[liveedit] reimplement frame restarting.

src/debug/debug.h

Index: src/debug/debug.h
diff --git a/src/debug/debug.h b/src/debug/debug.h
index b3bb3c46a64cfa2d673f69c4bac9b6dc5809aaeb..4d41f69112d229f2fbf097da4741f9299a7c2bbe 100644
--- a/src/debug/debug.h
+++ b/src/debug/debug.h
@@ -426,7 +426,6 @@ class Debug {
   // Internal logic
   bool Load();
   void Break(JavaScriptFrame* frame);
-  void SetAfterBreakTarget(JavaScriptFrame* frame);
 
   // Scripts handling.
   Handle<FixedArray> GetLoadedScripts();
@@ -494,8 +493,7 @@ class Debug {
   bool IsBreakAtReturn(JavaScriptFrame* frame);
 
   // Support for LiveEdit
-  void FramesHaveBeenDropped(StackFrame::Id new_break_frame_id,
-                             LiveEditFrameDropMode mode);
+  void UpdateBreakFrameId();
 
   // Threading support.
   char* ArchiveDebug(char* to);
@@ -538,10 +536,8 @@ class Debug {
   StackFrame::Id break_frame_id() { return thread_local_.break_frame_id_; }
   int break_id() { return thread_local_.break_id_; }
 
-  Handle<Object> return_value() { return thread_local_.return_value_; }
-  void set_return_value(Handle<Object> value) {
-    thread_local_.return_value_ = value;
-  }
+  Object* return_value() { return thread_local_.return_value_; }
+  void set_return_value(Object* value) { thread_local_.return_value_ = value; }
 
   // Support for embedding into generated code.
   Address is_active_address() {
@@ -552,10 +548,6 @@ class Debug {
     return reinterpret_cast<Address>(&hook_on_function_call_);
   }
 
-  Address after_break_target_address() {
-    return reinterpret_cast<Address>(&after_break_target_);
-  }
-
   Address last_step_action_address() {
     return reinterpret_cast<Address>(&thread_local_.last_step_action_);
   }
@@ -564,6 +556,15 @@ class Debug {
     return reinterpret_cast<Address>(&thread_local_.suspended_generator_);
   }
 
+  Address new_fp_address() {
+    return reinterpret_cast<Address>(&thread_local_.new_fp_);
+  }
+
+  void DropToFP(Address fp) {
+    // Drop to a lower a frame.

[jgruber, 2017/01/17 13:29:58]

Nit: 'a lower a'.

A diagram or brief description of how frame dropping works would be very helpful
since the code to handle it is pretty scattered (and this function doesn't
actually drop frames as its name suggests).

If I understood correctly, this function marks an upcoming drop, which then
actually takes place once we return from the DebugBreakStub. new_fp_ is only
reset when entering a new DebugBreak.

[Yang, 2017/01/18 07:49:08]

Done. Added comment.

+    if (fp > thread_local_.new_fp_) thread_local_.new_fp_ = fp;
+  }
+
   StepAction last_step_action() { return thread_local_.last_step_action_; }
 
   DebugFeatureTracker* feature_tracker() { return &feature_tracker_; }
@@ -697,11 +698,6 @@ class Debug {
   // List of active debug info objects.
   DebugInfoListNode* debug_info_list_;
 
-  // Storage location for jump when exiting debug break calls.
-  // Note that this address is not GC safe.  It should be computed immediately
-  // before returning to the DebugBreakCallHelper.
-  Address after_break_target_;
-
   // Used to collect histogram data on debugger feature usage.
   DebugFeatureTracker feature_tracker_;
 
@@ -738,10 +734,14 @@ class Debug {
 
     // Value of accumulator in interpreter frames. In non-interpreter frames
     // this value will be the hole.

[jgruber, 2017/01/17 13:29:58]

Is the comment about the hole still valid?

[Yang, 2017/01/18 07:49:07]

Done.

-    Handle<Object> return_value_;
+    Object* return_value_;
 
+    // The suspended generator object to track when stepping.
     Object* suspended_generator_;
 
+    // The new frame pointer to drop to when restarting a frame.
+    Address new_fp_;

[jgruber, 2017/01/17 13:29:58]

Nit: 'new_fp_' isn't very descriptive, maybe something like frame_drop_fp? Up to
you.

[Yang, 2017/01/18 07:49:08]

renamed to restart_fp_

+
     int async_task_count_;
   };
 
@@ -858,15 +858,10 @@ class DebugCodegen : public AllStatic {
   static void GenerateDebugBreakStub(MacroAssembler* masm,
                                      DebugBreakCallHelperMode mode);
 
-  // FrameDropper is a code replacement for a JavaScript frame with possibly
-  // several frames above.
-  // There is no calling conventions here, because it never actually gets
-  // called, it only gets returned to.
-  static void GenerateFrameDropperLiveEdit(MacroAssembler* masm);
-
-
   static void GenerateSlot(MacroAssembler* masm, RelocInfo::Mode mode);
 
+  static void GenerateFrameDropperTrampoline(MacroAssembler* masm);
+
   static void PatchDebugBreakSlot(Isolate* isolate, Address pc,
                                   Handle<Code> code);
   static bool DebugBreakSlotIsPatched(Address pc);