/debug/padded_neg.cc
>>   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
>>
>> --
>> 2.50.1
>>
>>
>
  

On 9/11/25 3:58 PM, Luc Grosheintz wrote:
> 
> 
> On 9/11/25 14:41, Tomasz Kaminski wrote:
>> On Thu, Sep 11, 2025 at 1:50 PM Luc Grosheintz <luc.grosheintz@gmail.com>
>> wrote:
>>
>>> Changes since v1:
>>>
>>>    - Use cmp_* when comparing two integers of differnt types.
>>>    - Remove the FTM submdspan and replace it with an unofficial
>>>    purely internal FTM called padded_layouts.
>>>    - Make __get_static_stride consteval.
>>>    - Make __is_*_padded_layout only available inside
>>>      __glibcxx_padded_layout regions.
>>>    - Use size_t for __least_multiple_at_least.
>>>    - Implement checks (mandate and prerequisite) for representable 
>>> padded
>>>      static stride.
>>>    - Implement checks for representable padded "size", i.e. the product
>>>      of padded extents. This product might be larger than the
>>>      required_span_size, but the standard requires the product to be
>>>      representable (not just required_span_size).
>>>    - New overload for __static_extents(__begin, __end).
>>>    - Remove the double checked prerequisite.
>>>    - Skip some calculations for static first stride (2x).
>>>    - Replace array<..., ...> with _Stride.
>>>    - s/_S_stride_type/_Stride
>>>    - Test additional cases for default ctor.
>>>    - Rename ConversionRule::Conventional to Regular.
>>>    - Remove empty file testsuite/mdspan/layouts/debug/padded_neg.cc.
>>>    - Ensure all tests are run.
>>>    - Test remaining prerequisite in mapping(layout_stride).
>>>
>>> I also noticed that some of the changes can be split into separate
>>> commits.
>>>
>> Hi,
>> The first four patches LGTM, however I will put them in one patch,
>> that just least all made changes, they are really small.
>> I do not think I will be able to review layout_left_padded today fully,
>> and I will continue review in two weeks. I will post a partial notes
>> marking them as so.
> 
> Thank you for the reviews. Sorry for reacting slowly, but it might be
> easiest to postpone the review. I'll likely be able to make progress
> on layout_right_padded (now that many things are cleared up).
> 
> The right-padded layout will likely require that I generalize certain
> concepts, e.g. a "padded stride" instead of a first/last stride.
> 

Earlier last week, I implemented the layout_right_padded. It's
essentially identical if one replaces
   s/0/rank-1/
   s/1/rank-2/

(more or less). One can avoid duplicating the entire logic
for checking mandates/prerequisites by creating a object
"storage" that contains the _M_stride and _M_extents.

The padded tests all get indented, because one needs to pass
in the layout template; and some other changes to make it
work for both layouts.

Since a lot of code changes/moves around, how would you like
to review it:

   1. A v3 with just the fixes for layout_left_padded. Then
   a refactor commit. Followed by layout_right_padded?

   2. Squash the refactoring into the v3 of layout_left_padded.
   Then add layout_right_padded?

>>
>>
>>>
>>> Luc Grosheintz (5):
>>>    libstdc++: Refactor layout mapping tests to use a concept.
>>>    libstdc++: Fix bug in layout mapping tests.
>>>    libstdc++: Prepare mapping layout tests for left padded.
>>>    libstdc++: Refactor __mdspan::__static_quotient.
>>>    libstdc++: Implement std::layout_left_padded.
>>>
>>>   libstdc++-v3/include/bits/version.def         |  10 +
>>>   libstdc++-v3/include/bits/version.h           |   9 +
>>>   libstdc++-v3/include/std/mdspan               | 541 +++++++++++++++-
>>>   libstdc++-v3/src/c++23/std.cc.in              |   8 +-
>>>   .../mdspan/layouts/class_mandate_neg.cc       |   1 +
>>>   .../23_containers/mdspan/layouts/ctors.cc     |  61 +-
>>>   .../mdspan/layouts/debug/padded_neg.cc        |  22 +
>>>   .../23_containers/mdspan/layouts/empty.cc     |  12 +-
>>>   .../23_containers/mdspan/layouts/mapping.cc   | 204 ++++--
>>>   .../23_containers/mdspan/layouts/padded.cc    | 611 ++++++++++++++++++
>>>   .../mdspan/layouts/padded_neg.cc              | 280 ++++++++
>>>   11 files changed, 1692 insertions(+), 67 deletions(-)
>>>   create mode 100644
>>> libstdc++-v3/testsuite/23_containers/mdspan/layouts/debug/padded_neg.cc
>>>   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
>>>
>>> -- 
>>> 2.50.1
>>>
>>>
>>
>
  

On Mon, Sep 22, 2025 at 7:40 AM Luc Grosheintz <luc.grosheintz@gmail.com>
wrote:

