progress towards thread support

This includes support for using the least significant bits of the class
pointer to indicate object state, which we'll use to indicate the
presence of a monitor pointer, among other things.

src/common.h

@@ -26,6 +26,9 @@ typedef void* object;
 const unsigned BytesPerWord = sizeof(uintptr_t);
 const unsigned BitsPerWord = BytesPerWord * 8;
 
+const uintptr_t PointerMask
+= ((~static_cast<uintptr_t>(0)) / BytesPerWord) * BytesPerWord;
+
 const unsigned LikelyPageSizeInBytes = 4 * 1024;
 
 inline unsigned
@@ -109,6 +112,13 @@ cast(object p, unsigned offset)
   return *reinterpret_cast<T*>(static_cast<uint8_t*>(p) + offset);
 }
 
+template <class T>
+inline T*
+mask(T* p)
+{
+  return reinterpret_cast<T*>(reinterpret_cast<uintptr_t>(p) & PointerMask);
+}
+
 }
 
 #endif//COMMON_H