In theory, irreversible computations have to produce heat, while reversible do not. In practice this is mostly irrelevant, because the heat involved is so minus

In theory, irreversible computations have to produce heat, while reversible do not. In practice this is mostly irrelevant, because the heat involved is so minuscule that there will always be other significantly larger sources of inefficiency involved. Also it is somewhat questionable whether one could actually construct physical realization of useful logic primitive that is truly reversible.