Cache current thread in a reserved register and use it in LoadIsolate

runtime/vm/thread.h

 // 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.
 
 #ifndef VM_THREAD_H_
 #define VM_THREAD_H_
 
 #include "vm/base_isolate.h"
 #include "vm/globals.h"
 #include "vm/os_thread.h"
@@ skipping 38 matching lines @@
   // Called at VM startup.
   static void InitOnce();
 
   // The topmost zone used for allocation in this thread.
   Zone* zone() {
     return reinterpret_cast<BaseIsolate*>(isolate())->current_zone();
   }
 
   // The isolate that this thread is operating on, or NULL if none.
   Isolate* isolate() const { return isolate_; }
+  static intptr_t isolate_offset() {
+    return OFFSET_OF(Thread, isolate_);
+  }
 
   // The (topmost) CHA for the compilation in the isolate of this thread.
   // TODO(23153): Move this out of Isolate/Thread.
   CHA* cha() const;
   void set_cha(CHA* value);
 
  private:
   static ThreadLocalKey thread_key_;
 
   Isolate* isolate_;
 
   Thread()
       : isolate_(NULL) {}
 
   static void SetCurrent(Thread* current);
 
   DISALLOW_COPY_AND_ASSIGN(Thread);
 };
 
 }  // namespace dart
 
 #endif  // VM_THREAD_H_