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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.
This commit is contained in:
Cassio Neri 2023-11-14 00:27:39 +00:00 committed by Jonathan Wakely
parent f6ce081d0f
commit f71352c71d
1 changed files with 2 additions and 2 deletions
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4
libstdc++-v3/include/std/chrono
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@ -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};
}
};