Minor fixes for background compilation

runtime/vm/stub_code.cc

 // Copyright (c) 2012, 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/stub_code.h"
 
 #include "platform/assert.h"
 #include "platform/globals.h"
 #include "vm/assembler.h"
 #include "vm/disassembler.h"
@@ skipping 106 matching lines @@
     return AllocateArray_entry()->code();
   }
   Code& stub = Code::Handle(zone, cls.allocation_stub());
   if (stub.IsNull()) {
     Assembler assembler;
     const char* name = cls.ToCString();
     StubCode::GenerateAllocationStubForClass(&assembler, cls);
 
     if (thread->IsMutatorThread()) {
       stub ^= Code::FinalizeCode(name, &assembler, false /* optimized */);
-      stub.set_owner(cls);
-      cls.set_allocation_stub(stub);
+      // Check if background compilation thread has not already added the stub.
+      if (cls.allocation_stub() == Code::null()) {
+        stub.set_owner(cls);
+        cls.set_allocation_stub(stub);
+      }
     } else {
       // This part of stub code generation must be at a safepoint.
       // Stop mutator thread before creating the instruction object and
       // installing code.
       // Mutator thread may not run code while we are creating the
       // instruction object, since the creation of instruction object
       // changes code page access permissions (makes them temporary not
       // executable).
       {
         SafepointOperationScope safepoint_scope(thread);
+        stub = cls.allocation_stub();
+        // Check if stub was already generated.
+        if (!stub.IsNull()) {
+          return stub.raw();
+        }
         // Do not Garbage collect during this stage and instead allow the
         // heap to grow.
         NoHeapGrowthControlScope no_growth_control;
         stub ^= Code::FinalizeCode(name, &assembler, false /* optimized */);
         stub.set_owner(cls);
         cls.set_allocation_stub(stub);
       }
       Isolate* isolate = thread->isolate();
       if (isolate->heap()->NeedsGarbageCollection()) {
         isolate->heap()->CollectAllGarbage();
@@ skipping 55 matching lines @@
   if ((name##_entry() != NULL) &&                                              \
       (entry_point == name##_entry()->EntryPoint())) {                         \
     return ""#name;                                                            \
   }
   VM_STUB_CODE_LIST(VM_STUB_CODE_TESTER);
 #undef VM_STUB_CODE_TESTER
   return NULL;
 }
 
 }  // namespace dart