... at the cost of erasing the result, and taking time proportional to the original computation? Clever optimization, but the net result seems suboptimal
The idea is that eventually we'll have to go reversible anyway. Computation will have to run twice in order to run at more than 2X the clock speed of running once. Getting easy garbage collection is a side effect, not a goal of the optimization.
And, the result will have to be copied off before rewinding.
Execute instructions. Some of those instructions effectively allocate memory. Reverse execution. Running backwards undoes the allocations.