Debugging Issues related to O3 Flag

I have a code that runs about 40% faster with the O3 flag on compared to O2 flag.

But recently I have started noticing some errors (non-physical results) in the code compiled with O3 flag.

I would like to keep using O3 but would like to isolate the part of code creating issue when using O3.

How can I figure out which part of the code is creating the issue when using the O3 flag?