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

src/debug/x64/debug-x64.cc

Index: src/debug/x64/debug-x64.cc
diff --git a/src/debug/x64/debug-x64.cc b/src/debug/x64/debug-x64.cc
index 094e7b608ece3dc54ac5f1f708a057b62854a9cf..c554d7a53a86eb0c107aeae6e4424c4c5685b457 100644
--- a/src/debug/x64/debug-x64.cc
+++ b/src/debug/x64/debug-x64.cc
@@ -69,9 +69,15 @@ void DebugCodegen::GenerateDebugBreakStub(MacroAssembler* masm,
     }
     __ 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()));
 
@@ -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 rax 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) {

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

If this is too hacky, I will just push and pop rax when mode ==
SAVE_ERSULT_REGISTER.

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

Ditto. I'm not sure it's necessary to push / pop, but making this loop
explicitly check when the register is rax and mode is SAVE_RESULT_REGISTER.

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

Done.

         Register reg = {JSCallerSavedCode(i)};
         __ 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);