-xO1

Does basic local optimization (peephole). 

-xO2

Does basic local and global optimization. This is induction variable elimination, local and global common subexpression elimination, algebraic simplification, copy propagation, constant propagation, loop-invariant optimization, register allocation, basic block merging, tail recursion elimination, dead code elimination, tail call elimination, and complex expression expansion. 

The -xO2 level does not assign global, external, or indirect references or definitions to registers. It treats these references and definitions as if they were declared volatile. In general, the -xO2 level results in minimum code size.

-xO3

Performs like -xO2, but also optimizes references or definitions for external variables. Loop unrolling and software pipelining are also performed. This level does not trace the effects of pointer assignments. When compiling either device drivers, or programs that modify external variables from within signal handlers, you may need to use the volatile type qualifier to protect the object from optimization. In general, the -xO3 level results in increased code size.

-xO4

Performs like -xO3, but also automatically inlines functions contained in the same file; this usually improves execution speed. If you want to control which functions are inlined, see B.2.91 -xinline=list.

This level traces the effects of pointer assignments, and usually results in increased code size. 

-xO5

Attempts to generate the highest level of optimization. Uses optimization algorithms that take more compilation time or that do not have as high a certainty of improving execution time. Optimization at this level is more likely to improve performance if it is done with profile feedback. See B.2.131 -xprofile=p.