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

src/debug/ia32/debug-ia32.cc

Index: src/debug/ia32/debug-ia32.cc
diff --git a/src/debug/ia32/debug-ia32.cc b/src/debug/ia32/debug-ia32.cc
index 7df18c7eb8f7bddd8768769058021cc153ca099c..b860e09f9bf742e1deaa4e1bc2880f19261fb800 100644
--- a/src/debug/ia32/debug-ia32.cc
+++ b/src/debug/ia32/debug-ia32.cc
@@ -68,9 +68,15 @@ void DebugCodegen::GenerateDebugBreakStub(MacroAssembler* masm,
     }
     __ 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())));
@@ -79,14 +85,14 @@ void DebugCodegen::GenerateDebugBreakStub(MacroAssembler* masm,
     __ 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);