Re: [committed] libstdc++: Improve performance of chrono::utc_clock::now()

[Jonathan Wakely <jwakely.gcc@gmail.com>]

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On Thu, 17 Nov 2022, 06:30 Daniel Krügler via Libstdc++, <
libstdc++@gcc.gnu.org> wrote:

> Am Mi., 16. Nov. 2022 um 22:00 Uhr schrieb Jonathan Wakely via
> Libstdc++ <libstdc++@gcc.gnu.org>:
> >
> > Tested x86_64-linux. Pushed to trunk.
> >
> > -- >8 --
> >
> > We can use an array instead of a std::vector, and we can avoid the
> > binary search for the common case of a time point after the most recent
> > leap second. On one system where I tested this, utc_clock::now() now
> > takes about 16ns instead of 31ns.
> >
> > libstdc++-v3/ChangeLog:
> >
> >         * include/std/chrono (get_leap_second_info): Optimize.
> > ---
> >  libstdc++-v3/include/std/chrono | 31 ++++++++++++++++++++++++-------
> >  1 file changed, 24 insertions(+), 7 deletions(-)
> >
> > diff --git a/libstdc++-v3/include/std/chrono
> b/libstdc++-v3/include/std/chrono
> > index 90b73f8198e..2468023f6c5 100644
> > --- a/libstdc++-v3/include/std/chrono
> > +++ b/libstdc++-v3/include/std/chrono
> > @@ -2747,9 +2747,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >        {
> >         if constexpr (is_same_v<_Duration, seconds>)
> >           {
> > -           // TODO move this function into the library and get leaps
> from tzdb.
> > -           vector<seconds::rep> __leaps
> > -           {
> > +           const seconds::rep __leaps[] {
> >                 78796800, // 1 Jul 1972
> >                 94694400, // 1 Jan 1973
> >                126230400, // 1 Jan 1974
> > @@ -2778,12 +2776,31 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >               1435708800, // 1 Jul 2015
> >               1483228800, // 1 Jan 2017
> >             };
> > +           // The list above is known to be valid until 2023-06-28
> 00:00:00 UTC
> > +           const seconds::rep __expires = 1687910400;
> > +           const seconds::rep __s = __ut.time_since_epoch().count();
> >
> > -           auto __s = __ut.time_since_epoch().count();
> > -           auto __pos = std::upper_bound(__leaps.begin(),
> __leaps.end(), __s);
> > +           const seconds::rep* __first = std::begin(__leaps);
> > +           const seconds::rep* __last = std::end(__leaps);
> > +
> > +           if (__s > __expires)
> > +             {
> > +               // TODO: use updated leap_seconds from tzdb
> > +#if 0
> > +               auto __db = get_tzdb_list().begin();
> > +               __first = __db->leap_seconds.data();
> > +               __last = __first + __db->leap_seconds.size();
> > +#endif
> > +             }
> > +
> > +           // Don't bother searching the list if we're after the last
> one.
> > +           if (__s > __last[-1])
> > +             return { false, seconds(__last - __first) };
> > +
> > +           auto __pos = std::upper_bound(__first, __last, __s);
> >             return {
> > -             __pos != __leaps.begin() && __pos[-1] == __s,
> > -             seconds{__pos - __leaps.begin()}
> > +             __pos != begin(__leaps) && __pos[-1] == __s,
>
> The inconsistency between usage of std::begin versus begin here seems
> odd and I'm wondering why instead of "begin(__leaps)" the above
> introduced "__first" variable is not used instead.
>

Because this code is going to be changed again soon, this is a partial
merge from a local branch with the TODO fixed. Yes, it's inconsistent, but
it works correctly and it's not my priority right now :-)