Ars Technica reports on tracing, a new optimization added to SpiderMonkey:
They are "getting ready to take JavaScript performance into the next tier" with a radically innovative optimization tactic called tracing that has already produced performance improvements ranging between 20 and 40 times faster in some cases.
Andreas Gal is the researcher behind the new optimization and writes about it:
Traces represent a single iteration through a loop, and can span multiple methods and program modules. If a function is invoked from inside a loop, we follow the function call and inline the instructions executed inside the called method. Function calls themselves are never actually recorded. We merely verify at runtime that the same conditions that caused that function to be activated still hold.