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