CSE2410 Fall 2014 Bounce

// Copyright 2012 the V8 project authors. All rights reserved.

// Redistribution and use in source and binary forms, with or without

// modification, are permitted provided that the following conditions are

// met:

//

//     * Redistributions of source code must retain the above copyright

//       notice, this list of conditions and the following disclaimer.

//     * Redistributions in binary form must reproduce the above

//       copyright notice, this list of conditions and the following

//       disclaimer in the documentation and/or other materials provided

//       with the distribution.

//     * Neither the name of Google Inc. nor the names of its

//       contributors may be used to endorse or promote products derived

//       from this software without specific prior written permission.

//

// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS

// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT

// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR

// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT

// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,

// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT

// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,

// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY

// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT

// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE

// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

#ifndef V8_ISOLATE_H_

#define V8_ISOLATE_H_

#include "../include/v8-debug.h"

#include "allocation.h"

#include "apiutils.h"

#include "assert-scope.h"

#include "atomicops.h"

#include "builtins.h"

#include "contexts.h"

#include "execution.h"

#include "frames.h"

#include "date.h"

#include "global-handles.h"

#include "handles.h"

#include "hashmap.h"

#include "heap.h"

#include "optimizing-compiler-thread.h"

#include "regexp-stack.h"

#include "runtime-profiler.h"

#include "runtime.h"

#include "zone.h"

namespace v8 {

namespace internal {

class Bootstrapper;

class CodeGenerator;

class CodeRange;

struct CodeStubInterfaceDescriptor;

class CompilationCache;

class ContextSlotCache;

class ContextSwitcher;

class Counters;

class CpuFeatures;

class CpuProfiler;

class DeoptimizerData;

class Deserializer;

class EmptyStatement;

class ExternalCallbackScope;

class ExternalReferenceTable;

class Factory;

class FunctionInfoListener;

class HandleScopeImplementer;

class HeapProfiler;

class HStatistics;

class HTracer;

class InlineRuntimeFunctionsTable;

class NoAllocationStringAllocator;

class InnerPointerToCodeCache;

class PreallocatedMemoryThread;

class RandomNumberGenerator;

class RegExpStack;

class SaveContext;

class UnicodeCache;

class ConsStringIteratorOp;

class StringTracker;

class StubCache;

class SweeperThread;

class ThreadManager;

class ThreadState;

class ThreadVisitor;  // Defined in v8threads.h

template <StateTag Tag> class VMState;

// 'void function pointer', used to roundtrip the

// ExternalReference::ExternalReferenceRedirector since we can not include

// assembler.h, where it is defined, here.

typedef void* ExternalReferenceRedirectorPointer();

#ifdef ENABLE_DEBUGGER_SUPPORT

class Debug;

class Debugger;

class DebuggerAgent;

#endif

#if !defined(__arm__) && V8_TARGET_ARCH_ARM || \

    !defined(__mips__) && V8_TARGET_ARCH_MIPS

class Redirection;

class Simulator;

#endif

// Static indirection table for handles to constants.  If a frame

// element represents a constant, the data contains an index into

// this table of handles to the actual constants.

// Static indirection table for handles to constants.  If a Result

// represents a constant, the data contains an index into this table

// of handles to the actual constants.

typedef ZoneList<Handle<Object> > ZoneObjectList;

#define RETURN_IF_SCHEDULED_EXCEPTION(isolate)            \

  do {                                                    \

    Isolate* __isolate__ = (isolate);                     \

    if (__isolate__->has_scheduled_exception()) {         \

      return __isolate__->PromoteScheduledException();    \

    }                                                     \

  } while (false)

#define RETURN_HANDLE_IF_SCHEDULED_EXCEPTION(isolate, T)  \

  do {                                                    \

    Isolate* __isolate__ = (isolate);                     \

    if (__isolate__->has_scheduled_exception()) {         \

      __isolate__->PromoteScheduledException();           \

      return Handle<T>::null();                           \

    }                                                     \

  } while (false)

#define RETURN_IF_EMPTY_HANDLE_VALUE(isolate, call, value) \

  do {                                                     \

    if ((call).is_null()) {                                \

      ASSERT((isolate)->has_pending_exception());          \

      return (value);                                      \

    }                                                      \

  } while (false)

#define CHECK_NOT_EMPTY_HANDLE(isolate, call)     \

  do {                                            \

    ASSERT(!(isolate)->has_pending_exception());  \

    CHECK(!(call).is_null());                     \

    CHECK(!(isolate)->has_pending_exception());   \

  } while (false)

#define RETURN_IF_EMPTY_HANDLE(isolate, call)                       \

  RETURN_IF_EMPTY_HANDLE_VALUE(isolate, call, Failure::Exception())

#define FOR_EACH_ISOLATE_ADDRESS_NAME(C)                \

  C(Handler, handler)                                   \

  C(CEntryFP, c_entry_fp)                               \

  C(Context, context)                                   \

  C(PendingException, pending_exception)                \

  C(ExternalCaughtException, external_caught_exception) \

  C(JSEntrySP, js_entry_sp)

// Platform-independent, reliable thread identifier.

class ThreadId {

 public:

  // Creates an invalid ThreadId.

  ThreadId() : id_(kInvalidId) {}

  // Returns ThreadId for current thread.

  static ThreadId Current() { return ThreadId(GetCurrentThreadId()); }

  // Returns invalid ThreadId (guaranteed not to be equal to any thread).

  static ThreadId Invalid() { return ThreadId(kInvalidId); }

  // Compares ThreadIds for equality.

  INLINE(bool Equals(const ThreadId& other) const) {

    return id_ == other.id_;

  }

  // Checks whether this ThreadId refers to any thread.

  INLINE(bool IsValid() const) {

    return id_ != kInvalidId;

  }

  // Converts ThreadId to an integer representation

  // (required for public API: V8::V8::GetCurrentThreadId).

  int ToInteger() const { return id_; }

  // Converts ThreadId to an integer representation

  // (required for public API: V8::V8::TerminateExecution).

  static ThreadId FromInteger(int id) { return ThreadId(id); }

 private:

  static const int kInvalidId = -1;

  explicit ThreadId(int id) : id_(id) {}

  static int AllocateThreadId();

  static int GetCurrentThreadId();

  int id_;

  static Atomic32 highest_thread_id_;

  friend class Isolate;

};

class ThreadLocalTop BASE_EMBEDDED {

 public:

  // Does early low-level initialization that does not depend on the

  // isolate being present.

