Extended Linear Scan: An Alternate Foundation for Global Register Allocation

Abstract

In this paper, we extend past work on Linear Scan register allocation, and propose two Extended Linear Scan (ELS) algorithms that retain the compile-time efficiency of past Linear Scan algorithms while delivering performance that can match or surpass that of Graph Coloring. Specifically, this paper makes the following contributions: - We highlight three fundamental theoretical limitations in using Graph Coloring as a foundation for global register allocation, and introduce a basic Extended Linear Scan algorithm, ELS0, which addresses all three limitations for the problem of Spill-Free Register Allocation. - We introduce the ELS1 algorithm which extends ELS0 to obtain a greedy algorithm for the problem of Register Allocation with Total Spills. - Finally, we present experimental results to compare the Graph Coloring and Extended Linear Scan algorithms. Our results show that the compile-time speedups for ELS1 relative to GC were significant, and varied from 15× to 68×. In addition, the resulting execution time improved by up to 5.8%, with an average improvement of 2.3%. Together, these results show that Extended Linear Scan is promising as an alternate foundation for global register allocation, compared to Graph Coloring, due to its compile-time scalability without loss of execution time performance.