| Message ID | 20231114002739.14809-1-cassio.neri@gmail.com |
|---|---|
| State | New |
| Headers | show |
| Series | libstdc++: Improve operator-(weekday x, weekday y). | expand |
On Tue, 14 Nov 2023 at 00:27, Cassio Neri <cassio.neri@gmail.com> wrote: > > 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? Pushed to trunk, thanks! > > 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 >
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}; } };