  ThreadLocalTop();

  // Initialize the thread data.

  void Initialize();

  // Get the top C++ try catch handler or NULL if none are registered.

  //

  // This method is not guarenteed to return an address that can be

  // used for comparison with addresses into the JS stack.  If such an

  // address is needed, use try_catch_handler_address.

  v8::TryCatch* TryCatchHandler();

  // Get the address of the top C++ try catch handler or NULL if

  // none are registered.

  //

  // This method always returns an address that can be compared to

  // pointers into the JavaScript stack.  When running on actual

  // hardware, try_catch_handler_address and TryCatchHandler return

  // the same pointer.  When running on a simulator with a separate JS

  // stack, try_catch_handler_address returns a JS stack address that

  // corresponds to the place on the JS stack where the C++ handler

  // would have been if the stack were not separate.

  inline Address try_catch_handler_address() {

    return try_catch_handler_address_;

  }

  // Set the address of the top C++ try catch handler.

  inline void set_try_catch_handler_address(Address address) {

    try_catch_handler_address_ = address;

  }

  void Free() {

    ASSERT(!has_pending_message_);

    ASSERT(!external_caught_exception_);

    ASSERT(try_catch_handler_address_ == NULL);

  }

  Isolate* isolate_;

  // The context where the current execution method is created and for variable

  // lookups.

  Context* context_;

  ThreadId thread_id_;

  MaybeObject* pending_exception_;

  bool has_pending_message_;

  bool rethrowing_message_;

  Object* pending_message_obj_;

  Object* pending_message_script_;

  int pending_message_start_pos_;

  int pending_message_end_pos_;

  // Use a separate value for scheduled exceptions to preserve the

  // invariants that hold about pending_exception.  We may want to

  // unify them later.

  MaybeObject* scheduled_exception_;

  bool external_caught_exception_;

  SaveContext* save_context_;

  v8::TryCatch* catcher_;

  // Stack.

  Address c_entry_fp_;  // the frame pointer of the top c entry frame

  Address handler_;   // try-blocks are chained through the stack

#ifdef USE_SIMULATOR

  Simulator* simulator_;

#endif

  Address js_entry_sp_;  // the stack pointer of the bottom JS entry frame

  // the external callback we're currently in

  ExternalCallbackScope* external_callback_scope_;

  StateTag current_vm_state_;

  // Generated code scratch locations.

  int32_t formal_count_;

  // Call back function to report unsafe JS accesses.

  v8::FailedAccessCheckCallback failed_access_check_callback_;

  // Head of the list of live LookupResults.

  LookupResult* top_lookup_result_;

  // Whether out of memory exceptions should be ignored.

  bool ignore_out_of_memory_;

 private:

  void InitializeInternal();

  Address try_catch_handler_address_;

};

class SystemThreadManager {

 public:

  enum ParallelSystemComponent {

    PARALLEL_SWEEPING,

    CONCURRENT_SWEEPING,

    PARALLEL_RECOMPILATION

  };

  static int NumberOfParallelSystemThreads(ParallelSystemComponent type);

  static const int kMaxThreads = 4;

};

#ifdef ENABLE_DEBUGGER_SUPPORT

#define ISOLATE_DEBUGGER_INIT_LIST(V)                                          \

  V(DebuggerAgent*, debugger_agent_instance, NULL)

#else

#define ISOLATE_DEBUGGER_INIT_LIST(V)

#endif

#ifdef DEBUG

#define ISOLATE_INIT_DEBUG_ARRAY_LIST(V)                                       \

  V(CommentStatistic, paged_space_comments_statistics,                         \

      CommentStatistic::kMaxComments + 1)

#else

#define ISOLATE_INIT_DEBUG_ARRAY_LIST(V)

#endif

#define ISOLATE_INIT_ARRAY_LIST(V)                                             \

  /* SerializerDeserializer state. */                                          \

  V(int32_t, jsregexp_static_offsets_vector, kJSRegexpStaticOffsetsVectorSize) \

  V(int, bad_char_shift_table, kUC16AlphabetSize)                              \

  V(int, good_suffix_shift_table, (kBMMaxShift + 1))                           \

  V(int, suffix_table, (kBMMaxShift + 1))                                      \

  V(uint32_t, private_random_seed, 2)                                          \

  ISOLATE_INIT_DEBUG_ARRAY_LIST(V)

typedef List<HeapObject*, PreallocatedStorageAllocationPolicy> DebugObjectCache;

#define ISOLATE_INIT_LIST(V)                                                   \

  /* SerializerDeserializer state. */                                          \

  V(int, serialize_partial_snapshot_cache_length, 0)                           \

  V(int, serialize_partial_snapshot_cache_capacity, 0)                         \

  V(Object**, serialize_partial_snapshot_cache, NULL)                          \

  /* Assembler state. */                                                       \

  /* A previously allocated buffer of kMinimalBufferSize bytes, or NULL. */    \

  V(byte*, assembler_spare_buffer, NULL)                                       \

  V(FatalErrorCallback, exception_behavior, NULL)                              \

  V(AllowCodeGenerationFromStringsCallback, allow_code_gen_callback, NULL)     \

  /* To distinguish the function templates, so that we can find them in the */ \

  /* function cache of the native context. */                                  \

  V(int, next_serial_number, 0)                                                \

  V(ExternalReferenceRedirectorPointer*, external_reference_redirector, NULL)  \

  V(bool, always_allow_natives_syntax, false)                                  \

  /* Part of the state of liveedit. */                                         \

  V(FunctionInfoListener*, active_function_info_listener, NULL)                \

  /* State for Relocatable. */                                                 \

  V(Relocatable*, relocatable_top, NULL)                                       \

  V(DebugObjectCache*, string_stream_debug_object_cache, NULL)                 \

  V(Object*, string_stream_current_security_token, NULL)                       \

  /* TODO(isolates): Release this on destruction? */                           \

  V(int*, irregexp_interpreter_backtrack_stack_cache, NULL)                    \

  /* Serializer state. */                                                      \

  V(ExternalReferenceTable*, external_reference_table, NULL)                   \

  /* AstNode state. */                                                         \

  V(int, ast_node_id, 0)                                                       \

  V(unsigned, ast_node_count, 0)                                               \

  V(bool, observer_delivery_pending, false)                                    \

  V(HStatistics*, hstatistics, NULL)                                           \

  V(HTracer*, htracer, NULL)                                                   \

  ISOLATE_DEBUGGER_INIT_LIST(V)

class Isolate {

  // These forward declarations are required to make the friend declarations in

  // PerIsolateThreadData work on some older versions of gcc.

