Error with GDC semaphore - ios

Question

The following Swift class is built to emulate Java Thread sleep and interrupt methods with GDC. The sleep method creates a semaphore, waits for it to be signaled and returns true when sleep ended prematurely. The interrupt method signals the semaphore to get the thread out of sleep:
public protocol GDCRunnable {
func run()
}
public class GDCThread {
private var semaphore : dispatch_semaphore_t?;
private let queue : dispatch_queue_t;
private let runnable : GDCRunnable?;
public init(_priority :Int = DISPATCH_QUEUE_PRIORITY_DEFAULT,
runnable : GDCRunnable? ) {
self.runnable = runnable
queue = dispatch_get_global_queue(_priority, 0)
}
public func start() {
dispatch_async(queue) {
self.run();
}
}
public func run() {
if runnable != nil {runnable!.run()}
}
public func sleep(_timeoutMillis : Int) -> Bool {
objc_sync_enter(self)
semaphore = dispatch_semaphore_create(1)
objc_sync_exit(self)
let signaled = (dispatch_semaphore_wait(semaphore,
dispatch_time(DISPATCH_TIME_NOW, Int64(_timeoutMillis*1000000))) != 0)
if !signaled {
dispatch_semaphore_signal(semaphore);
}
objc_sync_enter(self)
semaphore = nil;
objc_sync_exit(self)
return signaled
}
public func interrupt () {
objc_sync_enter(self)
if let currentSemaphore = semaphore {
dispatch_semaphore_signal(currentSemaphore)
}
objc_sync_exit(self)
}
}
As you can see, I put some objc_sync_enter and objc_sync_exit (though it's most probably redundant), but it didn't help: with iPhone 6
emulator it works great, but iPad Retina emulator gives BAD INSTRUCTION CODE on dispatch_semaphore_wait. Any suggestions?