Strange unsigned compare

[rfalke]

Version: 9a281a1

Subject: https://github.com/rfalke/decompiler-subjects/blob/master/from_holdec/branchless_arithmetic/arm64_macho/by_reko.c

Source:

u64b __attribute__((noinline)) sat_addu64b(u64b x, u64b y) {
  u64b res = x + y;
  res |= -(res < x);

  return res;
}

Output:

word64 _sat_addu64b(word64 x0, word64 x1)
{
	up64 x8_5 = x0 + x1;
	return x8_5 < 0x00 ? x8_5 : ~0x00;
}

I couldn't find a definition of up64 but I assume it is unsigned. But than x8_5 < 0x00 doesn't make sense.