Re: Saturation arithmetic woes.

[Jean-Pierre Rosen <rosen@adalog.fr>]

Niklas Holsti a �crit :
>> 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 (rosen@adalog.fr)
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