Fill the header of an Instructions object with null

runtime/vm/object.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/object.h"
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/assembler.h"
 #include "vm/cpu.h"
@@ skipping 1743 matching lines @@
 
 RawString* Object::DictionaryName() const {
   return String::null();
 }
 
 
 void Object::InitializeObject(uword address,
                               intptr_t class_id,
                               intptr_t size,
                               bool is_vm_object) {
-  uword initial_value = (class_id == kInstructionsCid)
-      ? Assembler::GetBreakInstructionFiller() : reinterpret_cast<uword>(null_);
+  const uword break_instruction_value = Assembler::GetBreakInstructionFiller();
+  const uword null_value = reinterpret_cast<uword>(null_);
+  const bool is_instructions = class_id == kInstructionsCid;
+  uword initial_value =
+      is_instructions ? break_instruction_value : null_value;
   uword cur = address;
   uword end = address + size;
+  if (is_instructions) {
+    // Fill the header with null. The remainder of the Instructions object will
+    // be filled with the break_instruction_value.
+    uword header_end = address + Instructions::HeaderSize();
+    while (cur < header_end) {
+      *reinterpret_cast<uword*>(cur) = null_value;
+      cur += kWordSize;
+    }
+  }
   while (cur < end) {
     *reinterpret_cast<uword*>(cur) = initial_value;
     cur += kWordSize;
   }
   uword tags = 0;
   ASSERT(class_id != kIllegalCid);
   tags = RawObject::ClassIdTag::update(class_id, tags);
   tags = RawObject::SizeTag::update(size, tags);
   tags = RawObject::VMHeapObjectTag::update(is_vm_object, tags);
   reinterpret_cast<RawObject*>(address)->tags_ = tags;
@@ skipping 19677 matching lines @@
   return tag_label.ToCString();
 }
 
 
 void UserTag::PrintJSONImpl(JSONStream* stream, bool ref) const {
   Instance::PrintJSONImpl(stream, ref);
 }
 
 
 }  // namespace dart