[PPL-devel] Fwd: PPL and FPU rounding mode

[Roberto Bagnara]

On 08/10/11 21:18, Benjamin Hiller wrote:
> Dear Prof. Roberto,
>
> thank you very much for your quick reply.
>
> Am 10.08.2011 18:38, schrieb Roberto Bagnara:
>> If you call restore_pre_PPL_rounding() reset the rounding mode to its
>> default (typically round-to-nearest) AND you use a PPL abstraction
>> based on floating point numbers without first calling
>> set_rounding_for_PPL() then the result computed by PPL will be bogus.
>> For example, you may have a non-empty box be mistaken for empty. Of
>> course, you may be lucky, but I would not count on that.
>
> Our data should be such that each polyhedron is far from being
> degenerated so I don't think they will become empty due to rounding.
>
>> Which PPL classes are you using? Many PPL classes are not based on
>> floating point numbers. If you only use those, then there is no problem
>> at all, whatever rounding mode is in effect.
>
> So far we are using the classes Variable, Variable_Set,
> Generator_System, Linear_Expression, Generator, C_Polytope,
> Constraint_System, Constraint. I'm not sure whether this is the
> information you need. As far as I know (I did not do the
> implementation myself), we scale everything such that we have
> integer points/constraint coefficients.

If by C_Polytope you mean C_Polyhedron and if the list of classes above
is otherwise complete, then your usage of the PPL does not depend
on the floating point rounding mode in effect.  So you should not worry.

> In any case, it is only slightly more inconvenient to turn on the
> appropriate rounding mode for PPL before using it so we might better
> do that.

This might be a wise thing if there is the danger that someone, one
day, may use other PPL abstractions based on machine floating-point
numbers.

> Best regards,
>
> Benjamin

Cheers,

   Roberto

P.S. Please direct all messages concerning the PPL to ppl-devel at cs.unipr.it

-- 
Prof. Roberto Bagnara
Applied Formal Methods Laboratory
Department of Mathematics, University of Parma, Italy
http://www.cs.unipr.it/~bagnara/
mailto:bagnara at cs.unipr.it