Register Allocation via Hierarchical Graph Coloring
چکیده
Register allocation is a vital stage in compiler optimization. It greatly impacts the effectiveness of other compiler optimization techniques. Graph coloring is the commonly used mechanism for register allocation. Since graph coloring is an NP-complete problem, heuristics are needed to find a practical solution. Several heuristics are available that perform well in practice. Commonly used heuristics, however, have a problem in that they do not include program structure information in register allocation. Without the information of program structure, poor spill decisions can be made. Callahan and Koblenz developed an algorithm that takes the program structure into account by performing register allocation hierarchically. Unfortunately, Callahan and Koblenz have not presented any experimental evidence to establish the effectiveness of their algorithm. This thesis examines the effectiveness of Callahan and Koblenz’s algorithm and shows that when register pressure is high, Callahan and Koblenz’s method generates worse code than Briggs’ method -the generally accepted method of graph coloring register allocation. The performance degradation of Callahan and Koblenz’s method is due to reserving registers in the graph coloring process. Also, by performing register allocation hierarchically, compensation move and jump instructions are required to ensure the correctness of program semantics. Furthermore, the experimental results suggest that without considering the degree of conflict in the spill cost, more values can be spilled than necessary.