[fpc-devel] Overflow checks and Trunc/Round functions

[Jonas Maebe]

On 13 Jun 2009, at 13:31, Alexander Klenin wrote:

> Currently, Round and Trunc functions ignore the state of  
> $OVERFLOWCHECKS
> compiler directive. The following code prints "-2 -2":
>
> {$MODE OBJFPC}{$OVERFLOWCHECKS ON}
> var
>  a: Integer;
>  b: Double;
> begin
>  b := MaxInt;
>  a := Round(2.0 * b);
>  Writeln(a);
>  a := Trunc(2.0 * b);
>  Writeln(a);
> end.
>
> This is Delphi-compatible.
> However, I think this is a bug, even if Delphi-inherited, and should
> be fixed in fpc/objfpc mode.
> What do others think?

In Borland-speak, these are not overflow errors, but range errors.  
Turn on range checking, and you will get an error.

The difference between range and overflow errors is as follows:
* range errors occur when an implicit type conversion occurs from one  
type to another (e.g., when assigning, or when using it as a  
parameter), and the value is not legal for the destination type
* overflow errors occur when performing a mathematical operation on a  
value and the result cannot be represented in the type used for  
performing this operation (this "operation type" depends both on the  
target platform and the types of the operands)


Jonas