Register Allocation

The code generated by a compiler must make effective use of the limited resources of the target processor. Among the most constrained resources is the set of hardware registers. Register use plays a major role in determining the performance of compiled code. At the same time, register allocation—the process of deciding which values to keep in registers—is a combinatorially hard problem. Most compilers decouple decisions about register allocation from other optimization decisions. Thus, most compilers include a separate pass for register allocation. This chapter begins with local register allocation, as a way to introduce the problem and the terminology. The bulk of the chapter focuses on global register allocation and assignment via graph coloring. The advanced topics section discusses some of the many variations on that technique that have been explored in research and employed in practice.