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

Frama-C Bug Tracking System - Frama-C

View Issue Details

Project

Category

View Status

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