# Frama-C-discuss mailing list archives

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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
• 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).
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.
> 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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