Abstractions to separate concerns in semi-regular grids

In various applications including atmospheric and ocean simulation programs, stencil computations occur on semi-regular grids where subdomains of the grid are regular (e.g., can be stored in an array) but boundaries between sub-domains connect in an irregular fashion. Implementations of stencils on semi-regular grids often have grid topology details tangled with stencil computation code. This tangling of details makes updating stencil code difficult as it requires the programmer to have full knowledge of the current grid topology. Existing libraries and tools for separating the concerns of stencil computations from grid connectivity have not dealt with semi-regular grids and instead have focused on purely regular grids with possible periodicity or purely irregular grids. In this paper we introduce programming abstractions for the class of semi-regular grids and describe a prototype Fortran 90+ library called GridLib that implements these abstractions. Implementing these abstractions requires solving issues involving nodes in the grid with a non-standard number of neighbors and determining the communication schedule given an orthogonal specification of the grid decomposition. We present solutions to these issues that work within the context of grids used in atmospheric and ocean simulations. We also show that to maintain the performance while still providing this separation of concerns, it is necessary for a source-to-source translator to perform inlining between user code and the GridLib run-time library code. We present performance results for a stencil computation extracted from the Parallel Ocean Program.