[sgen-bridge] New bridge implementation - currently is just a copy of the old one.

The bridge implementation can be picked by having bridge-implementation=old|new in
MONO_GC_PARAMS.

This commit just prepared the ground to land experimental improvements to the bridge
while retaining an working implementation by default.

mono/metadata/Makefile.am.in

@@ -220,6 +220,7 @@ sgen_sources = \
 	sgen-bridge.c		\
 	sgen-bridge.h		\
 	sgen-old-bridge.c		\
+	sgen-new-bridge.c		\
 	sgen-toggleref.c		\
 	sgen-toggleref.h		\
 	sgen-gc.h		\

mono/metadata/sgen-bridge.c

@@ -77,7 +77,19 @@ mono_gc_register_bridge_callbacks (MonoGCBridgeCallbacks *callbacks)
 
 	bridge_callbacks = *callbacks;
 
-	sgen_old_bridge_init (&bridge_processor);
+	if (!bridge_processor.reset_data)
+		sgen_old_bridge_init (&bridge_processor);
+}
+
+void
+sgen_set_bridge_implementation (const char *name)
+{
+	if (!strcmp ("old", name))
+		sgen_old_bridge_init (&bridge_processor);
+	else if (!strcmp ("new", name))
+		sgen_new_bridge_init (&bridge_processor);
+	else
+		g_warning ("Invalid value for bridge implementation, valid values are: 'new' and 'old'.");
 }
 
 gboolean

mono/metadata/sgen-gc.c

@@ -4731,6 +4731,11 @@ mono_gc_base_init (void)
 				sgen_register_test_bridge_callbacks (g_strdup (opt));
 				continue;
 			}
+			if (g_str_has_prefix (opt, "bridge-implementation=")) {
+				opt = strchr (opt, '=') + 1;
+				sgen_set_bridge_implementation (opt);
+				continue;
+			}
 			if (g_str_has_prefix (opt, "toggleref-test")) {
 				sgen_register_test_toggleref_callback ();
 				continue;

mono/metadata/sgen-gc.h

@@ -832,6 +832,8 @@ typedef struct {
 } SgenBridgeProcessor;
 
 void sgen_old_bridge_init (SgenBridgeProcessor *collector) MONO_INTERNAL;
+void sgen_new_bridge_init (SgenBridgeProcessor *collector) MONO_INTERNAL;
+void sgen_set_bridge_implementation (const char *name) MONO_INTERNAL;
 
 typedef mono_bool (*WeakLinkAlivePredicateFunc) (MonoObject*, void*);