Instruction Scheduling for Instruction-Level Parallel Processors

Nearly all personal computer and workstation processors, and virtually all high-performance embedded processor cores, now embody instruction-level parallel processing in the form of superscalar or VLIW architectures. ILP processors put much more of a burden on compilers; without "heroic" compiling techniques, most such processors fall far short of their performance goals. Those techniques are largely found in the high-level optimization phase and in the code generation phase; they are also collectively called instruction scheduling. This paper reviews the state-of-the- art in code generation for ILP parallel processors. Modern ILP code generation methods move code across basic block boundaries. These methods grew out of techniques for generating horizontal microcode, so our paper introduces the problem by describing its history. The current suite of approaches can be categorized by the shape of the schedule "region." Some of these regions are loops, and for those techniques known broadly as "software pipelining" are used. Software Pipelining techniques are covered elsewhere in this special issue, and so they are only considered here when there are issues relevant to the region-based techniques presented here. The selection of a type of region to use in this process is one of the most controversial questions in code generation; the paper surveys the best known alternatives. The paper then considers two questions: First, given a type of region, how does one pick specific regions of that type in the intermediate code. In conjunction with region selection, we consider region enlargement techniques such as unrolling and branch target expansion. The second question, how does one construct a schedule once regions have been selected, occupies the next section of the paper. Finally, schedule construction using recent, innovative resource modeling based on finite-state automata is then reexamined. The paper includes an extensive biography.