runtime/vm/stack_trace.cc

// Copyright (c) 2017, 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/stack_trace.h"

#include "vm/dart_api_impl.h"
#include "vm/object_store.h"
#include "vm/stack_frame.h"
#include "vm/symbols.h"

namespace dart {

// Keep in sync with:
// - sdk/lib/async/stream_controller.dart:_StreamController._STATE_SUBSCRIBED.
const intptr_t k_StreamController__STATE_SUBSCRIBED = 1;
// - sdk/lib/async/future_impl.dart:_FutureListener.stateThen.
const intptr_t k_FutureListener_stateThen = 1;
// - sdk/lib/async/future_impl.dart:_FutureListener.stateCatchError.
const intptr_t k_FutureListener_stateCatchError = 2;
// - sdk/lib/async/future_impl.dart:_FutureListener.stateWhenComplete.
const intptr_t k_FutureListener_stateWhenComplete = 8;

// Keep in sync with sdk/lib/async/future_impl.dart:_FutureListener.handleValue.
const intptr_t kNumArgsFutureListenerHandleValue = 1;

// Find current yield index from async closure.
// Async closures contains a variable, :await_jump_var that holds the index into
// async wrapper.
intptr_t GetYieldIndex(const Closure& receiver_closure) {
  const auto& function = Function::Handle(receiver_closure.function());
  if (!function.IsAsyncClosure() && !function.IsAsyncGenClosure()) {
    return UntaggedPcDescriptors::kInvalidYieldIndex;
  }
  const auto& await_jump_var =
      Object::Handle(Context::Handle(receiver_closure.context())
                         .At(Context::kAwaitJumpVarIndex));
  ASSERT(await_jump_var.IsSmi());
  return Smi::Cast(await_jump_var).Value();
}

intptr_t FindPcOffset(const PcDescriptors& pc_descs, intptr_t yield_index) {
  if (yield_index == UntaggedPcDescriptors::kInvalidYieldIndex) {
    return 0;
  }
  PcDescriptors::Iterator iter(pc_descs, UntaggedPcDescriptors::kAnyKind);
  while (iter.MoveNext()) {
    if (iter.YieldIndex() == yield_index) {
      return iter.PcOffset();
    }
  }
  UNREACHABLE();  // If we cannot find it we have a bug.
}

// Instance caches library and field references.
// This way we don't have to do the look-ups for every frame in the stack.
CallerClosureFinder::CallerClosureFinder(Zone* zone)
    : closure_(Closure::Handle(zone)),
      receiver_context_(Context::Handle(zone)),
      receiver_function_(Function::Handle(zone)),
      parent_function_(Function::Handle(zone)),
      context_entry_(Object::Handle(zone)),
      future_(Object::Handle(zone)),
      listener_(Object::Handle(zone)),
      callback_(Object::Handle(zone)),
      controller_(Object::Handle(zone)),
      state_(Object::Handle(zone)),
      var_data_(Object::Handle(zone)),
      callback_instance_(Object::Handle(zone)),
      future_impl_class(Class::Handle(zone)),
      future_listener_class(Class::Handle(zone)),
      async_start_stream_controller_class(Class::Handle(zone)),
      stream_controller_class(Class::Handle(zone)),
      async_stream_controller_class(Class::Handle(zone)),
      controller_subscription_class(Class::Handle(zone)),
      buffering_stream_subscription_class(Class::Handle(zone)),
      stream_iterator_class(Class::Handle(zone)),
      future_result_or_listeners_field(Field::Handle(zone)),
      callback_field(Field::Handle(zone)),
      future_listener_state_field(Field::Handle(zone)),
      future_listener_result_field(Field::Handle(zone)),
      controller_controller_field(Field::Handle(zone)),
      var_data_field(Field::Handle(zone)),
      state_field(Field::Handle(zone)),
      on_data_field(Field::Handle(zone)),
      state_data_field(Field::Handle(zone)) {
  const auto& async_lib = Library::Handle(zone, Library::AsyncLibrary());
  // Look up classes:
  // - async:
  future_impl_class = async_lib.LookupClassAllowPrivate(Symbols::FutureImpl());
  ASSERT(!future_impl_class.IsNull());
  future_listener_class =
      async_lib.LookupClassAllowPrivate(Symbols::_FutureListener());
  ASSERT(!future_listener_class.IsNull());
  // - async*:
  async_start_stream_controller_class =
      async_lib.LookupClassAllowPrivate(Symbols::_AsyncStarStreamController());
  ASSERT(!async_start_stream_controller_class.IsNull());
  stream_controller_class =
      async_lib.LookupClassAllowPrivate(Symbols::_StreamController());
  ASSERT(!stream_controller_class.IsNull());
  async_stream_controller_class =
      async_lib.LookupClassAllowPrivate(Symbols::_AsyncStreamController());
  ASSERT(!async_stream_controller_class.IsNull());
  controller_subscription_class =
      async_lib.LookupClassAllowPrivate(Symbols::_ControllerSubscription());
  ASSERT(!controller_subscription_class.IsNull());
  buffering_stream_subscription_class = async_lib.LookupClassAllowPrivate(
      Symbols::_BufferingStreamSubscription());
  ASSERT(!buffering_stream_subscription_class.IsNull());
  stream_iterator_class =
      async_lib.LookupClassAllowPrivate(Symbols::_StreamIterator());
  ASSERT(!stream_iterator_class.IsNull());

