Implement safepointing of threads

runtime/vm/gc_sweeper.cc

Index: runtime/vm/gc_sweeper.cc
diff --git a/runtime/vm/gc_sweeper.cc b/runtime/vm/gc_sweeper.cc
index a192138b4c6f5a72ddf92f2360cb4ea8c7ded1e4..25c00a098aa2b9fe214fd719016aedc1aa49833c 100644
--- a/runtime/vm/gc_sweeper.cc
+++ b/runtime/vm/gc_sweeper.cc
@@ -9,8 +9,8 @@
 #include "vm/heap.h"
 #include "vm/lockers.h"
 #include "vm/pages.h"
+#include "vm/safepoint.h"
 #include "vm/thread_pool.h"
-#include "vm/thread_registry.h"
 
 namespace dart {
 
@@ -116,13 +116,13 @@ class SweeperTask : public ThreadPool::Task {
   virtual void Run() {
     bool result = Thread::EnterIsolateAsHelper(task_isolate_);
     ASSERT(result);
+    Thread* thread = Thread::Current();
     GCSweeper sweeper;
 
     HeapPage* page = first_;
     HeapPage* prev_page = NULL;
 
     while (page != NULL) {
-      task_isolate_->thread_registry()->CheckSafepoint();
       HeapPage* next_page = page->next();
       ASSERT(page->type() == HeapPage::kData);
       bool page_in_use = sweeper.SweepPage(page, freelist_, false);
@@ -134,6 +134,7 @@ class SweeperTask : public ThreadPool::Task {
       {
         // Notify the mutator thread that we have added elements to the free
         // list or that more capacity is available.
+        TransitionVMToBlocked transition(thread);

[zra, 2016/01/08 23:32:07]

I think I'm a little confused by the meaning of the Blocked state. We
potentially block every time we try to take a lock, or Wait on a monitor, but we
don't always put the thread into the Blocked state.

[siva, 2016/01/12 21:26:22]

True this was a hack I did initially to get things going and forgot to chenge.
Transition to Blocked state is used only when we go into WaitWithSafepointCheck

I have added thread->CheckForSafepoint which I use here to see if the thread
needs wait for a safepoint operation.

         MonitorLocker ml(old_space_->tasks_lock());
         ml.Notify();
       }