Reversible computing needs a place to store waste entropy, ie a rather large memory that's initialized to known values and filled with junk as the computation r

Reversible computing needs a place to store waste entropy, ie a rather large memory that's initialized to known values and filled with junk as the computation runs. Venting entropy out of a system is not reversible. Clockless designs mean that you compute readiness information on the fly instead of having it precomputed at design time. This additional run-time computation is not free, and tooling for clocked designs is good enough that the extra slack they need is often cheaper.