Lecture Notes on Register Allocation 15-411: Compiler Design

In this lecture we discuss register allocation, which is one of the last steps in a compiler before code emission. Its task is to map the potentially unbounded numbers of variables or “temps” in pseudo-assembly to the actually available registers on the target machine. If not enough registers are available, some values must be saved to and restored from the stack, which is much less efficient than operating directly on registers. Register allocation is therefore of crucial importance in a compiler and has been the subject of much research. Register allocation is also covered thorougly in the textbook [App98, Chapter 11], but the algorithms described there are complicated and difficult to implement. We present here a simpler algorithm for register allocation based on chordal graph coloring due to Hack [Hac07] and Pereira and Palsberg [PP05]. Pereira and Palsberg have demonstrated that this algorithm performs well on typical programs even when the interference graph is not chordal. The fact that we target the x86-64 family of processors also helps, because it has 16 general registers so register allocation is less important than for the x86 with only 8 registers (ignoring floating-point and other special purpose registers). Most of material below is based on Pereira and Palsberg [PP05]1, where further background, references, details, empirical evaluation, and examples can be found.