Ensures that stubs added to each object pool on x64

runtime/vm/assembler_arm64.cc

 // Copyright (c) 2014, 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/globals.h"
 #if defined(TARGET_ARCH_ARM64)
 
 #include "vm/assembler.h"
 #include "vm/cpu.h"
 #include "vm/longjump.h"
@@ skipping 31 matching lines @@
     // object pool, but the HashMap uses 0 to indicate not found.
 
     object_pool_.Add(Bool::True(), Heap::kOld);
     patchable_pool_entries_.Add(kNotPatchable);
     object_pool_index_table_.Insert(ObjIndexPair(Bool::True().raw(), 1));
 
     object_pool_.Add(Bool::False(), Heap::kOld);
     patchable_pool_entries_.Add(kNotPatchable);
     object_pool_index_table_.Insert(ObjIndexPair(Bool::False().raw(), 2));
 
+    const Smi& vacant = Smi::Handle(Smi::New(0xfa >> kSmiTagShift));
+
     if (StubCode::UpdateStoreBuffer_entry() != NULL) {
       FindExternalLabel(&StubCode::UpdateStoreBufferLabel(), kNotPatchable);
     } else {
-      object_pool_.Add(Object::null_object(), Heap::kOld);
+      object_pool_.Add(vacant, Heap::kOld);
       patchable_pool_entries_.Add(kNotPatchable);
     }
 
     if (StubCode::CallToRuntime_entry() != NULL) {
       FindExternalLabel(&StubCode::CallToRuntimeLabel(), kNotPatchable);
     } else {
-      object_pool_.Add(Object::null_object(), Heap::kOld);
+      object_pool_.Add(vacant, Heap::kOld);
       patchable_pool_entries_.Add(kNotPatchable);
     }
 
     // Create fixed object pool entry for debugger stub.
     if (StubCode::BreakpointRuntime_entry() != NULL) {
       intptr_t index =
           FindExternalLabel(&StubCode::BreakpointRuntimeLabel(), kNotPatchable);
       ASSERT(index == kBreakpointRuntimeCPIndex);
     } else {
-      object_pool_.Add(Object::null_object(), Heap::kOld);
+      object_pool_.Add(vacant, Heap::kOld);
       patchable_pool_entries_.Add(kNotPatchable);
     }
   }
 }
 
 
 void Assembler::InitializeMemoryWithBreakpoints(uword data, intptr_t length) {
   ASSERT(Utils::IsAligned(data, 4));
   ASSERT(Utils::IsAligned(length, 4));
   const uword end = data + length;
@@ skipping 867 matching lines @@
     AddImmediate(temp_reg, temp_reg, class_offset, PP);
     ldr(TMP, Address(temp_reg, count_field_offset));
     AddImmediate(TMP, TMP, 1, PP);
     str(TMP, Address(temp_reg, count_field_offset));
   }
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_ARM64