[PATCH v2 2/2] libstdc++: Use __chrono_write and __empty_fmt for chrono ostream insertion
Tomasz Kaminski
tkaminsk@redhat.com
Tue Jun 30 10:09:30 GMT 2026
On Tue, Jun 30, 2026 at 12:04 PM Tomasz Kaminski <tkaminsk@redhat.com>
wrote:
>
>
> On Tue, Jun 30, 2026 at 11:18 AM Anlai Lu <agicy@qq.com> wrote:
>
>> Replace `os << format(...)` with a stack-buffer approach in all
>> chrono operator<< overloads. A new helper
>> __detail::__chrono_write formats into a 128-byte stack buffer via
>> __format::__do_vformat_to_n, then writes through __ostream_insert
>> which correctly handles sentry construction and width/fill padding.
>> If the output exceeds the buffer (which should not happen for typical
>> chrono output), the helper falls back to std::vformat with dynamic
>> allocation.
>>
>> A bridge function __detail::__empty_fmt uses basic_format_string to
>> extract a basic_string_view from __formatter_chrono::_S_empty_fs(),
>> which is now public. Types whose formatters already handle validation
>> (day, month, year, weekday, year_month_day) use __empty_fmt instead
>> of hand-rolled format strings with substr, eliminating manual ok()
>> checks.
>>
> This !ok() checks are already handled by internal debug mode, that is
enabled
if empty-spec is used, so are not needed.
Anyway, the type formatters if checking if format specifiers (like "%d")
can be used
in combination with specific type (and not particular object value), but
here we used
a fixed and always valid format string, so we can simply skip them by using
_Dynamic_format_string.
>
>> The weekday_indexed and weekday_last operator<< overloads now use
>> __chrono_write directly, per [time.cal.wdidx.nonmembers] and
>> [time.cal.wdlast.nonmembers]. There is no need for special failbit
>> handling because the standard specifies the output as a simple format
>> call, and __ostream_insert handles state via the standard sentry
>> mechanism.
>
>
>> This eliminates temporary std::string and std::stringstream allocations
>> while preserving formatted output function semantics.
>>
>> libstdc++-v3/ChangeLog:
>>
>> * include/bits/chrono_io.h (__formatter_chrono::_S_empty_fs):
>> Make public.
>> (__detail::__empty_fmt): New function template.
>> (__detail::__chrono_write): New function template.
>> (operator<<): Use __chrono_write and __empty_fmt consistently.
>> (operator<<(weekday_indexed)): Replace stringstream with
>> __chrono_write.
>> (operator<<(weekday_last)): Likewise.
>>
>> Signed-off-by: Anlai Lu <agicy@qq.com>
>> ---
>> libstdc++-v3/include/bits/chrono_io.h | 202 +++++++++++++++-----------
>> 1 file changed, 115 insertions(+), 87 deletions(-)
>>
>> diff --git a/libstdc++-v3/include/bits/chrono_io.h
>> b/libstdc++-v3/include/bits/chrono_io.h
>> index f8bb485d1..9e17c8ffd 100644
>> --- a/libstdc++-v3/include/bits/chrono_io.h
>> +++ b/libstdc++-v3/include/bits/chrono_io.h
>> @@ -884,11 +884,13 @@ namespace __format
>> static constexpr const _CharT* _S_minus_empty_spec = _S_chars + 17;
>> static constexpr const _CharT* _S_empty_spec = _S_chars + 18;
>>
>> + public:
>> [[__gnu__::__always_inline__]]
>> static _Dynamic_format_string<_CharT>
>> _S_empty_fs()
>> { return _Dynamic_format_string<_CharT>(_S_empty_spec); }
>>
>> + protected:
>> static constexpr const _CharT* _S_weekdays[]
>> {
>> _GLIBCXX_WIDEN("Sunday"),
>> @@ -3607,6 +3609,77 @@ namespace __detail
>> }
>> }
>>
>> + // Bridge to extract basic_string_view from _S_empty_fs().
>> + // Uses basic_format_string to safely convert, then discards it.