  // Look up fields:
  // - async:
  future_result_or_listeners_field =
      future_impl_class.LookupFieldAllowPrivate(Symbols::_resultOrListeners());
  ASSERT(!future_result_or_listeners_field.IsNull());
  callback_field =
      future_listener_class.LookupFieldAllowPrivate(Symbols::callback());
  ASSERT(!callback_field.IsNull());
  future_listener_state_field =
      future_listener_class.LookupFieldAllowPrivate(Symbols::state());
  ASSERT(!future_listener_state_field.IsNull());
  future_listener_result_field =
      future_listener_class.LookupFieldAllowPrivate(Symbols::result());
  ASSERT(!future_listener_result_field.IsNull());
  // - async*:
  controller_controller_field =
      async_start_stream_controller_class.LookupFieldAllowPrivate(
          Symbols::controller());
  ASSERT(!controller_controller_field.IsNull());
  state_field =
      stream_controller_class.LookupFieldAllowPrivate(Symbols::_state());
  ASSERT(!state_field.IsNull());
  var_data_field =
      stream_controller_class.LookupFieldAllowPrivate(Symbols::_varData());
  ASSERT(!var_data_field.IsNull());
  on_data_field = buffering_stream_subscription_class.LookupFieldAllowPrivate(
      Symbols::_onData());
  ASSERT(!on_data_field.IsNull());
  state_data_field =
      stream_iterator_class.LookupFieldAllowPrivate(Symbols::_stateData());
  ASSERT(!state_data_field.IsNull());
}

ClosurePtr CallerClosureFinder::GetCallerInFutureImpl(const Object& future) {
  ASSERT(!future.IsNull());
  ASSERT(future.GetClassId() == future_impl_class.id());
  // Since this function is recursive, we have to keep a local ref.
  auto& listener = Object::Handle(GetFutureFutureListener(future));
  if (listener.IsNull()) {
    return Closure::null();
  }
  return GetCallerInFutureListener(listener);
}

ClosurePtr CallerClosureFinder::FindCallerInAsyncGenClosure(
    const Context& receiver_context) {
  // Get the async* _StreamController.
  context_entry_ = receiver_context.At(Context::kControllerIndex);
  ASSERT(context_entry_.IsInstance());
  ASSERT(context_entry_.GetClassId() ==
         async_start_stream_controller_class.id());

  const Instance& controller = Instance::Cast(context_entry_);
  controller_ = controller.GetField(controller_controller_field);
  ASSERT(!controller_.IsNull());
  ASSERT(controller_.GetClassId() == async_stream_controller_class.id());

  // Get the _StreamController._state field.
  state_ = Instance::Cast(controller_).GetField(state_field);
  ASSERT(state_.IsSmi());
  if (Smi::Cast(state_).Value() != k_StreamController__STATE_SUBSCRIBED) {
    return Closure::null();
  }

  // Get the _StreamController._varData field.
  var_data_ = Instance::Cast(controller_).GetField(var_data_field);
  ASSERT(var_data_.GetClassId() == controller_subscription_class.id());

  // _ControllerSubscription<T>/_BufferingStreamSubscription.<T>_onData
  callback_ = Instance::Cast(var_data_).GetField(on_data_field);
  ASSERT(callback_.IsClosure());

