Compiler Warning: Threads may reach barrier out of order

Since the header of your loop depends on local id and each work-item in the work-group will traverse the loop for a different number of iterations, the compiler cannot determine the order at which the work-items exit the loop and reach the barrier after it at compile time and hence, gives you that warning. You don't have to fix this warning, but if you want to, you should write your code in a way that every work-item does the same number of operations before reaching the next barrier.