Precise sweeping makes zapping of garbage memory in UpdatePointer obsolete.

src/heap/mark-compact.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/v8.h"
 
 #include "src/base/atomicops.h"
 #include "src/code-stubs.h"
 #include "src/compilation-cache.h"
 #include "src/cpu-profiler.h"
@@ skipping 2927 matching lines @@
   // with NULL. In this case we do not need to transfer this entry to
   // the store buffer which we are rebuilding.
   // We perform the pointer update with a no barrier compare-and-swap. The
   // compare and swap may fail in the case where the pointer update tries to
   // update garbage memory which was concurrently accessed by the sweeper.
   if (new_addr != NULL) {
     base::NoBarrier_CompareAndSwap(
         reinterpret_cast<base::AtomicWord*>(address),
         reinterpret_cast<base::AtomicWord>(object),
         reinterpret_cast<base::AtomicWord>(HeapObject::FromAddress(new_addr)));
-  } else {
-    // We have to zap this pointer, because the store buffer may overflow later,
-    // and then we have to scan the entire heap and we don't want to find
-    // spurious newspace pointers in the old space.
-    // TODO(mstarzinger): This was changed to a sentinel value to track down
-    // rare crashes, change it back to Smi::FromInt(0) later.
-    base::NoBarrier_CompareAndSwap(
-        reinterpret_cast<base::AtomicWord*>(address),
-        reinterpret_cast<base::AtomicWord>(object),
-        reinterpret_cast<base::AtomicWord>(Smi::FromInt(0x0f100d00 >> 1)));
   }
 }
 
 
 static String* UpdateReferenceInExternalStringTableEntry(Heap* heap,
                                                          Object** p) {
   MapWord map_word = HeapObject::cast(*p)->map_word();
 
   if (map_word.IsForwardingAddress()) {
     return String::cast(map_word.ToForwardingAddress());
@@ skipping 1714 matching lines @@
   SlotsBuffer* buffer = *buffer_address;
   while (buffer != NULL) {
     SlotsBuffer* next_buffer = buffer->next();
     DeallocateBuffer(buffer);
     buffer = next_buffer;
   }
   *buffer_address = NULL;
 }
 }
 }  // namespace v8::internal