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[libc++][chrono] implements UTC clock.
While implementing this feature and its associated LWG issues it turns out - LWG3316 Correctly define epoch for utc_clock / utc_timepoint only added non-normative wording to the standard. Implements parts of: - P0355 Extending <chrono> to Calendars and Time Zones - P1361 Integration of chrono with text formatting - LWG3359 <chrono> leap second support should allow for negative leap seconds
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| Original file line number | Diff line number | Diff line change |
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| //===----------------------------------------------------------------------===// | ||
| // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. | ||
| // See https://llvm.org/LICENSE.txt for license information. | ||
| // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception | ||
| // | ||
| //===----------------------------------------------------------------------===// | ||
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| #include <chrono> | ||
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| #include "benchmark/benchmark.h" | ||
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| // Benchmarks the performance of the UTC <-> system time conversions. These | ||
| // operations determine the sum of leap second insertions at a specific time. | ||
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| static void BM_from_sys(benchmark::State& state) { | ||
| std::chrono::sys_days date{std::chrono::July / 1 / state.range(0)}; | ||
| for (auto _ : state) | ||
| benchmark::DoNotOptimize(std::chrono::utc_clock::from_sys(date)); | ||
| } | ||
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| BENCHMARK(BM_from_sys) | ||
| ->Arg(1970) // before the first leap seconds | ||
| ->Arg(1979) // in the first half of inserted leap seconds | ||
| ->Arg(1993) // in the second half of inserted leap seconds | ||
| ->Arg(2100); // after the last leap second | ||
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| BENCHMARK(BM_from_sys)->Arg(1970)->Arg(1979)->Arg(1993)->Arg(2100)->Threads(4); | ||
| BENCHMARK(BM_from_sys)->Arg(1970)->Arg(1979)->Arg(1993)->Arg(2100)->Threads(16); | ||
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| static void BM_to_sys(benchmark::State& state) { | ||
| // 59 sec offset means we pass th UTC offset for the leap second; assuming | ||
| // there won't be more than 59 leap seconds ever. | ||
| std::chrono::utc_seconds date{ | ||
| std::chrono::sys_days{std::chrono::July / 1 / state.range(0)}.time_since_epoch() + std::chrono::seconds{59}}; | ||
| for (auto _ : state) | ||
| benchmark::DoNotOptimize(std::chrono::utc_clock::to_sys(date)); | ||
| } | ||
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| BENCHMARK(BM_to_sys) | ||
| ->Arg(1970) // before the first leap seconds | ||
| ->Arg(1979) // in the first half of inserted leap seconds | ||
| ->Arg(1993) // in the second half of inserted leap seconds | ||
| ->Arg(2100); // after the last leap second | ||
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| BENCHMARK(BM_to_sys)->Arg(1970)->Arg(1979)->Arg(1993)->Arg(2100)->Threads(4); | ||
| BENCHMARK(BM_to_sys)->Arg(1970)->Arg(1979)->Arg(1993)->Arg(2100)->Threads(16); | ||
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| int main(int argc, char** argv) { | ||
| benchmark::Initialize(&argc, argv); | ||
| if (benchmark::ReportUnrecognizedArguments(argc, argv)) | ||
| return 1; | ||
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| benchmark::RunSpecifiedBenchmarks(); | ||
| } |
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| Original file line number | Diff line number | Diff line change |
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| // -*- C++ -*- | ||
| //===----------------------------------------------------------------------===// | ||
| // | ||
| // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. | ||
| // See https://llvm.org/LICENSE.txt for license information. | ||
| // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception | ||
| // | ||
| //===----------------------------------------------------------------------===// | ||
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| #ifndef _LIBCPP___CHRONO_UTC_CLOCK_H | ||
| #define _LIBCPP___CHRONO_UTC_CLOCK_H | ||
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| #include <version> | ||
| // Enable the contents of the header only when libc++ was built with experimental features enabled. | ||
| #if !defined(_LIBCPP_HAS_NO_INCOMPLETE_TZDB) | ||
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| # include <__chrono/duration.h> | ||
| # include <__chrono/system_clock.h> | ||
| # include <__chrono/time_point.h> | ||
| # include <__chrono/tzdb.h> | ||
| # include <__chrono/tzdb_list.h> | ||
| # include <__config> | ||
| # include <__type_traits/common_type.h> | ||
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| # if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER) | ||
| # pragma GCC system_header | ||
| # endif | ||
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| _LIBCPP_BEGIN_NAMESPACE_STD | ||
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| # if _LIBCPP_STD_VER >= 20 && !defined(_LIBCPP_HAS_NO_TIME_ZONE_DATABASE) && !defined(_LIBCPP_HAS_NO_FILESYSTEM) && \ | ||
| !defined(_LIBCPP_HAS_NO_LOCALIZATION) | ||
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| namespace chrono { | ||
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| class utc_clock; | ||
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| template <class _Duration> | ||
| using utc_time = time_point<utc_clock, _Duration>; | ||
| using utc_seconds = utc_time<seconds>; | ||
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| class utc_clock { | ||
| public: | ||
| using rep = system_clock::rep; | ||
