DASS: Combining Dynamic & Static Scheduling in High-Level Synthesis

A central task in high-level synthesis is
scheduling
: the allocation of operations to clock cycles. The classic approach to scheduling is
static
, in which each operation is mapped to a clock cycle at compile-time, but recent years have seen the emergence of
dynamic
scheduling, in which an operation’s clock cycle is only determined at runtime. Both approaches have their merits: static scheduling (SS) can lead to simpler circuitry and more resource sharing, while dynamic scheduling (DS) can lead to faster hardware when the computation has nontrivial control flow. In this work, we seek a scheduling approach that combines the best of both worlds. Our idea is to identify the parts of the input program, where DS does not bring any performance advantage and to use SS on those parts. These statically scheduled parts are then treated as black boxes when creating a dataflow circuit for the remainder of the program, which can benefit from the flexibility of DS. An empirical evaluation on a range of applications suggests that by using this approach, we can obtain 74% of the area savings that would be made by switching from DS to SS, and 135% of the performance benefits that would be made by switching from SS to DS.