  // If this is not the "_StreamIterator._onData" tear-off, we return the
  // callback we found.
  receiver_function_ = Closure::Cast(callback_).function();
  if (!receiver_function_.IsImplicitInstanceClosureFunction() ||
      receiver_function_.Owner() != stream_iterator_class.ptr()) {
    return Closure::Cast(callback_).ptr();
  }

  // All implicit closure functions (tear-offs) have the "this" receiver
  // captured.
  receiver_context_ = Closure::Cast(callback_).context();
  ASSERT(receiver_context_.num_variables() == 1);
  callback_instance_ = receiver_context_.At(0);
  ASSERT(callback_instance_.IsInstance());

  // If the async* stream is await-for'd:
  if (callback_instance_.GetClassId() == stream_iterator_class.id()) {
    // _StreamIterator._stateData
    future_ = Instance::Cast(callback_instance_).GetField(state_data_field);
    return GetCallerInFutureImpl(future_);
  }

  UNREACHABLE();  // If no onData is found we have a bug.
}

ClosurePtr CallerClosureFinder::GetCallerInFutureListener(
    const Object& future_listener) {
  closure_ = GetCallerInFutureListenerInternal(future_listener);
  return UnwrapAsyncThen(closure_);
}

ClosurePtr CallerClosureFinder::GetCallerInFutureListenerInternal(
    const Object& future_listener) {
  auto value = GetFutureListenerState(future_listener);

  // If the _FutureListener is a `then`, `catchError`, or `whenComplete`
  // listener, follow the Future being completed, `result`, instead of the
  // dangling whenComplete `callback`.
  if (value == k_FutureListener_stateThen ||
      value == k_FutureListener_stateCatchError ||
      value == k_FutureListener_stateWhenComplete) {
    future_ = GetFutureListenerResult(future_listener);
    return GetCallerInFutureImpl(future_);
  }

  // If no chained futures, fall back on _FutureListener.callback.
  return GetFutureListenerCallback(future_listener);
}

ClosurePtr CallerClosureFinder::FindCaller(const Closure& receiver_closure) {
  closure_ = FindCallerInternal(receiver_closure);
  return UnwrapAsyncThen(closure_);
}

ClosurePtr CallerClosureFinder::UnwrapAsyncThen(const Closure& closure) {
  if (closure.IsNull()) return closure.ptr();

  receiver_function_ = closure.function();
  receiver_function_ = receiver_function_.parent_function();
  if (receiver_function_.recognized_kind() ==
      MethodRecognizer::kAsyncThenWrapperHelper) {
    receiver_context_ = closure.context();
    RELEASE_ASSERT(receiver_context_.num_variables() == 1);
    return Closure::RawCast(receiver_context_.At(0));
  }
  return closure.ptr();
}

ClosurePtr CallerClosureFinder::FindCallerInternal(
    const Closure& receiver_closure) {
  receiver_function_ = receiver_closure.function();
  receiver_context_ = receiver_closure.context();

  if (receiver_function_.IsAsyncGenClosure()) {
    return FindCallerInAsyncGenClosure(receiver_context_);
  }

  if (receiver_function_.IsAsyncClosure()) {
    future_ = receiver_context_.At(Context::kAsyncFutureIndex);
    return GetCallerInFutureImpl(future_);
  }

  if (receiver_function_.HasParent()) {
    parent_function_ = receiver_function_.parent_function();
    if (parent_function_.recognized_kind() ==
        MethodRecognizer::kFutureTimeout) {
      ASSERT(IsolateGroup::Current()
                 ->object_store()
                 ->future_timeout_future_index() != Object::null());
      const intptr_t future_index =
          Smi::Value(IsolateGroup::Current()
                         ->object_store()
                         ->future_timeout_future_index());
      context_entry_ = receiver_context_.At(future_index);
      return GetCallerInFutureImpl(context_entry_);
    }

    if (parent_function_.recognized_kind() == MethodRecognizer::kFutureWait) {
      receiver_context_ = receiver_context_.parent();
      ASSERT(!receiver_context_.IsNull());
      ASSERT(
          IsolateGroup::Current()->object_store()->future_wait_future_index() !=
          Object::null());
      const intptr_t future_index = Smi::Value(
          IsolateGroup::Current()->object_store()->future_wait_future_index());
      context_entry_ = receiver_context_.At(future_index);
      return GetCallerInFutureImpl(context_entry_);
    }
  }

  return Closure::null();
}

ObjectPtr CallerClosureFinder::GetAsyncFuture(const Closure& receiver_closure) {
  // Closure -> Context -> _Future.
  receiver_context_ = receiver_closure.context();
  return receiver_context_.At(Context::kAsyncFutureIndex);
}