  class ThreadDataTable;

  class EntryStackItem;

 public:

  ~Isolate();

  // A thread has a PerIsolateThreadData instance for each isolate that it has

  // entered. That instance is allocated when the isolate is initially entered

  // and reused on subsequent entries.

  class PerIsolateThreadData {

   public:

    PerIsolateThreadData(Isolate* isolate, ThreadId thread_id)

        : isolate_(isolate),

          thread_id_(thread_id),

          stack_limit_(0),

          thread_state_(NULL),

#if !defined(__arm__) && V8_TARGET_ARCH_ARM || \

    !defined(__mips__) && V8_TARGET_ARCH_MIPS

          simulator_(NULL),

#endif

          next_(NULL),

          prev_(NULL) { }

    Isolate* isolate() const { return isolate_; }

    ThreadId thread_id() const { return thread_id_; }

    void set_stack_limit(uintptr_t value) { stack_limit_ = value; }

    uintptr_t stack_limit() const { return stack_limit_; }

    ThreadState* thread_state() const { return thread_state_; }

    void set_thread_state(ThreadState* value) { thread_state_ = value; }

#if !defined(__arm__) && V8_TARGET_ARCH_ARM || \

    !defined(__mips__) && V8_TARGET_ARCH_MIPS

    Simulator* simulator() const { return simulator_; }

    void set_simulator(Simulator* simulator) {

      simulator_ = simulator;

    }

#endif

    bool Matches(Isolate* isolate, ThreadId thread_id) const {

      return isolate_ == isolate && thread_id_.Equals(thread_id);

    }

   private:

    Isolate* isolate_;

    ThreadId thread_id_;

    uintptr_t stack_limit_;

    ThreadState* thread_state_;

#if !defined(__arm__) && V8_TARGET_ARCH_ARM || \

    !defined(__mips__) && V8_TARGET_ARCH_MIPS

    Simulator* simulator_;

#endif

    PerIsolateThreadData* next_;

    PerIsolateThreadData* prev_;

    friend class Isolate;

    friend class ThreadDataTable;

    friend class EntryStackItem;

    DISALLOW_COPY_AND_ASSIGN(PerIsolateThreadData);

  };

  enum AddressId {

#define DECLARE_ENUM(CamelName, hacker_name) k##CamelName##Address,

    FOR_EACH_ISOLATE_ADDRESS_NAME(DECLARE_ENUM)

#undef DECLARE_ENUM

    kIsolateAddressCount

  };

  // Returns the PerIsolateThreadData for the current thread (or NULL if one is

  // not currently set).

  static PerIsolateThreadData* CurrentPerIsolateThreadData() {

    return reinterpret_cast<PerIsolateThreadData*>(

        Thread::GetThreadLocal(per_isolate_thread_data_key_));

  }

  // Returns the isolate inside which the current thread is running.

  INLINE(static Isolate* Current()) {

    Isolate* isolate = reinterpret_cast<Isolate*>(

        Thread::GetExistingThreadLocal(isolate_key_));

    ASSERT(isolate != NULL);

    return isolate;

  }

  INLINE(static Isolate* UncheckedCurrent()) {

    return reinterpret_cast<Isolate*>(Thread::GetThreadLocal(isolate_key_));

  }

  // Usually called by Init(), but can be called early e.g. to allow

  // testing components that require logging but not the whole

  // isolate.

  //

  // Safe to call more than once.

  void InitializeLoggingAndCounters();

  bool Init(Deserializer* des);

  bool IsInitialized() { return state_ == INITIALIZED; }

  // True if at least one thread Enter'ed this isolate.

  bool IsInUse() { return entry_stack_ != NULL; }

  // Destroys the non-default isolates.

  // Sets default isolate into "has_been_disposed" state rather then destroying,

  // for legacy API reasons.

  void TearDown();

  static void GlobalTearDown();

  bool IsDefaultIsolate() const { return this == default_isolate_; }

  static void SetCrashIfDefaultIsolateInitialized();

  // Ensures that process-wide resources and the default isolate have been

  // allocated. It is only necessary to call this method in rare cases, for

  // example if you are using V8 from within the body of a static initializer.

  // Safe to call multiple times.

  static void EnsureDefaultIsolate();

  // Find the PerThread for this particular (isolate, thread) combination

  // If one does not yet exist, return null.

  PerIsolateThreadData* FindPerThreadDataForThisThread();

  // Find the PerThread for given (isolate, thread) combination

  // If one does not yet exist, return null.

  PerIsolateThreadData* FindPerThreadDataForThread(ThreadId thread_id);

#ifdef ENABLE_DEBUGGER_SUPPORT

  // Get the debugger from the default isolate. Preinitializes the

  // default isolate if needed.

  static Debugger* GetDefaultIsolateDebugger();

#endif

  // Get the stack guard from the default isolate. Preinitializes the

  // default isolate if needed.

  static StackGuard* GetDefaultIsolateStackGuard();

  // Returns the key used to store the pointer to the current isolate.

  // Used internally for V8 threads that do not execute JavaScript but still

  // are part of the domain of an isolate (like the context switcher).

  static Thread::LocalStorageKey isolate_key() {

    return isolate_key_;

  }

  // Returns the key used to store process-wide thread IDs.

  static Thread::LocalStorageKey thread_id_key() {

    return thread_id_key_;

  }

  static Thread::LocalStorageKey per_isolate_thread_data_key();

  // If a client attempts to create a Locker without specifying an isolate,

  // we assume that the client is using legacy behavior. Set up the current

  // thread to be inside the implicit isolate (or fail a check if we have

  // switched to non-legacy behavior).

  static void EnterDefaultIsolate();

  // Mutex for serializing access to break control structures.

