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

["Daniel Krügler" <daniel.kruegler@gmail.com>]

Am Do., 17. Nov. 2022 um 10:48 Uhr schrieb Jonathan Wakely <jwakely@redhat.com>:
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> On Thu, 17 Nov 2022 at 09:47, Jonathan Wakely <jwakely@redhat.com> wrote:
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>> On Thu, 17 Nov 2022 at 09:25, Daniel Krügler <daniel.kruegler@gmail.com> wrote:
>>>
>>> Am Do., 17. Nov. 2022 um 10:07 Uhr schrieb Jonathan Wakely
>>> <jwakely.gcc@gmail.com>:
>>> >
>>> >
>>> >
>>> > 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 :-)
>>>
>>> What about the suggestion to use the already existing "__first"
>>> variable instead of the begin call?
>>
>>
>> It's an array, the begin call is free.
>
> Do you really want me to stop working on the missing time zone support to test and commit that change?

I do not. I was reviewing and hoping to make a useful comment.

Thanks,

- Daniel