Demote re-registered nmethods (#336)

If an nmethod is in the mature roots and the nmethod is re-registered
due to code patching, the nmethod will appear in both the nursery roots
and the mature roots for one collection, causing some roots to be
scanned twice. This PR removes an nmethod from the mature roots, when
the nmethod is re-registered, so that the nmethod will only be present
in the nursery.

Author: no-defun-allowed

mmtk/src/api.rs

@@ -512,6 +512,13 @@ pub extern "C" fn mmtk_register_nmethod(nm: Address) {
     NMETHOD_SLOTS.with_borrow_mut(|slots| {
         if !slots.is_empty() {
             let mut roots = crate::NURSERY_CODE_CACHE_ROOTS.lock().unwrap();
+            let mut mature_roots = crate::MATURE_CODE_CACHE_ROOTS.lock().unwrap();
+            // The nmethod might already be in the mature roots, if we
+            // are re-registering the nmethod due to code patching.
+            // If the nmethod is already in the mature roots, we demote
+            // the nmethod to the nursery roots, as the nmethod might
+            // now refer to a young object.
+            mature_roots.remove(&nm);
             roots.insert(nm, std::mem::take(slots));
         }
     });