Loop unwinding might not bring any improvement in the speed because the compilers have been improved . Anyway, it's pleasant reading about the history :)
Loop unwinding [http://en.wikipedia.org]
Detailed explanation of SWITCH statement
Duff's device [tenaciousc.com]
A reusable duff device [drdobbs.com]
Duff device [www.lysator.liu.se/c/]
How does duffs device work [stackoverflow.com]