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

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

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #if V8_TARGET_ARCH_MIPS
 
 // Note on Mips implementation:
 //
 // The result_register() for mips is the 'v0' register, which is defined
 // by the ABI to contain function return values. However, the first
@@ skipping 1250 matching lines @@
                                                       TypeofMode typeof_mode,
                                                       Label* slow) {
   Register current = cp;
   Register next = a1;
   Register temp = a2;
 
   Scope* s = scope();
   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));
       // Walk the rest of the chain without clobbering cp.
       current = next;
     }
     // If no outer scope calls eval, we do not need to check more
     // context extensions.
     if (!s->outer_scope_calls_sloppy_eval() || s->is_eval_scope()) break;
     s = s->outer_scope();
   }
 
   if (s->is_eval_scope()) {
     Label loop, fast;
     if (!current.is(next)) {
       __ Move(next, current);
     }
     __ bind(&loop);
     // Terminate at native context.
     __ 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);
     __ bind(&fast);
   }
 
   // All extension objects were empty and it is safe to use a normal global
   // load machinery.
   EmitGlobalVariableLoad(proxy, typeof_mode);
 }
 
 
 MemOperand FullCodeGenerator::ContextSlotOperandCheckExtensions(Variable* var,
                                                                 Label* slow) {
   DCHECK(var->IsContextSlot());
   Register context = cp;
   Register next = a3;
   Register temp = t0;
 
   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
   // destroy the cp register).
   return ContextMemOperand(context, var->index());
 }
 
 
 void FullCodeGenerator::EmitDynamicLookupFastCase(VariableProxy* proxy,
                                                   TypeofMode typeof_mode,
@@ skipping 3626 matching lines @@
          reinterpret_cast<uint32_t>(
              isolate->builtins()->OsrAfterStackCheck()->entry()));
   return OSR_AFTER_STACK_CHECK;
 }
 
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_TARGET_ARCH_MIPS