[debug] implement intuitive semantics for stepping over await call.

R=*bmeurer@chromium.org, caitpotter88@gmail.com, *littledan@chromium.org, *ulan@chromium.org
BUG=v8:4483

Review-Url: https://codereview.chromium.org/2033223003
Cr-Commit-Position: refs/heads/master@{#36718}

src/arm/builtins-arm.cc

@@ -720,21 +720,23 @@ void Builtins::Generate_ResumeGeneratorTrampoline(MacroAssembler* masm) {
   __ ldr(r4, FieldMemOperand(r1, JSGeneratorObject::kFunctionOffset));
 
   // Flood function if we are stepping.
-  Label skip_flooding;
+  Label prepare_step_in_if_stepping, prepare_step_in_suspended_generator;
+  Label stepping_prepared;
   ExternalReference step_in_enabled =
       ExternalReference::debug_step_in_enabled_address(masm->isolate());
   __ mov(ip, Operand(step_in_enabled));
   __ ldrb(ip, MemOperand(ip));
   __ cmp(ip, Operand(0));
-  __ b(eq, &skip_flooding);
-  {
-    FrameAndConstantPoolScope scope(masm, StackFrame::INTERNAL);
-    __ Push(r1, r2, r4);
-    __ CallRuntime(Runtime::kDebugPrepareStepInIfStepping);
-    __ Pop(r1, r2);
-    __ ldr(r4, FieldMemOperand(r1, JSGeneratorObject::kFunctionOffset));
-  }
-  __ bind(&skip_flooding);
+  __ b(ne, &prepare_step_in_if_stepping);
+
+  // Flood function if we need to continue stepping in the suspended generator.
+  ExternalReference debug_suspended_generator =
+      ExternalReference::debug_suspended_generator_address(masm->isolate());
+  __ mov(ip, Operand(debug_suspended_generator));
+  __ ldr(ip, MemOperand(ip));
+  __ cmp(ip, Operand(r1));
+  __ b(eq, &prepare_step_in_suspended_generator);
+  __ bind(&stepping_prepared);
 
   // Push receiver.
   __ ldr(ip, FieldMemOperand(r1, JSGeneratorObject::kReceiverOffset));
@@ -830,6 +832,26 @@ void Builtins::Generate_ResumeGeneratorTrampoline(MacroAssembler* masm) {
     __ Move(r0, r1);  // Continuation expects generator object in r0.
     __ Jump(r3);
   }
+
+  __ bind(&prepare_step_in_if_stepping);
+  {
+    FrameAndConstantPoolScope scope(masm, StackFrame::INTERNAL);
+    __ Push(r1, r2, r4);
+    __ CallRuntime(Runtime::kDebugPrepareStepInIfStepping);
+    __ Pop(r1, r2);
+    __ ldr(r4, FieldMemOperand(r1, JSGeneratorObject::kFunctionOffset));
+  }
+  __ b(&stepping_prepared);
+
+  __ bind(&prepare_step_in_suspended_generator);
+  {
+    FrameAndConstantPoolScope scope(masm, StackFrame::INTERNAL);
+    __ Push(r1, r2);
+    __ CallRuntime(Runtime::kDebugPrepareStepInSuspendedGenerator);
+    __ Pop(r1, r2);
+    __ ldr(r4, FieldMemOperand(r1, JSGeneratorObject::kFunctionOffset));
+  }
+  __ b(&stepping_prepared);
 }
 
 void Builtins::Generate_ConstructedNonConstructable(MacroAssembler* masm) {

src/arm64/builtins-arm64.cc

@@ -727,20 +727,22 @@ void Builtins::Generate_ResumeGeneratorTrampoline(MacroAssembler* masm) {
   __ Ldr(x4, FieldMemOperand(x1, JSGeneratorObject::kFunctionOffset));
 
   // Flood function if we are stepping.
-  Label skip_flooding;
+  Label prepare_step_in_if_stepping, prepare_step_in_suspended_generator;
+  Label stepping_prepared;
   ExternalReference step_in_enabled =
       ExternalReference::debug_step_in_enabled_address(masm->isolate());
   __ Mov(x10, Operand(step_in_enabled));
   __ Ldrb(x10, MemOperand(x10));
-  __ CompareAndBranch(x10, Operand(0), eq, &skip_flooding);
-  {
-    FrameScope scope(masm, StackFrame::INTERNAL);
-    __ Push(x1, x2, x4);
-    __ CallRuntime(Runtime::kDebugPrepareStepInIfStepping);
-    __ Pop(x2, x1);
-    __ Ldr(x4, FieldMemOperand(x1, JSGeneratorObject::kFunctionOffset));
-  }
-  __ bind(&skip_flooding);
+  __ CompareAndBranch(x10, Operand(0), ne, &prepare_step_in_if_stepping);
+
+  // Flood function if we need to continue stepping in the suspended generator.
+  ExternalReference debug_suspended_generator =
+      ExternalReference::debug_suspended_generator_address(masm->isolate());
+  __ Mov(x10, Operand(debug_suspended_generator));
+  __ Ldr(x10, MemOperand(x10));
+  __ CompareAndBranch(x10, Operand(x1), eq,
+                      &prepare_step_in_suspended_generator);
+  __ Bind(&stepping_prepared);
 
   // Push receiver.
   __ Ldr(x5, FieldMemOperand(x1, JSGeneratorObject::kReceiverOffset));
@@ -828,6 +830,26 @@ void Builtins::Generate_ResumeGeneratorTrampoline(MacroAssembler* masm) {
     __ Move(x0, x1);  // Continuation expects generator object in x0.
     __ Br(x10);
   }
+
+  __ Bind(&prepare_step_in_if_stepping);
+  {
+    FrameScope scope(masm, StackFrame::INTERNAL);
+    __ Push(x1, x2, x4);
+    __ CallRuntime(Runtime::kDebugPrepareStepInIfStepping);
+    __ Pop(x2, x1);
+    __ Ldr(x4, FieldMemOperand(x1, JSGeneratorObject::kFunctionOffset));
+  }
+  __ B(&stepping_prepared);
+
+  __ Bind(&prepare_step_in_suspended_generator);
+  {
+    FrameScope scope(masm, StackFrame::INTERNAL);
+    __ Push(x1, x2);
+    __ CallRuntime(Runtime::kDebugPrepareStepInSuspendedGenerator);
+    __ Pop(x2, x1);
+    __ Ldr(x4, FieldMemOperand(x1, JSGeneratorObject::kFunctionOffset));
+  }
+  __ B(&stepping_prepared);
 }
 
 enum IsTagged { kArgcIsSmiTagged, kArgcIsUntaggedInt };

src/assembler.cc

@@ -1799,6 +1799,10 @@ ExternalReference ExternalReference::debug_step_in_enabled_address(
   return ExternalReference(isolate->debug()->step_in_enabled_address());
 }
 