  RecursiveMutex* break_access() { return &break_access_; }

  // Mutex for serializing access to debugger.

  RecursiveMutex* debugger_access() { return &debugger_access_; }

  Address get_address_from_id(AddressId id);

  // Access to top context (where the current function object was created).

  Context* context() { return thread_local_top_.context_; }

  void set_context(Context* context) {

    ASSERT(context == NULL || context->IsContext());

    thread_local_top_.context_ = context;

  }

  Context** context_address() { return &thread_local_top_.context_; }

  SaveContext* save_context() { return thread_local_top_.save_context_; }

  void set_save_context(SaveContext* save) {

    thread_local_top_.save_context_ = save;

  }

  // Access to current thread id.

  ThreadId thread_id() { return thread_local_top_.thread_id_; }

  void set_thread_id(ThreadId id) { thread_local_top_.thread_id_ = id; }

  // Interface to pending exception.

  MaybeObject* pending_exception() {

    ASSERT(has_pending_exception());

    return thread_local_top_.pending_exception_;

  }

  bool external_caught_exception() {

    return thread_local_top_.external_caught_exception_;

  }

  void set_external_caught_exception(bool value) {

    thread_local_top_.external_caught_exception_ = value;

  }

  void set_pending_exception(MaybeObject* exception) {

    thread_local_top_.pending_exception_ = exception;

  }

  void clear_pending_exception() {

    thread_local_top_.pending_exception_ = heap_.the_hole_value();

  }

  MaybeObject** pending_exception_address() {

    return &thread_local_top_.pending_exception_;

  }

  bool has_pending_exception() {

    return !thread_local_top_.pending_exception_->IsTheHole();

  }

  void clear_pending_message() {

    thread_local_top_.has_pending_message_ = false;

    thread_local_top_.pending_message_obj_ = heap_.the_hole_value();

    thread_local_top_.pending_message_script_ = heap_.the_hole_value();

  }

  v8::TryCatch* try_catch_handler() {

    return thread_local_top_.TryCatchHandler();

  }

  Address try_catch_handler_address() {

    return thread_local_top_.try_catch_handler_address();

  }

  bool* external_caught_exception_address() {

    return &thread_local_top_.external_caught_exception_;

  }

  v8::TryCatch* catcher() {

    return thread_local_top_.catcher_;

  }

  void set_catcher(v8::TryCatch* catcher) {

    thread_local_top_.catcher_ = catcher;

  }

  MaybeObject** scheduled_exception_address() {

    return &thread_local_top_.scheduled_exception_;

  }

  Address pending_message_obj_address() {

    return reinterpret_cast<Address>(&thread_local_top_.pending_message_obj_);

  }

  Address has_pending_message_address() {

    return reinterpret_cast<Address>(&thread_local_top_.has_pending_message_);

  }

  Address pending_message_script_address() {

    return reinterpret_cast<Address>(

        &thread_local_top_.pending_message_script_);

  }

  MaybeObject* scheduled_exception() {

    ASSERT(has_scheduled_exception());

    return thread_local_top_.scheduled_exception_;

  }

  bool has_scheduled_exception() {

    return thread_local_top_.scheduled_exception_ != heap_.the_hole_value();

  }

  void clear_scheduled_exception() {

    thread_local_top_.scheduled_exception_ = heap_.the_hole_value();

  }

  bool IsExternallyCaught();

  bool is_catchable_by_javascript(MaybeObject* exception) {

    return (!exception->IsOutOfMemory()) &&

        (exception != heap()->termination_exception());

  }

  // Serializer.

  void PushToPartialSnapshotCache(Object* obj);

  // JS execution stack (see frames.h).

  static Address c_entry_fp(ThreadLocalTop* thread) {

    return thread->c_entry_fp_;

  }

  static Address handler(ThreadLocalTop* thread) { return thread->handler_; }

  inline Address* c_entry_fp_address() {

    return &thread_local_top_.c_entry_fp_;

  }

  inline Address* handler_address() { return &thread_local_top_.handler_; }

  // Bottom JS entry.

  Address js_entry_sp() {

    return thread_local_top_.js_entry_sp_;

  }

  inline Address* js_entry_sp_address() {

    return &thread_local_top_.js_entry_sp_;

  }

  // Generated code scratch locations.

  void* formal_count_address() { return &thread_local_top_.formal_count_; }

  // Returns the global object of the current context. It could be

  // a builtin object, or a JS global object.

  Handle<GlobalObject> global_object() {

    return Handle<GlobalObject>(context()->global_object());

  }

  // Returns the global proxy object of the current context.

  Object* global_proxy() {

    return context()->global_proxy();

  }

  Handle<JSBuiltinsObject> js_builtins_object() {

    return Handle<JSBuiltinsObject>(thread_local_top_.context_->builtins());

  }

  static int ArchiveSpacePerThread() { return sizeof(ThreadLocalTop); }

  void FreeThreadResources() { thread_local_top_.Free(); }

  // This method is called by the api after operations that may throw

  // exceptions.  If an exception was thrown and not handled by an external

  // handler the exception is scheduled to be rethrown when we return to running

  // JavaScript code.  If an exception is scheduled true is returned.

  bool OptionalRescheduleException(bool is_bottom_call);

  class ExceptionScope {

   public:

    explicit ExceptionScope(Isolate* isolate) :

      // Scope currently can only be used for regular exceptions, not

      // failures like OOM or termination exception.

      isolate_(isolate),

      pending_exception_(isolate_->pending_exception()->ToObjectUnchecked(),

                         isolate_),

      catcher_(isolate_->catcher())

    { }

    ~ExceptionScope() {

      isolate_->set_catcher(catcher_);

      isolate_->set_pending_exception(*pending_exception_);

    }

   private:

    Isolate* isolate_;

    Handle<Object> pending_exception_;

    v8::TryCatch* catcher_;

  };

  void SetCaptureStackTraceForUncaughtExceptions(

      bool capture,

      int frame_limit,

      StackTrace::StackTraceOptions options);

  // Tells whether the current context has experienced an out of memory

  // exception.

  bool is_out_of_memory();

  bool ignore_out_of_memory() {

    return thread_local_top_.ignore_out_of_memory_;

  }

  void set_ignore_out_of_memory(bool value) {

    thread_local_top_.ignore_out_of_memory_ = value;

  }

  void PrintCurrentStackTrace(FILE* out);

  void PrintStackTrace(FILE* out, char* thread_data);

  void PrintStack(StringStream* accumulator);

  void PrintStack(FILE* out);

  void PrintStack();

  Handle<String> StackTraceString();

  NO_INLINE(void PushStackTraceAndDie(unsigned int magic,

                                      Object* object,

                                      Map* map,

                                      unsigned int magic2));

  Handle<JSArray> CaptureCurrentStackTrace(

      int frame_limit,

      StackTrace::StackTraceOptions options);

  Handle<JSArray> CaptureSimpleStackTrace(Handle<JSObject> error_object,

                                          Handle<Object> caller,

                                          int limit);

  void CaptureAndSetDetailedStackTrace(Handle<JSObject> error_object);

  // Returns if the top context may access the given global object. If

  // the result is false, the pending exception is guaranteed to be

  // set.

