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

runtime/vm/isolate.h

 // Copyright (c) 2013, 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_ISOLATE_H_
 #define VM_ISOLATE_H_
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/base_isolate.h"
@@ skipping 149 matching lines @@
 
   Dart_MessageNotifyCallback message_notify_callback() const {
     return message_notify_callback_;
   }
   void set_message_notify_callback(Dart_MessageNotifyCallback value) {
     message_notify_callback_ = value;
   }
 
   // A thread that operates on this isolate and may execute Dart code.
   // No other threads operating on this isolate may execute Dart code.
-  // TODO(koda): Remove after caching current thread in generated code.
+  // TODO(koda): Remove after pivoting to thread in NativeArguments.
   Thread* mutator_thread() {
     DEBUG_ASSERT(mutator_thread_ == NULL || IsIsolateOf(mutator_thread_));
     return mutator_thread_;
   }
+  // TODO(koda): Remove after pivoting to thread in JumpToExceptionHandler.
+  static intptr_t mutator_thread_offset() {
+    return OFFSET_OF(Isolate, mutator_thread_);
+  }
 #if defined(DEBUG)
   bool IsIsolateOf(Thread* thread);
 #endif  // DEBUG
 
   const char* name() const { return name_; }
   const char* debugger_name() const { return debugger_name_; }
   void set_debugger_name(const char* name);
 
   // TODO(koda): Move to Thread.
   class Log* Log() const;
@@ skipping 778 matching lines @@
   uint8_t* serialized_args_;
   intptr_t serialized_args_len_;
   uint8_t* serialized_message_;
   intptr_t serialized_message_len_;
   bool paused_;
 };
 
 }  // namespace dart
 
 #endif  // VM_ISOLATE_H_