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| author | Jonathan Wakely <jwakely@redhat.com> | 2020-11-25 17:59:44 +0000 |
|---|---|---|
| committer | Jonathan Wakely <jwakely@redhat.com> | 2020-11-25 18:24:13 +0000 |
| commit | dfc537e554afa98b42a4b203ffd08c0eddba746e (patch) | |
| tree | d683b7caeea89f9f590ed5f4e6df5b3362b7d956 | |
| parent | 7d2a98a7273c423842a3935de64b15a6d6cb33bc (diff) | |
| download | gcc-dfc537e554afa98b42a4b203ffd08c0eddba746e.zip gcc-dfc537e554afa98b42a4b203ffd08c0eddba746e.tar.gz gcc-dfc537e554afa98b42a4b203ffd08c0eddba746e.tar.bz2 | |
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.
| -rw-r--r-- | libstdc++-v3/include/bits/atomic_timed_wait.h | 40 |
1 files changed, 24 insertions, 16 deletions
diff --git a/libstdc++-v3/include/bits/atomic_timed_wait.h b/libstdc++-v3/include/bits/atomic_timed_wait.h index 9e44114..1c91c85 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 |