WebKit + Squirrel Extreme + Acid3 + Vuela!

How Tracing Works

In simple terms tracing works by watching for commonly-repeated actions (such as loops, function calls, or type checking) and tries to optimize their resulting native code into the lowest number of actions. The premise is rather simple - and it's an advance that we'll probably see proliferate to many interpreters and engines in the upcoming years.

Andreas Gal published a paper (PDF) on the subject and Brendan Eich has written up a TraceMonkey-specific explanation.

Some of the improvements made by tracing include:

    * Function Inlining: Removing the overhead of function calls by simply replacing them with their resulting native code.
    * Type Inference: Removing checks surrounding common operators (like "+") when the types contained within a variable are already known. This means that the engine will have already pre-determined, for example, that two strings need to be concated when it sees the "+" operator.
    * Looping: The overhead of looping has been grossly diminished. It's one of the most common areas of overhead in JavaScript applications (common repetition of a task) and the constant determining of bounds and the resulting inner code is made negligible.

The code for this project has come from a number of places - one of which is coming from some code contributed to Mozilla, from Adobe: Tamarin Tracing, specifically the nanojit code that's able to work a lot of this just-in-time magic.