Use a better hash function in IdentityMap.

src/identity-map.cc

Index: src/identity-map.cc
diff --git a/src/identity-map.cc b/src/identity-map.cc
index 2f5739dad99341031c4ade37182c82636dfe4c87..97b70ae2fd7a096bd55ce8943f9c71aeef23e624 100644
--- a/src/identity-map.cc
+++ b/src/identity-map.cc
@@ -4,7 +4,7 @@
 
 #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"
 
@@ -42,8 +42,7 @@ IdentityMapBase::RawEntry IdentityMapBase::Insert(Object* key) {
 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));
 }