[sgen] Add some bridge documentation.

diff --git a/mono/metadata/sgen-bridge.h b/mono/metadata/sgen-bridge.h
index 2a658f7870f091b8198a00074a614cb4a11d250f..28bf2caf759969be762332c9d9cc4eea504022fd 100644 (file)
--- a/mono/metadata/sgen-bridge.h
+++ b/mono/metadata/sgen-bridge.h
@@ -21,6 +21,41 @@
  * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
  */
 
+/*
+ * The bridge is a mechanism for SGen to let clients override the death of some
+ * unreachable objects.  We use it in monodroid to do garbage collection across
+ * the Mono and Java heaps.
+ *
+ * The client can designate some objects as "bridged", which means that they
+ * participate in the bridge processing step once SGen considers them
+ * unreachable, i.e., dead.  Bridged objects must be registered for
+ * finalization.
+ *
+ * When SGen is done marking, it puts together a list of all dead bridged
+ * objects and then does a strongly connected component analysis over their
+ * object graph.  That graph will usually contain non-bridged objects, too.
+ *
+ * The output of the SCC analysis is passed to the `cross_references()`
+ * callback.  It is expected to set the `is_alive` flag on those strongly
+ * connected components that it wishes to be kept alive.  Only bridged objects
+ * will be reported to the callback, i.e., non-bridged objects are removed from
+ * the callback graph.
+ *
+ * In monodroid each bridged object has a corresponding Java mirror object.  In
+ * the bridge callback it reifies the Mono object graph in the Java heap so that
+ * the full, combined object graph is now instantiated on the Java side.  Then
+ * it triggers a Java GC, waits for it to finish, and checks which of the Java
+ * mirror objects are still alive.  For those it sets the `is_alive` flag and
+ * returns from the callback.
+ *
+ * The SCC analysis is done while the world is stopped, but the callback is made
+ * with the world running again.  Weak links to bridged objects and other
+ * objects reachable from them are kept until the callback returns, at which
+ * point all links to bridged objects that don't have `is_alive` set are nulled.
+ * Note that weak links to non-bridged objects reachable from bridged objects
+ * are not nulled.  This might be considered a bug.
+ */
+
 #ifndef _MONO_SGEN_BRIDGE_H_
 #define _MONO_SGEN_BRIDGE_H_
 
@@ -56,7 +91,16 @@ typedef struct {
 
 typedef struct {
        int bridge_version;
+       /*
+        * Tells the runtime which classes to even consider when looking for
+        * bridged objects.  If subclasses are to be considered as well, the
+        * subclass check must be done in the callback.
+        */
        MonoGCBridgeObjectKind (*bridge_class_kind) (MonoClass *klass);
+       /*
+        * This is only called on objects for whose classes
+        * `bridge_class_kind()` returned `XXX_BRIDGE_CLASS`.
+        */
        mono_bool (*is_bridge_object) (MonoObject *object);
        void (*cross_references) (int num_sccs, MonoGCBridgeSCC **sccs, int num_xrefs, MonoGCBridgeXRef *xrefs);
 } MonoGCBridgeCallbacks;