Garbage collectors typically do not differentiate live-and-mutated from live-but-unmutated, which is what is needed here.
Generational collectors need to record when older generation objects are modified (by card marking, for example), so they can distinguish mutated from unmutated.
alt Hacker News
Generational collectors need to record when older generation objects are modified (by card marking, for example), so they can distinguish mutated from unmutated.