Increasing the Applicability of Scalar Replacement

Abstract

This paper describes an algorithm for scalar replacement, which replaces repeated accesses to an array element with a scalar temporary. The element is accessed from a register rather than memory, thereby eliminating unnecessary memory accesses. A previous approach to this problem combines scalar replacement with a loop transformation called unroll-and-jam, whereby outer loops in a nest are unrolled, and the resulting duplicate inner loop bodies are fused together. The effect of unroll-and-jam is to bring opportunities for scalar replacement into inner loop bodies. In this paper, we describe an alternative approach that can exploit reuse opportunities across multiple loops in a nest, and without requiring unroll-and-jam. We also use this technique to eliminate unnecessary writes back to memory. The approach described in this paper is particularly well-suited to architectures with large register files and efficient mechanisms for register-to-register transfer. From our experimental results mapping 5 multimedia kernels to an FPGA platform, assuming 32 registers, we observe a 58 to 90 percent of reduction in memory accesses and speedup 2.34 to 7.31 over original programs.KeywordsMemory AccessOuter LoopLoop NestLoop BodyDependence VectorThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.