Fixes slow object pool search on x64.

runtime/vm/assembler_x64.h

Index: runtime/vm/assembler_x64.h
===================================================================
--- runtime/vm/assembler_x64.h	(revision 27479)
+++ runtime/vm/assembler_x64.h	(working copy)
@@ -12,6 +12,7 @@
 #include "platform/assert.h"
 #include "platform/utils.h"
 #include "vm/constants_x64.h"
+#include "vm/hash_map.h"
 
 namespace dart {
 
@@ -837,6 +838,47 @@
   // Patchability of pool entries.
   GrowableArray<Patchability> patchable_pool_entries_;
 
+  // Pair type parameter for DirectChainedHashMap.
+  class ObjIndexPair {
+   public:
+    // TODO(zra): A WeakTable should be used here instead, but then it would
+    // also have to be possible to register and de-register WeakTables with the
+    // heap. Also, the Assembler would need to become a StackResource.
+    // Issue 13305. In the meantime...
+    // CAUTION: the RawObject* below is only safe because:
+    // The HashMap that will use this pair type will not contain any RawObject*
+    // keys that are not in the object_pool_ array. Since the keys will be
+    // visited by the GC when it visits the object_pool_, and since all objects
+    // in the object_pool_ are Old (and so will not be moved) the GC does not
+    // also need to visit the keys here in the HashMap.
+
+    // Typedefs needed for the DirectChainedHashMap template.
+    typedef RawObject* Key;

[Florian Schneider, 2013/09/16 09:09:48]

Is there a performance reason for using raw pointers here? If not, 
how about using const Object* instead of RawObject* as key? E.g. once there is a
compacting GC, the old-object argument does not hold anymore. The fewer places
that rely on old objects not moving, the better.

[Ivan Posva, 2013/09/16 15:02:21]

You cannot hash on handles when you are trying to get an identity map of
objects. You get many different handles for one object. See the TODO and issue
above...

+    typedef intptr_t Value;
+    typedef ObjIndexPair Pair;
+
+    ObjIndexPair(Key key, Value value) : key_(key), value_(value) { }
+
+    static Key KeyOf(Pair kv) { return kv.key_; }
+
+    static Value ValueOf(Pair kv) { return kv.value_; }
+
+    static intptr_t Hashcode(Key key) {
+      return reinterpret_cast<intptr_t>(key) >> kObjectAlignmentLog2;

[Florian Schneider, 2013/09/16 09:09:48]

Again, using the raw pointer as hash it not good if objects move.

How about something like String::Handle(String::New(key->ToCString())).Hash()?

+    }
+
+    static inline bool IsKeyEqual(Pair kv, Key key) {
+      return kv.key_ == key;

[Florian Schneider, 2013/09/16 09:09:48]

If you use handles as keys, this becomes

kv.key_->raw() == key->raw()

+    }
+
+   private:
+    Key key_;
+    Value value_;
+  };
+
+  // Hashmap for fast lookup in object pool.
+  DirectChainedHashMap<ObjIndexPair> object_pool_index_table_;
+
   int prologue_offset_;
 
   class CodeComment : public ZoneAllocated {