Selective just-in-time compilation for client-side mobile JavaScript engine

Smart phone's full web browsing requires a high-performance JavaScript engine because JavaScript execution takes a non-trivial portion of the loading time for many web sites. The current wisdom of speeding up JavaScript engine is simply turning on its just-in-time compilation (JITC), which compiles JavaScript code to machine code on the fly and executes it instead of interpretation. Unfortunately, we found that JITC actually increases the loading time tangibly for some JavaScript-heavy web pages compared to interpretation, while it can still reduce the running time for JavaScript benchmarks. We observed that the web page JavaScript behaves differently from the benchmark JavaScript in the sense that hot spots rarely exist. This would lower the reuse ratio of the compiled machine code, making the compilation overhead higher than its benefit. This is especially true for a JavaScript engine which compiles all executed functions at their first invocation, as the SFX engine in the WebKit. In order to overcome this problem, we introduce selective compilation to the SFX engine so as to compile only hot functions detected during interpretation. This reduces the slowdown of the SFX for web page JavaScript, while accelerating JavaScript benchmarks. However, selective compilation for web page JavaScript shows a different behavior from other environment, and we discuss it.