ObjectPtr CallerClosureFinder::GetFutureFutureListener(const Object& future) {
  ASSERT(future.GetClassId() == future_impl_class.id());
  auto& listener = Object::Handle(
      Instance::Cast(future).GetField(future_result_or_listeners_field));
  // This field can either hold a _FutureListener, Future, or the Future result.
  if (listener.GetClassId() != future_listener_class.id()) {
    return Closure::null();
  }
  return listener.ptr();
}

intptr_t CallerClosureFinder::GetFutureListenerState(
    const Object& future_listener) {
  ASSERT(future_listener.GetClassId() == future_listener_class.id());
  state_ =
      Instance::Cast(future_listener).GetField(future_listener_state_field);
  return Smi::Cast(state_).Value();
}

ClosurePtr CallerClosureFinder::GetFutureListenerCallback(
    const Object& future_listener) {
  ASSERT(future_listener.GetClassId() == future_listener_class.id());
  return Closure::RawCast(
      Instance::Cast(future_listener).GetField(callback_field));
}

ObjectPtr CallerClosureFinder::GetFutureListenerResult(
    const Object& future_listener) {
  ASSERT(future_listener.GetClassId() == future_listener_class.id());
  return Instance::Cast(future_listener).GetField(future_listener_result_field);
}

bool CallerClosureFinder::HasCatchError(const Object& future_listener) {
  ASSERT(future_listener.GetClassId() == future_listener_class.id());
  listener_ = future_listener.ptr();
  Object& result = Object::Handle();
  // Iterate through any `.then()` chain.
  while (!listener_.IsNull()) {
    if (GetFutureListenerState(listener_) == k_FutureListener_stateCatchError) {
      return true;
    }
    result = GetFutureListenerResult(listener_);
    RELEASE_ASSERT(!result.IsNull());
    listener_ = GetFutureFutureListener(result);
  }
  return false;
}

bool CallerClosureFinder::IsRunningAsync(const Closure& receiver_closure) {
  auto zone = Thread::Current()->zone();

  // The async* functions are never started synchronously, they start running
  // after the first `listen()` call to its returned `Stream`.
  const Function& receiver_function_ =
      Function::Handle(zone, receiver_closure.function());
  if (receiver_function_.IsAsyncGenClosure()) {
    return true;
  }
  ASSERT(receiver_function_.IsAsyncClosure());

  const Context& receiver_context_ =
      Context::Handle(zone, receiver_closure.context());
  const Object& is_sync =
      Object::Handle(zone, receiver_context_.At(Context::kIsSyncIndex));
  ASSERT(!is_sync.IsNull());
  ASSERT(is_sync.IsBool());
  // isSync indicates whether the future should be completed async. or sync.,
  // based on whether it has yielded yet.
  // isSync is true when the :async_op has yielded at least once.
  // I.e. isSync will be false even after :async_op has run, if e.g. it threw
  // an exception before yielding.
  return Bool::Cast(is_sync).value();
}

ClosurePtr StackTraceUtils::FindClosureInFrame(ObjectPtr* last_object_in_caller,
                                               const Function& function) {
  NoSafepointScope nsp;

  ASSERT(!function.IsNull());
  ASSERT(function.IsAsyncClosure() || function.IsAsyncGenClosure());

  // The callee has function signature
  //   :async_op(result_or_exception, stack)
  // so the "this" closure is the 3rd argument.
  ObjectPtr arg = last_object_in_caller[2];
  if (arg->IsHeapObject() && arg->GetClassId() == kClosureCid) {
    auto& closure = Closure::Handle();
    closure = Closure::RawCast(arg);
    if (closure.function() == function.ptr()) {
      return closure.ptr();
    }
  }
  ASSERT(arg == Symbols::OptimizedOut().ptr());
  return Closure::null();
}

