Simplified irregular loop
DO I = 1, N
X(IA(I)) = X(IA(I)) + Y(IB(I))
Inspector phase takes distribution of data arrays, global subscripts, and returns local subscripts and communication schedules.
Executor phase takes communication schedules. Collective GATHER fills ghost regions. Does local arithmetic. Collective SCATTER_ADD flushes ghost regions back to X array, accumulating.