1. A processor-implemented method comprising: receiving an atomic instruction block which includes a plurality of instructions; determining a characteristic of the atomic instruction block based at least in part on metadata of the atomic instruction block set by a compiler or on an identifier of the atomic instruction block; and switching between an out-of-order instruction issue mode of a processor and an in-order instruction issue mode of the processor based at least in part on the characteristic, wherein the out-of-order instruction issue mode and the in-order instruction issue mode are associated with different energy consumption characteristics.