MatrixFree setup: Reduce cost of vector access pattern computation #15867
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With the code introduced by #14343, we would also check the access pattern of unknowns located on faces, by referring to the unknowns on the corresponding cells on either side. However, there is a significant amount of repetition in this step, and it makes more sense to first determine all cells involved on cells and faces, and then check in that list of cells. This reduces the number of instructions for typical DG cases by a factor of 3 in the surrounding function that computes this pattern (or 5-10% of run-time cost in the overall indices setup of
MatrixFree), and has the additional benefit of a simpler code.