| using period = system_clock::period; | ||
| using duration = chrono::duration<rep, period>; | ||
| using time_point = chrono::time_point<utc_clock>; | ||
| static constexpr bool is_steady = false; // The system_clock is not steady. | ||
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| [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static time_point now() { return from_sys(system_clock::now()); } | ||
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| template <class _Duration> | ||
| [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static sys_time<common_type_t<_Duration, seconds>> | ||
| to_sys(const utc_time<_Duration>& __time); | ||
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| template <class _Duration> | ||
| [[nodiscard]] _LIBCPP_HIDE_FROM_ABI static utc_time<common_type_t<_Duration, seconds>> | ||
| from_sys(const sys_time<_Duration>& __time) { | ||
| using _Rp = utc_time<common_type_t<_Duration, seconds>>; | ||
| // TODO TZDB investigate optimizations. | ||
| // | ||
| // The leap second database stores all transitions, this mean to calculate | ||
| // the current number of leap seconds the code needs to iterate over all | ||
| // leap seconds to accumulate the sum. Then the sum can be used to determine | ||
| // the sys_time. Accessing the database involves acquiring a mutex. | ||
| // | ||
| // The historic entries in the database are immutable. Hard-coding these | ||
| // values in a table would allow: | ||
| // - To store the sum, allowing a binary search on the data. | ||
| // - Avoid acquiring a mutex. | ||
| // The disadvantage are: | ||
| // - A slightly larger code size. | ||
| // | ||
| // There are two optimization directions | ||
| // - hard-code the database and do a linear search for future entries. This | ||
| // search can start at the back, and should probably contain very few | ||
| // entries. (Adding leap seconds is quite rare and new release of libc++ | ||
| // can add the new entries; they are announced half a year before they are | ||
| // added.) | ||
| // - During parsing the leap seconds store an additional database in the | ||
| // dylib with the list of the sum of the leap seconds. In that case there | ||
| // can be a private function __get_utc_to_sys_table that returns the | ||
| // table. | ||
| // | ||
| // Note for to_sys there are no optimizations to be done; it uses | ||
| // get_leap_second_info. The function get_leap_second_info could benefit | ||
| // from optimizations as described above; again both options apply. | ||
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| // Both UTC and the system clock use the same epoch. The Standard | ||
| // specifies from 1970-01-01 even when UTC starts at | ||
| // 1972-01-01 00:00:10 TAI. So when the sys_time is before epoch we can be | ||
| // sure there both clocks return the same value. | ||
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| const tzdb& __tzdb = chrono::get_tzdb(); | ||
| _Rp __result{__time.time_since_epoch()}; | ||
| for (const auto& __leap_second : __tzdb.leap_seconds) { | ||
| if (__time < __leap_second) | ||
| return __result; | ||
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| __result += __leap_second.value(); | ||
| } | ||
| return __result; | ||
| } | ||
| }; | ||
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| struct leap_second_info { | ||
| bool is_leap_second; | ||
| seconds elapsed; | ||
| }; | ||
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| template <class _Duration> | ||
| [[nodiscard]] _LIBCPP_HIDE_FROM_ABI leap_second_info get_leap_second_info(const utc_time<_Duration>& __time) { | ||
| const tzdb& __tzdb = chrono::get_tzdb(); | ||
| if (__tzdb.leap_seconds.empty()) | ||
| return {false, chrono::seconds{0}}; | ||
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| sys_seconds __sys{chrono::floor<seconds>(__time).time_since_epoch()}; | ||
| seconds __elapsed{0}; | ||
| for (const auto& __leap_second : __tzdb.leap_seconds) { | ||
| if (__sys == __leap_second.date() + __elapsed) | ||
| return {__leap_second.value() > 0s, // only positive leap seconds are considered leap seconds | ||
| __elapsed + __leap_second.value()}; | ||
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| if (__sys < __leap_second.date() + __elapsed) | ||
| return {false, __elapsed}; | ||
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| __elapsed += __leap_second.value(); | ||
| } | ||
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| return {false, __elapsed}; | ||
| } | ||
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| template <class _Duration> | ||
| [[nodiscard]] _LIBCPP_HIDE_FROM_ABI sys_time<common_type_t<_Duration, seconds>> | ||
| utc_clock::to_sys(const utc_time<_Duration>& __time) { | ||
| using _Dp = common_type_t<_Duration, seconds>; | ||
| leap_second_info __info = get_leap_second_info(__time); | ||
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| sys_time<common_type_t<_Duration, seconds>> __result{__time.time_since_epoch() - __info.elapsed}; | ||
| if (__info.is_leap_second) | ||
| return chrono::floor<seconds>(__result) + chrono::seconds{1} - _Dp{1}; | ||
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| return __result; | ||
| } | ||
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| } // namespace chrono | ||
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| # endif // _LIBCPP_STD_VER >= 20 && !defined(_LIBCPP_HAS_NO_TIME_ZONE_DATABASE) && !defined(_LIBCPP_HAS_NO_FILESYSTEM) | ||
| // && !defined(_LIBCPP_HAS_NO_LOCALIZATION) | ||
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| _LIBCPP_END_NAMESPACE_STD | ||
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| #endif // !defined(_LIBCPP_HAS_NO_INCOMPLETE_TZDB) | ||
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| #endif // _LIBCPP___CHRONO_UTC_CLOCK_H |
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