Multi-dimensional Unrolling

[muellch]

Unrolling a stencil in 2 or even 3 dimensions (+ inlining) might have performance benefits over unrolling into one dimension (+ inlining).

Example in 2D:

• Assume for every grid point (i,j) the Laplacians for the 9 surrounding gridpoints have to be calculated (i-1) to (i+1) and (j-1) to (j+1).
• We now compare no unrolling to unrolling in one direction by 4 to unrolling in 2 dimensions by 2. We always look at how many Laplacians have to be calculated for 4 grid points.
• No Unrolling: 4*9 = 36
• Unrolling in one dimension by 4: 9 + 3*3 = 18
• Unrolling in two dimensions by 2: 9 + 3 + 4 = 16

Here the unrolling in two dimensions seems to be the best choice for performance

[gysit]

I close this issue since this has rather low priority at the moment. I think we should focus on features and correctness