I fail to understand what you are saying. Any speed gains of Duff's device aren't regarding the calculation of fractional components, or where to enter or leave the track at the odd place. That's only done once.

It's more like running tracks where each time you pass the finish line you have to stop, and decrease the lap counter. Running 1 mile on the smaller track means having to stop 4 times to decrease the counter, while running on the longer track means stopping once. That's loop unrolling. The trick of Duff's device is doing that instead of running on big track for N miles, and a couple of runs on the small track to make up for the fractional distance, you start somewhere on the big track.

It's the loop unrolling that saves time, it's Duff's trick of mixing switch/while that saves on tracks. Or rather, it saves on (duplicate) code.

Abigail