[cfe-dev] [StaticAnalyzer] False positive with loop handling

[Artem Dergachev via cfe-dev]

A false positive due to a known issue.

This happens because "nc = na + nb - 1" is too complicated for our 
extremely simple and fast solver (RangeConstraintManager) to handle. 
Because of that, we need to *conjure* a completely new symbol for "nc", 
and then it assumes that "nc" may be equal to 2 even when "na" and "nb" 
are equal to 1. Essentially, we can construct a correct SymSymExpr for 
"nc", but we'd need to do some bigger work to let the solver solve the 
equations.

I'm all for producing more SymSymExpr objects and simplifying them 
(through an alpha-renaming-like procedure) whenever their components get 
simplified out, just need time to implement this.

On 10/19/16 10:29 PM, Dominic Chen via cfe-dev wrote:
>
> Hello,
>
> I've been trying to track down a false positive with symbolic 
> execution of an array assignment within a loop, but I'm having some 
> trouble figuring out the root cause. In the attached loop.c file, the 
> analyzer identifies the LHS of the assignment equal operation on line 
> 11 to be a garbage value, but this is actually initialized on line 6.
>
> Initially, I thought that this was an issue with dead symbols being 
> incorrectly purged, because the trimmed ExplodedGraph shows that the 
> analyzer identifies the array "c" to be an ElementRegion "c: 
> &element{c, 0 S64b,int}" in an earlier node before 
> PreStmtPurgeDeadSymbols. But this is unchanged even after eliminating 
> calls to collectNode(), so I'm confused if the graph is representing 
> the actual symbolic execution exploration state, or just simply the 
> ordering of explored states from the worklist?
>
> The other place that I've been looking at is the branch condition 
> handling inside ExprEngine::processBranch(). When the symbolic 
> execution engine assumes either of the false/true branches, does that 
> also entail the body of the loop?
>
> Thanks,
>
> Dominic
>
>
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