Special case the "empty string" root so it doesn't constantly jump around

src/heap/heap.h

 // 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.
 
 #ifndef V8_HEAP_HEAP_H_
 #define V8_HEAP_HEAP_H_
 
 #include <cmath>
 
 #include "src/allocation.h"
@@ skipping 19 matching lines @@
   V(Map, free_space_map, FreeSpaceMap)                                         \
   V(Map, one_pointer_filler_map, OnePointerFillerMap)                          \
   V(Map, two_pointer_filler_map, TwoPointerFillerMap)                          \
   /* Cluster the most popular ones in a few cache lines here at the top.    */ \
   V(Smi, store_buffer_top, StoreBufferTop)                                     \
   V(Oddball, undefined_value, UndefinedValue)                                  \
   V(Oddball, the_hole_value, TheHoleValue)                                     \
   V(Oddball, null_value, NullValue)                                            \
   V(Oddball, true_value, TrueValue)                                            \
   V(Oddball, false_value, FalseValue)                                          \
+  V(String, empty_string, empty_string)                                        \
   V(Oddball, uninitialized_value, UninitializedValue)                          \
   V(Map, cell_map, CellMap)                                                    \
   V(Map, global_property_cell_map, GlobalPropertyCellMap)                      \
   V(Map, shared_function_info_map, SharedFunctionInfoMap)                      \
   V(Map, meta_map, MetaMap)                                                    \
   V(Map, heap_number_map, HeapNumberMap)                                       \
   V(Map, mutable_heap_number_map, MutableHeapNumberMap)                        \
   V(Map, native_context_map, NativeContextMap)                                 \
   V(Map, fixed_array_map, FixedArrayMap)                                       \
   V(Map, code_map, CodeMap)                                                    \
   V(Map, scope_info_map, ScopeInfoMap)                                         \
   V(Map, fixed_cow_array_map, FixedCOWArrayMap)                                \
   V(Map, fixed_double_array_map, FixedDoubleArrayMap)                          \
   V(Map, constant_pool_array_map, ConstantPoolArrayMap)                        \
   V(Map, weak_cell_map, WeakCellMap)                                           \
   V(Map, one_byte_string_map, OneByteStringMap)                                \
   V(Map, one_byte_internalized_string_map, OneByteInternalizedStringMap)       \
   V(Map, function_context_map, FunctionContextMap)                             \
   V(FixedArray, empty_fixed_array, EmptyFixedArray)                            \
   V(ByteArray, empty_byte_array, EmptyByteArray)                               \
   V(DescriptorArray, empty_descriptor_array, EmptyDescriptorArray)             \
   V(ConstantPoolArray, empty_constant_pool_array, EmptyConstantPoolArray)      \
   /* The roots above this line should be boring from a GC point of view.    */ \

[Yang, 2015/04/10 09:23:36]

I think there was a reason there were only 32 items before this comment. I don't
remember what the reason was though.

   /* This means they are never in new space and never on a page that is     */ \
   /* being compacted.                                                       */ \
   V(Oddball, no_interceptor_result_sentinel, NoInterceptorResultSentinel)      \
   V(Oddball, arguments_marker, ArgumentsMarker)                                \
   V(Oddball, exception, Exception)                                             \
   V(Oddball, termination_exception, TerminationException)                      \
   V(FixedArray, number_string_cache, NumberStringCache)                        \
   V(Object, instanceof_cache_function, InstanceofCacheFunction)                \
   V(Object, instanceof_cache_map, InstanceofCacheMap)                          \
   V(Object, instanceof_cache_answer, InstanceofCacheAnswer)                    \
@@ skipping 134 matching lines @@
   V(proto_string, "__proto__")                                 \
   V(arguments_string, "arguments")                             \
   V(Arguments_string, "Arguments")                             \
   V(caller_string, "caller")                                   \
   V(boolean_string, "boolean")                                 \
   V(Boolean_string, "Boolean")                                 \
   V(callee_string, "callee")                                   \
   V(constructor_string, "constructor")                         \
   V(dot_result_string, ".result")                              \
   V(eval_string, "eval")                                       \
-  V(empty_string, "")                                          \
   V(function_string, "function")                               \
   V(Function_string, "Function")                               \
   V(length_string, "length")                                   \
   V(name_string, "name")                                       \
   V(null_string, "null")                                       \
   V(number_string, "number")                                   \
   V(Number_string, "Number")                                   \
   V(nan_string, "NaN")                                         \
   V(source_string, "source")                                   \
   V(source_url_string, "source_url")                           \
@@ skipping 135 matching lines @@
   V(ScriptContextMap)                   \
   V(UndefinedMap)                       \
   V(TheHoleMap)                         \
   V(NullMap)                            \
   V(BooleanMap)                         \
   V(UninitializedMap)                   \
   V(ArgumentsMarkerMap)                 \
   V(JSMessageObjectMap)                 \
   V(ForeignMap)                         \
   V(NeanderMap)                         \
+  V(empty_string)                       \
   PRIVATE_SYMBOL_LIST(V)
 
 // Forward declarations.
 class HeapStats;
 class Isolate;
 class WeakObjectRetainer;
 
 
 typedef String* (*ExternalStringTableUpdaterCallback)(Heap* heap,
                                                       Object** pointer);
@@ skipping 2220 matching lines @@
   DisallowHeapAllocation no_allocation;  // i.e. no gc allowed.
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(PathTracer);
 };
 #endif  // DEBUG
 }
 }  // namespace v8::internal
 
 #endif  // V8_HEAP_HEAP_H_