To increase performance:

• Try compiling with -fpromote-loop-indices in addition to your usual optimization options. This experimental option significantly improves performance on some benchmarks. It is not yet enabled by default with -O3 because of its experimental nature.
• Try adding -funroll-loops to your optimization flags. Code size will increase, but you may get better performance.
• Try adding -fno-common to your optimization flags. For most bare-metal applications, this option will reduce code-size and improve performance.