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

[Tomasz Kaminski]

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.

>
> - 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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