>> + template<typename _CharT, typename _Tp>
>> + inline constexpr basic_string_view<_CharT>
>> + __empty_fmt()
>> + {
>> + using _Fmt = basic_format_string<_CharT, type_identity_t<_Tp>>;
>> + return
>> _Fmt(__format::__formatter_chrono<_CharT>::_S_empty_fs()).get();
>> + }
>> +
>> + // Format into stack buffer, write via __ostream_insert.
>> + template<typename _CharT, typename _Traits, typename... _Args>
>> + inline basic_ostream<_CharT, _Traits>&
>> + __chrono_write(basic_ostream<_CharT, _Traits>& __os,
>> + basic_string_view<_CharT> __fmt, const _Args&...
>> __args)
>>
> Passing a format string should not be necessary, for all formatter
> specializations,
> the "{}" is already equivalent to the output of the operator<<, so we
> could just pass
> ostream and locale.
>
>
>> + {
>> + constexpr size_t __bufsize = 128;
>> + _CharT __buf[__bufsize];
>> + auto __res
>> + = __format::__do_vformat_to_n(__buf, __bufsize, __fmt,
>> + std::make_format_args<
>> + __format::__format_context<_CharT>
>> + >(__args...));
>>
> Because __format_fs returns _Dynamic_format_string, format calls will not
> check the the format string at compile time, so I think we could replace
> that,
> with:
> template<typename _CharT, typename _Traits, typename _Arg, typename...
> OptLocale>
> inline basic_ostream<_CharT, _Traits>&
> __chrono_write(basic_ostream<_CharT, _Traits>& __os, const _Arg& __arg,
> const OptLocale... __loc...)
> {
> static_assert(sizeof...(OptLocale) <= 1);
>
> constexpr auto __fs =
> __format::__formatter_chrono<_CharT>::_S_empty_fs();
> constexpr size_t __bufsize = 128;
> _CharT __buf[__bufsize];
> auto __res = std::format_to_n(__buf, __buf_size, __loc..., __fs,
> __arg);
> if (static_cast<size_t>(__res.size) <= __bufsize) [[likely]]
> return std::__ostream_insert(__os, __buf, __res.size);
>
> auto __s = std::format(__loc..., __fs, __arg);
> return std::__ostream_insert(__os, __s.data(), __s.size());
> }
>
> And then call for every operator<< accepting xx except for duration
> __format::__chrono_write(__os, _xx);
> __format::__chrono_write(__os, _xx, __os.getloc());
>
> Did you run into any problems with the above?
>
>
> + if (static_cast<size_t>(__res.size) <= __bufsize) [[likely]]
>> + return std::__ostream_insert(__os, __buf, __res.size);
>> + auto __s = std::vformat(__fmt,
>> + std::make_format_args<
>> + __format::__format_context<_CharT>
>> + >(__args...));
>> + return std::__ostream_insert(__os, __s.data(), __s.size());
>> + }
>> +
>> + template<typename _CharT, typename _Traits, typename... _Args>
>> + inline basic_ostream<_CharT, _Traits>&
>> + __chrono_write(basic_ostream<_CharT, _Traits>& __os, const locale&
>> __loc,
>> + basic_string_view<_CharT> __fmt, const _Args&...
>> __args)
>> + {
>> + constexpr size_t __bufsize = 128;
>> + _CharT __buf[__bufsize];
>> + auto __res
>> + = __format::__do_vformat_to_n(__buf, __bufsize, __fmt,
>> + std::make_format_args<
>> + __format::__format_context<_CharT>
>> + >(__args...),
>> + &__loc);
>> + if (static_cast<size_t>(__res.size) <= __bufsize) [[likely]]
>> + return std::__ostream_insert(__os, __buf, __res.size);
>> + auto __s = std::vformat(__loc, __fmt,
>> + std::make_format_args<
>> +
>> __format::__format_context<_CharT>>(__args...));
>> + return std::__ostream_insert(__os, __s.data(), __s.size());
>> + }
>> +
>> + // Thin forwarders for const _CharT* (compile-time literals).
