From: Cassio Neri <cassio.neri@gmail.com>
Date: Tue, 14 Nov 2023 00:27:39 +0000 (+0000)
Subject: libstdc++: Improve operator-(weekday x, weekday y)
X-Git-Tag: releases/gcc-12.4.0~355
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=a491dd0eac360758dd20b29bef78b27d005547a1;p=thirdparty%2Fgcc.git

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.

(cherry picked from commit f71352c71d78ac977ea0e71a6900699a8cf09219)
---

diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono
index 0d3c08194394..3c5b425c7fe8 100644
--- a/libstdc++-v3/include/std/chrono
+++ b/libstdc++-v3/include/std/chrono
@@ -686,8 +686,8 @@ _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};
       }
 
       // TODO: operator<<, from_stream.