gcc optimize option Os

-Os Optimize for size.  -Os enables all -O2 optimizations that do
           not typically increase code size.

           -Os disables the following optimization flags:
           -falign-functions  -falign-jumps  -falign-loops
           -falign-labels  -fprefetch-loop-arrays

           It also enables -finline-functions, causes the compiler to
           tune for code size rather than execution speed, and performs
           further optimizations designed to reduce code size.