>> + template<typename _CharT, typename _Traits, typename... _Args>
>> + inline basic_ostream<_CharT, _Traits>&
>> + __chrono_write(basic_ostream<_CharT, _Traits>& __os,
>> + const _CharT* __fmt, const _Args&... __args)
>> + {
>> + return __chrono_write(__os, basic_string_view<_CharT>(__fmt),
>> __args...);
>> + }
>> +
>> + template<typename _CharT, typename _Traits, typename... _Args>
>> + inline basic_ostream<_CharT, _Traits>&
>> + __chrono_write(basic_ostream<_CharT, _Traits>& __os, const locale&
>> __loc,
>> + const _CharT* __fmt, const _Args&... __args)
>> + {
>> + return __chrono_write(__os, __loc,
>> basic_string_view<_CharT>(__fmt),
>> + __args...);
>> + }
>> +
>> } // namespace __detail
>> /// @endcond
>>
>> @@ -3629,14 +3702,9 @@ namespace __detail
>> inline basic_ostream<_CharT, _Traits>&
>> operator<<(basic_ostream<_CharT, _Traits>& __os, const day& __d)
>> {
>> - using _Ctx = __format::__format_context<_CharT>;
>> - using _Str = basic_string_view<_CharT>;
>> - _Str __s = _GLIBCXX_WIDEN("{:02d} is not a valid day");
>> - if (__d.ok())
>> - __s = __s.substr(0, 6);
>> - auto __u = (unsigned)__d;
>> - __os << std::vformat(__s, make_format_args<_Ctx>(__u));
>> - return __os;
>> + return __detail::__chrono_write(__os,
>> + __detail::__empty_fmt<_CharT,
>> day>(),
>> + __d);
>> }
>>
>> template<typename _CharT, typename _Traits,
>> @@ -3657,18 +3725,9 @@ namespace __detail
>> inline basic_ostream<_CharT, _Traits>&
>> operator<<(basic_ostream<_CharT, _Traits>& __os, const month& __m)
>> {
>> - using _Ctx = __format::__format_context<_CharT>;
>> - using _Str = basic_string_view<_CharT>;
>> - _Str __s = _GLIBCXX_WIDEN("{:L%b}{} is not a valid month");
>> - if (__m.ok())
>> - __os << std::vformat(__os.getloc(), __s.substr(0, 6),
>> - make_format_args<_Ctx>(__m));
>> - else
>> - {
>> - auto __u = (unsigned)__m;
>> - __os << std::vformat(__s.substr(6),
>> make_format_args<_Ctx>(__u));
>> - }
>> - return __os;
>> + return __detail::__chrono_write(__os, __os.getloc(),
>> + __detail::__empty_fmt<_CharT,
>> month>(),
>> + __m);
>> }
>>
>> template<typename _CharT, typename _Traits,
>> @@ -3689,18 +3748,9 @@ namespace __detail
>> inline basic_ostream<_CharT, _Traits>&
>> operator<<(basic_ostream<_CharT, _Traits>& __os, const year& __y)
>> {
>> - using _Ctx = __format::__format_context<_CharT>;
>> - using _Str = basic_string_view<_CharT>;
>> - _Str __s = _GLIBCXX_WIDEN("-{:04d} is not a valid year");
>> - if (__y.ok())
>> - __s = __s.substr(0, 7);
>> - int __i = (int)__y;
>> - if (__i >= 0) [[likely]]
>> - __s.remove_prefix(1);
>> - else
>> - __i = -__i;
>> - __os << std::vformat(__s, make_format_args<_Ctx>(__i));
>> - return __os;
>> + return __detail::__chrono_write(__os,
>> + __detail::__empty_fmt<_CharT,
>> year>(),
>> + __y);
>> }
>>
>> template<typename _CharT, typename _Traits,
>> @@ -3721,18 +3771,9 @@ namespace __detail
>> inline basic_ostream<_CharT, _Traits>&
>> operator<<(basic_ostream<_CharT, _Traits>& __os, const weekday& __wd)
>> {
>> - using _Ctx = __format::__format_context<_CharT>;
>> - using _Str = basic_string_view<_CharT>;
>> - _Str __s = _GLIBCXX_WIDEN("{:L%a}{} is not a valid weekday");
>> - if (__wd.ok())
>> - __os << std::vformat(__os.getloc(), __s.substr(0, 6),
>> - make_format_args<_Ctx>(__wd));
>> - else
>> - {
>> - auto __c = __wd.c_encoding();
>> - __os << std::vformat(__s.substr(6),
>> make_format_args<_Ctx>(__c));
>> - }
>> - return __os;
>> + return __detail::__chrono_write(__os, __os.getloc(),
>> + __detail::__empty_fmt<_CharT,
>> weekday>(),
>> + __wd);
>> }
>>
>> template<typename _CharT, typename _Traits,
>> @@ -3754,23 +3795,14 @@ namespace __detail
>> operator<<(basic_ostream<_CharT, _Traits>& __os,
>> const weekday_indexed& __wdi)
>> {
>> - // The standard says to format wdi.weekday() and wdi.index() using
>> - // either "{:L}[{}]" or "{:L}[{} is not a valid index]". The {:L}
>> spec
>> - // means to format the weekday using ostringstream, so just do
>> that.
