[pavlov]

This change only affects JavaScriptCore, the original WebKit JS engine. (Various iterations of JavaScriptCore have been promoted as "Squirrelfish" and "Squirrelfish Extreme" and marketed by Apple as "Nitro", so the nomenclature around this engine is a bit confusing.)

Chrome uses V8 instead of JavaScriptCore, and I don't think anything would make them change, since having their own VM is such a large asset for Google (for example, they can use V8 to promote Dart).

JavaScriptCore actually has several JITs. There was the simple JIT that translates JS bytecode to machine code (I think it's called "method JIT"). There's a more advanced JIT that does data flow analysis. This is fairly new; I think it shipped last year in 64-bit Safari.

This new development is about improving performance on code that may not need to be JIT-compiled. Instead of always going to the JIT, they now use a fast interpreter (called LLInt), and only do the native compilation if a code path is deemed hot.