>
>
> On 9/11/25 3:58 PM, Luc Grosheintz wrote:
> >
> >
> > On 9/11/25 14:41, Tomasz Kaminski wrote:
> >> On Thu, Sep 11, 2025 at 1:50 PM Luc Grosheintz <
> luc.grosheintz@gmail.com>
> >> wrote:
> >>
> >>> Changes since v1:
> >>>
> >>>    - Use cmp_* when comparing two integers of differnt types.
> >>>    - Remove the FTM submdspan and replace it with an unofficial
> >>>    purely internal FTM called padded_layouts.
> >>>    - Make __get_static_stride consteval.
> >>>    - Make __is_*_padded_layout only available inside
> >>>      __glibcxx_padded_layout regions.
> >>>    - Use size_t for __least_multiple_at_least.
> >>>    - Implement checks (mandate and prerequisite) for representable
> >>> padded
> >>>      static stride.
> >>>    - Implement checks for representable padded "size", i.e. the product
> >>>      of padded extents. This product might be larger than the
> >>>      required_span_size, but the standard requires the product to be
> >>>      representable (not just required_span_size).
> >>>    - New overload for __static_extents(__begin, __end).
> >>>    - Remove the double checked prerequisite.
> >>>    - Skip some calculations for static first stride (2x).
> >>>    - Replace array<..., ...> with _Stride.
> >>>    - s/_S_stride_type/_Stride
> >>>    - Test additional cases for default ctor.
> >>>    - Rename ConversionRule::Conventional to Regular.
> >>>    - Remove empty file testsuite/mdspan/layouts/debug/padded_neg.cc.
> >>>    - Ensure all tests are run.
> >>>    - Test remaining prerequisite in mapping(layout_stride).
> >>>
> >>> I also noticed that some of the changes can be split into separate
> >>> commits.
> >>>
> >> Hi,
> >> The first four patches LGTM, however I will put them in one patch,
> >> that just least all made changes, they are really small.
> >> I do not think I will be able to review layout_left_padded today fully,
> >> and I will continue review in two weeks. I will post a partial notes
> >> marking them as so.
> >
> > Thank you for the reviews. Sorry for reacting slowly, but it might be
> > easiest to postpone the review. I'll likely be able to make progress
> > on layout_right_padded (now that many things are cleared up).
> >
> > The right-padded layout will likely require that I generalize certain
> > concepts, e.g. a "padded stride" instead of a first/last stride.
> >
>
> Earlier last week, I implemented the layout_right_padded. It's
> essentially identical if one replaces
>    s/0/rank-1/
>    s/1/rank-2/
>
> (more or less). One can avoid duplicating the entire logic
> for checking mandates/prerequisites by creating a object
> "storage" that contains the _M_stride and _M_extents.
>
We could then also provide enough interface for this storage object,
so it could be used with indexing operators, if we make them templated
on just typename _Extents. I think just having an extent function that
would return
adjusted for stride will be sufficient.

I think we could do something similar with representable size and empty,
for storage object:
__static_extents -> drop the leftmost/rightmost extent
__dynamic_extents -> drop the leftmost/rightmost extent
__padded_extents -> return:
  * integral_constant<size_t, 1> for non-padded layouts, this way we can
always
    add it, but it will not affect computation
  * integral_constant<size_t, X> for padded with static strides
  * size_t for padded with dynamic stride
Or __padded_extents could return:
  extents<IndexType> const& for extents, just have constexpr inline global
  variable that we would return address off,
  extents<IndexType, X> const& for padded storage.


>
> The padded tests all get indented, because one needs to pass
> in the layout template; and some other changes to make it
> work for both layouts.
>
> Since a lot of code changes/moves around, how would you like
> to review it:
>
>    1. A v3 with just the fixes for layout_left_padded. Then
>    a refactor commit. Followed by layout_right_padded?
>
>    2. Squash the refactoring into the v3 of layout_left_padded.
>    Then add layout_right_padded?
>
If the suggestion above would allow us to merge a lot of the now separate
code,
I would prefer squashed refactoring of v3 of layout_left padded.


>
> >>
> >>
> >>>
> >>> Luc Grosheintz (5):
> >>>    libstdc++: Refactor layout mapping tests to use a concept.
> >>>    libstdc++: Fix bug in layout mapping tests.
> >>>    libstdc++: Prepare mapping layout tests for left padded.
> >>>    libstdc++: Refactor __mdspan::__static_quotient.
> >>>    libstdc++: Implement std::layout_left_padded.
> >>>
> >>>   libstdc++-v3/include/bits/version.def         |  10 +
> >>>   libstdc++-v3/include/bits/version.h           |   9 +
> >>>   libstdc++-v3/include/std/mdspan               | 541 +++++++++++++++-
> >>>   libstdc++-v3/src/c++23/std.cc.in              |   8 +-
> >>>   .../mdspan/layouts/class_mandate_neg.cc       |   1 +
> >>>   .../23_containers/mdspan/layouts/ctors.cc     |  61 +-
> >>>   .../mdspan/layouts/debug/padded_neg.cc        |  22 +
> >>>   .../23_containers/mdspan/layouts/empty.cc     |  12 +-
> >>>   .../23_containers/mdspan/layouts/mapping.cc   | 204 ++++--
> >>>   .../23_containers/mdspan/layouts/padded.cc    | 611
> ++++++++++++++++++
> >>>   .../mdspan/layouts/padded_neg.cc              | 280 ++++++++
> >>>   11 files changed, 1692 insertions(+), 67 deletions(-)
> >>>   create mode 100644
> >>> libstdc++-v3/testsuite/23_containers/mdspan/layouts/debug/padded_neg.cc
> >>>   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
> >>>
> >>> --
> >>> 2.50.1
> >>>
> >>>
> >>
> >
>
>