-O2 Optimize even more.  GCC performs nearly all supported optimizations that do not involve a space-speed
           tradeoff.  The compiler does not perform loop unrolling or function inlining when you specify -O2.  As
           compared to -O, this option increases both compilation time and the performance of the generated code.

           -O2 turns on all optimization flags specified by -O.  It also turns on the following optimization flags:
           -fthread-jumps -fcrossjumping -foptimize-sibling-calls -fcse-follow-jumps  -fcse-skip-blocks -fgcse
           -fgcse-lm -fexpensive-optimizations -fstrength-reduce -frerun-cse-after-loop  -frerun-loop-opt
           -fcaller-saves -fforce-mem -fpeephole2 -fschedule-insns  -fschedule-insns2 -fsched-interblock
           -fsched-spec -fregmove -fstrict-aliasing -fdelete-null-pointer-checks -freorder-blocks
           -freorder-functions -funit-at-a-time -falign-functions  -falign-jumps -falign-loops  -falign-labels
           -ftree-pre