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

src/full-codegen/ia32/full-codegen-ia32.cc

Index: src/full-codegen/ia32/full-codegen-ia32.cc
diff --git a/src/full-codegen/ia32/full-codegen-ia32.cc b/src/full-codegen/ia32/full-codegen-ia32.cc
index c3467062e161e59d34dc7a795c1f0d71f0b46c09..66ada7986ec6a794e1ebfa3cf2b80b7778bc70b7 100644
--- a/src/full-codegen/ia32/full-codegen-ia32.cc
+++ b/src/full-codegen/ia32/full-codegen-ia32.cc
@@ -1195,10 +1195,9 @@ void FullCodeGenerator::EmitLoadGlobalCheckExtensions(VariableProxy* proxy,
   while (s != NULL) {
     if (s->num_heap_slots() > 0) {
       if (s->calls_sloppy_eval()) {
-        // Check that extension is NULL.
-        __ cmp(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.
       __ mov(temp, ContextOperand(context, Context::PREVIOUS_INDEX));
@@ -1224,9 +1223,9 @@ void FullCodeGenerator::EmitLoadGlobalCheckExtensions(VariableProxy* proxy,
     __ cmp(FieldOperand(temp, HeapObject::kMapOffset),
            Immediate(isolate()->factory()->native_context_map()));
     __ j(equal, &fast, Label::kNear);
-    // Check that extension is NULL.
-    __ cmp(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.
     __ mov(temp, ContextOperand(temp, Context::PREVIOUS_INDEX));
     __ jmp(&next);
@@ -1248,19 +1247,18 @@ 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.
-        __ cmp(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);
       }
       __ mov(temp, ContextOperand(context, Context::PREVIOUS_INDEX));
       // Walk the rest of the chain without clobbering esi.
       context = temp;
     }
   }
-  // Check that last extension is NULL.
-  __ cmp(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 esi-based operand (the write barrier cannot be allowed to