Add some comments to zend_gc.c

[tstarling]

It took me a while to understand this code, so it makes sense to write some notes and share them.

[arnaud-lb]

Great!

[arnaud-lb]

Also, for each traversed node, this decrements the refcount of child nodes. The purpose of marking grey is to remember which nodes where already traversed during this step.

On a higher level this simulates the effect (on refcounts) of deleting roots. Nodes that end up with a refcount > 0, and nodes reachable from them, are proven to not be part of a garbage cycle, and to be live. gc_scan_roots() will then restore the refcount of live nodes and mark other ones as garbage.

[tstarling]

I clarified this.

[arnaud-lb]

/* For all roots marked purple, traverse the graph, decrementing the reference
 * count of visited nodes. Mark visited nodes grey so that their reference
 * counts will only be decremented once. See MarkRoots() in Bacon & Rajan. */

This is slightly incorrect as it can be understood that we are decrementing the refcount of non-grey nodes themselves. This would also imply that we decrement the refcount at most once per node. In reality we decrement the refcount of their child nodes ("internal reference counts" in the paper).

I suggest:

For all roots marked purple, traverse the graph, decrementing the reference
count of their child nodes. Mark visited nodes grey so that they are not
visited again. See MarkRoots() in Bacon & Rajan.