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From: Jonathan Wakely <redi@gcc.gnu.org> To: gcc-cvs@gcc.gnu.org, libstdc++-cvs@gcc.gnu.org Subject: [gcc r13-4110] libstdc++: Improve performance of chrono::utc_clock::now() Date: Wed, 16 Nov 2022 20:59:37 +0000 (GMT) [thread overview] Message-ID: <20221116205937.82AAA396DC27@sourceware.org> (raw) https://gcc.gnu.org/g:629897ed80512a8618e08673c03d8482cbc42eef commit r13-4110-g629897ed80512a8618e08673c03d8482cbc42eef Author: Jonathan Wakely <jwakely@redhat.com> Date: Wed Nov 16 15:35:23 2022 +0000 libstdc++: Improve performance of chrono::utc_clock::now() 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. Diff: --- 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(); + + 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 __s = __ut.time_since_epoch().count(); - auto __pos = std::upper_bound(__leaps.begin(), __leaps.end(), __s); + 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, + seconds{__pos - __first} }; } else
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