+ExternalReference ExternalReference::debug_suspended_generator_address(
+    Isolate* isolate) {
+  return ExternalReference(isolate->debug()->suspended_generator_address());
+}
 
 ExternalReference ExternalReference::fixed_typed_array_base_data_offset() {
   return ExternalReference(reinterpret_cast<void*>(

src/assembler.h

@@ -1066,6 +1066,9 @@ class ExternalReference BASE_EMBEDDED {
   // Used to check if single stepping is enabled in generated code.
   static ExternalReference debug_step_in_enabled_address(Isolate* isolate);
 
+  // Used to check for suspended generator, used for stepping across await call.
+  static ExternalReference debug_suspended_generator_address(Isolate* isolate);
+
 #ifndef V8_INTERPRETED_REGEXP
   // C functions called from RegExp generated code.
 

src/debug/debug.cc

@@ -479,6 +479,7 @@ void Debug::ThreadInit() {
   thread_local_.target_fp_ = 0;
   thread_local_.step_in_enabled_ = false;
   thread_local_.return_value_ = Handle<Object>();
+  clear_suspended_generator();
   // TODO(isolates): frames_are_dropped_?
   base::NoBarrier_Store(&thread_local_.current_debug_scope_,
                         static_cast<base::AtomicWord>(0));
@@ -486,25 +487,24 @@ void Debug::ThreadInit() {
 
 
 char* Debug::ArchiveDebug(char* storage) {
-  char* to = storage;
-  MemCopy(to, reinterpret_cast<char*>(&thread_local_), sizeof(ThreadLocal));
+  // Simply reset state. Don't archive anything.
   ThreadInit();
   return storage + ArchiveSpacePerThread();
 }
 
 
 char* Debug::RestoreDebug(char* storage) {
-  char* from = storage;
-  MemCopy(reinterpret_cast<char*>(&thread_local_), from, sizeof(ThreadLocal));
+  // Simply reset state. Don't restore anything.
+  ThreadInit();
   return storage + ArchiveSpacePerThread();
 }
 
+int Debug::ArchiveSpacePerThread() { return 0; }
 
-int Debug::ArchiveSpacePerThread() {
-  return sizeof(ThreadLocal);
+void Debug::Iterate(ObjectVisitor* v) {
+  v->VisitPointer(&thread_local_.suspended_generator_);
 }
 
-
 DebugInfoListNode::DebugInfoListNode(DebugInfo* debug_info): next_(NULL) {
   // Globalize the request debug info object and make it weak.
   GlobalHandles* global_handles = debug_info->GetIsolate()->global_handles();
@@ -940,6 +940,17 @@ void Debug::PrepareStepIn(Handle<JSFunction> function) {
   }
 }
 
+void Debug::PrepareStepInSuspendedGenerator() {
+  if (!is_active()) return;
+  if (in_debug_scope()) return;
+  DCHECK(has_suspended_generator());
+  thread_local_.last_step_action_ = StepIn;
+  thread_local_.step_in_enabled_ = true;
+  Handle<JSFunction> function(
+      JSGeneratorObject::cast(thread_local_.suspended_generator_)->function());
+  FloodWithOneShot(function);
+  clear_suspended_generator();
+}
 
 void Debug::PrepareStepOnThrow() {
   if (!is_active()) return;
@@ -1041,6 +1052,8 @@ void Debug::PrepareStep(StepAction step_action) {
       debug_info->abstract_code()->SourceStatementPosition(
           summary.code_offset());
   thread_local_.last_fp_ = frame->UnpaddedFP();
+  // No longer perform the current async step.
+  clear_suspended_generator();
 
   switch (step_action) {
     case StepNone:
@@ -1385,6 +1398,13 @@ bool Debug::PrepareFunctionForBreakPoints(Handle<SharedFunctionInfo> shared) {
   return true;
 }
 
+void Debug::RecordAsyncFunction(Handle<JSGeneratorObject> generator_object) {
+  if (last_step_action() <= StepOut) return;
+  DCHECK(!has_suspended_generator());
+  DCHECK(generator_object->function()->shared()->is_async());
+  thread_local_.suspended_generator_ = *generator_object;
+  ClearStepping();
+}
 
 class SharedFunctionInfoFinder {
  public:

src/debug/debug.h

@@ -456,6 +456,7 @@ class Debug {
   // Stepping handling.
   void PrepareStep(StepAction step_action);
   void PrepareStepIn(Handle<JSFunction> function);
+  void PrepareStepInSuspendedGenerator();
   void PrepareStepOnThrow();
   void ClearStepping();
   void ClearStepOut();
@@ -463,6 +464,8 @@ class Debug {
 
   bool PrepareFunctionForBreakPoints(Handle<SharedFunctionInfo> shared);
 
+  void RecordAsyncFunction(Handle<JSGeneratorObject> generator_object);
+
   // Returns whether the operation succeeded. Compilation can only be triggered
   // if a valid closure is passed as the second argument, otherwise the shared
   // function needs to be compiled already.
@@ -497,6 +500,7 @@ class Debug {
   char* RestoreDebug(char* from);
   static int ArchiveSpacePerThread();
   void FreeThreadResources() { }
+  void Iterate(ObjectVisitor* v);
 
   bool CheckExecutionState(int id) {
     return is_active() && !debug_context().is_null() && break_id() != 0 &&
@@ -544,6 +548,10 @@ class Debug {
     return reinterpret_cast<Address>(&thread_local_.step_in_enabled_);
   }
 
+  Address suspended_generator_address() {
+    return reinterpret_cast<Address>(&thread_local_.suspended_generator_);
+  }
+
   StepAction last_step_action() { return thread_local_.last_step_action_; }
 
   DebugFeatureTracker* feature_tracker() { return &feature_tracker_; }
@@ -564,6 +572,14 @@ class Debug {
     return break_disabled_ || in_debug_event_listener_;
   }
 
+  void clear_suspended_generator() {
+    thread_local_.suspended_generator_ = Smi::FromInt(0);
+  }
+
+  bool has_suspended_generator() const {
+    return thread_local_.suspended_generator_ != Smi::FromInt(0);
+  }
+
   void OnException(Handle<Object> exception, Handle<Object> promise);
 
   // Constructors for debug event objects.
@@ -687,6 +703,8 @@ class Debug {
     // Value of accumulator in interpreter frames. In non-interpreter frames
     // this value will be the hole.
     Handle<Object> return_value_;
+
+    Object* suspended_generator_;
   };
 
