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

src/debug/arm64/debug-arm64.cc

 // Copyright 2013 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.
 
 #if V8_TARGET_ARCH_ARM64
 
 #include "src/arm64/frames-arm64.h"
 #include "src/codegen.h"
 #include "src/debug/debug.h"
 
@@ skipping 76 matching lines @@
     FrameScope scope(masm, StackFrame::INTERNAL);
 
     // Load padding words on stack.
     __ Mov(scratch, Smi::FromInt(LiveEdit::kFramePaddingValue));
     __ PushMultipleTimes(scratch, LiveEdit::kFramePaddingInitialSize);
     __ Mov(scratch, Smi::FromInt(LiveEdit::kFramePaddingInitialSize));
     __ Push(scratch);
 
     if (mode == SAVE_RESULT_REGISTER) __ Push(x0);
 
-    __ Mov(x0, 0);  // No arguments.
+    // One argument. The hole in FCG. Value of accumulator in interpreter.
+    __ Mov(x0, 1);
+    __ Push(masm->isolate()->factory()->the_hole_value());
     __ Mov(x1, ExternalReference(Runtime::FunctionForId(Runtime::kDebugBreak),
                                  masm->isolate()));
 
     CEntryStub stub(masm->isolate(), 1);
     __ CallStub(&stub);
 
     if (FLAG_debug_code) {
       for (int i = 0; i < kNumJSCallerSaved; i++) {
         Register reg = Register::XRegFromCode(JSCallerSavedCode(i));
         __ Mov(reg, Operand(kDebugZapValue));
       }
     }
 
     // Restore the register values from the expression stack.
     if (mode == SAVE_RESULT_REGISTER) __ Pop(x0);

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

I thought we are going to use the return value of the runtime function to
restore x0. What we are doing now is using push/pop to persist x0, and passing
the-hole to debug break, which returns it.

Instead, we should push x0 if mode == SAVE_RESULT_REGISTER, and the-hole
otherwise. Runtime_DebugBreak would get the return value for the
Return_DebugBreak. 

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

Done. I am sorry, did not port it to arm earlier. 

 
     // Don't bother removing padding bytes pushed on the stack
     // as the frame is going to be restored right away.
 
     // Leave the internal frame.
   }
 
   // Now that the break point has been handled, resume normal execution by
   // jumping to the target address intended by the caller and that was
   // overwritten by the address of DebugBreakXXX.
@@ skipping 35 matching lines @@
   __ Br(scratch);
 }
 
 
 const bool LiveEdit::kFrameDropperSupported = true;
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_TARGET_ARCH_ARM64