Improve copying and moving weak refs.

Fixes a bug introduced (replacing a similar bug) in the last rewrite of
objc_moveWeak.  This version should now be correct.

objc_copyWeak was implemented in a naive way, which had a lot more
overhead than required.

arc.m

@@ -802,7 +802,24 @@ id objc_loadWeak(id* object)
 
 void objc_copyWeak(id *dest, id *src)
 {
-	objc_release(objc_initWeak(dest, objc_loadWeakRetained(src)));
+	// Don't retain or release.  While the weak ref lock is held, we know that
+	// the object can't be deallocated, so we just move the value and update
+	// the weak reference table entry to indicate the new address.
+	LOCK_FOR_SCOPE(&weakRefLock);
+	id obj;
+	WeakRef *srcRef;
+	WeakRef *dstRef;
+	loadWeakPointer(dest, &obj, &dstRef);
+	loadWeakPointer(src, &obj, &srcRef);
+	*dest = *src;
+	if (srcRef)
+	{
+		srcRef->weak_count++;
+	}
+	if (dstRef)
+	{
+		weakRefRelease(dstRef);
+	}
 }
 
 void objc_moveWeak(id *dest, id *src)
@@ -811,10 +828,16 @@ void objc_moveWeak(id *dest, id *src)
 	// the object can't be deallocated, so we just move the value and update
 	// the weak reference table entry to indicate the new address.
 	LOCK_FOR_SCOPE(&weakRefLock);
-	WeakRef *oldRef = weak_ref_table_get(weakRefs, *dest);
+	id obj;
+	WeakRef *oldRef;
+	// If the destination is a weak ref, free it.
+	loadWeakPointer(dest, &obj, &oldRef);
 	*dest = *src;
 	*src = nil;
-	weakRefRelease(oldRef);
+	if (oldRef != NULL)
+	{
+		weakRefRelease(oldRef);
+	}
 }
 
 void objc_destroyWeak(id* obj)