[PATCH v3 0/2] libstdc++: Optimize chrono ostream insertion via __chrono_write

[Tomasz Kaminski]

The changes I have requested are minimal, so I will do them locally before
pushing once the patches
are approved. No need to post new revision.

On Tue, Jun 30, 2026 at 3:52 PM Tomasz Kaminski <tkaminsk@redhat.com> wrote:

>
>
> On Tue, Jun 30, 2026 at 3:26 PM Tomasz Kaminski <tkaminsk@redhat.com>
> wrote:
>
>>
>>
>> On Tue, Jun 30, 2026 at 3:10 PM Tomasz Kaminski <tkaminsk@redhat.com>
>> wrote:
>>
>>>
>>>
>>> On Tue, Jun 30, 2026 at 2:52 PM Anlai Lu <agicy@qq.com> wrote:
>>>
>>>> This is v3 of the patch series.  Performance data: see v2 cover letter.
>>>> Results are unchanged -- same stack-buffer + __ostream_insert approach.
>>>>
>>>> Patch 1 is unchanged from v2.
>>>>
>>>> Changes in Patch 2 from v2:
>>>>
>>>> - Dropped the format string parameter from __chrono_write.
>>>>   _S_empty_fs() is used directly, per Tomasz Kaminski's suggestion.
>>>>   All chrono types now share the same two call forms:
>>>>     __detail::__chrono_write(__os, __arg);
>>>>     __detail::__chrono_write(__os, __arg, __os.getloc());
>>>>
>>>> - Removed __detail::__empty_fmt: no longer needed.
>>>>
>>>> - All remaining operator<< that used basic_stringstream now use
>>>>   __chrono_write (month_day, month_day_last, month_weekday,
>>>>   month_weekday_last, year_month, year_month_day_last,
>>>>   year_month_weekday, year_month_weekday_last, sys_days, local_time).
>>>>
>>>> - The only types not converted: duration (must forward stream flags
>>>>   and precision) and local_info (uses __formatter_chrono_info).
>>>>
>>> I think for local_info the __chrono_write would also give a correct
>>> result,
>>> the formatter<local_info> uses __formatter_chrono_info and calls to
>>> format_to
>>> will  pick it up.
>>>
>> However, now I wonder if it the output fits in 128B buffer, if it does I
>> think the reduced
>> duplication of the code would be beneficial (the _M_format_to overload
>> for local_info)
>> is esentially copy of operator<<.
>>
> However, with update to local_info we are no longer performing any
> allocations in
> operator<<, so  this will be strictly be a regression in performance. So I
> think you made
> a right call, to not change it. It would be nice to put summary of that in
> the commit message.
>
>
>> Previously std::format("{}", t) format was implemented by delegating
>> throu operator<<,
>> and I have reimplemented all the formatters (outside duration) to be
>> independed,
>> some time ago. And then flipping the logic to use operator<< instead of
>> formatter was
>> on my TODO list. So thank you for picking this up.
>>
>
>>
>>>
>>>> - Uses std::format_to_n (public API) instead of __do_vformat_to_n.
>>>>
>>>> Tested on x86_64-linux-gnu.  libstdc++ testsuite (std/time/*) clean.
>>>> All converted types verified byte-identical across C, en_US, de_DE,
>>>> fr_FR, and zh_CN locales.
>>>>
>>> Do you see same kind of improvements as with your original patch?
>>>
>>>>
>>>> Anlai Lu (2):
>>>>   libstdc++: Add stream state tests for chrono operator<<
>>>>   libstdc++: Use __chrono_write via _S_empty_fs for chrono ostream
>>>>     insertion
>>>>
>>>>  libstdc++-v3/include/bits/chrono_io.h         | 227 ++++--------------
>>>>  .../testsuite/std/time/ostream_insert.cc      | 163 +++++++++++++
>>>>  2 files changed, 212 insertions(+), 178 deletions(-)
>>>>  create mode 100644 libstdc++-v3/testsuite/std/time/ostream_insert.cc
>>>>
>>>> --
>>>> 2.34.1
>>>>
>>>>
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