Replay compilation: Improving debuggability of a just-in-time compiler

Abstract

The performance of Java has been tremendously improved by the advance of Just-in-Time (JIT) compilation technologies. However, debugging such a dynamic compiler is much harder than a static compiler. Recompiling the problematic method to produce a diagnostic output does not necessarily work as expected, because the compilation of a method depends on runtime information at the time of compilation. In this paper, we propose a new approach, called replay JIT compilation, which can reproduce the same compilation remotely by using two compilers, the state-saving compiler and the replaying compiler. The state-saving compiler is used in a normal run, and, while compiling a method, records into a log all of the input for the compiler. The replaying compiler is then used in a debugging run with the system dump, to recompile a method with the options for diagnostic output. We reduced the overhead to save the input by using the system dump and by categorizing the input based on how its value changes. In our experiment, the increase of the compilation time for saving the input was only 1%, and the size of the additional memory needed for saving the input was only 10% of the compiler-generated code. Copyright © 2006 ACM.