Demystifying on-the-fly spill code

Modulo scheduling is an effective code generation technique that exploits the parallelism in program loops by overlapping iterations. One drawback of this optimization is that register requirements increase significantly because values across different loop iterations can be live concurrently. One possible solution to reduce register pressure is to insert spill code to release registers. Spill code stores values to memory between the producer and consumer instructions. Spilling heuristics can be divided into two classes: 1) a posteri-ori approaches (spill code is inserted after scheduling the loop) or 2) on-the-fly approaches (spill code is inserted during loop scheduling). Recent studies have reported obtaining better results for spilling on-the-fly. In this work, we study both approaches and propose two new techniques, one for each approach. Our new algorithms try to address the drawbacks observed in previous proposals. We show that the new algorithms outperform previous techniques and, at the same time, reduce compilation time. We also show that much to our surprise, a posteriori spilling can be in fact slightly more effective than on-the-fly spilling.