>> - basic_stringstream<_CharT> __os2;
>> - __os2.imbue(__os.getloc());
>> - __os2 << __wdi.weekday();
>> - const auto __i = __wdi.index();
>> - basic_string_view<_CharT> __s
>> - = _GLIBCXX_WIDEN("[ is not a valid index]");
>> - __os2 << __s[0];
>> - __os2 << std::format(_GLIBCXX_WIDEN("{}"), __i);
>> - if (__i >= 1 && __i <= 5)
>> - __os2 << __s.back();
>> - else
>> - __os2 << __s.substr(1);
>> - __os << __os2.view();
>> - return __os;
>> + // [time.cal.wdidx.nonmembers]
>> + basic_string_view<_CharT> __s1 = _GLIBCXX_WIDEN("{:L}[{}]");
>> + basic_string_view<_CharT> __s2
>> + = _GLIBCXX_WIDEN("{:L}[{} is not a valid index]");
>> + auto __i = __wdi.index();
>> + return __detail::__chrono_write(__os, __os.getloc(),
>> + __i >= 1 && __i <= 5 ? __s1 : __s2,
>> + __wdi.weekday(), __i);
>>
> This should not be needed, simple "{}" should work.
>
>> }
>>
>> template<typename _CharT, typename _Traits>
>> @@ -3778,12 +3810,10 @@ namespace __detail
>> operator<<(basic_ostream<_CharT, _Traits>& __os,
>> const weekday_last& __wdl)
>> {
>> - // As above, just write straight to a stringstream, as if by
>> "{:L}[last]"
>> - basic_stringstream<_CharT> __os2;
>> - __os2.imbue(__os.getloc());
>> - __os2 << __wdl.weekday() << _GLIBCXX_WIDEN("[last]");
>> - __os << __os2.view();
>> - return __os;
>> + // [time.cal.wdlast.nonmembers]
>> + return __detail::__chrono_write(__os, __os.getloc(),
>> + _GLIBCXX_WIDEN("{:L}[last]"),
>> + __wdl.weekday());
>> }
>>
>> template<typename _CharT, typename _Traits>
>> @@ -3906,12 +3936,8 @@ namespace __detail
>> operator<<(basic_ostream<_CharT, _Traits>& __os,
>> const year_month_day& __ymd)
>> {
>> - using _Ctx = __format::__format_context<_CharT>;
>> - using _Str = basic_string_view<_CharT>;
>> - _Str __s = _GLIBCXX_WIDEN("{:%F} is not a valid date");
>> - __os << std::vformat(__ymd.ok() ? __s.substr(0, 5) : __s,
>> - make_format_args<_Ctx>(__ymd));
>> - return __os;
>> + return __detail::__chrono_write(
>> + __os, __detail::__empty_fmt<_CharT, year_month_day>(), __ymd);
>> }
>>
>> template<typename _CharT, typename _Traits,
>> @@ -3994,7 +4020,8 @@ namespace __detail
>> operator<<(basic_ostream<_CharT, _Traits>& __os,
>> const hh_mm_ss<_Duration>& __hms)
>> {
>> - return __os << format(__os.getloc(), _GLIBCXX_WIDEN("{:L%T}"),
>> __hms);
>> + return __detail::__chrono_write(__os, __os.getloc(),
>> + _GLIBCXX_WIDEN("{:L%T}"), __hms);
>> }
>>
>> #if _GLIBCXX_USE_CXX11_ABI || ! _GLIBCXX_USE_DUAL_ABI
>> @@ -4003,7 +4030,8 @@ namespace __detail
>> basic_ostream<_CharT, _Traits>&
>> operator<<(basic_ostream<_CharT, _Traits>& __os, const sys_info& __i)
>> {
>> - return __os << std::format(__os.getloc(), _GLIBCXX_WIDEN("{}"),
>> __i);
>> + return __detail::__chrono_write(
>> + __os, __os.getloc(), __detail::__empty_fmt<_CharT, sys_info>(),
>> __i);
>> }
>>
>> /// Writes a local_info object to an ostream in an unspecified format.
