[gcc r14-5470] libstdc++: Improve operator-(weekday x, weekday y)

[gcc r14-5470] libstdc++: Improve operator-(weekday x, weekday y)

[Jonathan Wakely]

https://gcc.gnu.org/g:f71352c71d78ac977ea0e71a6900699a8cf09219

commit r14-5470-gf71352c71d78ac977ea0e71a6900699a8cf09219
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.

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 bd21ec189a5..10bdd1c4ede 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};
       }
     };