0000325: Precise widening when a loop condition involves a char or short

Frama-C Bug Tracking System - Frama-C

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Date Submitted

Last Update

0000325

Frama-C

Plug-in > Eva

public

2009-11-07 12:05

2014-02-12 16:55

Reporter

pascal

Assigned To

pascal

Priority

normal

Severity

feature

Reproducibility

always

Status

closed

Resolution

fixed

Platform

OS Version

Product Version

Frama-C Beryllium-20090902

Target Version

Fixed in Version

Frama-C Boron-20100401

Summary

0000325: Precise widening when a loop condition involves a char or short

Description

> Depending the type of i, the range is not the same.
> The range of i in the loop is [0..10] (for int) or [0..15] for char or
> short.
_____

void main(void)
{
char i=0;
int j=0;
while (i<10) i++;
while (j<10) j++;
}

[value] ====== VALUES COMPUTED ======
[value] Values for function main:
i IN {10; 11; 12; 13; 14; 15; }
j IN {10; }
_____

Note that the AST for the two loops is different:
  i = (char)0;
  j = 0;
  while ((int )i < 10) {i = (char )((int )i + 1);}
  while (j < 10) {j ++;}

CIL transforms the code thus because the standard
specifies that operators such as ++ do not operate on
types smaller than int, and that values of these types
are implicitly promoted to int in these conditions.

Meanwhile, in the absence of any loop-related
option, the value analysis tries to keep computations
short at the price of precision by using a technique
called "widening". In order to limit the loss of precision,
however, various heuristics are used, including a
syntactic one for the j loop that recognizes that
j IN [0..10] is a good candidate for the loop invariant.

These heuristic does not currently recognize the condition
((int )i < 10) as one where it would be valuable to try
the same kind of invariant.

Steps To Reproduce

Additional Information