Safepoint interface and unit tests.

runtime/vm/thread_registry.cc

Index: runtime/vm/thread_registry.cc
diff --git a/runtime/vm/thread_registry.cc b/runtime/vm/thread_registry.cc
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+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+#include "vm/thread_registry.h"
+
+#include "vm/isolate.h"
+#include "vm/lockers.h"
+
+namespace dart {
+
+void ThreadRegistry::SafepointThreads() {
+  MonitorLocker ml(monitor_);
+  // First wait for any older rounds that are still in progress.
+  while (in_rendezvous_) {
+    CheckSafepointLocked();
+  }
+  // Start a new round.
+  in_rendezvous_ = true;
+  ++round_;  // Overflows after 240+ years @ 10^9 safepoints per second.
+  remaining_ = CountScheduledLocked();
+  Isolate* isolate = Isolate::Current();
+  // We only expect this method to be called from within the isolate itself.
+  ASSERT(isolate->thread_registry() == this);
+  // TODO(koda): Rename Thread::PrepareForGC and call it here?
+  --remaining_;  // Exclude this thread from the count.
+  // Ensure the main mutator will reach a safepoint (could be running Dart).
+  if (Thread::Current() != isolate->mutator_thread()) {
+    isolate->ScheduleInterrupts(Isolate::kVMInterrupt);
+  }
+  while (remaining_ > 0) {
+    ml.Wait(Monitor::kNoTimeout);
+  }
+}
+
+
+void ThreadRegistry::ResumeAllThreads() {
+  MonitorLocker ml(monitor_);
+  ASSERT(in_rendezvous_);
+  in_rendezvous_ = false;
+  ml.NotifyAll();
+}
+
+
+void ThreadRegistry::CheckSafepointLocked() {
+  int64_t last_round = -1;
+  while (in_rendezvous_) {
+    ASSERT(round_ >= last_round);
+    if (round_ != last_round) {
+      ASSERT((last_round == -1) || (round_ == (last_round + 1)));
+      last_round = round_;
+      // Participate in this round.
+      // TODO(koda): Rename Thread::PrepareForGC and call it here?
+      if (--remaining_ == 0) {
+        // Ensure the organizing thread is notified.
+        // TODO(koda): Use separate condition variables and plain 'Notify'.
+        monitor_->NotifyAll();
+      }
+    }
+    monitor_->Wait(Monitor::kNoTimeout);
+    // Note: Here, round_ is needed to detect and distinguish two cases:
+    // a) The old rendezvous is still in progress, so just keep waiting, or
+    // b) after ResumeAllThreads, another call to SafepointThreads was
+    // made before this thread got a chance to reaquire monitor_, thus this
+    // thread should (again) decrease remaining_ to indicate cooperation in
+    // this new round.
+  }
+}
+
+
+intptr_t ThreadRegistry::CountScheduledLocked() {
+  intptr_t count = 0;
+  for (int i = 0; i < entries_.length(); ++i) {
+    const Entry& entry = entries_[i];
+    if (entry.scheduled) {
+      ++count;
+    }
+  }
+  return count;
+}
+
+}  // namespace dart