[PATCH v2] libstdc++: Skip locale in chrono operator<< for integer seconds precision
Jonathan Wakely
jwakely.gcc@gmail.com
Sun Aug 30 11:33:18 GMT 2026
He's waiting for me to approve it.
On Sun, 30 Aug 2026, 10:16 Anlai Lu, <agicy@qq.com> wrote:
> Hi Tomasz,
>
> Thanks for merging. I checked trunk but couldn't find the patch. Could you
> confirm it was pushed, or give me the commit hash?
>
> Best regards,
> Anlai
>
> > On Aug 4, 2026, at 14:33, Tomasz Kaminski <tkaminsk@redhat.com> wrote:
> >
> > Hi Anlai,
> >
> > Patch was merged, thanks again for doing it.
> >
> > Regards,
> > Tomasz
> >
> > On Mon, Aug 3, 2026 at 6:17 PM Jonathan Wakely <jwakely@redhat.com>
> wrote:
> >
> >
> > On Mon, 27 Jul 2026 at 08:41, Tomasz Kaminski <tkaminsk@redhat.com>
> wrote:
> > Thanks this looks good to me. I will handle merging it once approved,
> > with adjustments to the changelog listed below:
> >
> > OK with those changes, thanks.
> >
> >
> > On Mon, Jul 27, 2026 at 5:29 AM Anlai Lu <agicy@qq.com> wrote:
> > When _Duration uses integer seconds precision (period::den == 1,
> > not floating-point), time_point formatting has no sub-second
> > digits and therefore no locale-dependent components. Add
> > __detail::__chrono_write_time that conditionally skips the locale
> > argument for integer-second time_points, eliminating the
> > format_to_n locale overload and the basic_format_context
> > locale member initialization.
> >
> > Also add [[__gnu__::__always_inline__]] to
> > __formatter_chrono::_M_subsecs to mitigate a small mixed-TU
> > inlining regression.
> >
> > libstdc++-v3/ChangeLog:
> >
> > * include/bits/chrono_io.h
> > (__detail::__chrono_write_time): New function that conditionally
> > skips locale for integer-second time_points.
> > (__formatter_chrono::_M_subsecs): Add __always_inline__.
> > (operator<< for hh_mm_ss): Use if constexpr to skip locale
> > for integer-second durations.
> > (operator<< for sys_time, utc_time, tai_time, gps_time,
> > file_time, local_time, zoned_time): Use __chrono_write_time.
> > Changed it to:
> > * include/bits/chrono_io.h (__detail::__chrono_write_time):
> > New function that conditionally skips locale for
> integer-second
> > time_points.
> > (__formatter_chrono::_M_subsecs): Add __always_inline__.
> > (operator<<(basic_ostream<...>, const hh_mm_ss<_Duration>&)):
> > Use if constexpr to skip locale for integer-second durations.
> > (operator<<(basic_ostream<...>, const
> zoned_time<_Duration>&))
> > (operator<<(basic_ostream<...>, const sys_time<_Duration>&))
> > (operator<<(basic_ostream<...>, const utc_time<_Duration>&))
> > (operator<<(basic_ostream<...>, const tai_time<_Duration>&))
> > (operator<<(basic_ostream<...>, const file_time<_Duration>&))
> > (operator<<(basic_ostream<...>, const
> local_time<_Duration>&)):
> > Use __chrono_write_time.
> > (operator<<(basic_ostream<...>, const sys_days<_Duration>&)):
> > Remove trailing semicolon.
