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And from the last link some detail that I am interested in:

New in JavaScript 1.8.1

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JavaScript 1.8.1 is a modest update syntactically to JavaScript; the main change in this release is the addition of the Tracemonkey just-in-time compiler, which improves performance.

[Ernest_at_the_Beach]

Getting nitty gritty:

Tracing JIT

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The tracing JIT in SpiderMonkey consists of a generic, low level component called nanojit which is co-maintained between Adobe and Mozilla, and a SpiderMonkey-specific high level component called jstracer. The nanojit component is language agnostic, and contains no knowledge about SpiderMonkey or any other part of the Mozilla codebase. The jstracer component consists of a monitor and a recorder. The monitor watches the executing SpiderMonkey interpreter. When the monitor determines that the interpreter has entered a region of code that would benefit from native compilation, the monitor activates the recorder. The recorder records the activity of the interpreter, using nanojit to build an efficient, native representation of the execution called a fragment. The monitor then calls into the native code stored in the fragment.

A schematic diagram of the components of the tracing JIT follows: