At the high optimization level, the compiler balances between size and speed optimizations. However, it is possible to fine-tune the optimizations explicitly for either size or speed. They only differ in what thresholds that are used—speed will trade size for speed, whereas size will trade speed for size.

If you use the optimization level High speed, the ‑‑no_size_constraints compiler option relaxes the normal restrictions for code size expansion and enables more aggressive optimizations.

You can choose an optimization goal for each module, or even individual functions, using command line options and pragma directives (see -O and optimize). For a small embedded application, this makes it possible to achieve acceptable speed performance while minimizing the code size—Typically, only a few places in the application need to be fast, such as the most frequently executed inner loops, or the interrupt handlers.

Rather than compiling the whole application with High (Balanced) optimization, you can use High (Size) in general, but override this to get High (Speed) optimization only for those functions where the application needs to be fast.