Become: Also forward identity hashes.

runtime/vm/become.cc

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/become.h"
 
 #include "platform/assert.h"
 #include "platform/utils.h"
 
 #include "vm/dart_api_state.h"
@@ skipping 243 matching lines @@
       FATAL("become: Cannot forward to multiple targets");
     }
     if (after_obj->IsForwardingCorpse()) {
       // The Smalltalk become does allow this, and for very special cases
       // it is important (shape changes to Class or Mixin), but as these
       // cases do not arise in Dart, better to prohibit it.
       FATAL("become: No indirect chains of forwarding");
     }
 
     ForwardObjectTo(before_obj, after_obj);
+
+    // Forward the identity hash too if it has one.
+    intptr_t hash = heap->GetHash(before_obj);
+    if (hash != 0) {
+      ASSERT(heap->GetHash(after_obj) == 0);
+      heap->SetHash(after_obj, hash);
+    }

[siva, 2016/08/19 00:08:15]

How does this work for internal VM objects, for example if before_obj and after
_obj are strings then it is not correct to get the hash of before_obj and set it
to the hash of after_obj?

[rmacnak, 2016/08/19 00:16:54]

A String's usual hash is the one inline in the object. This forwards identity
hashes in the weak tables.

   }
 
   {
     // Follow forwarding pointers.
 
     // C++ pointers
     ForwardPointersVisitor pointer_visitor(isolate);
     isolate->VisitObjectPointers(&pointer_visitor, true);
 
     // Weak persistent handles.
@@ skipping 17 matching lines @@
   }
 
 #if defined(DEBUG)
   for (intptr_t i = 0; i < before.Length(); i++) {
     ASSERT(before.At(i) == after.At(i));
   }
 #endif
 }
 
 }  // namespace dart