Dynamic Inter-Block Scheduling for HLS

Abstract

A recent theme in HLS research is the production of dynamically scheduled circuits, which are made up of components that use handshaking to schedule themselves at run time, as opposed to following a schedule determined statically at compile time. Dynamically scheduled circuits promise superior performance on ‘irregular’ source programs, such as those whose control flow depends on input data, at the cost of additional area. Current dynamic scheduling techniques are well able to exploit parallelism among instructions within each basic block (BB) of the source program, but parallelism between BBs is underexplored. Although current tools allow the operations of different BBs to overlap, they require the BBs to start in strict program order, thus limiting the achievable parallelism and overall performance. We seek to lift this restriction. Doing so involves developing a toolflow that tackles the following challenges: (1) finding consecutive subgraphs in the control-flow graph and using static analysis to identify those subgraphs that can be safely parallelised, and (2) adapting the circuit so that those subgraphs are executed in parallel while ensuring deterministic circuit behaviour and correct usage of memory interfaces. Using two benchmark sets from related works, we compare our proposed toolflow against a state-of-the-art dynamically scheduled HLS tool called Dynamatic. Our results show that after standard loop unrolling is applied, our toolflow yields a 4 x average speedup, with a negligible area overhead. This increases to a 7.3 x average speedup when our toolflow is further combined with C-slow pipelining.