Optimistic register coalescing

Graph-coloring register allocators eliminate copies by coalescing the source and target node of a copy if they do not interfere in the interference graph. Coalescing is, however, known to be harmful to the colorability of the graph because it tends to yield a graph with nodes of higher degrees. Unlike aggressive coalescing which coalesces any pair of non-interfering copy-related nodes, conservative coalescing or iterated coalescing perform safe coalescing that preserves the colorability. Unfortunately, these heuristics give up coalescing too early, losing many opportunities of coalescing that would turn out to be safe. Moreover, they ignore the fact that coalescing may even improve the colorability of the graph by reducing the degree of neighbor nodes that are interfering with both the source and target nodes being coalesced. This paper proposes a new heuristic called optimistic coalescing which optimistically performs aggressive coalescing, thus fully exploiting the positive impact of coalescing, yet when a coalesced node is to be spilled, it is split back into separate nodes; there as a better chance of coloring one of them which reduces the overall spill cost