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From: Jonathan Wakely <redi@gcc.gnu.org> To: gcc-cvs@gcc.gnu.org, libstdc++-cvs@gcc.gnu.org Subject: [gcc r12-10211] libstdc++: Improve operator-(weekday x, weekday y) Date: Wed, 13 Mar 2024 09:53:04 +0000 (GMT) [thread overview] Message-ID: <20240313095304.5FF273857C65@sourceware.org> (raw) https://gcc.gnu.org/g:a491dd0eac360758dd20b29bef78b27d005547a1 commit r12-10211-ga491dd0eac360758dd20b29bef78b27d005547a1 Author: Cassio Neri <cassio.neri@gmail.com> Date: Tue Nov 14 00:27:39 2023 +0000 libstdc++: Improve operator-(weekday x, weekday y) The current implementation calls __detail::__modulo which is relatively expensive. A better implementation is possible if we assume that x.ok() && y.ok() == true, so that n = x.c_encoding() - y.c_encoding() is in [-6, 6]. In this case, it suffices to return n >= 0 ? n : n + 7. The above is allowed by [time.cal.wd.nonmembers]/5: the returned value is unspecified when x.ok() || y.ok() == false. The assembly emitted for x86-64 and ARM can be seen in: https://godbolt.org/z/nMdc5vv9n. libstdc++-v3/ChangeLog: * include/std/chrono (operator-(const weekday&, const weekday&)): Optimize. (cherry picked from commit f71352c71d78ac977ea0e71a6900699a8cf09219) Diff: --- libstdc++-v3/include/std/chrono | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono index 0d3c0819439..3c5b425c7fe 100644 --- a/libstdc++-v3/include/std/chrono +++ b/libstdc++-v3/include/std/chrono @@ -686,8 +686,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION friend constexpr days operator-(const weekday& __x, const weekday& __y) noexcept { - auto __n = static_cast<long long>(__x._M_wd) - __y._M_wd; - return days{__detail::__modulo(__n, 7)}; + const auto __n = __x.c_encoding() - __y.c_encoding(); + return static_cast<int>(__n) >= 0 ? days{__n} : days{__n + 7}; } // TODO: operator<<, from_stream.
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