How to know what has been optimized

[da_chen]

I wrote a program using MCUXpresso.

I found my code performs quite differently in different optimization settings. Is there any way to know what has been optimized while compiling the program? 

Environment:

IDE: MCUXpresso 10.3.1

Compiler: arm-none-eabi-gcc

[converse]

This describes the optimisation levels

Using the GNU Compiler Collection (GCC): Optimize Options 

(Also available in the docs supplied with MCUXpresso)