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[Frama-c-discuss] problems with recognition of the variant

  • Subject: [Frama-c-discuss] problems with recognition of the variant
  • From: Patrick.Baudin at cea.fr (BAUDIN Patrick)
  • Date: Fri, 12 Dec 2014 09:13:29 +0100
  • In-reply-to: <548A97AE.7010808@cs.unipr.it>
  • References: <CAAEWojUFkWjLv9yDjp37=zP0coN1xnFxuuhNc0ER_570HpFpYQ@mail.gmail.com> <548A97AE.7010808@cs.unipr.it>
Your code was :

void testVariant(unsigned long n) {
   unsigned long i= n;
   /*@loop variant i;*/
   while (i >= 2UL) {
     i >>= 2UL;
     }
}

Replacing the shift by integer division is a good idea since it could 
help to get automatic proof (the use of the shift will require a coq proof).
That will help you in guessing the missing loop invariants.
They are needed for the proof of your loop variant.
Also, perhaps that "i/2" is a better variant.

Le 12/12/2014 08:22, Roberto Bagnara a ?crit :
> On 20/11/2014 18:03, ALESSIO BORTOLOTTI wrote:
>> While I was trying to prove a while loop containing shifts, I ran into a problem: the program couldn't recognize the variable on which I have done the shift as a variant.
> I stumbled upon the same issue.  Apparently the latest version
> of Frama-C is not able to prove that a nonzero unsigned quantity
> shifted to the right by a nonzero number of positions results
> in a strict decrease.  Replacing shift by integer division does
> not help.  Can someone on this list explain what is going on?
> Is there a workaround for this problem?
> Kind regards,
>
>      Roberto
>

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