In response to the rise of multicore processors, mainstream languages have begun to offer primitives for concurrent programming. To avoid the cost of inter-core synchronisation, the new C/C++ standard, C11 [2], offers weakly consistent relaxed operations, alongside traditional reads, writes and mutexes. When using relaxed operations, different threads may see different, apparently contradictory orders of events. C11 permits a particularly surprising kind of relaxed behaviour: cycles in

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... . Two conditional guards on different threads can be satisfied by writes down the other branch. Both branches execute, even though each appears to depend on the other.