Merge pull request #3252 from xmcclure/tarjan-opt-basic

Cleanup on gc bridge to support future optimizations

mono/metadata/metadata.h

@@ -301,7 +301,10 @@ typedef struct {
 
 struct _MonoArrayType {
 	MonoClass *eklass;
+	// Number of dimensions of the array
 	uint8_t rank;
+
+	// Arrays recording known upper and lower index bounds for each dimension
 	uint8_t numsizes;
 	uint8_t numlobounds;
 	int *sizes;

mono/metadata/sgen-bridge-internals.h

@@ -45,6 +45,8 @@ typedef struct {
 	void (*register_finalized_object) (GCObject *object);
 	void (*describe_pointer) (GCObject *object);
 	void (*enable_accounting) (void);
+
+	// Optional-- used for debugging
 	void (*set_dump_prefix) (const char *prefix);
 
 	/*

mono/metadata/sgen-bridge.h

@@ -9,20 +9,23 @@
  * 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
+ * The client (Monodroid) 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.
+ * objects.  This is passed to the bridge processor, which does an analysis to
+ * simplify the graph: It replaces strongly-connected components with single
+ * nodes, and then removes any nodes corresponding to components which do not
+ * contain bridged objects.
  *
- * 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.
+ * The output of the SCC analysis is passed to the client's `cross_references()`
+ * callback.  This consists of 2 arrays, an array of SCCs (MonoGCBridgeSCC),
+ * and an array of "xrefs" (edges between SCCs, MonoGCBridgeXRef).  Edges are
+ * encoded as pairs of "API indices", ie indexes in the SCC array.  The client
+ * is expected to set the `is_alive` flag on those strongly connected components
+ * that it wishes to be kept alive.
  *
  * 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
@@ -37,6 +40,10 @@
  * 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.
+ *
+ * There are three different implementations of the bridge processor, each of
+ * which implements 8 callbacks (see SgenBridgeProcessor).  The implementations
+ * differ in the algorithm they use to compute the "simplified" SCC graph.
  */
 
 #ifndef _MONO_SGEN_BRIDGE_H_