[PATCH v2] libstdc++: Lift locale initialization in main chrono format loop [PR110739]
Jonathan Wakely
jwakely@redhat.com
Wed Jul 2 09:27:03 GMT 2025
On 01/07/25 16:54 +0200, Tomasz Kamiński wrote:
>This patch lifts locale initialization from locale-specific handling methods
>into _M_format_to function, and pass the locale by const reference.
>To avoid unnecessary computation of locale::classic(), we use _Optional_locale,
>and emplace into it only for localized formatting (_M_spec._M_localized) or if
>chrono-spec contains locale specific specifiers (_M_spec._M_locale_specific).
>The later is constructs locale::classic() in more cases that strictly necessary,
>as only subset of locale specific specifiers (%a, %A, %b, %B, %c, %p, %r) needs
>locale, while _M_locale_specific is also set for %x,%X and when O/E modifiers are
>used. However, none of default outputs are affects, so I believe this is
>acceptable.
>
>In _M_S we no longer guard querying of numpuct facet, with check that requires
>potentially equally expensive construction of locale::classic. We also mark
>localized path as unlikely.
>
>The _M_locale method is no longer used in __formatter_chrono, and thus was
>moved to __formatter_duration.
>
>libstdc++-v3/ChangeLog:
>
> * include/bits/chrono_io.h (__formatter_chrono::_M_format_to):
> Compute locale and pass it to specifiers method.
> (__formatter_chrono::_M_a_A, __formatter_chrono::_M_b_B)
> (__formatter_chrono::_M_c, __formatter_chrono::_M_p)
> (__formatter_chrono::_M_r): Accept locale instead of format context.
> (__formatter_chrono::_M_subsecs): Call __ctx.locale() directly,
> instead of _M_locale and do not compare with locale::classic().
> Add [[unlikely]] attributes.
> (__formatter_chrono::_M_locale): Move to __formatter_duration.
> (__formatter_duration::_M_locale): Moved from __formatter_chrono.
>---
>v2 updates the commit message text only in hope to make it more readable.
>
> libstdc++-v3/include/bits/chrono_io.h | 71 ++++++++++++++++-----------
> 1 file changed, 43 insertions(+), 28 deletions(-)
>
>diff --git a/libstdc++-v3/include/bits/chrono_io.h b/libstdc++-v3/include/bits/chrono_io.h
>index bcf9830fb9e..a25cb9ada01 100644
>--- a/libstdc++-v3/include/bits/chrono_io.h
>+++ b/libstdc++-v3/include/bits/chrono_io.h
>@@ -964,10 +964,16 @@ namespace __format
> return std::move(__out);
> };
>
>+ _Optional_locale __loc;
We could add:
bool __loc_is_classic = false;
>+ if (_M_spec._M_localized)
>+ __loc = __fc.locale();
and set it to __loc == locale::classic() here,
>+ else if (_M_spec._M_locale_specific)
>+ __loc = locale::classic();
and set it to true here.
>+
> struct tm __tm{};
> bool __use_locale_fmt = false;
> if (_M_spec._M_localized && _M_spec._M_locale_specific)
>- if (__fc.locale() != locale::classic())
>+ if (__loc.value() != locale::classic())
Then we could just test __loc_is_classic here. That would avoid a
second call to classic() for the case where we explicitly set __loc to
classic().
