[debugger] simplify debug stepping.

src/debug/debug.cc

Index: src/debug/debug.cc
diff --git a/src/debug/debug.cc b/src/debug/debug.cc
index 214d72e7c27abd486a994b5f33b7f7149d29071a..0b7b8421f1fc9bcfa25fb2c3c06c7eeef0090735 100644
--- a/src/debug/debug.cc
+++ b/src/debug/debug.cc
@@ -477,7 +477,6 @@ void Debug::ThreadInit() {
   thread_local_.last_statement_position_ = RelocInfo::kNoPosition;
   thread_local_.last_fp_ = 0;
   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_?
@@ -935,9 +934,7 @@ void Debug::PrepareStepIn(Handle<JSFunction> function) {
   if (!is_active()) return;
   if (last_step_action() < StepIn) return;
   if (in_debug_scope()) return;
-  if (thread_local_.step_in_enabled_) {
-    FloodWithOneShot(function);
-  }
+  FloodWithOneShot(function);
 }
 
 void Debug::PrepareStepInSuspendedGenerator() {
@@ -945,7 +942,6 @@ void Debug::PrepareStepInSuspendedGenerator() {
   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);
@@ -1005,10 +1001,7 @@ void Debug::PrepareStep(StepAction step_action) {
 
   feature_tracker()->Track(DebugFeatureTracker::kStepping);
 
-  // Remember this step action and count.
   thread_local_.last_step_action_ = step_action;
-  STATIC_ASSERT(StepFrame > StepIn);
-  thread_local_.step_in_enabled_ = (step_action >= StepIn);
 
   // If the function on the top frame is unresolved perform step out. This will
   // be the case when calling unknown function and having the debugger stopped
@@ -1043,10 +1036,11 @@ void Debug::PrepareStep(StepAction step_action) {
   BreakLocation location =
       BreakLocation::FromCodeOffset(debug_info, call_offset);
 
-  // Any step at a return is a step-out.
-  if (location.IsReturn()) step_action = StepOut;
-  // A step-next at a tail call is a step-out.
-  if (location.IsTailCall() && step_action == StepNext) step_action = StepOut;
+  // Any step at a return or a step-next at a tail call is a step-out.
+  if (location.IsReturn() ||
+      (location.IsTailCall() && thread_local_.last_step_action_ == StepNext)) {
+    thread_local_.last_step_action_ = StepOut;
+  }
 
   thread_local_.last_statement_position_ =
       debug_info->abstract_code()->SourceStatementPosition(
@@ -1055,7 +1049,7 @@ void Debug::PrepareStep(StepAction step_action) {
   // No longer perform the current async step.
   clear_suspended_generator();
 
-  switch (step_action) {
+  switch (thread_local_.last_step_action_) {
     case StepNone:
       UNREACHABLE();
       break;
@@ -1070,11 +1064,7 @@ void Debug::PrepareStep(StepAction step_action) {
         Deoptimizer::DeoptimizeFunction(frames_it.frame()->function());
         frames_it.Advance();
       }
-      if (frames_it.done()) {
-        // Stepping out to the embedder. Disable step-in to avoid stepping into
-        // the next (unrelated) call that the embedder makes.
-        thread_local_.step_in_enabled_ = false;
-      } else {
+      if (!frames_it.done()) {
         // Fill the caller function to return to with one-shot break points.
         Handle<JSFunction> caller_function(frames_it.frame()->function());
         FloodWithOneShot(caller_function);
@@ -1142,7 +1132,6 @@ void Debug::ClearStepping() {
   ClearOneShot();
 
   thread_local_.last_step_action_ = StepNone;
-  thread_local_.step_in_enabled_ = false;
   thread_local_.last_statement_position_ = RelocInfo::kNoPosition;
   thread_local_.last_fp_ = 0;
   thread_local_.target_fp_ = 0;
@@ -1167,12 +1156,6 @@ void Debug::ClearOneShot() {
 }
 
 
-void Debug::EnableStepIn() {
-  STATIC_ASSERT(StepFrame > StepIn);
-  thread_local_.step_in_enabled_ = (last_step_action() >= StepIn);
-}
-
-
 bool MatchingCodeTargets(Code* target1, Code* target2) {
   if (target1 == target2) return true;
   if (target1->kind() != target2->kind()) return false;