Control-flow versus data-flow-based scheduling: combining both approaches in an adaptive scheduling system

Abstract

As high-level synthesis techniques gain acceptance among designers, it is important to be able to provide a robust system which can handle large designs in short execution times, producing high-quality results. Scheduling is one of the most complex tasks in high-level synthesis, and although many algorithms exist for solving the scheduling problem, it remains a main source of inefficiency by either not producing high-quality results, not taking into account realistic design requirements, or requiring unacceptable execution times. One of the main problems in scheduling is the dichotomy between control and data. Many algorithms to date have been able to provide scheduling solutions by looking only at either the data part or the control part of the design. This has been done in order to simplify the problem; however, it has resulted in many algorithms unable to handle efficiently large designs with complex control and data functionality. This paper presents algorithms for combining dataflow and control-flow techniques into a robust scheduling system. The main characteristics of this system are as follows: 1) it uses path-based techniques for efficient handling of control and mutual exclusiveness (for resource sharing), 2) it allows operation reordering and parallelism extraction within the context of pathbased scheduling, 3) it contains a control partitioning algorithm for design space exploration as well as for reducing the number of control paths, and 4) it combines the above algorithms into an adaptive scheduling system which is capable of trading optimality for execution time on-the-fly. Results involving billions of paths are presented and analyzed.

Coordinated parallelizing compiler optimizations and high-level synthesis

“…Early high-level synthesis work focused on data-flow designs and applied optimizations such as algebraic transformations, re-timing and code motions across multiplexors for improved synthesis results [13,14]. More recent work -during the last decadehas presented speculative code motions for mixed control-data flow type of designs and demonstrated their effects on schedule lengths [15,16,17,18,19,20,21,22,23].…”

A comprehensive high-level synthesis system for control-flow intensive behaviors

“…While the CFG model is well suited for capturing execution of instructions on a general-purpose uniprocessor, it has been shown to be inadequate in exploiting the parallelism inherent in typical control-flow intensive applications. Existing CDFG models [7,8] are successful in preserving the parallelism in the behavior. However, the implementation complexity of using such a CDFG is much higher compared to that of using a CFG.…”

Hierarchical conditional dependency graphs as a unifying design representation in the CODESIS high-level synthesis system

“…One important advantage of list scheduling is that its quality depends on the choice of the priority function [1]. In [22] we exploit the guard hierarchy to define a probabilistic priority function that better accounts for the conditional nature of the design.…”

“…The topic of efficiently scheduling conditional behaviors having a complex conditional structure, has been thoroughly investigated in previous research work mainly because traditional DFG based heuristics do not efficiently handle this kind of descriptions [1].…”

“…Several better adapted heuristics were proposed ( [1], [2], [3], [4], [5], [6]). The quality of their results depends heavily on the ability to exploit conditional resource sharing ( [2], [4], [6], [7]) and speculative execution ( [3], [5], [16], [17]) possibilities as well as shorten path lengths using node duplication techniques [3].…”