[dbg_nsk]

The problem is that it all depends on the liveness of the variables. The same value on the stack can be a root at the begining of the method as the corresponding variable is still alive, and later it becomes useless as the variable is already dead. So, you still need to know a location of each live reference at every safepoint (and that means several stack-maps for each method).

[willvarfar]

You can zero them when they are not live.

[dbg_nsk]

Yeah, we've tried that. It was one of the attempts of improving conservative GC (no dead values on the stack => no false roots). Unfortunately, it causes noticeable performance degradation, so it is easier to consult with stack maps about liveness of the variable.