Improving Memory Hierarchy Performance through Combined Loop Interchange and Multi-Level Fusion

Because of the increasing gap between the speeds of processors and main memories, compilers must enhance the locality of applications to achieve high performance. Loop fusion enhances locality by fusing loops that access similar sets of data. Typically, it is applied to loops at the same level after loop interchange, which first attains the best nesting order for each local loop nest. However, since loop interchange cannot foresee the overall optimization effect, it often selects the wrong loops to be placed outermost for fusion, achieving suboptimal performance globally. Building on traditional unimodular transformations on perfectly nested loops, we present a novel transformation, dependence hoisting, that effectively combines interchange and fusion for arbitrarily nested loops. We present techniques to simultaneously interchange and fuse loops at multiple levels. By evaluating the compound optimization effect beforehand, we have achieved better performance than was possible by previous techniques, which apply interchange and fusion separately.