Tell me more about how object reachability is determined in GC.

Object reachability in the garbage collector (GC) is determined based on an object graph, where roots are objects accessible directly from the call stack, static fields, registers, and other "live" entry points.

The algorithm is roughly as follows:

1. Starting from the roots, traverse all objects that are referenced.
2. All objects reachable through reference chains from the roots are considered alive.
3. Objects that are not reachable from the roots are considered unreachable and are subject to garbage collection.

For example, if you have an object A referenced by a local variable in the current method (a root), and object B referenced by A, then both are considered reachable. If object C is not connected to any reachable object, it is considered unreachable.

This approach allows for efficient memory freeing by removing objects that can no longer be used by the program.