[PATCH v2 0/8] libstdc++: Sequence of improvements to <mdspan>.

[Luc Grosheintz]

On 8/4/25 17:42, Tomasz Kaminski wrote:
> On Mon, Aug 4, 2025 at 1:14 PM Tomasz Kaminski <tkaminsk@redhat.com> wrote:
> 
>>
>>
>> On Mon, Aug 4, 2025 at 1:08 PM Luc Grosheintz <luc.grosheintz@gmail.com>
>> wrote:
>>
>>> Hi Tomasz,
>>>
>>> Thank you for the review! Sorry about the missing parens, even after
>>> "Ctrl+F"ing each of the emails I can't find the requires clause (the
>>> two I found both need the parens).
>>>
>> I was mentioning this one, but it is possible that I am wrong on this one,
>> I haven't yet checked it, as I was planning to do it during full review.
>>      template<array _Extents>
>>        requires (__all_dynamic<_Extents>())
>>        class _StaticExtents<_Extents>;
>> But regardless, I will make that change locally if it does work, so you do
>> not
>> need to update patch series for it.
>>
> Parenthesis seem to be indeed required here, so my comment was a red
> herring.

I'm pretty sure that this is the reason why I wrongly concluded
that the pattern is `requires(cond)`, i.e. the parens are always
needed.

> 
>>
>>> Do you think it's possible to merge this series before the other two
>>> outstanding patch series? There's some risk of collision; making changes
>>> to this patch series is much more time consuming than rebasing (and
>>> retesting the other two patch series).
>>>
>> This sounds very reasonable. I will prepare a stack of patches in your
>> suggested
>> order locally, and push them once they are approved. But it still make
>>
>>>
>>> Thank you,
>>>
>> Thank you for your continued contributions.
>>
>>> Luc
>>>
>>> On 8/4/25 12:43, Tomasz Kaminski wrote:
>>>> Hi,
>>>>
>>>> I this time I made a quick pass through all changes, before commenting
>>> on
>>>> first commits,
>>>> and they look solid to me, and I haven't noticed anything I would like
>>> to
>>>> change (except parentheses
>>>> around requires, but I will handle that locally). I will try to do a
>>> full
>>>> review during this week.
>>>>
>>>> Regards,
>>>> Tomasz
>>>>
>>>> On Sun, Aug 3, 2025 at 10:59 PM Luc Grosheintz <
>>> luc.grosheintz@gmail.com>
>>>> wrote:
>>>>
>>>>> The combined effect of this sequence of change is:
>>>>>
>>>>>     * a reduction in the number of template instantiations, by
>>>>>       - avoiding needless dependency of IndexType,
>>>>>       - special formulas for low-rank extents,
>>>>>       - special formulas for (nearly) fully dynamic extents.
>>>>>
>>>>>     * improved code quality, by
>>>>>       - precomputing partial products of the static extents,
>>>>>       - special cases for low-rank extents,
>>>>>       - rewriting the condition E[i] == dynamic_extent in a more
>>>>>         optimizer friendly manner.
>>>>>       - effectively loop-unrolling extents::operator==.
>>>>>
>>>>> While simplistic micro-benchmarking shows the effectiveness of these
>>>>> changes, likely the stronger argument is presented in each commit:
>>>>>     a) each change removes needless complexity,
>>>>>     b) before/after examples of generated code show the effectiveness.
>>>>>
>>>>> Luc Grosheintz (8):
>>>>>     libstdc++: Reduce template instantiations in <mdspan>.
>>>>>     libstdc++: Precompute products of static extents.
>>>>>     libstdc++: Improve low-rank layout_{left,right}::stride.
>>>>>     libstdc++: Improve fully dynamic extents in mdspan.
>>>>>     libstdc++: Improve nearly fully dynamic extents in mdspan.
>>>>>     libstdc++: Reduce indirection in extents::extent.
>>>>>     libstdc++: Improve extents::operator==.
>>>>>     libstdc++: Replace numeric_limit with __int_traits in mdspan.
>>>>>
>>>>>    libstdc++-v3/include/std/mdspan               | 282
>>> +++++++++++++-----
>>>>>    .../mdspan/extents/class_mandates_neg.cc      |   3 +
>>>>>    2 files changed, 208 insertions(+), 77 deletions(-)
>>>>>
>>>>> --
>>>>> 2.50.0
>>>>>
>>>>>
>>>>
>>>
>>>
>