Need advice about coding style

A slightly quirky one, I agree. However, the following code is confusing Quartus:

  IF ci = '0' THEN
   co <= aab;
  ELSIF ci = '1' THEN
   aob := axb XOR aab; -- the same as (a OR b)
   co <= aob;
  END IF;

Using 'ELSE' covers all other cases. By using 'ELSIF' Quartus believes there may be some other paths it must handle - even though there aren't in this case.

Change these lines to:

  IF ci = '0' THEN
   co <= aab;
  ELSE
   aob := axb XOR aab; -- the same as (a OR b)
   co <= aob;
  END IF;

I recommend you always use an ELSE at the end of conditional statements that infer combinational logic.

Cheers,

Alex