0    Turn off all optimizations.

1    Turn on local optimizations that can be done quickly. Do peephole optimizations and instruction scheduling.

2    Turn on extensive optimization. This is the default.
The optimizations at this level are generally conservative, in the sense that they are virtually always beneficial and avoid changes which affect such things as floating point accuracy. In addition to the level 1 optimizations, do inner loop unrolling, if-conversion, two passes of instruction scheduling, global register allocation, dead store elimination, instruction scheduling across basic blocks, and partial redundancy elimination.

3    Turn on aggressive optimization.
The optimizations at this level are distinguished from -O2 by their aggressiveness, generally seeking highest-quality generated code even if it requires extensive compile time. They may include optimizations that are generally beneficial but may hurt performance.
This includes but is not limited to turning on the Loop Nest Optimizer, -LNO:opt=1, and setting -OPT:roundoff=1:IEEE_arithmetic=2:Olimit=9000:reorg_common=ON.

s    Specify that code size is to be given priority in tradeoffs with execution time.