Re: [PATCH v2] libstdc++: Skip locale in chrono operator<< for integer seconds precision

On Mon, 27 Jul 2026 at 08:41, Tomasz Kaminski <[email protected]> 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 <[email protected]> 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 <[email protected]>
>>
> Changed the above to:
>  Reviewed-by: Tomasz Kamiński <[email protected]>
>
>> Signed-off-by: Anlai Lu <[email protected]>
>> ---
>> 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
>>
>>