[gcc r13-8424] libstdc++: Fix UB in weekday::weekday(sys_days) and add test

[Jonathan Wakely]

https://gcc.gnu.org/g:d1472711efc77d5ddc2fa6d5eff57baca584c8ef

commit r13-8424-gd1472711efc77d5ddc2fa6d5eff57baca584c8ef
Author: Cassio Neri <cassio.neri@gmail.com>
Date:   Sun Nov 12 01:33:52 2023 +0000

    libstdc++: Fix UB in weekday::weekday(sys_days) and add test
    
    The following has undefined behaviour (signed overflow) [1]:
        weekday max{sys_days{days{numeric_limits<days::rep>::max()}}};
    
    The issue is in this line when __n is very large and __n + 4 overflows:
        return weekday(__n >= -4 ? (__n + 4) % 7 : (__n + 5) % 7 + 6);
    
    In addition to fixing this bug, the new implementation makes the compiler emit
    shorter and branchless code for x86-64 and ARM [2].
    
    [1] https://godbolt.org/z/1s5bv7KfT
    [2] https://godbolt.org/z/zKsabzrhs
    
    libstdc++-v3/ChangeLog:
    
            * include/std/chrono (weekday::_S_from_days): Fix UB.
            * testsuite/std/time/weekday/1.cc: Add test for overflow.
    
    (cherry picked from commit f6ce081d0ffb5f25d71eb2f30fcfdff7f20dba22)

Diff:
---
 libstdc++-v3/include/std/chrono              | 11 +++++++++--
 libstdc++-v3/testsuite/std/time/weekday/1.cc |  9 +++++++++
 2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono
index ac7febbaa2c..fb8d6c82e8a 100644
--- a/libstdc++-v3/include/std/chrono
+++ b/libstdc++-v3/include/std/chrono
@@ -936,8 +936,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       static constexpr weekday
       _S_from_days(const days& __d)
       {
-	auto __n = __d.count();
-	return weekday(__n >= -4 ? (__n + 4) % 7 : (__n + 5) % 7 + 6);
+	using _Rep = days::rep;
+	using _URep = make_unsigned_t<_Rep>;
+	const auto __n = __d.count();
+	const auto __m = static_cast<_URep>(__n);
+
+	// 1970-01-01 (__n =  0, __m = 0        ) -> Thursday (4)
+	// 1969-31-12 (__n = -1, __m = _URep(-1)) -> Wednesday (3)
+	const auto __offset = __n >= 0 ? _URep(4) : 3 - _URep(-1) % 7 - 7;
+	return weekday((__m + __offset) % 7);
       }
 
     public:
diff --git a/libstdc++-v3/testsuite/std/time/weekday/1.cc b/libstdc++-v3/testsuite/std/time/weekday/1.cc
index 1e018eaa3e0..bfc617c4cc8 100644
--- a/libstdc++-v3/testsuite/std/time/weekday/1.cc
+++ b/libstdc++-v3/testsuite/std/time/weekday/1.cc
@@ -21,6 +21,7 @@
 // Class template day [time.cal.weekday]
 
 #include <chrono>
+#include <limits>
 
 constexpr void
 constexpr_weekday()
@@ -38,6 +39,14 @@ constexpr_weekday()
   static_assert(weekday{3}[2].weekday() == weekday{3});
   static_assert(weekday{3}[last].weekday() == weekday{3});
 
+  // Test for UB (overflow).
+  {
+    using rep = days::rep;
+    using std::numeric_limits;
+    constexpr weekday max{sys_days{days{numeric_limits<rep>::max()}}};
+    constexpr weekday min{sys_days{days{numeric_limits<rep>::min()}}};
+  }
+
   static_assert(weekday{sys_days{1900y/January/1}} == Monday);
   static_assert(weekday{sys_days{1970y/January/1}} == Thursday);
   static_assert(weekday{sys_days{2020y/August/21}} == Friday);