Compiler transformations for effectively exploiting a zero overhead loop buffer

Abstract

A Zero Overhead Loop Buffer (ZOLB) is an architectural feature that is commonly found in DSP processors. This buffer can be viewed as a compiler managed cache that contains a sequence of instructions that will be executed a specified number of times without incurring any loop overhead. Unlike loop unrolling, a loop buffer can be used to minimize loop overhead without the penalty of increasing code size. In addition, a ZOLB requires relatively little space and power, which are both important considerations for most DSP applications. This paper describes strategies for generating code to effectively use a ZOLB. We have found that many common code improving transformations used by optimizing compiler on conventional architectures can be easily used to (1) allow more loops to be placed in a ZOLB, (2) further reduce loop overhead of the loops placed in a ZOLB, and (3) avoid redundant loading of ZOLB loops. The results given in this paper demonstrate that this architectural feature can often be exploited with substantial improvements in execution time and slight reductions in code size for various signal processing applications.