An incremental basic block instruction scheduler

Abstract

Incremental methods for the earlier phases of compilation such as lexical analysis, parsing, semantic analysis and machine independent optimization have been well studied. In this paper, we describe an approach towards incremental machine dependent optimization, in particular, basic block instruction scheduling. First we describe a simple algorithm for the incremental construction of the dependency directed acyclic graph (DDAG). We then target basic block scheduling based on the longest path length criterion. In this context, we apply an existing incremental shortest path algorithm for computing the lengths of the longest paths incrementally. The techniques described have been implemented in the form of an incremental basic block instruction scheduler for the IBM RISC System/6000 processor. Compile-time measurements on a few sample programs indicate that this approach results in significant reduction in the time taken for instruction scheduling when compared with that taken by the non-incremental approach.