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

src/debug/x64/debug-x64.cc

 // Copyright 2012 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_X64
 
 #include "src/assembler.h"
 #include "src/codegen.h"
 #include "src/debug/debug.h"
 
@@ skipping 51 matching lines @@
   // Enter an internal frame.
   {
     FrameScope scope(masm, StackFrame::INTERNAL);
 
     // Load padding words on stack.
     for (int i = 0; i < LiveEdit::kFramePaddingInitialSize; i++) {
       __ Push(Smi::FromInt(LiveEdit::kFramePaddingValue));
     }
     __ Push(Smi::FromInt(LiveEdit::kFramePaddingInitialSize));
 
-    if (mode == SAVE_RESULT_REGISTER) __ Push(rax);
-
-    __ Set(rax, 0);  // No arguments (argc == 0).
+    // Push arguments for DebugBreak call.
+    if (mode == SAVE_RESULT_REGISTER) {
+      // Break on return.
+      __ Push(rax);
+    } else {
+      // Non-return breaks.
+      __ Push(masm->isolate()->factory()->the_hole_value());
+    }
+    __ Set(rax, 1);
     __ Move(rbx, ExternalReference(Runtime::FunctionForId(Runtime::kDebugBreak),
                                    masm->isolate()));
 
     CEntryStub ceb(masm->isolate(), 1);
     __ CallStub(&ceb);
 
     if (FLAG_debug_code) {
       for (int i = 0; i < kNumJSCallerSaved; ++i) {
         Register reg = {JSCallerSavedCode(i)};
-        __ Set(reg, kDebugZapValue);
+        // Do not clobber rax if mode is SAVE_RESULT_REGISTER. It will
+        // contain return value of the function.
+        if (!(reg.is(rax) && (mode == SAVE_RESULT_REGISTER))) {
+          __ Set(reg, kDebugZapValue);
+        }
       }
     }
 
-    if (mode == SAVE_RESULT_REGISTER) __ Pop(rax);
-
     // Read current padding counter and skip corresponding number of words.
     __ Pop(kScratchRegister);
     __ SmiToInteger32(kScratchRegister, kScratchRegister);
     __ leap(rsp, Operand(rsp, kScratchRegister, times_pointer_size, 0));
 
     // Get rid of the internal frame.
   }
 
   // This call did not replace a call , so there will be an unwanted
   // return address left on the stack. Here we get rid of that.
@@ skipping 38 matching lines @@
 }
 
 const bool LiveEdit::kFrameDropperSupported = true;
 
 #undef __
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_TARGET_ARCH_X64