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
std::chrono::system_clock.
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 | 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 47ecd329ea9..a35020aef4f 100644
--- a/libstdc++-v3/include/bits/atomic_futex.h
+++ b/libstdc++-v3/include/bits/atomic_futex.h
@@ -71,7 +71,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
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;
@@ -169,7 +169,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
unsigned
_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);
@@ -229,7 +229,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
_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;
@@ -241,7 +240,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
template<typename _Duration>
_GLIBCXX_ALWAYS_INLINE bool
_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)
--
2.11.0
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