Safepointing in GC

runtime/vm/thread.h

Index: runtime/vm/thread.h
diff --git a/runtime/vm/thread.h b/runtime/vm/thread.h
index 69cc4ecd17b2f70a429bd02bd899675a274b5496..8a57c2b18c49b5ebcd49f2e93b650989236e1fbf 100644
--- a/runtime/vm/thread.h
+++ b/runtime/vm/thread.h
@@ -38,6 +38,9 @@ class Zone;
   CACHED_VM_OBJECTS_LIST(V)                                                    \
   CACHED_ADDRESSES_LIST(V)
 
+// TODO(koda): Merge thread.h and thread_registry.h.
+typedef int64_t SafepointId;
+static const SafepointId kNoSafepointId = -1;
 
 // A VM thread; may be executing Dart code or performing helper tasks like
 // garbage collection or compilation. The Thread structure associated with
@@ -63,7 +66,13 @@ class Thread {
   // "helper" to gain limited concurrent access to the isolate. One example is
   // SweeperTask (which uses the class table, which is copy-on-write).
   // TODO(koda): Properly synchronize heap access to expand allowed operations.
-  static void EnterIsolateAsHelper(Isolate* isolate);
+  //
+  // If 'pass_safepoint' is set to the current rendezvous (returned by
+  // ThreadRegistry::SafepointThreads), the thread will be allowed free passage
+  // into the isolate while this rendezvous is in progress.
+  static void EnterIsolateAsHelper(
+      Isolate* isolate,
+      SafepointId pass_safepoint = kNoSafepointId);
   static void ExitIsolateAsHelper();
 
   // Called when the current thread transitions from mutator to collector.
@@ -233,6 +242,7 @@ CACHED_CONSTANTS_LIST(DEFINE_OFFSET_METHOD)
   Heap* heap_;
   State state_;
   StoreBufferBlock* store_buffer_block_;
+  SafepointId pass_safepoint_;
 #define DECLARE_MEMBERS(type_name, member_name, expr, default_init_value)      \
   type_name member_name;
 CACHED_CONSTANTS_LIST(DECLARE_MEMBERS)