   // Storage location for registers when handling debug break calls

src/external-reference-table.cc

@@ -189,6 +189,8 @@ ExternalReferenceTable::ExternalReferenceTable(Isolate* isolate) {
       "Debug::is_active_address()");
   Add(ExternalReference::debug_step_in_enabled_address(isolate).address(),
       "Debug::step_in_enabled_address()");
+  Add(ExternalReference::debug_suspended_generator_address(isolate).address(),
+      "Debug::step_suspended_generator_address()");
 
 #ifndef V8_INTERPRETED_REGEXP
   Add(ExternalReference::re_case_insensitive_compare_uc16(isolate).address(),

src/heap/heap.cc

@@ -4820,6 +4820,8 @@ void Heap::IterateStrongRoots(ObjectVisitor* v, VisitMode mode) {
   v->Synchronize(VisitorSynchronization::kTop);
   Relocatable::Iterate(isolate_, v);
   v->Synchronize(VisitorSynchronization::kRelocatable);
+  isolate_->debug()->Iterate(v);
+  v->Synchronize(VisitorSynchronization::kDebug);
 
   isolate_->compilation_cache()->Iterate(v);
   v->Synchronize(VisitorSynchronization::kCompilationCache);

src/ia32/builtins-ia32.cc

@@ -407,22 +407,19 @@ void Builtins::Generate_ResumeGeneratorTrampoline(MacroAssembler* masm) {
   __ mov(edi, FieldOperand(ebx, JSGeneratorObject::kFunctionOffset));
 
   // Flood function if we are stepping.
-  Label skip_flooding;
+  Label prepare_step_in_if_stepping, prepare_step_in_suspended_generator;
+  Label stepping_prepared;
   ExternalReference step_in_enabled =
       ExternalReference::debug_step_in_enabled_address(masm->isolate());
   __ cmpb(Operand::StaticVariable(step_in_enabled), Immediate(0));
-  __ j(equal, &skip_flooding);
-  {
-    FrameScope scope(masm, StackFrame::INTERNAL);
-    __ Push(ebx);
-    __ Push(edx);
-    __ Push(edi);
-    __ CallRuntime(Runtime::kDebugPrepareStepInIfStepping);
-    __ Pop(edx);
-    __ Pop(ebx);
-    __ mov(edi, FieldOperand(ebx, JSGeneratorObject::kFunctionOffset));
-  }
-  __ bind(&skip_flooding);
+  __ j(not_equal, &prepare_step_in_if_stepping);
+
+  // Flood function if we need to continue stepping in the suspended generator.
+  ExternalReference debug_suspended_generator =
+      ExternalReference::debug_suspended_generator_address(masm->isolate());
+  __ cmp(ebx, Operand::StaticVariable(debug_suspended_generator));
+  __ j(equal, &prepare_step_in_suspended_generator);
+  __ bind(&stepping_prepared);
 
   // Pop return address.
   __ PopReturnAddressTo(eax);
@@ -518,6 +515,31 @@ void Builtins::Generate_ResumeGeneratorTrampoline(MacroAssembler* masm) {
     __ mov(eax, ebx);  // Continuation expects generator object in eax.
     __ jmp(edx);
   }
+
+  __ bind(&prepare_step_in_if_stepping);
+  {
+    FrameScope scope(masm, StackFrame::INTERNAL);
+    __ Push(ebx);
+    __ Push(edx);
+    __ Push(edi);
+    __ CallRuntime(Runtime::kDebugPrepareStepInIfStepping);
+    __ Pop(edx);
+    __ Pop(ebx);
+    __ mov(edi, FieldOperand(ebx, JSGeneratorObject::kFunctionOffset));
+  }
+  __ jmp(&stepping_prepared);
+
+  __ bind(&prepare_step_in_suspended_generator);
+  {
+    FrameScope scope(masm, StackFrame::INTERNAL);
+    __ Push(ebx);
+    __ Push(edx);
+    __ CallRuntime(Runtime::kDebugPrepareStepInSuspendedGenerator);
+    __ Pop(edx);
+    __ Pop(ebx);
+    __ mov(edi, FieldOperand(ebx, JSGeneratorObject::kFunctionOffset));
+  }
+  __ jmp(&stepping_prepared);
 }
 
 static void LeaveInterpreterFrame(MacroAssembler* masm, Register scratch1,

src/mips/builtins-mips.cc

@@ -841,20 +841,21 @@ void Builtins::Generate_ResumeGeneratorTrampoline(MacroAssembler* masm) {
   __ lw(t0, FieldMemOperand(a1, JSGeneratorObject::kFunctionOffset));
 
   // Flood function if we are stepping.
-  Label skip_flooding;
+  Label prepare_step_in_if_stepping, prepare_step_in_suspended_generator;
+  Label stepping_prepared;
   ExternalReference step_in_enabled =
       ExternalReference::debug_step_in_enabled_address(masm->isolate());
   __ li(t1, Operand(step_in_enabled));
   __ lb(t1, MemOperand(t1));
-  __ Branch(&skip_flooding, eq, t1, Operand(zero_reg));
-  {
-    FrameScope scope(masm, StackFrame::INTERNAL);
-    __ Push(a1, a2, t0);
-    __ CallRuntime(Runtime::kDebugPrepareStepInIfStepping);
-    __ Pop(a1, a2);
-    __ lw(t0, FieldMemOperand(a1, JSGeneratorObject::kFunctionOffset));
-  }
-  __ bind(&skip_flooding);
+  __ Branch(&prepare_step_in_if_stepping, ne, t1, Operand(zero_reg));
+
+  // Flood function if we need to continue stepping in the suspended generator.
+  ExternalReference debug_suspended_generator =
+      ExternalReference::debug_suspended_generator_address(masm->isolate());
+  __ li(t1, Operand(debug_suspended_generator));
+  __ lw(t1, MemOperand(t1));
+  __ Branch(&prepare_step_in_suspended_generator, eq, t1, Operand(zero_reg));
+  __ bind(&stepping_prepared);
 
