From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 2181) id 82AAA396DC27; Wed, 16 Nov 2022 20:59:37 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 82AAA396DC27 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org; s=default; t=1668632377; bh=2H/hvhPfxu8ck1G7PA6+7WSel0qadvFX/0fOGu1Kbws=; h=From:To:Subject:Date:From; b=jSuZ7BKJl4bAZ8ya/efGuimEitJTg9q3hnzwzqu423RBja+2gOh2pbPFYf283Q7Lm KMIzZtfrbVSkPrQ+aMJD10PXGjLMUmET60rBreAmwwTA0ZGNl/VWX+grxZowVWhwaY xNfiMKnsKHoP38uo/BbP+KDdZ2eh8RK8Fm/Pw1Uw= MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="utf-8" From: Jonathan Wakely To: gcc-cvs@gcc.gnu.org, libstdc++-cvs@gcc.gnu.org Subject: [gcc r13-4110] libstdc++: Improve performance of chrono::utc_clock::now() X-Act-Checkin: gcc X-Git-Author: Jonathan Wakely X-Git-Refname: refs/heads/master X-Git-Oldrev: 2f5c071860ba3f8ef67d0b9d8291a73766ce0a44 X-Git-Newrev: 629897ed80512a8618e08673c03d8482cbc42eef Message-Id: <20221116205937.82AAA396DC27@sourceware.org> Date: Wed, 16 Nov 2022 20:59:37 +0000 (GMT) List-Id: https://gcc.gnu.org/g:629897ed80512a8618e08673c03d8482cbc42eef commit r13-4110-g629897ed80512a8618e08673c03d8482cbc42eef Author: Jonathan Wakely 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 __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