> >
> >
> > Suggested-by: Tomasz Kamiński <tkaminsk@redhat.com>
> > Changed the above to:
> > Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
> > Signed-off-by: Anlai Lu <agicy@qq.com>
> > ---
> > Performance data (Xeon, -O2, core pinned, turbo off):
> >
> > den==1 types instructions branches
> > --------------- ------------ --------
> > sys_time_s -5.0% -11.3%
> > utc_time_s -5.0% -11.3%
> > tai_time_s -5.0% -11.3%
> > gps_time_s -5.0% -11.3%
> > file_time_s -5.0% -11.3%
> > local_time_s -5.1% -11.3%
> > zoned_time_s -2.8% -5.3%
> > hh_mm_ss_s -7.1% -15.7%
> >
> > den!=1:
> > sys_time_ms +0.3% +0.6%
> > utc_time_ms +0.3% +0.6%
> > local_time_ms +0.3% +0.6%
> > hh_mm_ss_ms +0.4% +0.7%
> >
> > ---
> > libstdc++-v3/include/bits/chrono_io.h | 44 +++++++++++++++++++++------
> > 1 file changed, 34 insertions(+), 10 deletions(-)
> >
> > diff --git a/libstdc++-v3/include/bits/chrono_io.h
> b/libstdc++-v3/include/bits/chrono_io.h
> > index c5170368f..2cbd61ccd 100644
> > --- a/libstdc++-v3/include/bits/chrono_io.h
> > +++ b/libstdc++-v3/include/bits/chrono_io.h
> > @@ -1656,6 +1656,7 @@ namespace __format
> > }
> >
> > template<typename _OutIter, typename _FormatContext>
> > + [[__gnu__::__always_inline__]]
> > _OutIter
> > _M_subsecs(const _ChronoData<_CharT>& __t, _OutIter __out,
> > _FormatContext& __ctx) const
> > @@ -3635,6 +3636,23 @@ namespace __detail
> > return std::__ostream_insert(__os, __s.data(), __s.size());
> > }
> >
> > + // Wrapper around __chrono_write that skips locale for
> > + // integer-second time_points.
> > + template<size_t _BufSize, typename _TimePoint, typename _CharT,
> > + typename _Traits>
> > + [[__gnu__::__always_inline__]]
> > + inline basic_ostream<_CharT, _Traits>&
> > + __chrono_write_time(basic_ostream<_CharT, _Traits>& __os,
> > + const _TimePoint& __tp)
> > + {
> > + using _Duration = typename _TimePoint::duration;
> > + if constexpr (!treat_as_floating_point_v<typename _Duration::rep>
> > + && _Duration::period::den == 1)
> > + return __chrono_write<_BufSize>(__os, __tp);
> > + else
> > + return __chrono_write<_BufSize>(__os, __tp, __os.getloc());
> > + }
> > +
> > } // namespace __detail
> > /// @endcond
> >
> > @@ -3740,7 +3758,7 @@ namespace __detail
> > operator<<(basic_ostream<_CharT, _Traits>& __os,
> > const weekday_last& __wdl)
> > { return __detail::__chrono_write<128>(__os, __wdl, __os.getloc());
> }
> > -
> > +
> > template<typename _CharT, typename _Traits>
> > inline basic_ostream<_CharT, _Traits>&
> > operator<<(basic_ostream<_CharT, _Traits>& __os, const month_day&
> __md)
> > @@ -3846,7 +3864,13 @@ namespace __detail
> > inline basic_ostream<_CharT, _Traits>&
> > operator<<(basic_ostream<_CharT, _Traits>& __os,
> > const hh_mm_ss<_Duration>& __hms)
> > - { return __detail::__chrono_write<64>(__os, __hms, __os.getloc()); }
> > + {
> > + if constexpr (!treat_as_floating_point_v<typename _Duration::rep>
> > + && _Duration::period::den == 1)
> > + return __detail::__chrono_write<64>(__os, __hms);
> > + else
> > + return __detail::__chrono_write<64>(__os, __hms, __os.getloc());
> > + }
> >
> > #if _GLIBCXX_USE_CXX11_ABI || ! _GLIBCXX_USE_DUAL_ABI
> > /// Writes a sys_info object to an ostream in an unspecified format.