ClosurePtr StackTraceUtils::ClosureFromFrameFunction(
    Zone* zone,
    CallerClosureFinder* caller_closure_finder,
    const DartFrameIterator& frames,
    StackFrame* frame,
    bool* skip_frame,
    bool* is_async) {
  auto& closure = Closure::Handle(zone);
  auto& function = Function::Handle(zone);

  function = frame->LookupDartFunction();
  if (function.IsNull()) {
    return Closure::null();
  }

  if (function.IsAsyncClosure() || function.IsAsyncGenClosure()) {
    // Next, look up caller's closure on the stack and walk backwards
    // through the yields.
    ObjectPtr* last_caller_obj =
        reinterpret_cast<ObjectPtr*>(frame->GetCallerSp());
    closure = FindClosureInFrame(last_caller_obj, function);
    if (closure.IsNull()) return Closure::null();

    // If this async function hasn't yielded yet, we're still dealing with a
    // normal stack. Continue to next frame as usual.
    if (!caller_closure_finder->IsRunningAsync(closure)) {
      return Closure::null();
    }

    *is_async = true;

    // Skip: Already handled this as a sync. frame.
    return caller_closure_finder->FindCaller(closure);
  }

  // May have been called from `_FutureListener.handleValue`, which means its
  // receiver holds the Future chain.
  DartFrameIterator future_frames(frames);
  if (function.recognized_kind() == MethodRecognizer::kRootZoneRunUnary) {
    frame = future_frames.NextFrame();
    function = frame->LookupDartFunction();
    if (function.recognized_kind() !=
        MethodRecognizer::kFutureListenerHandleValue) {
      return Closure::null();
    }
  }
  if (function.recognized_kind() ==
      MethodRecognizer::kFutureListenerHandleValue) {
    *is_async = true;
    *skip_frame = true;

    // The _FutureListener receiver is at the top of the previous frame, right
    // before the arguments to the call.
    Object& receiver =
        Object::Handle(*(reinterpret_cast<ObjectPtr*>(frame->GetCallerSp()) +
                         kNumArgsFutureListenerHandleValue));
    if (receiver.ptr() == Symbols::OptimizedOut().ptr()) {
      // In the very rare case that _FutureListener.handleValue has deoptimized
      // it may override the receiver slot in the caller frame with "<optimized
      // out>" due to the `this` no longer being needed.
      return Closure::null();
    }

    return caller_closure_finder->GetCallerInFutureListener(receiver);
  }

  return Closure::null();
}

void StackTraceUtils::UnwindAwaiterChain(
    Zone* zone,
    const GrowableObjectArray& code_array,
    GrowableArray<uword>* pc_offset_array,
    CallerClosureFinder* caller_closure_finder,
    const Closure& leaf_closure) {
  auto& code = Code::Handle(zone);
  auto& function = Function::Handle(zone);
  auto& closure = Closure::Handle(zone, leaf_closure.ptr());
  auto& pc_descs = PcDescriptors::Handle(zone);

  // Inject async suspension marker.
  code_array.Add(StubCode::AsynchronousGapMarker());
  pc_offset_array->Add(0);

  // Traverse the trail of async futures all the way up.
  for (; !closure.IsNull();
       closure = caller_closure_finder->FindCaller(closure)) {
    function = closure.function();
    if (function.IsNull()) {
      continue;
    }
    // In hot-reload-test-mode we sometimes have to do this:
    code = function.EnsureHasCode();
    RELEASE_ASSERT(!code.IsNull());
    code_array.Add(code);
    pc_descs = code.pc_descriptors();
    const intptr_t pc_offset = FindPcOffset(pc_descs, GetYieldIndex(closure));
    // Unlike other sources of PC offsets, the offset may be 0 here if we
    // reach a non-async closure receiving the yielded value.
    ASSERT(pc_offset >= 0);
    pc_offset_array->Add(pc_offset);

    // Inject async suspension marker.
    code_array.Add(StubCode::AsynchronousGapMarker());
    pc_offset_array->Add(0);
  }
}

void StackTraceUtils::CollectFramesLazy(
    Thread* thread,
    const GrowableObjectArray& code_array,
    GrowableArray<uword>* pc_offset_array,
    int skip_frames,
    std::function<void(StackFrame*)>* on_sync_frames,
    bool* has_async) {
  if (has_async != nullptr) {
    *has_async = false;
  }
  Zone* zone = thread->zone();
  DartFrameIterator frames(thread, StackFrameIterator::kNoCrossThreadIteration);
  StackFrame* frame = frames.NextFrame();

  // If e.g. the isolate is paused before executing anything, we might not get
  // any frames at all. Bail:
  if (frame == nullptr) {
    return;
  }

  auto& code = Code::Handle(zone);
  auto& closure = Closure::Handle(zone);

