Software pipelining of nested loops

Abstract

Software pipelining is a technique to improve the perfor- mance of a loop by overlapping the execution of several iterations. The execution of a software-pipelined loop goes through three phases: prolog, kernel, and epilog. Software pipelining works best if most of the time is spent in the kernel phase rather than in the prolog or epilog phases. This can happen only if the trip count of a pipelined loop is large enough to amortize the overhead of prolog and epilog phases. When a software- pipelined loop is part of a loop nest, the overhead of filling and draining the pipeline is incurred for every iteration of the outer loop. This pa- per introduces two novel methods to minimize the overhead of software- pipeline fill/drain in nested loops. In effect, these methods overlap the draining of the software pipeline corresponding to one outer loop iter- ation with the filling of the software pipeline corresponding to one or more subsequent outer loop iterations. This results in better instruction- level parallelism (ILP) for the loop nest, particularly for loop nests in which the trip counts of inner loops are small. These methods exploit ItaniumTM architecture software pipelining features such as predication, register rotation, and explicit epilog stage control, to minimize the code size overhead associated with such a transformation. However, the key idea behind these methods is applicable to other architectures as well. These methods have been prototyped in the Intel optimizing compiler for the ItaniumTM processor. Experimental results on SPEC2000 bench- mark programs are presented.