In src/share/vm/oops/oop.inline.hpp size_given_klass():
...
#ifdef _M_IA64
      // The Windows Itanium Aug 2002 SDK hoists this load above
      // the check for s < 0. An oop at the end of the heap will
      // cause an access violation if this load is performed on a non
      // array oop. Making the reference volatile prohibits this.
      // (%%% please explain by what magic the length is actually fetched!)
      volatile int *array_length;
      array_length = (volatile int *)( (intptr_t)this +
                          arrayOopDesc::length_offset_in_bytes() );
      assert(array_length > 0, "Integer arithmetic problem somewhere");
      // Put into size_t to avoid overflow.
      size_in_bytes = (size_t) array_length;
      size_in_bytes = size_in_bytes << Klass::layout_helper_log2_element_size(lh);
#else

This has been broken since 2005. As pointed out by [~kbarrett]

And there's other code in os_windows.cpp specific to IA64.