  CallerClosureFinder caller_closure_finder(zone);

  // Start by traversing the sync. part of the stack.
  for (; frame != nullptr; frame = frames.NextFrame()) {
    if (skip_frames > 0) {
      skip_frames--;
      continue;
    }

    // If we encounter a known part of the async/Future mechanism, unwind the
    // awaiter chain from the closures.
    bool skip_frame = false;
    bool is_async = false;
    closure = ClosureFromFrameFunction(zone, &caller_closure_finder, frames,
                                       frame, &skip_frame, &is_async);

    // This isn't a special (async) frame we should skip.
    if (!skip_frame) {
      // Add the current synchronous frame.
      code = frame->LookupDartCode();
      code_array.Add(code);
      const uword pc_offset = frame->pc() - code.PayloadStart();
      ASSERT(pc_offset > 0 && pc_offset <= code.Size());
      pc_offset_array->Add(pc_offset);
      // Callback for sync frame.
      if (on_sync_frames != nullptr) {
        (*on_sync_frames)(frame);
      }
    }

    // This frame is running async.
    // Note: The closure might still be null in case it's an unawaited future.
    if (is_async) {
      UnwindAwaiterChain(zone, code_array, pc_offset_array,
                         &caller_closure_finder, closure);
      if (has_async != nullptr) {
        *has_async = true;
      }
      // Ignore the rest of the stack; already unwound all async calls.
      return;
    }
  }

  return;
}

intptr_t StackTraceUtils::CountFrames(Thread* thread,
                                      int skip_frames,
                                      const Function& async_function,
                                      bool* sync_async_end) {
  Zone* zone = thread->zone();
  intptr_t frame_count = 0;
  DartFrameIterator frames(thread, StackFrameIterator::kNoCrossThreadIteration);
  StackFrame* frame = frames.NextFrame();
  ASSERT(frame != nullptr);  // We expect to find a dart invocation frame.
  Function& function = Function::Handle(zone);
  Code& code = Code::Handle(zone);
  Closure& closure = Closure::Handle(zone);
  const bool async_function_is_null = async_function.IsNull();

  ASSERT(async_function_is_null || sync_async_end != nullptr);

  for (; frame != nullptr; frame = frames.NextFrame()) {
    if (skip_frames > 0) {
      skip_frames--;
      continue;
    }
    code = frame->LookupDartCode();
    function = code.function();

    frame_count++;

    const bool function_is_null = function.IsNull();

    if (!async_function_is_null && !function_is_null &&
        function.parent_function() != Function::null()) {
      if (async_function.ptr() == function.parent_function()) {
        if (function.IsAsyncClosure() || function.IsAsyncGenClosure()) {
          ObjectPtr* last_caller_obj =
              reinterpret_cast<ObjectPtr*>(frame->GetCallerSp());
          closure = FindClosureInFrame(last_caller_obj, function);
          if (!closure.IsNull() &&
              CallerClosureFinder::IsRunningAsync(closure)) {
            *sync_async_end = false;
            return frame_count;
          }
        }
        break;
      }
    }
  }

  if (!async_function_is_null) {
    *sync_async_end = true;
  }

  return frame_count;
}

intptr_t StackTraceUtils::CollectFrames(Thread* thread,
                                        const Array& code_array,
                                        const TypedData& pc_offset_array,
                                        intptr_t array_offset,
                                        intptr_t count,
                                        int skip_frames) {
  Zone* zone = thread->zone();
  DartFrameIterator frames(thread, StackFrameIterator::kNoCrossThreadIteration);
  StackFrame* frame = frames.NextFrame();
  ASSERT(frame != NULL);  // We expect to find a dart invocation frame.
  Code& code = Code::Handle(zone);
  intptr_t collected_frames_count = 0;
  for (; (frame != NULL) && (collected_frames_count < count);
       frame = frames.NextFrame()) {
    if (skip_frames > 0) {
      skip_frames--;
      continue;
    }
    code = frame->LookupDartCode();
    const intptr_t pc_offset = frame->pc() - code.PayloadStart();
    code_array.SetAt(array_offset, code);
    pc_offset_array.SetUintPtr(array_offset * kWordSize, pc_offset);
    array_offset++;
    collected_frames_count++;
  }
  return collected_frames_count;
}

}  // namespace dart