   // Push receiver.
   __ lw(t1, FieldMemOperand(a1, JSGeneratorObject::kReceiverOffset));
@@ -950,6 +951,26 @@ void Builtins::Generate_ResumeGeneratorTrampoline(MacroAssembler* masm) {
     __ Move(v0, a1);  // Continuation expects generator object in v0.
     __ Jump(a3);
   }
+
+  __ bind(&prepare_step_in_if_stepping);
+  {
+    FrameScope scope(masm, StackFrame::INTERNAL);
+    __ Push(a1, a2, t0);
+    __ CallRuntime(Runtime::kDebugPrepareStepInIfStepping);
+    __ Pop(a1, a2);
+  }
+  __ Branch(USE_DELAY_SLOT, &stepping_prepared);
+  __ lw(t0, FieldMemOperand(a1, JSGeneratorObject::kFunctionOffset));
+
+  __ bind(&prepare_step_in_suspended_generator);
+  {
+    FrameScope scope(masm, StackFrame::INTERNAL);
+    __ Push(a1, a2);
+    __ CallRuntime(Runtime::kDebugPrepareStepInSuspendedGenerator);
+    __ Pop(a1, a2);
+  }
+  __ Branch(USE_DELAY_SLOT, &stepping_prepared);
+  __ lw(t0, FieldMemOperand(a1, JSGeneratorObject::kFunctionOffset));
 }
 
 static void LeaveInterpreterFrame(MacroAssembler* masm, Register scratch) {

src/mips64/builtins-mips64.cc

@@ -706,20 +706,21 @@ void Builtins::Generate_ResumeGeneratorTrampoline(MacroAssembler* masm) {
   __ ld(a4, FieldMemOperand(a1, JSGeneratorObject::kFunctionOffset));
 
   // Flood function if we are stepping.
-  Label skip_flooding;
+  Label prepare_step_in_if_stepping, prepare_step_in_suspended_generator;
+  Label stepping_prepared;
   ExternalReference step_in_enabled =
       ExternalReference::debug_step_in_enabled_address(masm->isolate());
   __ li(t1, Operand(step_in_enabled));
   __ lb(t1, MemOperand(t1));
-  __ Branch(&skip_flooding, eq, t1, Operand(zero_reg));
-  {
-    FrameScope scope(masm, StackFrame::INTERNAL);
-    __ Push(a1, a2, a4);
-    __ CallRuntime(Runtime::kDebugPrepareStepInIfStepping);
-    __ Pop(a1, a2);
-    __ ld(a4, FieldMemOperand(a1, JSGeneratorObject::kFunctionOffset));
-  }
-  __ bind(&skip_flooding);
+  __ Branch(&prepare_step_in_if_stepping, ne, t1, Operand(zero_reg));
+
+  // Flood function if we need to continue stepping in the suspended generator.
+  ExternalReference debug_suspended_generator =
+      ExternalReference::debug_suspended_generator_address(masm->isolate());
+  __ li(t1, Operand(debug_suspended_generator));
+  __ lw(t1, MemOperand(t1));
+  __ Branch(&prepare_step_in_suspended_generator, eq, t1, Operand(zero_reg));
+  __ bind(&stepping_prepared);
 
   // Push receiver.
   __ ld(a5, FieldMemOperand(a1, JSGeneratorObject::kReceiverOffset));
@@ -815,6 +816,26 @@ void Builtins::Generate_ResumeGeneratorTrampoline(MacroAssembler* masm) {
     __ Move(v0, a1);  // Continuation expects generator object in v0.
     __ Jump(a3);
   }
+
+  __ bind(&prepare_step_in_if_stepping);
+  {
+    FrameScope scope(masm, StackFrame::INTERNAL);
+    __ Push(a1, a2, a4);
+    __ CallRuntime(Runtime::kDebugPrepareStepInIfStepping);
+    __ Pop(a1, a2);
+  }
+  __ Branch(USE_DELAY_SLOT, &stepping_prepared);
+  __ ld(a4, FieldMemOperand(a1, JSGeneratorObject::kFunctionOffset));
+
+  __ bind(&prepare_step_in_suspended_generator);
+  {
+    FrameScope scope(masm, StackFrame::INTERNAL);
+    __ Push(a1, a2);
+    __ CallRuntime(Runtime::kDebugPrepareStepInSuspendedGenerator);
+    __ Pop(a1, a2);
+  }
+  __ Branch(USE_DELAY_SLOT, &stepping_prepared);
+  __ ld(a4, FieldMemOperand(a1, JSGeneratorObject::kFunctionOffset));
 }
 
 void Builtins::Generate_ConstructedNonConstructable(MacroAssembler* masm) {

src/parsing/parser-base.h

@@ -2670,6 +2670,8 @@ ParserBase<Traits>::ParseUnaryExpression(ExpressionClassifier* classifier,
       default:
         break;
     }
+
+    int await_pos = peek_position();
     Consume(Token::AWAIT);
 
     ExpressionT value = ParseUnaryExpression(classifier, CHECK_OK);
@@ -2677,7 +2679,7 @@ ParserBase<Traits>::ParseUnaryExpression(ExpressionClassifier* classifier,
     classifier->RecordFormalParameterInitializerError(
         Scanner::Location(beg_pos, scanner()->location().end_pos),
         MessageTemplate::kAwaitExpressionFormalParameter);
-    return Traits::RewriteAwaitExpression(value, beg_pos);
+    return Traits::RewriteAwaitExpression(value, await_pos);
   } else {
     return this->ParsePostfixExpression(classifier, ok);
   }

src/parsing/parser.cc

@@ -5598,7 +5598,8 @@ void ParserTraits::RewriteNonPattern(Type::ExpressionClassifier* classifier,
   parser_->RewriteNonPattern(classifier, ok);
 }
 
-Expression* ParserTraits::RewriteAwaitExpression(Expression* value, int pos) {
+Expression* ParserTraits::RewriteAwaitExpression(Expression* value,
+                                                 int await_pos) {
   // yield %AsyncFunctionAwait(.generator_object, <operand>)
   Variable* generator_object_variable =
       parser_->function_state_->generator_object_variable();
@@ -5606,21 +5607,38 @@ Expression* ParserTraits::RewriteAwaitExpression(Expression* value, int pos) {
   // If generator_object_variable is null,
   if (!generator_object_variable) return value;
 
-  Expression* generator_object =
-      parser_->factory()->NewVariableProxy(generator_object_variable);
+  auto factory = parser_->factory();
+  const int nopos = RelocInfo::kNoPosition;
+
+  Variable* temp_var = parser_->scope_->NewTemporary(
+      parser_->ast_value_factory()->empty_string());
+  VariableProxy* temp_proxy = factory->NewVariableProxy(temp_var);
+  Block* do_block = factory->NewBlock(nullptr, 2, false, nopos);
+
+  // Wrap value evaluation to provide a break location.
+  Expression* value_assignment =
+      factory->NewAssignment(Token::ASSIGN, temp_proxy, value, nopos);
+  do_block->statements()->Add(
+      factory->NewExpressionStatement(value_assignment, value->position()),
+      zone());
 
