X64: Align start of remembered set to a 4-byte boundary

src/spaces.h

 // Copyright 2006-2008 the V8 project authors. 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
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 212 matching lines @@
 #ifdef V8_TARGET_ARCH_X64
   static const int kRSetOffset = 4 * kPointerSize;  // Room for four pointers.
 #else
   static const int kRSetOffset = 0;
 #endif
   // The end offset of the remembered set in a page
   // (heaps are aligned to pointer size).
   static const int kRSetEndOffset = kRSetOffset + kPageSize / kBitsPerPointer;
 
   // The start offset of the object area in a page.
-  static const int kObjectStartOffset = kRSetEndOffset;
+  // This needs to be at least (bits per uint32_t) * kBitsPerPointer,
+  // to align start of rset to a uint32_t address.
+  static const int kObjectStartOffset = 256;
 
   // The start offset of the remembered set in a page.
   static const int kRSetStartOffset = kRSetOffset +
       kObjectStartOffset / kBitsPerPointer;
 
   // Object area size in bytes.
   static const int kObjectAreaSize = kPageSize - kObjectStartOffset;
 
   // Maximum object size that fits in a page.
   static const int kMaxHeapObjectSize = kObjectAreaSize;
@@ skipping 1495 matching lines @@
 
  private:
   LargeObjectChunk* current_;
   HeapObjectCallback size_func_;
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_SPACES_H_