Map::Hash() calculation made deterministic in predictable mode.

src/objects.cc

 // Copyright 2013 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/allocation-site-scopes.h"
 #include "src/api.h"
 #include "src/arguments.h"
@@ skipping 8981 matching lines @@
 
 
 void String::PrintOn(FILE* file) {
   int length = this->length();
   for (int i = 0; i < length; i++) {
     PrintF(file, "%c", Get(i));
   }
 }
 
 
+inline static uint32_t ObjectAddressForHashing(Object* object) {
+  uint32_t value = static_cast<uint32_t>(reinterpret_cast<uintptr_t>(object));
+  return value & MemoryChunk::kAlignmentMask;
+}
+
+
 int Map::Hash() {
   // For performance reasons we only hash the 3 most variable fields of a map:
   // constructor, prototype and bit_field2.
 
   // Shift away the tag.
-  int hash = (static_cast<uint32_t>(
-        reinterpret_cast<uintptr_t>(constructor())) >> 2);
+  int hash = ObjectAddressForHashing(constructor()) >> 2;
 
   // XOR-ing the prototype and constructor directly yields too many zero bits
   // when the two pointers are close (which is fairly common).
   // To avoid this we shift the prototype 4 bits relatively to the constructor.
-  hash ^= (static_cast<uint32_t>(
-        reinterpret_cast<uintptr_t>(prototype())) << 2);
+  hash ^= ObjectAddressForHashing(prototype()) << 2;

[Jakob Kummerow, 2014/09/29 10:38:21]

To document our offline discussion for posterity: I think it would make sense to
shift this value further left, so all 32 hash bits are (potentially) filled with
input bits.

 
   return hash ^ (hash >> 16) ^ bit_field2();
 }
 
 
 static bool CheckEquivalent(Map* first, Map* second) {
   return
     first->constructor() == second->constructor() &&
     first->prototype() == second->prototype() &&
     first->instance_type() == second->instance_type() &&
@@ skipping 7338 matching lines @@
   Handle<DependentCode> codes =
       DependentCode::Insert(handle(cell->dependent_code(), info->isolate()),
                             DependentCode::kPropertyCellChangedGroup,
                             info->object_wrapper());
   if (*codes != cell->dependent_code()) cell->set_dependent_code(*codes);
   info->dependencies(DependentCode::kPropertyCellChangedGroup)->Add(
       cell, info->zone());
 }
 
 } }  // namespace v8::internal