- Document the store barrier.

runtime/vm/assembler_ia32.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_IA32)
 
 #include "vm/assembler.h"
 #include "vm/heap.h"
 #include "vm/memory_region.h"
@@ skipping 1366 matching lines @@
       buffer_.EmitObject(object);
     } else {
       EmitUint8(0x81);
       EmitOperand(7, Operand(reg));
       buffer_.EmitObject(object);
     }
   }
 }
 
 
+// Destroys the value register.
 void Assembler::StoreIntoObjectFilter(Register object,
                                       Register value,
                                       Label* no_update) {
-  andl(value, Immediate(0x5));
+  // 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(0x7));
-  cmpl(value, Immediate(0x4));
+  // 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) {
   movl(dest, value);
   Label done;
   pushl(value);
@@ skipping 326 matching lines @@
     comments.SetCommentAt(i, comments_[i]->comment());
   }
 
   return comments;
 }
 
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_IA32