Is there some resource that I can refer to about the type of things that will work without -O3 flag but will fail with it. Thanks

Hi everyone, I am trying to run a mpi c code with -O3 flag and getting a segfault. The thing is that the same is running fine without the -O3 flag and the only warnings I am getting with -O3 are of type "ignoring return value of fread/system".

Is there some resource that I can refer to about the type of things that will work without -O3 flag but will fail with it.
Thanks