debug: turn off sparse bitmap pointer alignment.

third_party/WebKit/Source/platform/heap/HeapPage.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 545 matching lines @@
     size_t pageSize = availablePages->size();
 #if DEBUG_HEAP_COMPACTION
     if (!freedPageCount)
       LOG_HEAP_COMPACTION("Releasing:");
     LOG_HEAP_COMPACTION(" [%p, %p]", availablePages, availablePages + pageSize);
 #endif
     freedSize += pageSize;
     freedPageCount++;
     BasePage* nextPage;
     availablePages->unlink(&nextPage);
+    // Clear out the page before adding it to the free page pool, which
+    // decommits it. Recommitting the page must find a zeroed page later.
+    // We cannot assume that the OS will hand back a zeroed page across
+    // its "decommit" operation.
+    DCHECK(!availablePages->isLargeObjectPage());
+    NormalPage* unusedPage = reinterpret_cast<NormalPage*>(availablePages);
+    memset(unusedPage->payload(), 0, unusedPage->payloadSize());
     availablePages->removeFromHeap();
     availablePages = static_cast<NormalPage*>(nextPage);
   }
   if (freedPageCount)
     LOG_HEAP_COMPACTION("\n");
   heap.compaction()->finishedArenaCompaction(this, freedPageCount, freedSize);
 }
 
 #if ENABLE(ASSERT)
 bool NormalPageArena::isConsistentForGC() {
@@ skipping 921 matching lines @@
     headerAddress += size;
     markedObjectSize += size;
     allocationPoint += size;
     DCHECK(allocationPoint <= currentPage->payloadSize());
   }
   if (markedObjectSize)
     pageArena->getThreadState()->increaseMarkedObjectSize(markedObjectSize);
 
 #if ENABLE(ASSERT) || defined(LEAK_SANITIZER) || defined(ADDRESS_SANITIZER) || \
     defined(MEMORY_SANITIZER)
-  // Zap the page, which is now available and will either be compacted into
-  // or freed.
+  // Zap the unused portion, until it is either compacted into or freed.
   if (currentPage != this) {
     FreeList::zapFreedMemory(payload(), payloadSize());
   } else {
     FreeList::zapFreedMemory(payload() + allocationPoint,
                              payloadSize() - allocationPoint);
   }
 #endif
 }
 
 void NormalPage::makeConsistentForGC() {
@@ skipping 381 matching lines @@
 
   m_hasEntries = true;
   size_t index = hash(address);
   ASSERT(!(index & 1));
   Address cachePage = roundToBlinkPageStart(address);
   m_entries[index + 1] = m_entries[index];
   m_entries[index] = cachePage;
 }
 
 }  // namespace blink