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: --- OK for trunk? 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 10e868e5a03..6131e7e97b3 100644 --- a/libstdc++-v3/include/std/chrono +++ b/libstdc++-v3/include/std/chrono @@ -1036,8 +1036,8 @@ _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}; } }; -- 2.41.0
