Forwarding Pointers
A forwarding pointer redirects from an old version of an object to a new version. Forwarding pointers are created temporarily by Local Garbage Collection (LGC) when copying live objects into fresh heaps.
Assuming nothing has gone terribly wrong, forwarding pointers should be “invisible” to the source program. In particular, during an LGC, the thread that owns the heaps in-scope of the collection is paused. Any other thread which attempts to access an object in-scope of the collection is protected against witnessing a forwarding pointer by the Read Barrier.
To forward an object, we:
- copy the object to a new location, and
- overwrite the (old) object’s Header with a pointer to the new location.
Note that forwarding pointers are stored in place of the Header. We can distinguish between a header and a forwarding pointer by checking the least-significant bit. Every header will have a 1 in the least-significant position; forwarding pointers will always have a 0 in this position, due to alignment constraints.
