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

src/full-codegen/mips/full-codegen-mips.cc

Index: src/full-codegen/mips/full-codegen-mips.cc
diff --git a/src/full-codegen/mips/full-codegen-mips.cc b/src/full-codegen/mips/full-codegen-mips.cc
index 88fffa0f1af39dea1e6a3a31661638083f4c5ba0..92236cbd877eb1285fada18f0622e24a6e361a38 100644
--- a/src/full-codegen/mips/full-codegen-mips.cc
+++ b/src/full-codegen/mips/full-codegen-mips.cc
@@ -1268,9 +1268,9 @@ void FullCodeGenerator::EmitLoadGlobalCheckExtensions(VariableProxy* proxy,
   while (s != NULL) {
     if (s->num_heap_slots() > 0) {
       if (s->calls_sloppy_eval()) {
-        // Check that extension is NULL.
+        // Check that extension is "the hole".
         __ lw(temp, ContextMemOperand(current, Context::EXTENSION_INDEX));
-        __ Branch(slow, ne, temp, Operand(zero_reg));
+        __ JumpIfNotRoot(temp, Heap::kTheHoleValueRootIndex, slow);
       }
       // Load next context in chain.
       __ lw(next, ContextMemOperand(current, Context::PREVIOUS_INDEX));
@@ -1293,9 +1293,9 @@ void FullCodeGenerator::EmitLoadGlobalCheckExtensions(VariableProxy* proxy,
     __ lw(temp, FieldMemOperand(next, HeapObject::kMapOffset));
     __ LoadRoot(t0, Heap::kNativeContextMapRootIndex);
     __ Branch(&fast, eq, temp, Operand(t0));
-    // Check that extension is NULL.
+    // Check that extension is "the hole".
     __ lw(temp, ContextMemOperand(next, Context::EXTENSION_INDEX));
-    __ Branch(slow, ne, temp, Operand(zero_reg));
+    __ JumpIfNotRoot(temp, Heap::kTheHoleValueRootIndex, slow);
     // Load next context in chain.
     __ lw(next, ContextMemOperand(next, Context::PREVIOUS_INDEX));
     __ Branch(&loop);
@@ -1318,18 +1318,18 @@ 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.
+        // Check that extension is "the hole".
         __ lw(temp, ContextMemOperand(context, Context::EXTENSION_INDEX));
-        __ Branch(slow, ne, temp, Operand(zero_reg));
+        __ JumpIfNotRoot(temp, Heap::kTheHoleValueRootIndex, slow);
       }
       __ lw(next, ContextMemOperand(context, Context::PREVIOUS_INDEX));
       // Walk the rest of the chain without clobbering cp.
       context = next;
     }
   }
-  // Check that last extension is NULL.
+  // Check that last extension is "the hole".
   __ lw(temp, ContextMemOperand(context, Context::EXTENSION_INDEX));
-  __ Branch(slow, ne, temp, Operand(zero_reg));
+  __ JumpIfNotRoot(temp, Heap::kTheHoleValueRootIndex, slow);
 
   // This function is used only for loads, not stores, so it's safe to
   // return an cp-based operand (the write barrier cannot be allowed to