[PATCH] [libstdc++] Cleanup atomic timed wait implementation

Thomas Rodgers rodgert@appliantology.com
Fri Jun 4 22:29:13 GMT 2021


This cleans up the implementation of atomic_timed_wait.h and fixes the
accidental pessimization of spinning after waiting in
__timed_waiter_pool::_M_do_wait_until.

libstdc++-v3/ChangeLog:

	* include/bits/atomic_timed_wait.h (__wait_clock_t): Define
	conditionally.
	(__cond_wait_until_impl): Define conditionally.
	(__cond_wait_until): Define conditionally. Simplify clock
	type detection/conversion.
	(__timed_waiter_pool::_M_do_wait_until): Move the spin above
	the wait.

---
 libstdc++-v3/include/bits/atomic_timed_wait.h | 26 ++++++++++---------
 1 file changed, 14 insertions(+), 12 deletions(-)

diff --git a/libstdc++-v3/include/bits/atomic_timed_wait.h b/libstdc++-v3/include/bits/atomic_timed_wait.h
index ec7ff51cdbc..19386e5806a 100644
--- a/libstdc++-v3/include/bits/atomic_timed_wait.h
+++ b/libstdc++-v3/include/bits/atomic_timed_wait.h
@@ -51,7 +51,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   namespace __detail
   {
+#ifdef _GLIBCXX_HAVE_LINUX_FUTEX || _GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT
     using __wait_clock_t = chrono::steady_clock;
+#else
+    using __wait_clock_t = chrono::system_clock;
+#endif
 
     template<typename _Clock, typename _Dur>
       __wait_clock_t::time_point
@@ -133,11 +137,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	    return false;
 	  }
       }
-#else
+// #elsif <some other platform mechanism,>
 // define _GLIBCXX_HAVE_PLATFORM_TIMED_WAIT and implement __platform_wait_until()
 // if there is a more efficient primitive supported by the platform
 // (e.g. __ulock_wait())which is better than pthread_cond_clockwait
-#endif // ! PLATFORM_TIMED_WAIT
+#else
+// Use wait on condition variable
 
     // Returns true if wait ended before timeout.
     // _Clock must be either steady_clock or system_clock.
@@ -173,12 +178,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       __cond_wait_until(__condvar& __cv, mutex& __mx,
 	  const chrono::time_point<_Clock, _Dur>& __atime)
       {
-#ifdef _GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT
-	if constexpr (is_same_v<_Clock, chrono::steady_clock>)
-	  return __detail::__cond_wait_until_impl(__cv, __mx, __atime);
-	else
-#endif
-	if constexpr (is_same_v<_Clock, chrono::system_clock>)
+	if constexpr (is_same_v<__wait_clock_t, _Clock>)
 	  return __detail::__cond_wait_until_impl(__cv, __mx, __atime);
 	else
 	  {
@@ -194,6 +194,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	    return false;
 	  }
       }
+#endif // ! PLATFORM_TIMED_WAIT
 
     struct __timed_waiter_pool : __waiter_pool_base
     {
@@ -300,17 +301,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 			  const chrono::time_point<_Clock, _Dur>&
 							      __atime) noexcept
 	  {
+
 	    for (auto __now = _Clock::now(); __now < __atime;
 		  __now = _Clock::now())
 	      {
+		if (__base_type::_M_do_spin(__pred, __val,
+			       __timed_backoff_spin_policy(__atime, __now)))
+		  return true;
+
 		if (__base_type::_M_w._M_do_wait_until(
 		      __base_type::_M_addr, __val, __atime)
 		    && __pred())
 		  return true;
-
-		if (__base_type::_M_do_spin(__pred, __val,
-			       __timed_backoff_spin_policy(__atime, __now)))
-		  return true;
 	      }
 	    return false;
 	  }
-- 
2.26.2



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