Esterel is a design language for the representation of embedded systems. Based on the synchronous reactive paradigm, its execution relies on a clear distinction of instants of computation. As a consequence, deciding whether a piece of a program may or may not run instantaneously is central to any compilation scheme, both for correctness and efficiency. In general, this information can be obtained by an exhaustive exploration of all possible execution paths, which is expensive. Most compilers approximate it through algorithmic methods amenable to static analysis. In our contribution, we first formalize the analysis involved in detecting statements that may run instantaneously. Then, we identify statements that may terminate and be instantaneously reentered. This allows us to model precisely these compilers front-end activities with a clear mathematical specification and led us to uncover inefficiencies in the Esterel v5 academic compiler from Ecole des Mines and INRIA.