Optimization options?

User's Guide 6.2 says: "So, when we want very fast code, we use: -O -fvia-C." but I've seen programs which use the (apparently undocumented) -O2-for-C flag too. In fact, generally speaking anyone of: -O -fvia-C -O2 -fvia-C -O -fvia-C -O2-for-C -O2 -fvia-C -O2-for-C produces different output... So, which is the best choice? Juanma