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

src/debug/ia32/debug-ia32.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_IA32
 
 #include "src/codegen.h"
 #include "src/debug/debug.h"
 #include "src/ia32/frames-ia32.h"
 
@@ skipping 50 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(Immediate(Smi::FromInt(LiveEdit::kFramePaddingValue)));
     }
     __ push(Immediate(Smi::FromInt(LiveEdit::kFramePaddingInitialSize)));
 
-    if (mode == SAVE_RESULT_REGISTER) __ push(eax);
-
-    __ Move(eax, Immediate(0));  // No arguments.
+    // Push arguments for DebugBreak call.
+    if (mode == SAVE_RESULT_REGISTER) {
+      // Break on return.
+      __ push(eax);
+    } else {
+      // Non-return breaks.
+      __ Push(masm->isolate()->factory()->the_hole_value());
+    }
+    __ Move(eax, Immediate(1));
     __ mov(ebx,
            Immediate(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) {
+      // Do not clobber eax if SAVE_RESULT_REGISTER is set. It will
+      // contain return value of the function.
+      for (int i = SAVE_RESULT_REGISTER ? 1 : 0; i < kNumJSCallerSaved; ++i) {

[rmcilroy, 2016/03/22 11:32:34]

This is relying on JSCallerSavedCode(0) being eax, this is a bit brittle - could
you make this more explicit.

[mythria, 2016/03/22 12:08:09]

Thanks, I did not realize we could check if a register is eax. Done.

         Register reg = {JSCallerSavedCode(i)};
         __ Move(reg, Immediate(kDebugZapValue));
       }
     }
 
-    if (mode == SAVE_RESULT_REGISTER) __ pop(eax);
-
     __ pop(ebx);
     // We divide stored value by 2 (untagging) and multiply it by word's size.
     STATIC_ASSERT(kSmiTagSize == 1 && kSmiShiftSize == 0);
     __ lea(esp, Operand(esp, ebx, times_half_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_IA32