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| author | Cassio Neri <cassio.neri@gmail.com> | 2023-11-14 00:27:39 +0000 |
|---|---|---|
| committer | Jonathan Wakely <jwakely@redhat.com> | 2023-11-14 22:32:09 +0000 |
| commit | f71352c71d78ac977ea0e71a6900699a8cf09219 (patch) | |
| tree | 717f7ee03699fb6ad845634e4503edbf1f358cc0 /libstdc++-v3 | |
| parent | f6ce081d0ffb5f25d71eb2f30fcfdff7f20dba22 (diff) | |
| download | gcc-f71352c71d78ac977ea0e71a6900699a8cf09219.zip gcc-f71352c71d78ac977ea0e71a6900699a8cf09219.tar.gz gcc-f71352c71d78ac977ea0e71a6900699a8cf09219.tar.bz2 | |
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.
Diffstat (limited to 'libstdc++-v3')
| -rw-r--r-- | libstdc++-v3/include/std/chrono | 4 |
1 files changed, 2 insertions, 2 deletions
diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono index bd21ec1..10bdd1c 100644 --- a/libstdc++-v3/include/std/chrono +++ b/libstdc++-v3/include/std/chrono @@ -1043,8 +1043,8 @@ _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}; } }; |