Use a better hash function in IdentityMap.

src/identity-map.cc

 // Copyright 2015 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/identity-map.h"
 
-#include "src/heap/heap.h"
+#include "src/base/functional.h"
 #include "src/heap/heap-inl.h"
 #include "src/zone-containers.h"
 
 namespace v8 {
 namespace internal {
 
 static const int kInitialIdentityMapSize = 4;
 static const int kResizeFactor = 4;
 
 IdentityMapBase::~IdentityMapBase() { Clear(); }
@@ skipping 17 matching lines @@
 IdentityMapBase::RawEntry IdentityMapBase::Insert(Object* key) {
   int index = InsertIndex(key);
   DCHECK_GE(index, 0);
   return &values_[index];
 }
 
 
 int IdentityMapBase::Hash(Object* address) {
   CHECK_NE(address, heap_->not_mapped_symbol());
   uintptr_t raw_address = reinterpret_cast<uintptr_t>(address);
-  // Xor some of the upper bits, since the lower 2 or 3 are usually aligned.

[titzer, 2016/02/16 13:02:27]

What about the original comment? Objects will typically have the low-order two
bits set to "01" or "001" depending on the architecture. Does the hash utility
deal with this?

[rmcilroy, 2016/02/16 14:00:55]

It uses 64-bit mix functions:
https://gist.github.com/badboy/6267743#64-bit-mix-functions
This does a bunch of shifting from upper to lower bits, so it should still be
good for aligned pointers. 

-  return static_cast<int>((raw_address >> 11) ^ raw_address);
+  return static_cast<int>(hasher_(raw_address));
 }
 
 
 int IdentityMapBase::LookupIndex(Object* address) {
   int start = Hash(address) & mask_;
   Object* not_mapped = heap_->not_mapped_symbol();
   for (int index = start; index < size_; index++) {
     if (keys_[index] == address) return index;  // Found.
     if (keys_[index] == not_mapped) return -1;  // Not found.
   }
@@ skipping 131 matching lines @@
     DCHECK_GE(index, 0);
     values_[index] = old_values[i];
   }
 
   // Unregister old keys and register new keys.
   heap_->UnregisterStrongRoots(old_keys);
   heap_->RegisterStrongRoots(keys_, keys_ + size_);
 }
 }  // namespace internal
 }  // namespace v8