static double convert_to_double_core(const uint64_t* limb, int sz, int exp, bool sign) {
  uint64_t lsbits = 0;
  for (int i = 0; i < sz - 1; ++i)
    lsbits |= limb[i];
  uint64_t msw = limb[sz-1];
  lsbits |= msw & 1;

  // Don't use uint64_t -> double conversion, because there are compilers,
  // in particular MS VisualC++, that for some versions on some platform (I saw it on VS2013/x64/avx)
  // does not implement this conversion well when the argument is >= 2^63, i.e. exactly in our case.
  // On the other hand int64_t -> double conversion appears to be robust on all platforms that I tested

  int64_t m = msw >> 1;
  if (lsbits) m |= 1;
  if (sign) m = -m;
  return ldexp(m, exp-62);
}