Optimization coaching for Spidermonkey

In this video, one of Mozilla's most recent batch of interns talks about his experiences teaching the Spidermonkey JavaScript engine to list directly actionable optimization suggestions for arbitrary user code.

It's been a while since I did anything performance-sensitive in JavaScript, but if I ever do such a thing I very much hope I'd have access to these kinds of suggestions written and maintained by the compiler writers, rather than hearsay and ritual.