Frama-C-discuss mailing list archives

This page gathers the archives of the old Frama-C-discuss archives, that was hosted by Inria's gforge before its demise at the end of 2020. To search for mails newer than September 2020, please visit the page of the new mailing list on Renater.

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

[Frama-c-discuss] Small function with Shifting

  • Subject: [Frama-c-discuss] Small function with Shifting
  • From: michaelschausten at googlemail.com (Michael Schausten)
  • Date: Thu, 20 May 2010 13:24:27 +0200
Hello,

I'm not successfull verifying the following piece of code:


#include <math.h>

#pragma JessieIntegerModel(math)

typedef struct s1 { unsigned int mantissa; int exponent; };

/*@ requires (0 < (*e).mantissa < 10) && (10 < (*e).exponent < 20);
    ensures ((*e).mantissa * (1 << (*e).exponent)) == 
(\old((*e).mantissa) * (1 << \old((*e).exponent)));
*/
int sizetype_normalize(struct s1* e) {
   
    unsigned int min_mantissa = 1 << 21;
   
    if ((*e).mantissa < min_mantissa) {
        (*e).mantissa <<= 8;
        (*e).exponent -= 8;
    }

    return 0;
}


The computation actually is mantissa * 2^exponent. When i shift the 
mantissa 8 bits to the left, but therefore decrease the exponent by 8, 
the result should be the same as before. However, Alt-Ergo can't prove 
this. What am I doing wrong?