How does optimization level affect performance and memory usage ?

[Jordanstanley]

I’d like to understand the practical trade-offs — for example, how each level influences execution speed, flash usage, and RAM consumption. Also, are there any best practices or common pitfalls when choosing an optimization level for STM32 projects?

[Andrew Neil]

This is not really specific to STM32 - there are plenty of general references on this.

https://en.wikipedia.org/wiki/Optimizing_compiler

It will, of course, depend on the particular compiler.

For GCC, see:

https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html

You will see that some options target code size, and others target execution speed.

A big problem with optimisation is that it makes debugging harder - the higher the optimisation, the harder it gets to debug.

Probably the biggest pitfall is that optimisation is likely to show up flaws in your code - you'll find loads of forum posts on the lines of "my code was working fine, then I turned on optimisation, and it wouldn't work any more".

A common example is where you haven't put 'volatile' on variables which should have it. You may get away with that with no optimisation, but not with optimisation.