- Remember the fact that an object has been added to the

runtime/vm/stub_code_x64.cc

 // Copyright (c) 2013, 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_X64)
 
 #include "vm/assembler.h"
 #include "vm/compiler.h"
 #include "vm/dart_entry.h"
@@ skipping 930 matching lines @@
 DECLARE_LEAF_RUNTIME_ENTRY(void, StoreBufferBlockProcess, Isolate* isolate);
 
 // Helper stub to implement Assembler::StoreIntoObject.
 // Input parameters:
 //   RAX: Address being stored
 void StubCode::GenerateUpdateStoreBufferStub(Assembler* assembler) {
   // Save registers being destroyed.
   __ pushq(RDX);
   __ pushq(RCX);
 
+  Label add_to_buffer;
+  // Check whether this object has already been remembered. Skip adding to the
+  // store buffer if the object is in the store buffer already.
+  // Spilled: EDX, ECX
+  // EAX: Address being stored
+  __ movq(RCX, FieldAddress(RAX, Object::tags_offset()));
+  __ testq(RCX, Immediate(1 << RawObject::kRememberedBit));
+  __ j(EQUAL, &add_to_buffer, Assembler::kNearJump);
+  __ popq(RCX);
+  __ popq(RDX);
+  __ ret();
+
+  __ Bind(&add_to_buffer);
+  __ orq(RCX, Immediate(1 << RawObject::kRememberedBit));
+  __ movq(FieldAddress(RAX, Object::tags_offset()), RCX);
+
   // Load the isolate out of the context.
   // RAX: Address being stored
   __ movq(RDX, FieldAddress(CTX, Context::isolate_offset()));
 
   // Load top_ out of the StoreBufferBlock and add the address to the pointers_.
   // RAX: Address being stored
   // RDX: Isolate
   intptr_t store_buffer_offset = Isolate::store_buffer_block_offset();
   __ movl(RCX,
           Address(RDX, store_buffer_offset + StoreBufferBlock::top_offset()));
@@ skipping 1093 matching lines @@
   __ cmpq(left, right);
   __ Bind(&done);
   __ popq(right);
   __ popq(left);
   __ ret();
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_X64