#include <cassert>
#include <chrono>
#include <limits>
+#include <type_traits>
#include <boost/noncopyable.hpp>
return rv == FutexResult::AWOKEN;
}
- /** Similar to futexWait but also accepts a timeout that gives the time until
- * when the call can block (time is the absolute time i.e time since epoch).
- * Allowed clock types: std::chrono::system_clock, std::chrono::steady_clock.
- * Returns one of FutexResult values.
+ /** Similar to futexWait but also accepts a deadline until when the wait call
+ * may block.
*
+ * Optimal clock types: std::chrono::system_clock, std::chrono::steady_clock.
* NOTE: On some systems steady_clock is just an alias for system_clock,
- * and is not actually steady.*/
+ * and is not actually steady.
+ *
+ * For any other clock type, now() will be invoked twice. */
template <class Clock, class Duration = typename Clock::duration>
FutexResult futexWaitUntil(
- uint32_t expected,
- const std::chrono::time_point<Clock, Duration>& absTime,
- uint32_t waitMask = -1) {
- using std::chrono::duration_cast;
- using std::chrono::nanoseconds;
- using std::chrono::seconds;
- using std::chrono::steady_clock;
- using std::chrono::system_clock;
- using std::chrono::time_point;
-
- static_assert(
- (std::is_same<Clock, system_clock>::value ||
- std::is_same<Clock, steady_clock>::value),
- "futexWaitUntil only knows std::chrono::{system_clock,steady_clock}");
- assert((std::is_same<Clock, system_clock>::value) || Clock::is_steady);
-
- // We launder the clock type via a std::chrono::duration so that we
- // can compile both the true and false branch. Tricky case is when
- // steady_clock has a higher precision than system_clock (Xcode 6,
- // for example), for which time_point<system_clock> construction
- // refuses to do an implicit duration conversion. (duration is
- // happy to implicitly convert its denominator causing overflow, but
- // refuses conversion that might cause truncation.) We use explicit
- // duration_cast to work around this. Truncation does not actually
- // occur (unless Duration != Clock::duration) because the missing
- // implicit conversion is in the untaken branch.
- Duration absTimeDuration = absTime.time_since_epoch();
- if (std::is_same<Clock, system_clock>::value) {
- time_point<system_clock> absSystemTime(
- duration_cast<system_clock::duration>(absTimeDuration));
- return futexWaitImpl(expected, &absSystemTime, nullptr, waitMask);
- } else {
- time_point<steady_clock> absSteadyTime(
- duration_cast<steady_clock::duration>(absTimeDuration));
- return futexWaitImpl(expected, nullptr, &absSteadyTime, waitMask);
- }
+ uint32_t expected,
+ std::chrono::time_point<Clock, Duration> const& deadline,
+ uint32_t waitMask = -1) {
+ using Target = typename std::conditional<
+ Clock::is_steady,
+ std::chrono::steady_clock,
+ std::chrono::system_clock>::type;
+ auto const converted = time_point_conv<Target>(deadline);
+ return futexWaitImpl(expected, converted, waitMask);
}
/** Wakens up to count waiters where (waitMask & wakeMask) !=
uint32_t wakeMask = -1);
private:
+ /** Optimal when TargetClock is the same type as Clock.
+ *
+ * Otherwise, both Clock::now() and TargetClock::now() must be invoked. */
+ template <typename TargetClock, typename Clock, typename Duration>
+ static typename TargetClock::time_point time_point_conv(
+ std::chrono::time_point<Clock, Duration> const& time) {
+ using std::chrono::duration_cast;
+ using TargetDuration = typename TargetClock::duration;
+ using TargetTimePoint = typename TargetClock::time_point;
+ if (std::is_same<Clock, TargetClock>::value) {
+ // in place of time_point_cast, which cannot compile without if-constexpr
+ auto const delta = time.time_since_epoch();
+ return TargetTimePoint(duration_cast<TargetDuration>(delta));
+ } else {
+ // different clocks with different epochs, so non-optimal case
+ auto const delta = time - Clock::now();
+ return TargetClock::now() + duration_cast<TargetDuration>(delta);
+ }
+ }
+
+ template <typename Deadline>
+ typename std::enable_if<Deadline::clock::is_steady, FutexResult>::type
+ futexWaitImpl(
+ uint32_t expected,
+ Deadline const& deadline,
+ uint32_t waitMask) {
+ return futexWaitImpl(expected, nullptr, &deadline, waitMask);
+ }
+
+ template <typename Deadline>
+ typename std::enable_if<!Deadline::clock::is_steady, FutexResult>::type
+ futexWaitImpl(
+ uint32_t expected,
+ Deadline const& deadline,
+ uint32_t waitMask) {
+ return futexWaitImpl(expected, &deadline, nullptr, waitMask);
+ }
/** Underlying implementation of futexWait and futexWaitUntil.
* At most one of absSystemTime and absSteadyTime should be non-null.