Probabilistic Scheduling in High-Level Synthesis

Jianyi Cheng, John Wickerson, George A. Constantinides

Abstract: High-level synthesis (HLS) tools automatically transform a high-level program, for example in C/C++, into a low- level hardware description. A key challenge in HLS tools is scheduling, i.e. determining the start time of all the operations in the untimed program. There are three approaches to scheduling: static, dynamic and hybrid. A major shortcoming of existing approaches to scheduling is that the tools either assume the worst- case timing behaviour, which can cause significant performance loss or area overhead, or use simulation-based approaches, which take a long time to explore enough program traces.In this paper, we propose a probabilistic model that allows HLS tools to efficiently explore the timing behaviour of hardware generated from all these scheduling approaches. We capture the performance of the hardware using Petri nets, allowing us to leverage off-the-shelf Petri net analysis tools to make HLS decisions.We demonstrate the utility of our approach by using it to automatically infer the optimal initiation interval (II) for statically scheduled components that form part of a larger dynamically scheduled circuit. An empirical evaluation on a range of benchmarks suggests that by using this approach, on average we incur a 2% overhead in area-delay product (ADP) compared to optimal designs. In contrast, the static analysis in Vitis HLS incurs a 112% ADP overhead, while the throughput analysis in the dynamically scheduled Dynamatic tool incurs a 17% ADP overhead.