[heap] All immutable roots are either smis or immovable heap objects.

src/heap/heap.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.
 
 #include "src/v8.h"
 
 #include "src/accessors.h"
 #include "src/api.h"
 #include "src/base/bits.h"
 #include "src/base/once.h"
@@ skipping 3298 matching lines @@
     case kStringTableRootIndex:
       return true;
 
     default:
       return false;
   }
 }
 
 
 bool Heap::RootCanBeTreatedAsConstant(RootListIndex root_index) {
-  return !RootCanBeWrittenAfterInitialization(root_index) &&
-         !InNewSpace(roots_array_start()[root_index]);
+  if (!RootCanBeWrittenAfterInitialization(root_index)) {
+    DCHECK(!InNewSpace(roots_array_start()[root_index]));
+    return true;
+  }
+  return false;
 }
 
 
 Object* RegExpResultsCache::Lookup(Heap* heap, String* key_string,
                                    Object* key_pattern, ResultsCacheType type) {
   FixedArray* cache;
   if (!key_string->IsInternalizedString()) return Smi::FromInt(0);
   if (type == STRING_SPLIT_SUBSTRINGS) {
     DCHECK(key_pattern->IsString());
     if (!key_pattern->IsInternalizedString()) return Smi::FromInt(0);
@@ skipping 3285 matching lines @@
     *object_type = "CODE_TYPE";                                                \
     *object_sub_type = "CODE_AGE/" #name;                                      \
     return true;
     CODE_AGE_LIST_COMPLETE(COMPARE_AND_RETURN_NAME)
 #undef COMPARE_AND_RETURN_NAME
   }
   return false;
 }
 }  // namespace internal
 }  // namespace v8