When you download an MPLAB XC C compiler from the Microchip website, the compiler executes as a PRO license for 60 days and then reverts to a Free license. You can purchase the PRO license if you want to continue to use the most optimizations.
To determine if you need to buy a PRO license, you can run a comparison tool for your application in MPLAB X IDE. On the Run toolbar, you can select to either “Build with PRO comparison” or “Clean and Build with PRO comparison.” An example output is below.
These options build the project normally, in whatever optimization is selected, but also create a non-working PRO build and then compare the two results. You still get the original build as the output. Any parts of the PRO build are removed.