  // TODO(yangguo): temporary wrappers

  bool MayNamedAccessWrapper(Handle<JSObject> receiver,

                             Handle<Object> key,

                             v8::AccessType type) {

    return MayNamedAccess(*receiver, *key, type);

  }

  bool MayIndexedAccessWrapper(Handle<JSObject> receiver,

                               uint32_t index,

                               v8::AccessType type) {

    return MayIndexedAccess(*receiver, index, type);

  }

  bool MayNamedAccess(JSObject* receiver,

                      Object* key,

                      v8::AccessType type);

  bool MayIndexedAccess(JSObject* receiver,

                        uint32_t index,

                        v8::AccessType type);

  void SetFailedAccessCheckCallback(v8::FailedAccessCheckCallback callback);

  void ReportFailedAccessCheck(JSObject* receiver, v8::AccessType type);

  // Exception throwing support. The caller should use the result

  // of Throw() as its return value.

  Failure* Throw(Object* exception, MessageLocation* location = NULL);

  // Re-throw an exception.  This involves no error reporting since

  // error reporting was handled when the exception was thrown

  // originally.

  Failure* ReThrow(MaybeObject* exception);

  void ScheduleThrow(Object* exception);

  // Re-set pending message, script and positions reported to the TryCatch

  // back to the TLS for re-use when rethrowing.

  void RestorePendingMessageFromTryCatch(v8::TryCatch* handler);

  void ReportPendingMessages();

  // Return pending location if any or unfilled structure.

  MessageLocation GetMessageLocation();

  Failure* ThrowIllegalOperation();

  // Promote a scheduled exception to pending. Asserts has_scheduled_exception.

  Failure* PromoteScheduledException();

  void DoThrow(Object* exception, MessageLocation* location);

  // Checks if exception should be reported and finds out if it's

  // caught externally.

  bool ShouldReportException(bool* can_be_caught_externally,

                             bool catchable_by_javascript);

  // Attempts to compute the current source location, storing the

  // result in the target out parameter.

  void ComputeLocation(MessageLocation* target);

  // Override command line flag.

  void TraceException(bool flag);

  // Out of resource exception helpers.

  Failure* StackOverflow();

  Failure* TerminateExecution();

  void CancelTerminateExecution();

  // Administration

  void Iterate(ObjectVisitor* v);

  void Iterate(ObjectVisitor* v, ThreadLocalTop* t);

  char* Iterate(ObjectVisitor* v, char* t);

  void IterateThread(ThreadVisitor* v, char* t);

  // Returns the current native and global context.

  Handle<Context> native_context();

  Handle<Context> global_context();

  // Returns the native context of the calling JavaScript code.  That

  // is, the native context of the top-most JavaScript frame.

  Handle<Context> GetCallingNativeContext();

  void RegisterTryCatchHandler(v8::TryCatch* that);

  void UnregisterTryCatchHandler(v8::TryCatch* that);

  char* ArchiveThread(char* to);

  char* RestoreThread(char* from);

  static const char* const kStackOverflowMessage;

  static const int kUC16AlphabetSize = 256;  // See StringSearchBase.

  static const int kBMMaxShift = 250;        // See StringSearchBase.

  // Accessors.

#define GLOBAL_ACCESSOR(type, name, initialvalue)                       \

  inline type name() const {                                            \

    ASSERT(OFFSET_OF(Isolate, name##_) == name##_debug_offset_);        \

    return name##_;                                                     \

  }                                                                     \

  inline void set_##name(type value) {                                  \

    ASSERT(OFFSET_OF(Isolate, name##_) == name##_debug_offset_);        \

    name##_ = value;                                                    \

  }

  ISOLATE_INIT_LIST(GLOBAL_ACCESSOR)

#undef GLOBAL_ACCESSOR

#define GLOBAL_ARRAY_ACCESSOR(type, name, length)                       \

  inline type* name() {                                                 \

    ASSERT(OFFSET_OF(Isolate, name##_) == name##_debug_offset_);        \

    return &(name##_)[0];                                               \

  }

  ISOLATE_INIT_ARRAY_LIST(GLOBAL_ARRAY_ACCESSOR)

#undef GLOBAL_ARRAY_ACCESSOR

#define NATIVE_CONTEXT_FIELD_ACCESSOR(index, type, name)            \

  Handle<type> name() {                                             \

    return Handle<type>(context()->native_context()->name(), this); \

  }                                                                 \

  bool is_##name(type* value) {                                     \

    return context()->native_context()->is_##name(value);           \

  }

  NATIVE_CONTEXT_FIELDS(NATIVE_CONTEXT_FIELD_ACCESSOR)

#undef NATIVE_CONTEXT_FIELD_ACCESSOR

  Bootstrapper* bootstrapper() { return bootstrapper_; }

  Counters* counters() {

    // Call InitializeLoggingAndCounters() if logging is needed before

    // the isolate is fully initialized.

    ASSERT(counters_ != NULL);

    return counters_;

  }

  CodeRange* code_range() { return code_range_; }

  RuntimeProfiler* runtime_profiler() { return runtime_profiler_; }

  CompilationCache* compilation_cache() { return compilation_cache_; }

  Logger* logger() {

    // Call InitializeLoggingAndCounters() if logging is needed before

    // the isolate is fully initialized.

