Disclosed approaches involve simulating a circuit design specified in a hardware description language (HDL). During simulation, a thread is started at an edge of a simulation clock signal for evaluation of states of a finite state machine (FSM) that represent a series of events specified in a statement in the HDL. The thread transitions from one state to a next state in the FSM in response to evaluation of the one state. In response to encountering a fork state in the FSM, the thread is forked into two threads during simulation. The fork state represents a composite operator in the statement, and the FSM has a branch from the fork state for each operand of the composite operator. In response to encountering a join state in the FSM by the two threads, the two threads are joined into one thread.