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Mozilla plans to give some relief to web developers struggling to create a "desktop-like" experience for web applications: relief in the form of huge increases in speed. On Friday, the father of JavaScript, Brendan Eich, announced the launch of a new JIT (Just In Time) JavaScript compiler called TraceMonkey that will be included in Firefox 3.1.

Depending on the benchmark, TraceMonkey currently improves JavaScript performance in Firefox 3.1 by about 2 to 37 times over Firefox 3.0. The average performance improvement is predicted to be about 4.6 times faster.

TraceMonkey is an evolution of Mozilla's current JavaScript engine, SpiderMonkey, that also draws from the Adobe-contributed Tamarin Tracing project. It is based on the concept of tracing (or trace-based compilation), which takes the approach of monitoring bytecode interpretation, following frequently-executed backwards branches to a "loop start point", analyzing the linear sequence of instructions in what is called a "trace" (using data structures called Trace Trees), and natively compiling these code paths. This has the advantage over more traditional JIT compilers--that keep track of which methods get called most frequently, and do whole-method analysis and compilation--of having to analyze and natively compile only the performance-critical parts of the code. It also has the advantage of being more mobile-friendly as this translates to a much lighter memory footprint. Andreas Gal, the principal architect of the TraceMonkey project, discusses the technique in his blog.

According to Gal, Firefox now has the "fastest JavaScript engine in the world". So what can you do when your browser's JavaScript is that fast? How about online photo editing...