    ASSERT(logger_ != NULL);

    return logger_;

  }

  StackGuard* stack_guard() { return &stack_guard_; }

  Heap* heap() { return &heap_; }

  StatsTable* stats_table();

  StubCache* stub_cache() { return stub_cache_; }

  DeoptimizerData* deoptimizer_data() { return deoptimizer_data_; }

  ThreadLocalTop* thread_local_top() { return &thread_local_top_; }

  TranscendentalCache* transcendental_cache() const {

    return transcendental_cache_;

  }

  MemoryAllocator* memory_allocator() {

    return memory_allocator_;

  }

  KeyedLookupCache* keyed_lookup_cache() {

    return keyed_lookup_cache_;

  }

  ContextSlotCache* context_slot_cache() {

    return context_slot_cache_;

  }

  DescriptorLookupCache* descriptor_lookup_cache() {

    return descriptor_lookup_cache_;

  }

  v8::ImplementationUtilities::HandleScopeData* handle_scope_data() {

    return &handle_scope_data_;

  }

  HandleScopeImplementer* handle_scope_implementer() {

    ASSERT(handle_scope_implementer_);

    return handle_scope_implementer_;

  }

  Zone* runtime_zone() { return &runtime_zone_; }

  UnicodeCache* unicode_cache() {

    return unicode_cache_;

  }

  InnerPointerToCodeCache* inner_pointer_to_code_cache() {

    return inner_pointer_to_code_cache_;

  }

  ConsStringIteratorOp* write_iterator() { return write_iterator_; }

  GlobalHandles* global_handles() { return global_handles_; }

  EternalHandles* eternal_handles() { return eternal_handles_; }

  ThreadManager* thread_manager() { return thread_manager_; }

  ContextSwitcher* context_switcher() { return context_switcher_; }

  void set_context_switcher(ContextSwitcher* switcher) {

    context_switcher_ = switcher;

  }

  StringTracker* string_tracker() { return string_tracker_; }

  unibrow::Mapping<unibrow::Ecma262UnCanonicalize>* jsregexp_uncanonicalize() {

    return &jsregexp_uncanonicalize_;

  }

  unibrow::Mapping<unibrow::CanonicalizationRange>* jsregexp_canonrange() {

    return &jsregexp_canonrange_;

  }

  ConsStringIteratorOp* objects_string_compare_iterator_a() {

    return &objects_string_compare_iterator_a_;

  }

  ConsStringIteratorOp* objects_string_compare_iterator_b() {

    return &objects_string_compare_iterator_b_;

  }

  StaticResource<ConsStringIteratorOp>* objects_string_iterator() {

    return &objects_string_iterator_;

  }

  RuntimeState* runtime_state() { return &runtime_state_; }

  void set_fp_stubs_generated(bool value) {

    fp_stubs_generated_ = value;

  }

  bool fp_stubs_generated() { return fp_stubs_generated_; }

  Builtins* builtins() { return &builtins_; }

  void NotifyExtensionInstalled() {

    has_installed_extensions_ = true;

  }

  bool has_installed_extensions() { return has_installed_extensions_; }

  unibrow::Mapping<unibrow::Ecma262Canonicalize>*

      regexp_macro_assembler_canonicalize() {

    return &regexp_macro_assembler_canonicalize_;

  }

  RegExpStack* regexp_stack() { return regexp_stack_; }

  unibrow::Mapping<unibrow::Ecma262Canonicalize>*

      interp_canonicalize_mapping() {

    return &interp_canonicalize_mapping_;

  }

  void* PreallocatedStorageNew(size_t size);

  void PreallocatedStorageDelete(void* p);

  void PreallocatedStorageInit(size_t size);

  inline bool IsCodePreAgingActive();

#ifdef ENABLE_DEBUGGER_SUPPORT

  Debugger* debugger() {

    if (!NoBarrier_Load(&debugger_initialized_)) InitializeDebugger();

    return debugger_;

  }

  Debug* debug() {

    if (!NoBarrier_Load(&debugger_initialized_)) InitializeDebugger();

    return debug_;

  }

#endif

  inline bool IsDebuggerActive();

  inline bool DebuggerHasBreakPoints();

  CpuProfiler* cpu_profiler() const { return cpu_profiler_; }

  HeapProfiler* heap_profiler() const { return heap_profiler_; }

#ifdef DEBUG

  HistogramInfo* heap_histograms() { return heap_histograms_; }

  JSObject::SpillInformation* js_spill_information() {

    return &js_spill_information_;

  }

  int* code_kind_statistics() { return code_kind_statistics_; }

#endif

#if V8_TARGET_ARCH_ARM && !defined(__arm__) || \

    V8_TARGET_ARCH_MIPS && !defined(__mips__)

  bool simulator_initialized() { return simulator_initialized_; }

  void set_simulator_initialized(bool initialized) {

    simulator_initialized_ = initialized;

  }

  HashMap* simulator_i_cache() { return simulator_i_cache_; }

  void set_simulator_i_cache(HashMap* hash_map) {

    simulator_i_cache_ = hash_map;

  }

  Redirection* simulator_redirection() {

    return simulator_redirection_;

  }

  void set_simulator_redirection(Redirection* redirection) {

    simulator_redirection_ = redirection;

  }

#endif

  Factory* factory() { return reinterpret_cast<Factory*>(this); }

  static const int kJSRegexpStaticOffsetsVectorSize = 128;

  ExternalCallbackScope* external_callback_scope() {

    return thread_local_top_.external_callback_scope_;

  }

  void set_external_callback_scope(ExternalCallbackScope* scope) {

    thread_local_top_.external_callback_scope_ = scope;

  }

  StateTag current_vm_state() {

    return thread_local_top_.current_vm_state_;

  }

  void set_current_vm_state(StateTag state) {

    thread_local_top_.current_vm_state_ = state;

  }

  void SetData(void* data) { embedder_data_ = data; }

  void* GetData() { return embedder_data_; }

  LookupResult* top_lookup_result() {

    return thread_local_top_.top_lookup_result_;

  }

  void SetTopLookupResult(LookupResult* top) {

    thread_local_top_.top_lookup_result_ = top;

  }

  bool IsDead() { return has_fatal_error_; }

  void SignalFatalError() { has_fatal_error_ = true; }

  bool use_crankshaft() const { return use_crankshaft_; }

  bool initialized_from_snapshot() { return initialized_from_snapshot_; }

  double time_millis_since_init() {

    return OS::TimeCurrentMillis() - time_millis_at_init_;

