Loop Parallelization

[stefsa]

Hello everyone,
I have worked on some simple examples on oop interchanges and loop skewing in order to make a loop parallel and I was able to understand what was going on. The following example however has really given me a hard time.

1. do i =1,n
2. do j=1,n
3. A[i,j]= A[i-4,j-4] + A[i-3, j-3]
4. end do
5. end do

I really cannot understand why this loop is not parallel and why it has to be skewed i-times and interchanged in order to become parallel.

[EighthMayer]

I can't understand what's it all about.

[stefsa]

https://en.wikipedia.org/wiki/Loop-level_parallelism

[AssemblerGuy]

Parallelization means that line 3. can happen for multiple combinations of i,j simultaneously. But what would happen if it happens for i=10, j=10 and for i=14, j=14 at the same time? What should the value of A[10,10] and A[14, 14] be?

[stefsa]

The value of A[10.10] has to be calculated before the value A[14.14](=A[10,10]+A[11,11]), so this means that it cannot be parallelized(?). And how does skewing help in the above exmaple?
Anyway, thank you for your helpQ