Merge branch 'ubq' into 'master'

Improve code readability of UnboundedBlockingMpscQueue

See merge request i4/manycore/emper!52

emper/UnboundedBlockingMpscQueue.hpp

@@ -33,13 +33,14 @@ class UnboundedBlockingMpscQueue : public Blockable<LogSubsystem::U_B_MPSC_Q> {
 		// Therefore if we tryToGet a new 't' while 't' is available the current 't'
 		// would be overridden and dropped.
 		assert(!tPopped);
+
 		std::lock_guard<std::mutex> lock(queueMutex);
-		if (!mpscQueue.empty()) {
-			t = mpscQueue.front();
-			mpscQueue.pop();
-			tPopped = true;
-			postRetrieve();
-		}
+		if (mpscQueue.empty()) return;
+
+		t = mpscQueue.front();
+		mpscQueue.pop();
+		tPopped = true;
+		postRetrieve();
 	}
 
  public:
@@ -54,7 +55,7 @@ class UnboundedBlockingMpscQueue : public Blockable<LogSubsystem::U_B_MPSC_Q> {
 
 		// Micro optimization, see if there is a blocked context
 		// before performing the atomic exchange operation.
-		if (blockedContext.load() != nullptr) {
+		if (blockedContext != nullptr) {
 			tryToWakeupBlockedContext<callerEnvironment>();
 		}
 	}
@@ -82,10 +83,16 @@ class UnboundedBlockingMpscQueue : public Blockable<LogSubsystem::U_B_MPSC_Q> {
 			});
 			if (!tPopped) {
 				std::lock_guard<std::mutex> lock(queueMutex);
+				// If we observed tPopped to be false without holding the
+				// queueMutex, then the same must be true when holding the
+				// mutex.
+				assert(!tPopped);
+
 				// If 't' isn't already set, then mspcQueue.get() MUST
 				// return an element. Note that a non-lineralizabe
 				// queue may break this invariant.
 				assert(!mpscQueue.empty());
+
 				t = mpscQueue.front();
 				mpscQueue.pop();
 				postRetrieve();