  }

  DateCache* date_cache() {

    return date_cache_;

  }

  void set_date_cache(DateCache* date_cache) {

    if (date_cache != date_cache_) {

      delete date_cache_;

    }

    date_cache_ = date_cache;

  }

  Map* get_initial_js_array_map(ElementsKind kind);

  bool IsFastArrayConstructorPrototypeChainIntact();

  CodeStubInterfaceDescriptor*

      code_stub_interface_descriptor(int index);

  void IterateDeferredHandles(ObjectVisitor* visitor);

  void LinkDeferredHandles(DeferredHandles* deferred_handles);

  void UnlinkDeferredHandles(DeferredHandles* deferred_handles);

#ifdef DEBUG

  bool IsDeferredHandle(Object** location);

#endif  // DEBUG

  OptimizingCompilerThread* optimizing_compiler_thread() {

    return optimizing_compiler_thread_;

  }

  // PreInits and returns a default isolate. Needed when a new thread tries

  // to create a Locker for the first time (the lock itself is in the isolate).

  // TODO(svenpanne) This method is on death row...

  static v8::Isolate* GetDefaultIsolateForLocking();

  SweeperThread** sweeper_threads() {

    return sweeper_thread_;

  }

  int id() const { return static_cast<int>(id_); }

  HStatistics* GetHStatistics();

  HTracer* GetHTracer();

  FunctionEntryHook function_entry_hook() { return function_entry_hook_; }

  void set_function_entry_hook(FunctionEntryHook function_entry_hook) {

    function_entry_hook_ = function_entry_hook;

  }

  void* stress_deopt_count_address() { return &stress_deopt_count_; }

  inline RandomNumberGenerator* random_number_generator();

  // Given an address occupied by a live code object, return that object.

  Object* FindCodeObject(Address a);

 private:

  Isolate();

  friend struct GlobalState;

  friend struct InitializeGlobalState;

  enum State {

    UNINITIALIZED,    // Some components may not have been allocated.

    INITIALIZED       // All components are fully initialized.

  };

  // These fields are accessed through the API, offsets must be kept in sync

  // with v8::internal::Internals (in include/v8.h) constants. This is also

  // verified in Isolate::Init() using runtime checks.

  State state_;  // Will be padded to kApiPointerSize.

  void* embedder_data_;

  Heap heap_;

  // The per-process lock should be acquired before the ThreadDataTable is

  // modified.

  class ThreadDataTable {

   public:

    ThreadDataTable();

    ~ThreadDataTable();

    PerIsolateThreadData* Lookup(Isolate* isolate, ThreadId thread_id);

    void Insert(PerIsolateThreadData* data);

    void Remove(PerIsolateThreadData* data);

    void RemoveAllThreads(Isolate* isolate);

   private:

    PerIsolateThreadData* list_;

  };

  // These items form a stack synchronously with threads Enter'ing and Exit'ing

  // the Isolate. The top of the stack points to a thread which is currently

  // running the Isolate. When the stack is empty, the Isolate is considered

  // not entered by any thread and can be Disposed.

  // If the same thread enters the Isolate more then once, the entry_count_

  // is incremented rather then a new item pushed to the stack.

  class EntryStackItem {

   public:

    EntryStackItem(PerIsolateThreadData* previous_thread_data,

                   Isolate* previous_isolate,

                   EntryStackItem* previous_item)

        : entry_count(1),

          previous_thread_data(previous_thread_data),

          previous_isolate(previous_isolate),

          previous_item(previous_item) { }

    int entry_count;

    PerIsolateThreadData* previous_thread_data;

    Isolate* previous_isolate;

    EntryStackItem* previous_item;

   private:

    DISALLOW_COPY_AND_ASSIGN(EntryStackItem);

  };

  // This mutex protects highest_thread_id_, thread_data_table_ and

  // default_isolate_.

  static Mutex process_wide_mutex_;

  static Thread::LocalStorageKey per_isolate_thread_data_key_;

  static Thread::LocalStorageKey isolate_key_;

  static Thread::LocalStorageKey thread_id_key_;

  static Isolate* default_isolate_;

  static ThreadDataTable* thread_data_table_;

  // A global counter for all generated Isolates, might overflow.

  static Atomic32 isolate_counter_;

  void Deinit();

  static void SetIsolateThreadLocals(Isolate* isolate,

                                     PerIsolateThreadData* data);

  // Find the PerThread for this particular (isolate, thread) combination.

  // If one does not yet exist, allocate a new one.

  PerIsolateThreadData* FindOrAllocatePerThreadDataForThisThread();

  // Initializes the current thread to run this Isolate.

  // Not thread-safe. Multiple threads should not Enter/Exit the same isolate

  // at the same time, this should be prevented using external locking.

  void Enter();

  // Exits the current thread. The previosuly entered Isolate is restored

  // for the thread.

  // Not thread-safe. Multiple threads should not Enter/Exit the same isolate

  // at the same time, this should be prevented using external locking.

  void Exit();

  void PreallocatedMemoryThreadStart();

  void PreallocatedMemoryThreadStop();

  void InitializeThreadLocal();

  void PrintStackTrace(FILE* out, ThreadLocalTop* thread);

  void MarkCompactPrologue(bool is_compacting,

                           ThreadLocalTop* archived_thread_data);

  void MarkCompactEpilogue(bool is_compacting,

                           ThreadLocalTop* archived_thread_data);

  void FillCache();

  void PropagatePendingExceptionToExternalTryCatch();

  void InitializeDebugger();

  // Traverse prototype chain to find out whether the object is derived from

  // the Error object.

  bool IsErrorObject(Handle<Object> obj);

  Atomic32 id_;

  EntryStackItem* entry_stack_;

  int stack_trace_nesting_level_;

  StringStream* incomplete_message_;

  // The preallocated memory thread singleton.

