[runtime] Use "the hole" instead of smi 0 as sentinel for context extension.

src/full-codegen/x64/full-codegen-x64.cc

Index: src/full-codegen/x64/full-codegen-x64.cc
diff --git a/src/full-codegen/x64/full-codegen-x64.cc b/src/full-codegen/x64/full-codegen-x64.cc
index 98fd6a699e66e53f211c2fee0f4b1121fc99ce12..5da01e689c9dab0298e9c1bf975d36d756bdaa18 100644
--- a/src/full-codegen/x64/full-codegen-x64.cc
+++ b/src/full-codegen/x64/full-codegen-x64.cc
@@ -1222,10 +1222,9 @@ void FullCodeGenerator::EmitLoadGlobalCheckExtensions(VariableProxy* proxy,
   while (s != NULL) {
     if (s->num_heap_slots() > 0) {
       if (s->calls_sloppy_eval()) {
-        // Check that extension is NULL.
-        __ cmpp(ContextOperand(context, Context::EXTENSION_INDEX),
-                Immediate(0));
-        __ j(not_equal, slow);
+        // Check that extension is "the hole".
+        __ JumpIfNotRoot(ContextOperand(context, Context::EXTENSION_INDEX),
+                         Heap::kTheHoleValueRootIndex, slow);
       }
       // Load next context in chain.
       __ movp(temp, ContextOperand(context, Context::PREVIOUS_INDEX));
@@ -1252,9 +1251,9 @@ void FullCodeGenerator::EmitLoadGlobalCheckExtensions(VariableProxy* proxy,
     // Terminate at native context.
     __ cmpp(kScratchRegister, FieldOperand(temp, HeapObject::kMapOffset));
     __ j(equal, &fast, Label::kNear);
-    // Check that extension is NULL.
-    __ cmpp(ContextOperand(temp, Context::EXTENSION_INDEX), Immediate(0));
-    __ j(not_equal, slow);
+    // Check that extension is "the hole".
+    __ JumpIfNotRoot(ContextOperand(temp, Context::EXTENSION_INDEX),
+                     Heap::kTheHoleValueRootIndex, slow);
     // Load next context in chain.
     __ movp(temp, ContextOperand(temp, Context::PREVIOUS_INDEX));
     __ jmp(&next);
@@ -1276,19 +1275,18 @@ MemOperand FullCodeGenerator::ContextSlotOperandCheckExtensions(Variable* var,
   for (Scope* s = scope(); s != var->scope(); s = s->outer_scope()) {
     if (s->num_heap_slots() > 0) {
       if (s->calls_sloppy_eval()) {
-        // Check that extension is NULL.
-        __ cmpp(ContextOperand(context, Context::EXTENSION_INDEX),
-                Immediate(0));
-        __ j(not_equal, slow);
+        // Check that extension is "the hole".
+        __ JumpIfNotRoot(ContextOperand(context, Context::EXTENSION_INDEX),
+                         Heap::kTheHoleValueRootIndex, slow);
       }
       __ movp(temp, ContextOperand(context, Context::PREVIOUS_INDEX));
       // Walk the rest of the chain without clobbering rsi.
       context = temp;
     }
   }
-  // Check that last extension is NULL.
-  __ cmpp(ContextOperand(context, Context::EXTENSION_INDEX), Immediate(0));
-  __ j(not_equal, slow);
+  // Check that last extension is "the hole".
+  __ JumpIfNotRoot(ContextOperand(context, Context::EXTENSION_INDEX),
+                   Heap::kTheHoleValueRootIndex, slow);
 
   // This function is used only for loads, not stores, so it's safe to
   // return an rsi-based operand (the write barrier cannot be allowed to