--- layout: fc_discuss_archives title: Message 14 from Frama-C-discuss on November 2009 ---
Hello St?phane, > 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