[PATCH v5 4/8] libstdc++ atomic_futex: Use std::chrono::steady_clock as reference clock

Mike Crowe mac@mcrowe.com
Fri May 29 06:17:30 GMT 2020

The user-visible effect of this change is that std::future::wait_for now
uses std::chrono::steady_clock to determine the timeout.  This makes it
immune to changes made to the system clock.  It also means that anyone
using their own clock types with std::future::wait_until will have the
timeout converted to std::chrono::steady_clock rather than

Now that use of both std::chrono::steady_clock and
std::chrono::system_clock are correctly supported for the wait timeout, I
believe that std::chrono::steady_clock is a better choice for the reference
clock that all other clocks are converted to since it is guaranteed to
advance steadily.  The previous behaviour of converting to
std::chrono::system_clock risks timeouts changing dramatically when the
system clock is changed.

	* libstdc++-v3/include/bits/atomic_futex.h:
	(__atomic_futex_unsigned): Change __clock_t typedef to use
	steady_clock so that unknown clocks are synced to it rather than
	system_clock. Change existing __clock_t overloads of
	_M_load_and_text_until_impl and _M_load_when_equal_until to use
	system_clock explicitly. Remove comment about DR 887 since these
	changes address that problem as best as we currently able.
 libstdc++-v3/include/bits/atomic_futex.h | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/libstdc++-v3/include/bits/atomic_futex.h b/libstdc++-v3/include/bits/atomic_futex.h
index 507c5c9..4375129 100644
--- a/libstdc++-v3/include/bits/atomic_futex.h
+++ b/libstdc++-v3/include/bits/atomic_futex.h
   template <unsigned _Waiter_bit = 0x80000000>
   class __atomic_futex_unsigned : __atomic_futex_unsigned_base
-    typedef chrono::system_clock __clock_t;
+    typedef chrono::steady_clock __clock_t;
     // This must be lock-free and at offset 0.
     atomic<unsigned> _M_data;
     _M_load_and_test_until_impl(unsigned __assumed, unsigned __operand,
 	bool __equal, memory_order __mo,
-	const chrono::time_point<__clock_t, _Dur>& __atime)
+	const chrono::time_point<std::chrono::system_clock, _Dur>& __atime)
       auto __s = chrono::time_point_cast<chrono::seconds>(__atime);
       auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s);
       _M_load_when_equal_until(unsigned __val, memory_order __mo,
 	  const chrono::time_point<_Clock, _Duration>& __atime)
-	// DR 887 - Sync unknown clock to known clock.
 	const typename _Clock::time_point __c_entry = _Clock::now();
 	const __clock_t::time_point __s_entry = __clock_t::now();
 	const auto __delta = __atime - __c_entry;
     template<typename _Duration>
     _M_load_when_equal_until(unsigned __val, memory_order __mo,
-	const chrono::time_point<__clock_t, _Duration>& __atime)
+	const chrono::time_point<std::chrono::system_clock, _Duration>& __atime)
       unsigned __i = _M_load(__mo);
       if ((__i & ~_Waiter_bit) == __val)
git-series 0.9.1

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