Word mantissa = Word.fromInt(descriptor>>>VM_MANTISSA_SHIFT);
Word mantissa = Word.fromIntSignExtend(descriptor>>>VM_MANTISSA_SHIFT);
  public static Address getStart(int descriptor) throws InlinePragma {    if (Assert.VERIFY_ASSERTIONS) Assert._assert(isContiguous(descriptor));    Word mantissa = Word.fromInt(descriptor>>>VM_MANTISSA_SHIFT);    int exponent = (descriptor & VM_EXPONENT_MASK)>>>VM_EXPONENT_SHIFT;    return mantissa.lsh(VM_BASE_EXPONENT+exponent).toAddress();  }