[avr-gcc-list] -O1 vs. -O3

[Marlin Unruh]

Excuse me, but my amateurishness is showing!  :)

What is the difference between -O1, -O2, -O3 ???

Optimization of course, but why not always as optimized as possible? Why not
always use -O1?

I had never change the setting till today, and my code size drop 112 bytes
using -O1 instead of -O3.

Everything has an equal and opposite reaction so, what's the trade-off?
Speed? Reliability?

Regards,

- Marlin