Because GHC's GC is generational it needs a way to handle heap objects from older generations that point into younger generations.  This only happens when an older object is mutated to point to a younger object.  GHC solves this by invoking the GC write barrier (not to be confused with write barriers for memory synchronization) `dirty_MUT_VAR`.  This will add that mutable object to a mutable list that will be traversed in minor GCs along with young generation roots.  Additionally the write barrier will mark the heap object as "dirty" to avoid adding it to the list more than once.

Ryan