What's Wrong with Graph Coloring?

Graph coloring is the de facto standard technique for register allocation within a compiler. In this paper we examine the intuition that a better coloring algorithm results in better register allocation. By replacing the coloring phase of the gcc compiler's register allocator with an optimal coloring algorithm, we demonstrate both the importance of extending the graph coloring model to better express the costs of allocation decisions and the unsuitability of a pure graph coloring model of register allocation.