[Interpreter] Adds support to fetch return value on break at return.

src/debug/debug.cc

Index: src/debug/debug.cc
diff --git a/src/debug/debug.cc b/src/debug/debug.cc
index b255bb81a1dd80db2b46599382491d05d74a2b99..c8bd707d31c5106d6a14b86f71cf239f78c59f58 100644
--- a/src/debug/debug.cc
+++ b/src/debug/debug.cc
@@ -474,6 +474,7 @@ void Debug::ThreadInit() {
   thread_local_.last_fp_ = 0;
   thread_local_.target_fp_ = 0;
   thread_local_.step_in_enabled_ = false;
+  thread_local_.return_value_ = Handle<Object>();
   // TODO(isolates): frames_are_dropped_?
   base::NoBarrier_Store(&thread_local_.current_debug_scope_,
                         static_cast<base::AtomicWord>(0));
@@ -563,7 +564,11 @@ void Debug::Unload() {
 
 void Debug::Break(Arguments args, JavaScriptFrame* frame) {
   HandleScope scope(isolate_);
-  DCHECK(args.length() == 0);
+  DCHECK(args.length() == 1);
+
+  // args[0] contains the return value (eax/rax/r0/accumulator) if we break
+  // on return.
+  thread_local_.return_value_ = args.at<Object>(0);

[Yang, 2016/03/22 12:22:57]

Please introduce a method to set and retrieve the return value, and use it in
Runtime_DebugBreak.

 
   // Initialize LiveEdit.
   LiveEdit::InitializeThreadLocal(this);
@@ -1586,7 +1591,7 @@ Object* Debug::SetAfterBreakTarget(JavaScriptFrame* frame) {
       // Continue just after the slot.
       after_break_target_ = frame->pc();
     }
-    return isolate_->heap()->undefined_value();
+    return *thread_local_.return_value_;

[mythria, 2016/03/21 16:24:29]

Returns the actual return value to avoid pushing and popping rax/eax in the
DebugBreakStub.

[Yang, 2016/03/22 12:22:58]

I forgot about this part. For the interpreter, the return value of this function
is the after-break-target. For FCG, we store it in a memory location.

Let's split Runtime_DebugBreak into two runtime functions. One to cater to FCG.
It would set the return value before calling Debug::Break, and return the return
value. The other caters to Ignition, it sets the return value before calling
Debug::Break, and returns the debug break bytecode handler.

Let's also move the code in SetAfterBreakTarget for the interpreter case into
the runtime function and only call SetAfterBreakTarget for FCG.

[mythria, 2016/03/22 15:11:44]

Done.

   }
 }
 
@@ -2327,9 +2332,10 @@ DebugScope::DebugScope(Debug* debug)
   base::NoBarrier_Store(&debug_->thread_local_.current_debug_scope_,
                         reinterpret_cast<base::AtomicWord>(this));
 
-  // Store the previous break id and frame id.
+  // Store the previous break id, frame id and return value.
   break_id_ = debug_->break_id();
   break_frame_id_ = debug_->break_frame_id();
+  return_value_ = debug_->get_return_value();
 
   // Create the new break info. If there is no JavaScript frames there is no
   // break frame id.
@@ -2367,6 +2373,7 @@ DebugScope::~DebugScope() {
   // Restore to the previous break state.
   debug_->thread_local_.break_frame_id_ = break_frame_id_;
   debug_->thread_local_.break_id_ = break_id_;
+  debug_->thread_local_.return_value_ = return_value_;
 
   debug_->UpdateState();
 }