> {
> __use_locale_fmt = true;
>
>@@ -1004,7 +1010,7 @@ namespace __format
> {
> _CharT __c = *__first++;
> if (__use_locale_fmt && _S_localized_spec(__c, __mod)) [[unlikely]]
>- __out = _M_locale_fmt(std::move(__out), __fc.locale(),
>+ __out = _M_locale_fmt(std::move(__out), __loc.value(),
> __tm, __c, __mod);
> else switch (__c)
> {
>@@ -1014,15 +1020,17 @@ namespace __format
> break;
> case 'a':
> case 'A':
>- __out = _M_a_A(__t._M_weekday, std::move(__out), __fc, __c == 'A');
>+ __out = _M_a_A(__t._M_weekday, std::move(__out),
>+ __loc.value(), __c == 'A');
> break;
> case 'b':
> case 'h':
> case 'B':
>- __out = _M_b_B(__t._M_month, std::move(__out), __fc, __c == 'B');
>+ __out = _M_b_B(__t._M_month, std::move(__out),
>+ __loc.value(), __c == 'B');
> break;
> case 'c':
>- __out = _M_c(__t, std::move(__out), __fc);
>+ __out = _M_c(__t, std::move(__out), __loc.value());
> break;
> case 'C':
> case 'y':
>@@ -1058,7 +1066,7 @@ namespace __format
> __out = _M_M(__t._M_minutes, __print_sign());
> break;
> case 'p':
>- __out = _M_p(__t._M_hours, std::move(__out), __fc);
>+ __out = _M_p(__t._M_hours, std::move(__out), __loc.value());
> break;
> case 'q':
> __out = _M_q(__t._M_unit_suffix, std::move(__out));
>@@ -1067,7 +1075,7 @@ namespace __format
> __out = _M_Q(__t, __print_sign(), __fc);
> break;
> case 'r':
>- __out = _M_r(__t, __print_sign(), __fc);
>+ __out = _M_r(__t, __print_sign(), __loc.value());
> break;
> case 'R':
> case 'X':
>@@ -1146,10 +1154,10 @@ namespace __format
> return std::move(__out);
> }
>
>- template<typename _OutIter, typename _FormatContext>
>+ template<typename _OutIter>
> _OutIter
> _M_a_A(chrono::weekday __wd, _OutIter __out,
>- _FormatContext& __ctx, bool __full) const
>+ const locale& __loc, bool __full) const
> {
> // %a Locale's abbreviated weekday name.
> // %A Locale's full weekday name.
>@@ -1165,7 +1173,6 @@ namespace __format
> __string_view(_GLIBCXX_WIDEN(" is not a valid weekday")));
> }
>
>- locale __loc = _M_locale(__ctx);
> const auto& __tp = use_facet<__timepunct<_CharT>>(__loc);
> const _CharT* __days[7];
> if (__full)
>@@ -1176,10 +1183,10 @@ namespace __format
> return _M_write(std::move(__out), __loc, __str);
> }
>
>- template<typename _OutIter, typename _FormatContext>
>+ template<typename _OutIter>
> _OutIter
> _M_b_B(chrono::month __m, _OutIter __out,
>- _FormatContext& __ctx, bool __full) const
>+ const locale& __loc, bool __full) const
> {
> // %b Locale's abbreviated month name.
> // %B Locale's full month name.
>@@ -1195,7 +1202,6 @@ namespace __format
> __string_view(_GLIBCXX_WIDEN(" is not a valid month")));
> }
>
>- locale __loc = _M_locale(__ctx);
> const auto& __tp = use_facet<__timepunct<_CharT>>(__loc);
> const _CharT* __months[12];
> if (__full)
>@@ -1206,17 +1212,17 @@ namespace __format
> return _M_write(std::move(__out), __loc, __str);
> }
>
>- template<typename _OutIter, typename _FormatContext>
>+ template<typename _OutIter>
> _OutIter
> _M_c(const _ChronoData<_CharT>& __t, _OutIter __out,
>- _FormatContext& __ctx) const
>+ const locale& __loc) const
> {
> // %c Locale's date and time representation, for C-locale: %a %b %e %T %Y
> // %Ec Locale's alternate date and time representation, for C-locale same as above
>
>- __out = _M_a_A(__t._M_weekday, std::move(__out), __ctx, false);
>+ __out = _M_a_A(__t._M_weekday, std::move(__out), __loc, false);
> *__out = _S_space;
>- __out = _M_b_B(__t._M_month, std::move(++__out), __ctx, false);
>+ __out = _M_b_B(__t._M_month, std::move(++__out), __loc, false);
> *__out = _S_space;
> __out = _M_d_e(__t._M_day, std::move(++__out), 'e');
> *__out = _S_space;
>@@ -1458,16 +1464,15 @@ namespace __format
> return __format::__write(std::move(__out), _S_two_digits(__i));
> }
>
>- template<typename _OutIter, typename _FormatContext>
>+ template<typename _OutIter>
> _OutIter
>- _M_p(chrono::hours __h, _OutIter __out, _FormatContext& __ctx) const
>+ _M_p(chrono::hours __h, _OutIter __out, const locale& __loc) const
> {
> // %p The locale's equivalent of the AM/PM designations.
