Avoid unnecessary code target changes during code compaction.

src/mark-compact.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 2297 matching lines @@
     UpdatePointer(p);
   }
 
   void VisitPointers(Object** start, Object** end) {
     for (Object** p = start; p < end; p++) UpdatePointer(p);
   }
 
   void VisitEmbeddedPointer(RelocInfo* rinfo) {
     ASSERT(rinfo->rmode() == RelocInfo::EMBEDDED_OBJECT);
     Object* target = rinfo->target_object();
+    Object* old_target = target;
     VisitPointer(&target);
-    rinfo->set_target_object(target);
+    // Avoid unnecessary changes that might unnecessary flush the instruction
+    // cache.
+    if (target != old_target) {
+      rinfo->set_target_object(target);
+    }
   }
 
   void VisitCodeTarget(RelocInfo* rinfo) {
     ASSERT(RelocInfo::IsCodeTarget(rinfo->rmode()));
     Object* target = Code::GetCodeFromTargetAddress(rinfo->target_address());
     VisitPointer(&target);
     rinfo->set_target_address(Code::cast(target)->instruction_start());
   }
 
   void VisitDebugTarget(RelocInfo* rinfo) {
@@ skipping 1425 matching lines @@
   while (buffer != NULL) {
     SlotsBuffer* next_buffer = buffer->next();
     DeallocateBuffer(buffer);
     buffer = next_buffer;
   }
   *buffer_address = NULL;
 }
 
 
 } }  // namespace v8::internal