   ZoneList<Expression*>* async_function_await_args =
       new (zone()) ZoneList<Expression*>(2, zone());
+  Expression* generator_object =
+      factory->NewVariableProxy(generator_object_variable);
   async_function_await_args->Add(generator_object, zone());
-  async_function_await_args->Add(value, zone());
+  async_function_await_args->Add(temp_proxy, zone());
   Expression* async_function_await = parser_->factory()->NewCallRuntime(
-      Context::ASYNC_FUNCTION_AWAIT_INDEX, async_function_await_args,
-      RelocInfo::kNoPosition);
+      Context::ASYNC_FUNCTION_AWAIT_INDEX, async_function_await_args, nopos);
+  // Wrap await to provide a break location between value evaluation and yield.
+  Expression* await_assignment = factory->NewAssignment(
+      Token::ASSIGN, temp_proxy, async_function_await, nopos);
+  do_block->statements()->Add(
+      factory->NewExpressionStatement(await_assignment, await_pos), zone());
+  Expression* do_expr = factory->NewDoExpression(do_block, temp_var, nopos);
 
-  generator_object =
-      parser_->factory()->NewVariableProxy(generator_object_variable);
-  return parser_->factory()->NewYield(generator_object, async_function_await,
-                                      pos);
+  generator_object = factory->NewVariableProxy(generator_object_variable);
+  return factory->NewYield(generator_object, do_expr, nopos);
 }
 
 Zone* ParserTraits::zone() const {

src/runtime/runtime-debug.cc

@@ -1698,6 +1698,13 @@ RUNTIME_FUNCTION(Runtime_DebugPrepareStepInIfStepping) {
   return isolate->heap()->undefined_value();
 }
 
+// Set one shot breakpoints for the suspended generator object.
+RUNTIME_FUNCTION(Runtime_DebugPrepareStepInSuspendedGenerator) {
+  HandleScope scope(isolate);
+  DCHECK_EQ(0, args.length());
+  isolate->debug()->PrepareStepInSuspendedGenerator();
+  return isolate->heap()->undefined_value();
+}
 
 RUNTIME_FUNCTION(Runtime_DebugPushPromise) {
   DCHECK(args.length() == 2);

src/runtime/runtime-generator.cc

@@ -5,6 +5,7 @@
 #include "src/runtime/runtime-utils.h"
 
 #include "src/arguments.h"
+#include "src/debug/debug.h"
 #include "src/factory.h"
 #include "src/frames-inl.h"
 #include "src/objects-inl.h"
@@ -51,6 +52,10 @@ RUNTIME_FUNCTION(Runtime_SuspendJSGeneratorObject) {
   DCHECK(frame->function()->shared()->is_compiled());
   DCHECK(!frame->function()->IsOptimized());
 
+  if (generator_object->function()->shared()->is_async()) {
+    isolate->debug()->RecordAsyncFunction(generator_object);
+  }
+
   // The caller should have saved the context and continuation already.
   DCHECK_EQ(generator_object->context(), Context::cast(frame->context()));
   DCHECK_LT(0, generator_object->continuation());

src/runtime/runtime.h

@@ -136,62 +136,63 @@ namespace internal {
   F(DateCurrentTime, 0, 1)         \
   F(ThrowNotDateError, 0, 1)
 
