Dynamically Scheduled High-level Synthesis

Abstract

High-level synthesis (HLS) tools almost universally generate statically scheduled datapaths. Static scheduling implies that circuits out of HLS tools have a hard time exploiting parallelism in code with potential memory dependencies, with control-dependent dependencies in inner loops, or where performance is limited by long latency control decisions. The situation is essentially the same as in computer architecture between Very-Long Instruction Word (VLIW) processors and dynamically scheduled superscalar processors; the former display the best performance per cost in highly regular embedded applications, but general purpose, irregular, and control-dominated computing tasks require the runtime flexibility of dynamic scheduling. In this work, we show that high-level synthesis of dynamically scheduled circuits is perfectly feasible by describing the implementation of a prototype synthesizer which generates a particular form of latency-insensitive synchronous circuits. Compared to a commercial HLS tool, the result is a different trade-off between performance and circuit complexity, much as superscalar processors represent a different trade-off compared to VLIW processors: in demanding applications, the performance is very significantly improved at an affordable cost. We here demonstrate only the first steps towards more performant high-level synthesis tools adapted to emerging FPGA applications and the demands of computing in broader application domains.