Scripts can have associated counters measure JIT activity, stub call count, and type information at a per-PC granularity, and which can be updated by JM-compiled code. These counters are a handy way to get fine grained performance info, see what optimizations are being applied and quickly identify perf faults. This info can be accessed in the browser, https://addons.mozilla.org/addon/jit-inspector/. It would be good if IM also updated these counters and used this infrastructure, a bit trickier to do than JM due to SSA transformations, but if counters metadata can be attached to MIR nodes and accessed during codegen then this shouldn't be too hard.