Inline instance object hash code into object header on 64 bit.

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index 2c6422e7d0e5d68d8799ed3ba5034b6b3bc99174..435f6a42891b241c34b77e393155644602ba46f7 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -911,6 +911,21 @@ void Object::InitOnce(Isolate* isolate) {
   cls = void_class_;
   *void_type_ = Type::NewNonParameterizedType(cls);
 
+#if defined(HASH_IN_OBJECT_HEADER)

[Vyacheslav Egorov (Google), 2017/05/30 15:25:00]

How do we make sure that this list is exhaustive? 

For example what happens if I call:

(1).runtimeType.hash

?

[erikcorry, 2017/06/02 15:34:04]

Good point that does not work.

Instead, the new version uses the visitor to give hash codes to all objects in
the VM isolate.  PTAL.

+  // These need to be set now because they can end up in read-only memory where
+  // they can't be overwritten.  Some of them actually have hashCode methods,
+  // so the value won't be used once the VM is fully booted.
+  Object::SetCachedHash(sentinel_->raw(), 1);
+  Object::SetCachedHash(transition_sentinel_->raw(), 2);
+  Object::SetCachedHash(unknown_constant_->raw(), 3);
+  Object::SetCachedHash(non_constant_->raw(), 4);
+  Object::SetCachedHash(empty_array_->raw(), 256);
+  Object::SetCachedHash(zero_array_->raw(), 257);
+  Object::SetCachedHash(dynamic_type_->raw(), 103);
+  Object::SetCachedHash(void_type_->raw(), 104);
+  Object::SetCachedHash(null_instance_->raw(), 42);
+#endif
+
   // Allocate and initialize singleton true and false boolean objects.
   cls = Class::New<Bool>();
   isolate->object_store()->set_bool_class(cls);