Reuse analysis of indirectly indexed arrays

Abstract

We propose techniques for identifying and exploiting spatial and temporal reuse for indirectly indexed arrays. Indirectly indexed arrays are those arrays which are, typically, accessed inside multilevel loop nests and whose index expression includes not only loop iterators and constants but arrays as well. Existing techniques for improving locality are quite sophisticated in the case of directly indexed arrays. But, unfortunately, they are inadequate for handling indirectly indexed arrays. In this article we therefore extend the existing framework and techniques of directly indexed to indirectly indexed arrays. The concepts of reuse subspace, dependence vector, self, and group reuse are extended and applied in this new context. Also, lately scratch-pad memory has become an attractive alternative to data-cache, specially in the embedded multimedia community. This is because embedded systems are very sensitive to area and energy and the scratch-pad is smaller in area and consumes less energy on a per access basis compared to the cache of the same capacity. Several techniques have been proposed in the past for the efficient exploitation of the scratch-pad for directly indexed arrays. We extend these techniques by presenting a method for scratch-pad mapping of indirectly indexed arrays. This enables the scratch-pad to be used in a larger context than was possible before.