C.: Optimization of two Jacobi smoother kernels by domain-specific program transformation

Our aim is to apply program transformations to stencil codes, in order to yield highest possible performance. We observe memory bandwidth as a major limitation in stencil code performance. We conducted a small study in which we ap-plied optimizing transformations to two Jacobi smoother kernels: one 3D 1st-grade 7-point stencil and one 3D 3rd-grade 19-point stencil. To obtain highest performance, the optimizations have to be customized for the execution plat-form at hand. We illustrate this by experiments on two x86 architectures and one BlueGene/Q architecture. A compiler with specific knowledge about stencil codes and execution platforms should be able to apply our transformations auto-matically. We are working towards such a compiler in the DFG-funded project ExaStencils. 1