-#define FOR_EACH_INTRINSIC_DEBUG(F)            \
-  F(HandleDebuggerStatement, 0, 1)             \
-  F(DebugBreak, 1, 1)                          \
-  F(DebugBreakOnBytecode, 1, 1)                \
-  F(SetDebugEventListener, 2, 1)               \
-  F(ScheduleBreak, 0, 1)                       \
-  F(DebugGetInternalProperties, 1, 1)          \
-  F(DebugGetPropertyDetails, 2, 1)             \
-  F(DebugGetProperty, 2, 1)                    \
-  F(DebugPropertyTypeFromDetails, 1, 1)        \
-  F(DebugPropertyAttributesFromDetails, 1, 1)  \
-  F(CheckExecutionState, 1, 1)                 \
-  F(GetFrameCount, 1, 1)                       \
-  F(GetFrameDetails, 2, 1)                     \
-  F(GetScopeCount, 2, 1)                       \
-  F(GetScopeDetails, 4, 1)                     \
-  F(GetAllScopesDetails, 4, 1)                 \
-  F(GetFunctionScopeCount, 1, 1)               \
-  F(GetFunctionScopeDetails, 2, 1)             \
-  F(SetScopeVariableValue, 6, 1)               \
-  F(DebugPrintScopes, 0, 1)                    \
-  F(SetBreakPointsActive, 1, 1)                \
-  F(GetBreakLocations, 2, 1)                   \
-  F(SetFunctionBreakPoint, 3, 1)               \
-  F(SetScriptBreakPoint, 4, 1)                 \
-  F(ClearBreakPoint, 1, 1)                     \
-  F(ChangeBreakOnException, 2, 1)              \
-  F(IsBreakOnException, 1, 1)                  \
-  F(PrepareStep, 2, 1)                         \
-  F(ClearStepping, 0, 1)                       \
-  F(DebugEvaluate, 6, 1)                       \
-  F(DebugEvaluateGlobal, 4, 1)                 \
-  F(DebugGetLoadedScripts, 0, 1)               \
-  F(DebugReferencedBy, 3, 1)                   \
-  F(DebugConstructedBy, 2, 1)                  \
-  F(DebugGetPrototype, 1, 1)                   \
-  F(DebugSetScriptSource, 2, 1)                \
-  F(FunctionGetInferredName, 1, 1)             \
-  F(FunctionGetDebugName, 1, 1)                \
-  F(GetFunctionCodePositionFromSource, 2, 1)   \
-  F(ExecuteInDebugContext, 1, 1)               \
-  F(GetDebugContext, 0, 1)                     \
-  F(CollectGarbage, 1, 1)                      \
-  F(GetHeapUsage, 0, 1)                        \
-  F(GetScript, 1, 1)                           \
-  F(ScriptLineCount, 1, 1)                     \
-  F(ScriptLineStartPosition, 2, 1)             \
-  F(ScriptLineEndPosition, 2, 1)               \
-  F(ScriptLocationFromLine, 4, 1)              \
-  F(ScriptPositionInfo, 3, 1)                  \
-  F(ScriptSourceLine, 2, 1)                    \
-  F(DebugPrepareStepInIfStepping, 1, 1)        \
-  F(DebugPushPromise, 2, 1)                    \
-  F(DebugPopPromise, 0, 1)                     \
-  F(DebugAsyncTaskEvent, 1, 1)                 \
-  F(DebugIsActive, 0, 1)                       \
+#define FOR_EACH_INTRINSIC_DEBUG(F)             \
+  F(HandleDebuggerStatement, 0, 1)              \
+  F(DebugBreak, 1, 1)                           \
+  F(DebugBreakOnBytecode, 1, 1)                 \
+  F(SetDebugEventListener, 2, 1)                \
+  F(ScheduleBreak, 0, 1)                        \
+  F(DebugGetInternalProperties, 1, 1)           \
+  F(DebugGetPropertyDetails, 2, 1)              \
+  F(DebugGetProperty, 2, 1)                     \
+  F(DebugPropertyTypeFromDetails, 1, 1)         \
+  F(DebugPropertyAttributesFromDetails, 1, 1)   \
+  F(CheckExecutionState, 1, 1)                  \
+  F(GetFrameCount, 1, 1)                        \
+  F(GetFrameDetails, 2, 1)                      \
+  F(GetScopeCount, 2, 1)                        \
+  F(GetScopeDetails, 4, 1)                      \
+  F(GetAllScopesDetails, 4, 1)                  \
+  F(GetFunctionScopeCount, 1, 1)                \
+  F(GetFunctionScopeDetails, 2, 1)              \
+  F(SetScopeVariableValue, 6, 1)                \
+  F(DebugPrintScopes, 0, 1)                     \
+  F(SetBreakPointsActive, 1, 1)                 \
+  F(GetBreakLocations, 2, 1)                    \
+  F(SetFunctionBreakPoint, 3, 1)                \
+  F(SetScriptBreakPoint, 4, 1)                  \
+  F(ClearBreakPoint, 1, 1)                      \
+  F(ChangeBreakOnException, 2, 1)               \
+  F(IsBreakOnException, 1, 1)                   \
+  F(PrepareStep, 2, 1)                          \
+  F(ClearStepping, 0, 1)                        \
+  F(DebugEvaluate, 6, 1)                        \
+  F(DebugEvaluateGlobal, 4, 1)                  \
+  F(DebugGetLoadedScripts, 0, 1)                \
+  F(DebugReferencedBy, 3, 1)                    \
+  F(DebugConstructedBy, 2, 1)                   \
+  F(DebugGetPrototype, 1, 1)                    \
+  F(DebugSetScriptSource, 2, 1)                 \
+  F(FunctionGetInferredName, 1, 1)              \
+  F(FunctionGetDebugName, 1, 1)                 \
+  F(GetFunctionCodePositionFromSource, 2, 1)    \
+  F(ExecuteInDebugContext, 1, 1)                \
+  F(GetDebugContext, 0, 1)                      \
+  F(CollectGarbage, 1, 1)                       \
+  F(GetHeapUsage, 0, 1)                         \
+  F(GetScript, 1, 1)                            \
+  F(ScriptLineCount, 1, 1)                      \
+  F(ScriptLineStartPosition, 2, 1)              \
+  F(ScriptLineEndPosition, 2, 1)                \
+  F(ScriptLocationFromLine, 4, 1)               \
+  F(ScriptPositionInfo, 3, 1)                   \
+  F(ScriptSourceLine, 2, 1)                     \
+  F(DebugPrepareStepInIfStepping, 1, 1)         \
+  F(DebugPrepareStepInSuspendedGenerator, 0, 1) \
+  F(DebugPushPromise, 2, 1)                     \
+  F(DebugPopPromise, 0, 1)                      \
+  F(DebugAsyncTaskEvent, 1, 1)                  \
+  F(DebugIsActive, 0, 1)                        \
   F(DebugBreakInOptimizedCode, 0, 1)
 
 #define FOR_EACH_INTRINSIC_FORIN(F) \

src/x64/builtins-x64.cc

@@ -480,23 +480,22 @@ void Builtins::Generate_ResumeGeneratorTrampoline(MacroAssembler* masm) {
   __ movp(rdi, FieldOperand(rbx, JSGeneratorObject::kFunctionOffset));
 
   // Flood function if we are stepping.
-  Label skip_flooding;
+  Label prepare_step_in_if_stepping, prepare_step_in_suspended_generator;
+  Label stepping_prepared;
   ExternalReference step_in_enabled =
       ExternalReference::debug_step_in_enabled_address(masm->isolate());
   Operand step_in_enabled_operand = masm->ExternalOperand(step_in_enabled);
   __ cmpb(step_in_enabled_operand, Immediate(0));
-  __ j(equal, &skip_flooding);
-  {
-    FrameScope scope(masm, StackFrame::INTERNAL);
-    __ Push(rbx);
-    __ Push(rdx);
-    __ Push(rdi);
-    __ CallRuntime(Runtime::kDebugPrepareStepInIfStepping);
-    __ Pop(rdx);
-    __ Pop(rbx);
-    __ movp(rdi, FieldOperand(rbx, JSGeneratorObject::kFunctionOffset));
-  }
-  __ bind(&skip_flooding);
+  __ j(not_equal, &prepare_step_in_if_stepping);
+
+  // Flood function if we need to continue stepping in the suspended generator.
+  ExternalReference debug_suspended_generator =
+      ExternalReference::debug_suspended_generator_address(masm->isolate());
+  Operand debug_suspended_generator_operand =
+      masm->ExternalOperand(debug_suspended_generator);
+  __ cmpp(rbx, debug_suspended_generator_operand);
+  __ j(equal, &prepare_step_in_suspended_generator);
+  __ bind(&stepping_prepared);
 
   // Pop return address.
   __ PopReturnAddressTo(rax);
@@ -596,6 +595,31 @@ void Builtins::Generate_ResumeGeneratorTrampoline(MacroAssembler* masm) {
     __ movp(rax, rbx);  // Continuation expects generator object in rax.
     __ jmp(rdx);
   }
+
+  __ bind(&prepare_step_in_if_stepping);
+  {
+    FrameScope scope(masm, StackFrame::INTERNAL);
+    __ Push(rbx);
+    __ Push(rdx);
+    __ Push(rdi);
+    __ CallRuntime(Runtime::kDebugPrepareStepInIfStepping);
+    __ Pop(rdx);
+    __ Pop(rbx);
+    __ movp(rdi, FieldOperand(rbx, JSGeneratorObject::kFunctionOffset));
+  }
+  __ jmp(&stepping_prepared);
+
+  __ bind(&prepare_step_in_suspended_generator);
+  {
+    FrameScope scope(masm, StackFrame::INTERNAL);
+    __ Push(rbx);
+    __ Push(rdx);
+    __ CallRuntime(Runtime::kDebugPrepareStepInSuspendedGenerator);
+    __ Pop(rdx);
+    __ Pop(rbx);
+    __ movp(rdi, FieldOperand(rbx, JSGeneratorObject::kFunctionOffset));
+  }
+  __ jmp(&stepping_prepared);
 }
 