> auto __hi = __h.count();
> if (__hi >= 24) [[unlikely]]
> __hi %= 24;
>
>- locale __loc = _M_locale(__ctx);
> const auto& __tp = use_facet<__timepunct<_CharT>>(__loc);
> const _CharT* __ampm[2];
> __tp._M_am_pm(__ampm);
>@@ -1491,10 +1496,10 @@ namespace __format
> return std::vformat_to(std::move(__out), _S_empty_spec, __t._M_ereps);
> }
>
>- template<typename _OutIter, typename _FormatContext>
>+ template<typename _OutIter>
> _OutIter
> _M_r(const _ChronoData<_CharT>& __t, _OutIter __out,
>- _FormatContext& __ctx) const
>+ const locale& __loc) const
> {
> // %r Locale's 12-hour clock time, for C-locale: %I:%M:%S %p
> auto __hi = __t._M_hours.count() % 12;
>@@ -1511,7 +1516,7 @@ namespace __format
>
> __string_view __sv(__buf, 9);
> __out = __format::__write(std::move(__out), __sv);
>- return _M_p(__t._M_hours, std::move(__out), __ctx);
>+ return _M_p(__t._M_hours, std::move(__out), __loc);
> }
>
> template<typename _OutIter>
>@@ -1575,9 +1580,9 @@ namespace __format
> return __out;
>
> _CharT __dot = _S_dot;
>- if (_M_spec._M_localized)
>- if (auto __loc = __ctx.locale(); __loc != locale::classic())
>+ if (_M_spec._M_localized) [[unlikely]]
> {
>+ auto __loc = __ctx.locale();
> const auto& __np = use_facet<numpunct<_CharT>>(__loc);
> __dot = __np.decimal_point();
> }
>@@ -1587,8 +1592,8 @@ namespace __format
> if (_M_spec._M_floating_point_rep)
> {
> _Str_sink<_CharT> __sink;
>- if (_M_spec._M_localized && _M_spec._M_custom_rep)
>- std::vformat_to(__sink.out(), _M_locale(__ctx),
>+ if (_M_spec._M_localized && _M_spec._M_custom_rep) [[unlikely]]
>+ std::vformat_to(__sink.out(), __ctx.locale(),
> _GLIBCXX_WIDEN("{1:0.{2}Lf}"), __t._M_ereps);
> else
> std::vformat_to(__sink.out(),
>@@ -1846,7 +1851,6 @@ namespace __format
> };
>
> using __formatter_chrono<_CharT>::__formatter_chrono;
>- using __formatter_chrono<_CharT>::_M_locale;
> using __formatter_chrono<_CharT>::_M_spec;
>
> template<typename _Duration, typename _ParseContext>
>@@ -1876,6 +1880,17 @@ namespace __format
> return __res;
> }
>
>+ // Return the formatting locale.
>+ template<typename _FormatContext>
>+ std::locale
>+ _M_locale(_FormatContext& __fc) const
>+ {
>+ if (!_M_spec._M_localized)
>+ return std::locale::classic();
>+ else
>+ return __fc.locale();
>+ }
It looks like you've copied this to __formatter_duration but not
removed it from __formatter_chrono, so it's duplicated now (and not
needed in __formatter_chrono).
>+
> // Format duration for empty chrono-specs, e.g. "{}" (C++20 [time.format] p6).
> template<typename _Rep, typename _Period, typename _FormatContext>
> typename _FormatContext::iterator
>--
>2.49.0
>
>
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