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

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

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.

- Daniel