Fast-path loop unrolling of non-counted loops to enable subsequent compiler optimizations

Java programs can contain non-counted loops, that is, loops for which the iteration count can neither be determined at compile time nor at run time. State-of-the-art compilers do not aggressively optimize them, since unrolling non-counted loops often involves duplicating also a loop's exit condition, which thus only improves run-time performance if subsequent compiler optimizations can optimize the unrolled code. This paper presents an unrolling approach for non-counted loops that uses simulation at run time to determine whether unrolling such loops enables subsequent compiler optimizations. Simulating loop unrolling allows the compiler to determine performance and code size effects for each potential transformation prior to performing it. We implemented our approach on top of the GraalVM, a high-performance virtual machine for Java, and evaluated it with a set of Java and JavaScript benchmarks in terms of peak performance, compilation time and code size increase. We show that our approach can improve performance by up to 150% while generating a median code size and compile-time increase of not more than 25%. Our results indicate that fast-path unrolling of non-counted loops can be used in practice to increase the performance of Java applications.