  PreallocatedMemoryThread* preallocated_memory_thread_;

  Address isolate_addresses_[kIsolateAddressCount + 1];  // NOLINT

  NoAllocationStringAllocator* preallocated_message_space_;

  Bootstrapper* bootstrapper_;

  RuntimeProfiler* runtime_profiler_;

  CompilationCache* compilation_cache_;

  Counters* counters_;

  CodeRange* code_range_;

  RecursiveMutex break_access_;

  Atomic32 debugger_initialized_;

  RecursiveMutex debugger_access_;

  Logger* logger_;

  StackGuard stack_guard_;

  StatsTable* stats_table_;

  StubCache* stub_cache_;

  DeoptimizerData* deoptimizer_data_;

  ThreadLocalTop thread_local_top_;

  bool capture_stack_trace_for_uncaught_exceptions_;

  int stack_trace_for_uncaught_exceptions_frame_limit_;

  StackTrace::StackTraceOptions stack_trace_for_uncaught_exceptions_options_;

  TranscendentalCache* transcendental_cache_;

  MemoryAllocator* memory_allocator_;

  KeyedLookupCache* keyed_lookup_cache_;

  ContextSlotCache* context_slot_cache_;

  DescriptorLookupCache* descriptor_lookup_cache_;

  v8::ImplementationUtilities::HandleScopeData handle_scope_data_;

  HandleScopeImplementer* handle_scope_implementer_;

  UnicodeCache* unicode_cache_;

  Zone runtime_zone_;

  PreallocatedStorage in_use_list_;

  PreallocatedStorage free_list_;

  bool preallocated_storage_preallocated_;

  InnerPointerToCodeCache* inner_pointer_to_code_cache_;

  ConsStringIteratorOp* write_iterator_;

  GlobalHandles* global_handles_;

  EternalHandles* eternal_handles_;

  ContextSwitcher* context_switcher_;

  ThreadManager* thread_manager_;

  RuntimeState runtime_state_;

  bool fp_stubs_generated_;

  Builtins builtins_;

  bool has_installed_extensions_;

  StringTracker* string_tracker_;

  unibrow::Mapping<unibrow::Ecma262UnCanonicalize> jsregexp_uncanonicalize_;

  unibrow::Mapping<unibrow::CanonicalizationRange> jsregexp_canonrange_;

  ConsStringIteratorOp objects_string_compare_iterator_a_;

  ConsStringIteratorOp objects_string_compare_iterator_b_;

  StaticResource<ConsStringIteratorOp> objects_string_iterator_;

  unibrow::Mapping<unibrow::Ecma262Canonicalize>

      regexp_macro_assembler_canonicalize_;

  RegExpStack* regexp_stack_;

  DateCache* date_cache_;

  unibrow::Mapping<unibrow::Ecma262Canonicalize> interp_canonicalize_mapping_;

  CodeStubInterfaceDescriptor* code_stub_interface_descriptors_;

  RandomNumberGenerator* random_number_generator_;

  // True if fatal error has been signaled for this isolate.

  bool has_fatal_error_;

  // True if we are using the Crankshaft optimizing compiler.

  bool use_crankshaft_;

  // True if this isolate was initialized from a snapshot.

  bool initialized_from_snapshot_;

  // Time stamp at initialization.

  double time_millis_at_init_;

#if V8_TARGET_ARCH_ARM && !defined(__arm__) || \

    V8_TARGET_ARCH_MIPS && !defined(__mips__)

  bool simulator_initialized_;

  HashMap* simulator_i_cache_;

  Redirection* simulator_redirection_;

#endif

#ifdef DEBUG

  // A static array of histogram info for each type.

  HistogramInfo heap_histograms_[LAST_TYPE + 1];

  JSObject::SpillInformation js_spill_information_;

  int code_kind_statistics_[Code::NUMBER_OF_KINDS];

#endif

#ifdef ENABLE_DEBUGGER_SUPPORT

  Debugger* debugger_;

  Debug* debug_;

#endif

  CpuProfiler* cpu_profiler_;

  HeapProfiler* heap_profiler_;

  FunctionEntryHook function_entry_hook_;

#define GLOBAL_BACKING_STORE(type, name, initialvalue)                         \

  type name##_;

  ISOLATE_INIT_LIST(GLOBAL_BACKING_STORE)

#undef GLOBAL_BACKING_STORE

#define GLOBAL_ARRAY_BACKING_STORE(type, name, length)                         \

  type name##_[length];

  ISOLATE_INIT_ARRAY_LIST(GLOBAL_ARRAY_BACKING_STORE)

#undef GLOBAL_ARRAY_BACKING_STORE

#ifdef DEBUG

  // This class is huge and has a number of fields controlled by

  // preprocessor defines. Make sure the offsets of these fields agree

  // between compilation units.

#define ISOLATE_FIELD_OFFSET(type, name, ignored)                              \

  static const intptr_t name##_debug_offset_;

  ISOLATE_INIT_LIST(ISOLATE_FIELD_OFFSET)

  ISOLATE_INIT_ARRAY_LIST(ISOLATE_FIELD_OFFSET)

#undef ISOLATE_FIELD_OFFSET

#endif

  DeferredHandles* deferred_handles_head_;

  OptimizingCompilerThread* optimizing_compiler_thread_;

  SweeperThread** sweeper_thread_;

  // Counts deopt points if deopt_every_n_times is enabled.

  unsigned int stress_deopt_count_;

  friend class ExecutionAccess;

  friend class HandleScopeImplementer;

  friend class IsolateInitializer;

  friend class OptimizingCompilerThread;

  friend class SweeperThread;

  friend class ThreadManager;

  friend class Simulator;

  friend class StackGuard;

  friend class ThreadId;

  friend class TestMemoryAllocatorScope;

  friend class TestCodeRangeScope;

  friend class v8::Isolate;

  friend class v8::Locker;

  friend class v8::Unlocker;

  DISALLOW_COPY_AND_ASSIGN(Isolate);

};

// If the GCC version is 4.1.x or 4.2.x an additional field is added to the

// class as a work around for a bug in the generated code found with these

// versions of GCC. See V8 issue 122 for details.

class SaveContext BASE_EMBEDDED {

 public:

  inline explicit SaveContext(Isolate* isolate);

  ~SaveContext() {

    isolate_->set_context(context_.is_null() ? NULL : *context_);

    isolate_->set_save_context(prev_);

  }

  Handle<Context> context() { return context_; }

  SaveContext* prev() { return prev_; }

  // Returns true if this save context is below a given JavaScript frame.

  bool IsBelowFrame(JavaScriptFrame* frame) {