0000414: imprecision in widening/narrowing for char and short index

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0000414

Frama-C

Plug-in > Eva

public

2010-02-19 14:37

2010-04-13 15:33

Reporter

sduprat

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

0000414: imprecision in widening/narrowing for char and short index

Description

Hello Stéphane,

> 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; }
_____

It's interesting that you noticed this behavior,
because I fixed a comparable problem on short
notice this summer for an intern who had to demo
his plug-in the following week.

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 C 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.

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

I have filed this issue as "feature wish" in the Bug Tracking
System, so that it is not forgotten.
http://bts.frama-c.com/view.php?id=325 [^]

Pascal

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duplicate of

0000325

closed

pascal

Precise widening when a loop condition involves a char or short

Notes
(0000703) pascal (reporter) 2010-02-19 16:10	Stéphane, quand je dis que je mets une idée dans le BTS et que je donne le numéro, pas la peine de re-soumettre la même pour me rappeler, il suffit de t'abonner à ce bug report ou de voter dessus.

Issue History
Date Modified	Username	Field	Change
2010-02-19 14:37	sduprat	New Issue
2010-02-19 14:37	sduprat	Status	new => assigned
2010-02-19 14:37	sduprat	Assigned To	=> pascal
2010-02-19 14:46	pascal	Relationship added	duplicate of 0000325
2010-02-19 16:10	pascal	Note Added: 0000703
2010-02-19 16:11	pascal	Status	assigned => resolved
2010-02-19 16:11	pascal	Resolution	open => fixed
2010-04-13 15:30	signoles	Status	resolved => new
2010-04-13 15:31	signoles	Status	new => closed
2010-04-13 15:33	signoles	Fixed in Version	=> Frama-C Boron
2018-01-12 14:26	signoles	Category	Plug-in > value analysis => Plug-in > Eva