ILP-based modulo scheduling for high-level synthesis

In high-level synthesis, loop pipelining is a technique to improve the throughput and utilisation of hardware datapaths by starting new loop iterations after a fixed amount of time, called the initiation interval (II), allowing to overlap subsequent iterations. The problem is to find the smallest II and corresponding operation schedule that fulfils all data dependencies and resource constraints, both of which are usually found by modulo scheduling. We propose Moovac1, a novel integer linear program (ILP) formulation of the modulo scheduling problem based on overlap variables to model exact resource constraints. Given enough time, Moovac will find a mimimum-II solution. This is in contrast to Canis' state-of-the-art Modulo SDC approach, which requires heuristic simplifications of the resource constraints. Moovac can thus be used as a reference to evaluate heuristics, or in a time-limited mode as a heuristic itself to provide a best-so-far solution. We schedule kernels from the CHStone and MachSuite benchmarks for loop pipelining with Moovac, Modulo SDC and a prior exact formulation by Eichenberger. Moovac has competitive performance in its time-limited mode, and delivers better results faster than the Modulo SDC scheduler for some loops. Often its structure leads to quicker solution times than Eichenberger's formulation. Using the Moovac-computed optimal solutions as a reference, we can confirm that the Modulo SDC heuristic is indeed capable of finding optimal or near-optimal solutions for the majority of smallto medium-sized loops. However, for larger loops the two algorithms begin to diverge, with Moovac often being significantly faster to prove the infeasibility of a candidate II. This can be exploited by running both schedulers synergistically, leading to a quicker convergence to the final II.