TraceMonkey, the JavaScript accelerator for Firefox

Mozilla is accelerating JavaScript functions in pre-release versions of Firefox 3.1. Using the TraceMonkey just-in-time compiler the execution time for some functions is reduced to as little as five percent of previous version's execution time – the developers give some examples in the JavaScript Roadmap Blog.

In their blog, developers Andreas Gal and Michael Franz explain how the tracing mechanism in TraceMonkey works. One of the main tricks performed by Tracemonkey is that, where branching occurs, it generates not only the required code, but also considers and compiles the other branches and saves the code for subsequent use. The just-in-time compiler can generate code for x86 and ARM systems, which should also be a boost for the in-development Mobile Firefox.

Because Firefox implements many internal functions using JavaScript, the acceleration should have a general effect on Firefox's operation speed, beyond just speeding up web page Javascript.

Firefox 3.1 is scheduled for release at the end of this year. The new acceleration technology is currently disabled in the pre-release version and users will need to activate it via the javascript.options.jit.content preference.

(djwm)