Advanced conservative and optimistic register coalescing

• Authors: Florent Bouchez;Alain Darte;Fabrice Rastello
• Venue: CASES '08 Proceedings of the 2008 international conference on Compilers, architectures and synthesis for embedded systems
• Year: 2008

In the context of embedded systems, it is crucial to minimize memory transfers to reduce latency and power consumption. Stack access due to variable spilling during register allocation can be significantly reduced using aggressive live-range splitting. However, without a good (conservative) coalescing technique, the benefit of a good spilling may be lost due to the many register-to-register moves introduced. The challenge is thus to find a good trade-off between a too aggressive strategy that could make the interference graph uncolorable without more spilling, and a too conservative strategy that preserves colorability but leaves unnecessary moves. Two main approaches are "iterated register coalescing" by George and Appel and "optimistic coalescing" by Park and Moon. The first coalesces moves one by one conservatively. The second coalesces moves regardless of the colorability, then undo coalescings to reduce spilling. Focusing on greedy-k-colorable graphs---which are usually obtained after all spill decisions and, possibly, some split decisions---we show how these two approaches can be improved, optimistic coalescing with a more involved de-coalescing phase, incremental coalescing with a less pessimistic conservative technique. Unlike previous experiments, our results show that optimistic strategies do not outperform conservative ones. Our incremental conservative coalescing performs even better than our improved de-coalescing scheme and leads to about 15% improvement compared to the state-of-the-art optimistic coalescing.