-O
   -O1 Optimize.
        Optimizing compilation takes somewhat more time, and a lot more
        memory for a large function. With -O, the compiler tries to reduce
        code size and execution time, without performing any optimizations
        that take a great deal of compilation time.
   -O2 Optimize even more.
        GCC performs nearly all supported optimizations that do not involve a
        space-speed tradeoff. As compared to -O, this option increases both
        compilation time and the performance of the generated code.
   -O3 Optimize yet more.
       -O3 turns on all optimizations specified by -O2 and also turns on the
       -finline-functions, -funswitch-loops, -fpredictive-commoning,
       -fgcse-after-reload, -ftree-vectorize and -fipa-cp-clone options.