# ifdef __ia64__

  // On ia64 virtual address space looks like one of:

  //   virt_addr_64 = [ <63..61> | <unimplemented> | L3 | L2 | L1 | offset ]

  //   virt_addr_64 = [ <63..61> | <unimplemented> | L4 | L3 | L2 | L1 | offset ]

  // where L{1..L4} are page tables. Each page table (except top-level L3 or L4)

  // is itself a page-size entry and can store PageSize / 8 entries. Top-level

  // entry is 1/8 of of L1/L2 (as 3 upper bits are part of <63..61> address part).

  // Note: that makes addressable size directly depend on page size.

  //

  // We conservatively assume 3 levels of page tables here. This makes the

  // following formula:

  //   L3     = log2(PAGE / 8 / 8) = log2(PAGE / 8) - 3

  //   L2                          = log2(PAGE / 8)

  //   L1                          = log2(PAGE / 8)

  //   offset = log2(PAGE)         = log2(PAGE / 8) + 3

  // thus

  //  L3 + L2 + L1 + offset = 4 * log2(PAGE / 8)

  // For more details see http://www.ia64-linux.org/doc/IA64linuxkernel.PDF

  // (slide 19: "user regions").

  static uint64_t ia64_virt_bits = std::min<uint64_t>(

      4 * (mozilla::FloorLog2(gc::SystemPageSize() / 8)),

      46);

  rand >>= (64 - ia64_virt_bits);

# else

# endif