Comment by emilfihlman

I mean, isn't the trivial answer to just collapse the if else tree into just math that's evaluated always?

  u32 a = (code <= 0x7F);
  u32 b = (code <= 0x07FF);
  u32 c = ((code < 0xD800) || 
  (0xDFFF < code));
  u32 d = (code <= 0xFFFF) * c;
  u32 e = (code <= 0x10FFFF);
  u32 v = (c && e);
  return(-1 * !v + v * (4 - a - b - d));

Highly likely easy to optimise.