 static void LeaveInterpreterFrame(MacroAssembler* masm, Register scratch1,

test/mjsunit/harmony/async-debug-step-abort-at-break.js

@@ -0,0 +1,55 @@
+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --expose-debug-as debug --allow-natives-syntax --harmony-async-await
+
+var Debug = debug.Debug;
+var step_count = 0;
+
+function listener(event, execState, eventData, data) {
+  if (event != Debug.DebugEvent.Break) return;
+  try {
+    var line = execState.frame(0).sourceLineText();
+    print(line);
+    var [match, expected_count, step] = /\/\/ B(\d) (\w+)$/.exec(line);
+    assertEquals(step_count++, parseInt(expected_count));
+    if (step != "Continue") execState.prepareStep(Debug.StepAction[step]);
+  } catch (e) {
+    print(e, e.stack);
+    quit(1);
+  }
+}
+
+Debug.setListener(listener);
+
+var late_resolve;
+
+function g() {
+  return new Promise( // B3 StepOut
+    function(res, rej) {
+      late_resolve = res;
+    }
+  );
+}
+
+async function f() {
+  var a = 1;
+  debugger;          // B0 StepNext
+  a +=               // B1 StepNext
+       await         // B4 StepNext
+             g();    // B2 StepIn
+  return a;
+}
+
+f();
+
+// Starting a new step action at an intermediate break point
+// means that we will abort the current async step.
+debugger;            // B5 StepNext
+
+late_resolve(3);     // B6 Continue
+
+%RunMicrotasks();
+
+assertEquals(7, step_count);

test/mjsunit/harmony/async-debug-step-continue-at-break.js

@@ -0,0 +1,55 @@
+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --expose-debug-as debug --allow-natives-syntax --harmony-async-await
+
+var Debug = debug.Debug;
+var step_count = 0;
+
+function listener(event, execState, eventData, data) {
+  if (event != Debug.DebugEvent.Break) return;
+  try {
+    var line = execState.frame(0).sourceLineText();
+    print(line);
+    var [match, expected_count, step] = /\/\/ B(\d) (\w+)$/.exec(line);
+    assertEquals(step_count++, parseInt(expected_count));
+    if (step != "Continue") execState.prepareStep(Debug.StepAction[step]);
+  } catch (e) {
+    print(e, e.stack);
+    quit(1);
+  }
+}
+
+Debug.setListener(listener);
+
+var late_resolve;
+
+function g() {
+  return new Promise( // B3 StepOut
+    function(res, rej) {
+      late_resolve = res;
+    }
+  );
+}
+
+async function f() {
+  var a = 1;
+  debugger;          // B0 StepNext
+  a +=               // B1 StepNext
+       await         // B4 StepNext
+             g();    // B2 StepIn
+  return a;          // B6 StepNext
+}                    // B7 Continue
+
+f();
+
+// Continuing at an intermediate break point means that we will
+// carry on with the current async step.
+debugger;            // B5 Continue
+
+late_resolve(3);
+
+%RunMicrotasks();
+
+assertEquals(8, step_count);

test/mjsunit/harmony/async-debug-step-in-and-out.js

@@ -0,0 +1,51 @@
+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --expose-debug-as debug --allow-natives-syntax --harmony-async-await
+
+var Debug = debug.Debug;
+var step_count = 0;
+
+function listener(event, execState, eventData, data) {
+  if (event != Debug.DebugEvent.Break) return;
+  try {
+    var line = execState.frame(0).sourceLineText();
+    print(line);
+    var [match, expected_count, step] = /\/\/ B(\d) (\w+)$/.exec(line);
+    assertEquals(step_count++, parseInt(expected_count));
+    if (step != "Continue") execState.prepareStep(Debug.StepAction[step]);
+  } catch (e) {
+    print(e, e.stack);
+    quit(1);
+  }
+}
+
+Debug.setListener(listener);
+
+var late_resolve;
+
+function g() {
+  return new Promise(  // B3 StepOut
+    function(res, rej) {
+      late_resolve = res;
+    }
+  );
+}
+
+async function f() {
+  var a = 1;
+  debugger;            // B0 StepNext
+  a +=                 // B1 StepNext
+       await           // B4 StepNext
+             g();      // B2 StepIn
+  return a;            // B5 StepNext
+}                      // B6 Continue
+
+f();
+
+late_resolve(3);
+
+%RunMicrotasks();
+
+assertEquals(7, step_count);

test/mjsunit/harmony/async-debug-step-in-out-out.js

@@ -0,0 +1,51 @@
+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --expose-debug-as debug --allow-natives-syntax --harmony-async-await
+
+var Debug = debug.Debug;
+var step_count = 0;
+
+function listener(event, execState, eventData, data) {
+  if (event != Debug.DebugEvent.Break) return;
+  try {
+    var line = execState.frame(0).sourceLineText();
+    print(line);
+    var [match, expected_count, step] = /\/\/ B(\d) (\w+)$/.exec(line);
+    assertEquals(step_count++, parseInt(expected_count));
+    if (step != "Continue") execState.prepareStep(Debug.StepAction[step]);
+  } catch (e) {
+    print(e, e.stack);
+    quit(1);
+  }
+}
+
+Debug.setListener(listener);
+
+var late_resolve;
+
+function g() {
+  return new Promise( // B3 StepOut
+    function(res, rej) {
+      late_resolve = res;
+    }
+  );
+}
+
+async function f() {
+  var a = 1;
+  debugger;        // B0 StepNext
+  a +=             // B1 StepNext
+       await       // B4 StepOut
+             g();  // B2 StepIn
+  return a;
+}
+
+f();
+
+late_resolve(3);   // B5 Continue
+
+%RunMicrotasks();
+
+assertEquals(6, step_count);

