Those are optimization parameters. O3 is very aggresive, and requires user-engaging-brain before using (you may want to search for info on pointer aliasing). fast-math will make some floating point operations faster, at the cost of losing strict IEEE floating point compliance (no biggie for most purposes, but again, user needs to know what he's doing). tree-vectorize will attempt to use parallelizing instructions (sse2, neon, etc.)