- Document the store barrier.

runtime/vm/assembler_x64.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/globals.h"
 #if defined(TARGET_ARCH_X64)
 
 #include "vm/assembler.h"
 #include "vm/heap.h"
 #include "vm/memory_region.h"
@@ skipping 1393 matching lines @@
   if (object.IsSmi()) {
     cmpq(reg, Immediate(reinterpret_cast<int64_t>(object.raw())));
   } else {
     ASSERT(reg != TMP);
     LoadObject(TMP, object);
     cmpq(reg, TMP);
   }
 }
 
 
-// Destroys the incoming value.
+// Destroys the value register.
 void Assembler::StoreIntoObjectFilter(Register object,
                                       Register value,
                                       Label* no_update) {
-  andl(value, Immediate(0x9));
+  // For the value we are only interested in the new/old bit and the tag bit.
+  andl(value, Immediate(kNewObjectAlignmentOffset | kHeapObjectTag));
+  // Shift the tag bit into the carry.
   shrl(value, Immediate(1));
+  // Add the tag bits together, if the value is not a Smi the addition will
+  // overflow into the next bit, leaving us with a zero low bit.
   adcl(value, object);
-  andl(value, Immediate(0xf));
-  cmpl(value, Immediate(0x6));
+  // Mask out higher, uninteresting bits which were polluted by dest.
+  andl(value, Immediate(kObjectAlignment - 1));
+  // Compare with the expected bit pattern.
+  cmpl(value, Immediate(
+      (kNewObjectAlignmentOffset >> 1) + kHeapObjectTag +
+      kOldObjectAlignmentOffset + kHeapObjectTag));
   j(NOT_ZERO, no_update, Assembler::kNearJump);
 }
 
 
 void Assembler::StoreIntoObject(Register object,
                                 const FieldAddress& dest,
                                 Register value) {
   movq(dest, value);
 
   Label done;
@@ skipping 299 matching lines @@
     comments.SetCommentAt(i, comments_[i]->comment());
   }
 
   return comments;
 }
 
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_X64