[PATCH v1] libstdc++: Implement std::layout_left_padded.

[Luc Grosheintz]

On 9/11/25 9:40 AM, Tomasz Kaminski wrote:
>>>>
>>>>     ---------------------------------------------------------
>>>>
>>>> The tests contain a function:
>>>>
>>>>       constexpr bool
>>>>       is_same_mapping(const auto& lhs, const auto& rhs)
>>>>         // ...
>>>>         for (size_t i = 0; i < lhs.extents().rank(); ++i)
>>>>           if (lhs.stride(i) != rhs.stride(i))
>>>>             return false;
>>>>         return true;
>>>>       }
>>>>
>>>> The above is accidentally quadratic, because computing all strides can
>>>> be done in linear time, but not if one computes them one by one, then
>>>> it's quadratic. It's annoyingly hard to do in linear time, because it
>>>> requires both precise knowledge of the definition of `stride` for each
>>>> layout; and ideally it also requires access to implementation details
>>>> related to how we separate static and dynamic extents to be able to
>>>> compute static products at compile time.
>>>>
>>> I think this is fine for tests, as long as they run in reasonable time,
>>> which seems to be the case one.
>>
>> Fully agree. However, it seems like computing all strides
>> in one go is a "common" operation that with the current API
>> users are force to do a) manually and b) in quadratic time.
>> Moreover, some layouts have a strides() member, others don't.
>> It would be easier if they all had one :)
>>
> I think requiring them for all mappings (including user-defined ones)
> is no go. However, we could add them for standard defined ones, however
> That is a design change and would require a separate committee paper.
> 

What are the chances of such a paper succeeding? I suspect it would
be a short paper.

>>>> Do we need a member strides() for all strided layout mappings to avoid
>>>> the issue?
>>
> We cannot add them, if the standard does not specify them.
> 

Yes, I should have been clearer on that point. Add referred to getting
it standardized, i.e. add to the standard and then add to libstdc++.