Re: Saturation arithmetic woes.

[Martin <martin.dowie@btopenworld.com>]

On Jul 30, 10:09 am, Jean-Pierre Rosen <ro...@adalog.fr> wrote:
> Niklas Holsti a écrit :
>
> >> package body Saturation is
>
> >>   function "+"
> >>     (Left  : Saturated_Type;
> >>      Right : Saturated_Type) return Saturated_Type
> >>   is
> >>     Temp_Left  : constant Discrete_Type := Discrete_Type (Left);
> >>     Temp_Right : constant Discrete_Type := Discrete_Type (Right);
> >>   begin
> >>     if Temp_Left + Temp_Right > Discrete_Type'Last then
>
> > Depending on the actual type associated with Discrete_Type (and assuming
> > you use the predefined "+" for that type), you risk getting an overflow
> > on Temp_Left + Temp_Right. If you have overflow checks disabled (as Gnat
> > has by default) the sum may "wrap around" and seem to be less than
> > Discrete_Type'Last, giving you the wrong result. If you have overflow
> > checks enabled, you get a Constraint_Error instead of
> > Discrete_Type'Last. Ditto for underflow and Discrete_Type'First.
>
> But of course, the simplest way to check is to write:
>    if Temp_Left > Discrete_Type'Last - Temp_Right then
>
> No overflow, ever! Of course, you need a "-" operator, and I assume it
> is consistent with "+"...
> --
> ---------------------------------------------------------
>            J-P. Rosen (ro...@adalog.fr)
> Visit Adalog's web site athttp://www.adalog.fr

Temp_Right = -1 ?

Cheers
-- Martin