Remove fake isolate from concurrent sweeper and extend lifetime of VM threads.

runtime/vm/thread.cc

 // 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.h"
 
 #include "vm/isolate.h"
 #include "vm/os_thread.h"
 #include "vm/profiler.h"
 #include "vm/thread_interrupter.h"
@@ skipping 20 matching lines @@
 
 
 void Thread::EnsureInit() {
   if (Thread::Current() == NULL) {
     SetCurrent(new Thread());
   }
 }
 
 
 void Thread::CleanUp() {
-  // We currently deallocate the Thread, to ensure that embedder threads don't
-  // leak the Thread structure. An alternative approach would be to clear and
-  // reuse it, but register a destructor at the OS level.
   Thread* current = Current();
   if (current != NULL) {
     delete current;
   }
   SetCurrent(NULL);
 }
 
 
 void Thread::EnterIsolate(Isolate* isolate) {
   EnsureInit();
@@ skipping 13 matching lines @@
   Profiler::BeginExecution(isolate);
   isolate->set_thread_state(thread_state);
   isolate->set_vm_tag(VMTag::kVMTagId);
   thread->isolate_ = isolate;
 }
 
 
 void Thread::ExitIsolate() {
   Thread* thread = Thread::Current();
   // TODO(koda): Audit callers; they should know whether they're in an isolate.
-  if (thread == NULL) return;
+  if (thread == NULL || thread->isolate() == NULL) return;
   Isolate* isolate = thread->isolate();
-  ASSERT(isolate != NULL);
   if (isolate->is_runnable()) {
     isolate->set_vm_tag(VMTag::kIdleTagId);
   } else {
     isolate->set_vm_tag(VMTag::kLoadWaitTagId);
   }
   isolate->set_thread_state(NULL);
   Profiler::EndExecution(isolate);
   isolate->set_mutator_thread(NULL);
   thread->isolate_ = NULL;
   ASSERT(Isolate::Current() == NULL);
-  CleanUp();
+}
+
+
+void Thread::EnterIsolateAsHelper(Isolate* isolate) {
+  EnsureInit();
+  Thread* thread = Thread::Current();
+  ASSERT(thread->isolate() == NULL);
+  thread->isolate_ = isolate;
+  // Do not update isolate->mutator_thread, but perform sanity check:
+  // this thread should not be both the main mutator and helper.
+  ASSERT(isolate->mutator_thread() != thread);
+}
+
+
+void Thread::ExitIsolateAsHelper() {
+  Thread* thread = Thread::Current();
+  Isolate* isolate = thread->isolate();
+  ASSERT(isolate != NULL);
+  thread->isolate_ = NULL;
+  ASSERT(isolate->mutator_thread() != thread);
 }
 
 
 CHA* Thread::cha() const {
   ASSERT(isolate_ != NULL);
   return isolate_->cha_;
 }
 
 
 void Thread::set_cha(CHA* value) {
   ASSERT(isolate_ != NULL);
   isolate_->cha_ = value;
 }
 
 }  // namespace dart