> keep counters of how often each block of code runs ... and then any block of code running more than a few thousand times run through a far more complex yet sl

> keep counters of how often each block of code runs ... and then any block of code running more than a few thousand times run through a far more complex yet slower optimizing jit. That's just all JITs. Sometimes its counters for going from interpreter -> JIT rather than levels of JITs, but this idea is as old as JITs.