>> @@ -4033,8 +4061,8 @@ namespace __detail
>> operator<<(basic_ostream<_CharT, _Traits>& __os,
>> const zoned_time<_Duration, _TimeZonePtr>& __t)
>> {
>> - __os << format(__os.getloc(), _GLIBCXX_WIDEN("{:L%F %T %Z}"), __t);
>> - return __os;
>> + return __detail::__chrono_write(__os, __os.getloc(),
>> + _GLIBCXX_WIDEN("{:L%F %T %Z}"),
>> __t);
>> }
>> #endif
>>
>> @@ -4045,8 +4073,8 @@ namespace __detail
>> operator<<(basic_ostream<_CharT, _Traits>& __os,
>> const sys_time<_Duration>& __tp)
>> {
>> - __os << std::format(__os.getloc(), _GLIBCXX_WIDEN("{:L%F %T}"),
>> __tp);
>> - return __os;
>> + return __detail::__chrono_write(__os, __os.getloc(),
>> + _GLIBCXX_WIDEN("{:L%F %T}"), __tp);
>> }
>>
>> template<typename _CharT, typename _Traits>
>> @@ -4090,8 +4118,8 @@ namespace __detail
>> operator<<(basic_ostream<_CharT, _Traits>& __os,
>> const utc_time<_Duration>& __t)
>> {
>> - __os << std::format(__os.getloc(), _GLIBCXX_WIDEN("{:L%F %T}"),
>> __t);
>> - return __os;
>> + return __detail::__chrono_write(__os, __os.getloc(),
>> + _GLIBCXX_WIDEN("{:L%F %T}"), __t);
>> }
>>
>> template<typename _CharT, typename _Traits, typename _Duration,
>> @@ -4125,8 +4153,8 @@ namespace __detail
>> operator<<(basic_ostream<_CharT, _Traits>& __os,
>> const tai_time<_Duration>& __t)
>> {
>> - __os << std::format(__os.getloc(), _GLIBCXX_WIDEN("{:L%F %T}"),
>> __t);
>> - return __os;
>> + return __detail::__chrono_write(__os, __os.getloc(),
>> + _GLIBCXX_WIDEN("{:L%F %T}"), __t);
>> }
>>
>> template<typename _CharT, typename _Traits, typename _Duration,
>> @@ -4164,8 +4192,8 @@ namespace __detail
>> operator<<(basic_ostream<_CharT, _Traits>& __os,
>> const gps_time<_Duration>& __t)
>> {
>> - __os << std::format(__os.getloc(), _GLIBCXX_WIDEN("{:L%F %T}"),
>> __t);
>> - return __os;
>> + return __detail::__chrono_write(__os, __os.getloc(),
>> + _GLIBCXX_WIDEN("{:L%F %T}"), __t);
>> }
>>
>> template<typename _CharT, typename _Traits, typename _Duration,
>> @@ -4202,8 +4230,8 @@ namespace __detail
>> operator<<(basic_ostream<_CharT, _Traits>& __os,
>> const file_time<_Duration>& __t)
>> {
>> - __os << std::format(__os.getloc(), _GLIBCXX_WIDEN("{:L%F %T}"),
>> __t);
>> - return __os;
>> + return __detail::__chrono_write(__os, __os.getloc(),
>> + _GLIBCXX_WIDEN("{:L%F %T}"), __t);
>> }
>>
>> template<typename _CharT, typename _Traits, typename _Duration,
>> --
>> 2.34.1
>>
>>
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