Looking at mxr, it appears that there's a lot of JSTracer stuff left in the tree. Shouldn't it all be gone now?
This is some unfortunate overloading, the JSTracer class keeps track of state during GC marking and doesn't have anything to do with the trace JIT.
JSTracer is used for tracing roots for the GC. We always had this kind of ambiguity, luckily the tracing jit is gone now.
Brain beat me to it ;)
Oops, sorry, Ryan, I should have remembered that when you brought up JSTracer in IRC, I just totally spaced out there...