For the case where a timeout is specified using the system_clock we
perform a conversion to the preferred clock (which is either
steady_clock or system_clock itself), wait using __cond_wait_until_impl,
and then check the time by that clock again to see if it was reached.
This is entirely redundant, as we can just call __cond_wait_until_impl
directly. It will wait using the specified clock, and there's no need to
check the time twice. For the no_timeout case this removes two
unnecessary calls to the clock's now() function, and for the timeout
case it removes three calls.
libstdc++-v3/ChangeLog:
* include/bits/atomic_timed_wait.h (__cond_wait_until): Do not
perform redundant conversions to the same clock.
Tested powerpc-aix and sparc-solaris2.11.
Committed to trunk.
commit dfc537e554afa98b42a4b203ffd08c0eddba746e
Author: Jonathan Wakely <jwak...@redhat.com>
Date: Wed Nov 25 17:59:44 2020
libstdc++: Remove redundant clock conversions in atomic waits
For the case where a timeout is specified using the system_clock we
perform a conversion to the preferred clock (which is either
steady_clock or system_clock itself), wait using __cond_wait_until_impl,
and then check the time by that clock again to see if it was reached.
This is entirely redundant, as we can just call __cond_wait_until_impl
directly. It will wait using the specified clock, and there's no need to
check the time twice. For the no_timeout case this removes two
unnecessary calls to the clock's now() function, and for the timeout
case it removes three calls.
libstdc++-v3/ChangeLog:
* include/bits/atomic_timed_wait.h (__cond_wait_until): Do not
perform redundant conversions to the same clock.
diff --git a/libstdc++-v3/include/bits/atomic_timed_wait.h
b/libstdc++-v3/include/bits/atomic_timed_wait.h
index 9e44114dd5b6..1c91c858ce7c 100644
--- a/libstdc++-v3/include/bits/atomic_timed_wait.h
+++ b/libstdc++-v3/include/bits/atomic_timed_wait.h
@@ -166,24 +166,32 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
__cond_wait_until(__condvar& __cv, mutex& __mx,
const chrono::time_point<_Clock, _Duration>& __atime)
{
-#ifdef _GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT
- using __clock_t = chrono::steady_clock;
-#else
+#ifndef _GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT
using __clock_t = chrono::system_clock;
+#else
+ using __clock_t = chrono::steady_clock;
+ if constexpr (is_same_v<_Clock, chrono::steady_clock>)
+ return __detail::__cond_wait_until_impl(__cv, __mx, __atime);
+ else
#endif
- 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;
- const auto __s_atime = __s_entry + __delta;
- if (__detail::__cond_wait_until_impl(__cv, __mx, __s_atime)
- == __atomic_wait_status::no_timeout)
- return __atomic_wait_status::no_timeout;
- // We got a timeout when measured against __clock_t but
- // we need to check against the caller-supplied clock
- // to tell whether we should return a timeout.
- if (_Clock::now() < __atime)
- return __atomic_wait_status::no_timeout;
- return __atomic_wait_status::timeout;
+ if constexpr (is_same_v<_Clock, chrono::system_clock>)
+ return __detail::__cond_wait_until_impl(__cv, __mx, __atime);
+ else
+ {
+ 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;
+ const auto __s_atime = __s_entry + __delta;
+ if (__detail::__cond_wait_until_impl(__cv, __mx, __s_atime)
+ == __atomic_wait_status::no_timeout)
+ return __atomic_wait_status::no_timeout;
+ // We got a timeout when measured against __clock_t but
+ // we need to check against the caller-supplied clock
+ // to tell whether we should return a timeout.
+ if (_Clock::now() < __atime)
+ return __atomic_wait_status::no_timeout;
+ return __atomic_wait_status::timeout;
+ }
}
#endif // FUTEX
