> On Jul 24, 2026, at 22:27, 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 an
> overload of __detail::__chrono_write that conditionally skips
> the locale argument, eliminating the format_to_n locale overload
> and the basic_format_context locale member initialization.
>
> libstdc++-v3/ChangeLog:
>
> * include/bits/chrono_io.h
> (__detail::__chrono_write): New overload skipping locale
> for integer-second durations.
> (operator<< for hh_mm_ss, sys_time, utc_time, tai_time,
> gps_time, file_time, local_time, zoned_time): Use it.
>
> Suggested-by: Tomasz Kamiński <[email protected]>
> Signed-off-by: Anlai Lu <[email protected]>
> ---
>
> Performance data (Xeon, -O2, core pinned, turbo off, 50M iters):
>
> den==1 types instructions branches
> --------------- ------------ --------
> sys_time_s -4.2% -9.6%
> utc_time_s -3.8% -8.7%
> tai_time_s -4.2% -9.6%
> gps_time_s -4.2% -9.6%
> file_time_s -4.2% -9.6%
> local_time_s -4.2% -9.6%
> zoned_time_s -2.4% -4.6%
> hh_mm_ss_s -6.0% -13.6%
>
> den!=1 (control):
> sys_time_ms +1.0% +1.1%
> utc_time_ms +1.0% +1.1%
> local_time_ms +1.0% +1.1%
> hh_mm_ss_ms +1.3% +1.4%
>
> The den==1 improvement comes from compile-time removal of the
> format_to_n locale overload and basic_format_context locale
> member. The den!=1 +1.0% is a known measurement artifact from
Quick follow-up. Not a measurement artifact, a codegen artifact.
The mechanism is a GCC codegen side effect from mixed-TU compilation:
when den==1 and den!=1 instantiations coexist in the same translation
unit, GCC's inlining heuristics may shift slightly, de-inlining
__formatter_chrono::_M_subsecs (and a few other helpers).
This adds call/return overhead for those functions. Isolated
single-type TUs are indeed byte-identical to the baseline.
The regression can be reduced from +1.0% to +0.3% with
[[gnu::always_inline]] on _M_subsecs. This patch does not include that
change.
> mixing 12 time_point instantiations in one translation unit;
> isolated single-type tests show zero difference.
>
>
> libstdc++-v3/include/bits/chrono_io.h | 35 +++++++++++++++++++--------
> 1 file changed, 25 insertions(+), 10 deletions(-)
>
> diff --git a/libstdc++-v3/include/bits/chrono_io.h
> b/libstdc++-v3/include/bits/chrono_io.h
> index c5170368f..fa40290a8 100644
> --- a/libstdc++-v3/include/bits/chrono_io.h
> +++ b/libstdc++-v3/include/bits/chrono_io.h
> @@ -3635,6 +3635,21 @@ namespace __detail
> return std::__ostream_insert(__os, __s.data(), __s.size());
> }
>
> + // Overload of __chrono_write that conditionally skips locale for
> + // integer-second time_points, hh_mm_ss, and zoned_time.
> + template<size_t _BufSize, typename _Duration, typename _CharT,
> + typename _Traits, typename _Tp>
> + inline basic_ostream<_CharT, _Traits>&
> + __chrono_write(basic_ostream<_CharT, _Traits>& __os,
> + const _Tp& __val)
> + {
> + if constexpr (!treat_as_floating_point_v<typename _Duration::rep>
> + && _Duration::period::den == 1)
> + return __chrono_write<_BufSize>(__os, __val);
> + else
> + return __chrono_write<_BufSize>(__os, __val, __os.getloc());
> + }
> +
> } // namespace __detail
> /// @endcond
>
> @@ -3740,7 +3755,7 @@ 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 +3861,7 @@ 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()); }
> + { return __detail::__chrono_write<64, _Duration>(__os, __hms); }
>
> #if _GLIBCXX_USE_CXX11_ABI || ! _GLIBCXX_USE_DUAL_ABI
> /// Writes a sys_info object to an ostream in an unspecified format.
> @@ -3866,7 +3881,7 @@ 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<128, _Duration>(__os, __t); }
> #endif
>
> template<typename _CharT, typename _Traits, typename _Duration>
> @@ -3875,12 +3890,12 @@ 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<64, _Duration>(__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 +3929,7 @@ 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<64, _Duration>(__os, __t); }
>
> template<typename _CharT, typename _Traits, typename _Duration,
> typename _Alloc = allocator<_CharT>>
> @@ -3946,7 +3961,7 @@ 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<64, _Duration>(__os, __t); }
>
> template<typename _CharT, typename _Traits, typename _Duration,
> typename _Alloc = allocator<_CharT>>
> @@ -3982,7 +3997,7 @@ 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<64, _Duration>(__os, __t); }
>
> template<typename _CharT, typename _Traits, typename _Duration,
> typename _Alloc = allocator<_CharT>>
> @@ -4017,7 +4032,7 @@ 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<64, _Duration>(__os, __t); }
>
> template<typename _CharT, typename _Traits, typename _Duration,
> typename _Alloc = allocator<_CharT>>
> @@ -4040,7 +4055,7 @@ 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<64, _Duration>(__os, __lt); }
>
> template<typename _CharT, typename _Traits, typename _Duration,
> typename _Alloc = allocator<_CharT>>
> --
> 2.34.1
