Am Do., 17. Nov. 2022 um 10:07 Uhr schrieb Jonathan Wakely
<jwakely....@gmail.com>:
>
>
>
> On Thu, 17 Nov 2022, 06:30 Daniel Krügler via Libstdc++,
> <libstd...@gcc.gnu.org> wrote:
>>
>> Am Mi., 16. Nov. 2022 um 22:00 Uhr schrieb Jonathan Wakely via
>> Libstdc++ <libstd...@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?
Thanks,
- Daniel
