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* \file
* Copyright 2011 Novell, Inc.
* Licensed under the MIT license. See LICENSE file in the project root for full license information.
* 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 (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. This is passed to the bridge processor, which does an analysis to
* simplify the graph: It replaces strongly-connected components with single
* nodes, and may remove nodes corresponding to components which do not contain
* bridged objects.
* 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
* 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.
* 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.
#include <mono/utils/mono-publib.h>
enum {
typedef enum {
/* Instances of this class should be scanned when computing the transitive dependency among bridges. E.g. List<object>*/
/* Instances of this class should not be scanned when computing the transitive dependency among bridges. E.g. String*/
/* Instances of this class should be bridged and have their dependency computed. */
/* Instances of this class should be bridged but no dependencies should not be calculated. */
} MonoGCBridgeObjectKind;
typedef struct {
mono_bool is_alive; /* to be set by the cross reference callback */
int num_objs;
MonoObject *objs [MONO_ZERO_LEN_ARRAY];
} MonoGCBridgeSCC;
typedef struct {
int src_scc_index;
int dst_scc_index;
} MonoGCBridgeXRef;
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;
* Note: This may be called at any time, but cannot be called concurrently
* with (during and on a separate thread from) sgen init. Callers are
* responsible for enforcing this.
MONO_API void mono_gc_register_bridge_callbacks (MonoGCBridgeCallbacks *callbacks);
MONO_API void mono_gc_wait_for_bridge_processing (void);