Re: [PPL-devel] Fwd: PPL and FPU rounding mode
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@cs.unipr.it
Hello PPL developers,
continuing a discussion I had with Prof. Bagnara:
Am 10.08.2011 21:38, schrieb Roberto Bagnara:
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.
Where in the documentation can I find the information which classes/operations are safe to use (ie not based on floats) without rounding and which not? I skimmed the online docs and was not able to figure this out myself.
Best regards,
Benjamin
On 08/12/11 13:53, Benjamin Hiller wrote:
Hello PPL developers,
continuing a discussion I had with Prof. Bagnara:
Am 10.08.2011 21:38, schrieb Roberto Bagnara:
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.
Where in the documentation can I find the information which classes/operations are safe to use (ie not based on floats) without rounding and which not? I skimmed the online docs and was not able to figure this out myself.
Hi Benjamin.
The templatic abstractions (such as Box, BD_Shape, Octagonal_Shape) instantiated with floating point types (float, double, long double) are based on floats. And so are all the abstractions that contain the above (such as powersets and products). All the other abstractions are not based on floats. Cheers,
Roberto
participants (2)
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Benjamin Hiller -
Roberto Bagnara