test/mjsunit/harmony/async-debug-step-in.js

@@ -0,0 +1,51 @@
+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --expose-debug-as debug --allow-natives-syntax --harmony-async-await
+
+var Debug = debug.Debug;
+var step_count = 0;
+
+function listener(event, execState, eventData, data) {
+  if (event != Debug.DebugEvent.Break) return;
+  try {
+    var line = execState.frame(0).sourceLineText();
+    print(line);
+    var [match, expected_count, step] = /\/\/ B(\d) (\w+)$/.exec(line);
+    assertEquals(step_count++, parseInt(expected_count));
+    if (step != "Continue") execState.prepareStep(Debug.StepAction[step]);
+  } catch (e) {
+    print(e, e.stack);
+    quit(1);
+  }
+}
+
+Debug.setListener(listener);
+
+var late_resolve;
+
+function g() {
+  return new Promise(  // B3 StepIn
+    function(res, rej) {
+      late_resolve = res;  // B4 StepIn
+    }                      // B5 StepIn
+  );
+}                      // B6 StepIn
+
+async function f() {
+  var a = 1;
+  debugger;            // B0 StepNext
+  a +=                 // B1 StepIn
+       await           // B7 StepIn
+             g();      // B2 StepIn
+  return a;            // B8 StepIn
+}                      // B9 Continue
+
+f().then(value => assertEquals(4, value));
+
+late_resolve(3);
+
+%RunMicrotasks();
+
+assertEquals(10, step_count);

test/mjsunit/harmony/async-debug-step-nested.js

@@ -0,0 +1,58 @@
+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --expose-debug-as debug --allow-natives-syntax --harmony-async-await
+
+var Debug = debug.Debug;
+var step_count = 0;
+
+function listener(event, execState, eventData, data) {
+  if (event != Debug.DebugEvent.Break) return;
+  try {
+    var line = execState.frame(0).sourceLineText();
+    print(line);
+    var [match, expected_count, step] = /\/\/ B(\d) (\w+)$/.exec(line);
+    assertEquals(step_count++, parseInt(expected_count));
+    if (step != "Continue") execState.prepareStep(Debug.StepAction[step]);
+  } catch (e) {
+    print(e, e.stack);
+    quit(1);
+  }
+}
+
+Debug.setListener(listener);
+
+var late_resolve;
+
+function g() {
+  return new Promise( // B4 StepOut
+    function(res, rej) {
+      late_resolve = res;
+    }
+  );
+}
+
+async function f1() {
+  var a = 1;
+  debugger;          // B0 StepNext
+  a +=               // B1 StepNext
+       await         // B6 StepNext
+             f2();   // B2 StepIn
+  return a;          // B7 StepNext
+}                    // B8 Continue
+
+async function f2() {
+  var b =
+          await      // B5 StepOut
+                g(); // B3 StepIn
+  return b;
+}
+
+f1();
+
+late_resolve(3);
+
+%RunMicrotasks();
+
+assertEquals(9, step_count);

test/mjsunit/harmony/async-debug-step-next-constant.js

@@ -0,0 +1,39 @@
+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --expose-debug-as debug --allow-natives-syntax --harmony-async-await
+
+var Debug = debug.Debug;
+var step_count = 0;
+
+function listener(event, execState, eventData, data) {
+  if (event != Debug.DebugEvent.Break) return;
+  try {
+    var line = execState.frame(0).sourceLineText();
+    print(line);
+    var [match, expected_count, step] = /\/\/ B(\d) (\w+)$/.exec(line);
+    assertEquals(step_count++, parseInt(expected_count));
+    if (step != "Continue") execState.prepareStep(Debug.StepAction[step]);
+  } catch (e) {
+    print(e, e.stack);
+    quit(1);
+  }
+}
+
+Debug.setListener(listener);
+
+async function f() {
+  var a = 1;
+  debugger;          // B0 StepNext
+  a +=               // B1 StepNext
+       await         // B3 StepNext
+             5;      // B2 StepNext
+  return a;          // B4 StepNext
+}                    // B5 Continue
+
+f();
+
+%RunMicrotasks();
+
+assertEquals(6, step_count);

test/mjsunit/harmony/async-debug-step-next.js

@@ -0,0 +1,51 @@
+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --expose-debug-as debug --allow-natives-syntax --harmony-async-await
+
+var Debug = debug.Debug;
+var step_count = 0;
+
+function listener(event, execState, eventData, data) {
+  if (event != Debug.DebugEvent.Break) return;
+  try {
+    var line = execState.frame(0).sourceLineText();
+    print(line);
+    var [match, expected_count, step] = /\/\/ B(\d) (\w+)$/.exec(line);
+    assertEquals(step_count++, parseInt(expected_count));
+    if (step != "Continue") execState.prepareStep(Debug.StepAction[step]);
+  } catch (e) {
+    print(e, e.stack);
+    quit(1);
+  }
+}
+
+Debug.setListener(listener);
+
+var late_resolve;
+
+function g() {
+  return new Promise(
+    function(res, rej) {
+      late_resolve = res;
+    }
+  );
+}
+
+async function f() {
+  var a = 1;
+  debugger;        // B0 StepNext
+  a +=             // B1 StepNext
+       await       // B3 StepNext
+             g();  // B2 StepNext
+  return a;        // B4 StepNext
+}                  // B5 Continue
+
+f();
+
+late_resolve(3);
+
+%RunMicrotasks();
+
+assertEquals(6, step_count);

test/mjsunit/harmony/async-debug-step-out.js

@@ -0,0 +1,49 @@
+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --expose-debug-as debug --allow-natives-syntax --harmony-async-await
+
+var Debug = debug.Debug;
+var step_count = 0;
+
+function listener(event, execState, eventData, data) {
+  if (event != Debug.DebugEvent.Break) return;
+  try {
+    var line = execState.frame(0).sourceLineText();
+    print(line);
+    var [match, expected_count, step] = /\/\/ B(\d) (\w+)$/.exec(line);
+    assertEquals(step_count++, parseInt(expected_count));
+    if (step != "Continue") execState.prepareStep(Debug.StepAction[step]);
+  } catch (e) {
+    print(e, e.stack);
+    quit(1);
+  }
+}
+
+Debug.setListener(listener);
+
+var late_resolve;
+
+function g() {
+  return new Promise(
+    function(res, rej) {
+      late_resolve = res;
+    }
+  );
+}
+
+async function f() {
+  var a = 1;
+  debugger;        // B0 StepNext
+  a += await g();  // B1 StepOut
+  return a;
+}
+
+f();
+
+late_resolve(3);   // B2 Continue
+
+%RunMicrotasks();
+
+assertEquals(3, step_count);

test/mjsunit/harmony/async-function-debug-scopes.js

@@ -501,7 +501,6 @@ function CheckFastAllScopes(scopes, exec_state) {
 // Check that the scope chain contains the expected types of scopes.
 function CheckScopeChain(scopes, exec_state) {
   var all_scopes = exec_state.frame().allScopes();
-  assertEquals(scopes.length, exec_state.frame().scopeCount());
   assertEquals(
       scopes.length, all_scopes.length, "FrameMirror.allScopes length");
   for (var i = 0; i < scopes.length; i++) {