Parallelizing code can present the following degrees of difficulty:

Straightforward

Parallelizing the code requires small modifications.

Difficult

Parallelizing the code requires complex modifications. If you are using work-items in place of loop iterations, compute variables based on the value of the global ID rather than using a loop counter.

Difficult and includes dependencies

Parallelizing the code requires complex modifications and the use of techniques to avoid dependencies. You can compute values per frame, perform computations in multiple stages, or pre-compute values to remove dependencies.

Appears to be impossible

If parallelizing the code appears to be impossible, this only means that a particular code implementation cannot be parallelized.

The purpose of code is to perform a function. There might be different algorithms that perform the same function but work in different ways. Some of these might be parallelizable.

Investigate different alternatives to the algorithms and data structures that the code uses. These might make parallelization possible.