> > @@ -3866,7 +3890,7 @@ namespace __detail
> > inline basic_ostream<_CharT, _Traits>&
> > operator<<(basic_ostream<_CharT, _Traits>& __os,
> > const zoned_time<_Duration, _TimeZonePtr>& __t)
> > - { return __detail::__chrono_write<128>(__os, __t, __os.getloc()); }
> > + { return __detail::__chrono_write_time<128>(__os, __t); }
> > #endif
> >
> > template<typename _CharT, typename _Traits, typename _Duration>
> > @@ -3875,12 +3899,12 @@ namespace __detail
> > inline basic_ostream<_CharT, _Traits>&
> > operator<<(basic_ostream<_CharT, _Traits>& __os,
> > const sys_time<_Duration>& __tp)
> > - { return __detail::__chrono_write<64>(__os, __tp, __os.getloc()); }
> > + { return __detail::__chrono_write_time<64>(__os, __tp); }
> >
> > template<typename _CharT, typename _Traits>
> > inline basic_ostream<_CharT, _Traits>&
> > operator<<(basic_ostream<_CharT, _Traits>& __os, const sys_days&
> __dp)
> > - { return __detail::__chrono_write<32>(__os, __dp); };
> > + { return __detail::__chrono_write<32>(__os, __dp); }
> >
> > template<typename _CharT, typename _Traits, typename _Duration,
> > typename _Alloc = allocator<_CharT>>
> > @@ -3914,7 +3938,7 @@ namespace __detail
> > inline basic_ostream<_CharT, _Traits>&
> > operator<<(basic_ostream<_CharT, _Traits>& __os,
> > const utc_time<_Duration>& __t)
> > - { return __detail::__chrono_write<64>(__os, __t, __os.getloc()); }
> > + { return __detail::__chrono_write_time<64>(__os, __t); }
> >
> > template<typename _CharT, typename _Traits, typename _Duration,
> > typename _Alloc = allocator<_CharT>>
> > @@ -3946,7 +3970,7 @@ namespace __detail
> > inline basic_ostream<_CharT, _Traits>&
> > operator<<(basic_ostream<_CharT, _Traits>& __os,
> > const tai_time<_Duration>& __t)
> > - { return __detail::__chrono_write<64>(__os, __t, __os.getloc()); }
> > + { return __detail::__chrono_write_time<64>(__os, __t); }
> >
> > template<typename _CharT, typename _Traits, typename _Duration,
> > typename _Alloc = allocator<_CharT>>
> > @@ -3982,7 +4006,7 @@ namespace __detail
> > inline basic_ostream<_CharT, _Traits>&
> > operator<<(basic_ostream<_CharT, _Traits>& __os,
> > const gps_time<_Duration>& __t)
> > - { return __detail::__chrono_write<64>(__os, __t, __os.getloc()); }
> > + { return __detail::__chrono_write_time<64>(__os, __t); }
> >
> > template<typename _CharT, typename _Traits, typename _Duration,
> > typename _Alloc = allocator<_CharT>>
> > @@ -4017,7 +4041,7 @@ namespace __detail
> > inline basic_ostream<_CharT, _Traits>&
> > operator<<(basic_ostream<_CharT, _Traits>& __os,
> > const file_time<_Duration>& __t)
> > - { return __detail::__chrono_write<64>(__os, __t, __os.getloc()); }
> > + { return __detail::__chrono_write_time<64>(__os, __t); }
> >
> > template<typename _CharT, typename _Traits, typename _Duration,
> > typename _Alloc = allocator<_CharT>>
> > @@ -4040,7 +4064,7 @@ namespace __detail
> > // _GLIBCXX_RESOLVE_LIB_DEFECTS
> > // 4257. Stream insertion for chrono::local_time should be
> constrained
> > requires requires(const sys_time<_Duration>& __st) { __os << __st; }
> > - { return __detail::__chrono_write<64>(__os, __lt, __os.getloc()); }
> > + { return __detail::__chrono_write_time<64>(__os, __lt); }
> >
> > template<typename _CharT, typename _Traits, typename _Duration,
> > typename _Alloc = allocator<_CharT>>
> > --
> > 2.34.1
> >
>
>
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