[cfe-dev] Handling of loops in the Clang Static Analyzer

[Venugopal Raghavan via cfe-dev]

Hi,

Many thanks for all your replies. I am trying to see how best I can change
my checker/or the way I run my checker taking into account your
suggestions. However, I am afraid, I have one more question. Consider the
following test case:

test(int q, int z) {
   int b;
   int x;
   int p;

   if (z > 0) {        // S1
     p = q + 1;
     x = p + 1;
   }

   if (x <= (q + 1)) {   // S2
     b = 0;                 // S3
   }
   else {
     b = 1;
   }
   printf("%d\n", b);
}

If (z > 0) is true in statement S1 above, x gets the value q + 2
(propagating through p). In the exploded graph, I do see this being the
case. Hence, shouldn't the constaint solver recognize the fact that the
condition (x <= (q + 1)) in statement S2 is always false? However, in the
exploded graph, I do not see this condition and the assignment of 0 to b in
S3 getting pruned away when I follow the path in which (z > 0) is true.
Can't the constraint solver handle this case? Or have I goofed up in my
reasoning with the example (hopefully, my reasoning about the mutual
exclusivity of the statements holds true irrespective of the signedness
etc.)

Thanks.

Regards,
Venu.


On Mon, Feb 27, 2017 at 4:48 PM, Sean Eveson <eveson.sean at gmail.com> wrote:

> Hi Venugopal,
>
> > Sean Eveson (cc’d) did some initial work on loop widening to mitigate
> this problem.
>
> I started to work on this, but have unfortunately not had time to take the
> next steps. There is a mode which does 'loop widening' which is off by
> default as the state after the loop is completely cleared which can lead to
> false positives. The idea is to improve on the widening, by changing it to
> only clear state that might be affected by the loop.
>
> To enable the loop widening pass: "-analyzer-config widen-loops=true".
>
> > Maybe -analyzer-max-loop is interesting. It tells Clang analyzer how
> many times you want to go through loops. The bigger value the better
> analysis, but slower analysis.
>
> An issue you will run in to when changing analyzer-max-loop is that it
> also affects variable-bound loops. If you set the max loop higher to work
> around your problem you will see some slow down for the concrete-bound
> loops, but you will see a lot more for the variable-bound ones.
>
> Example:
> // Where the value of j is unknown.
> for (i = 0; i < j; ++i) {
>   // ...
> }
> foo(i);
>
> As far as I remember, with a max loop of 1000 and no other limits on the
> analyzer, `foo` will be called for all the values between 0..~1000. This is
> because the analyzer will branch each time it reaches the loop condition.
>
> Regards,
> Sean Eveson
>
> Sean Eveson
> SN Systems - Sony Computer Entertainment Group
>
> On Fri, Feb 24, 2017 at 7:54 PM, Devin Coughlin <dcoughlin at apple.com>
> wrote:
>
>> Hi Venugopal,
>>
>> > On Feb 23, 2017, at 7:07 PM, Venugopal Raghavan via cfe-dev <
>> cfe-dev at lists.llvm.org> wrote:
>> >
>> > Hi,
>> >
>> > I am re-sending the question I asked under a different thread so that
>> the subject is more relevant to the topic.
>> >
>> > I did not quite realize it earlier but it seems that the static
>> analyzer unrolls a loop up to a certain number of times and then stops
>> exploring paths beyond that. In the checker I have written, I get the
>> message "Block count exceeded" and then state exploration stops. As a
>> result, my checker give false positives and does not achieve what it sets
>> out to do.
>>
>> The analyzer makes no promises about exhaustivity and in many cases will
>> drop flows and stop path exploration. If your analysis depends on full path
>> exploration to prevent false positives (as compared to false negatives)
>> then it is going to be an uphill battle to eliminate false positives.
>>
>> As you noted, for concrete-bound loops stopping path exploration can be
>> particularly pernicious because after unrolling the loop N times the
>> analyze simply stops. Any code dominated by the loop exit will simply not
>> be explored, leading to false negatives.
>>
>> Sean Eveson (cc’d) did some initial work on loop widening to mitigate
>> this problem. The basic idea there was rather than simply stopping, the
>> analyzer would “forget” any specific information about a particular
>> iteration through the loop and proceed analyzing after the loop without any
>> assumptions about how many times the loop was unrolled. This would lose
>> some precision but gain coverage for code after the loop. This feature not
>> complete and is off by default, but you can see the beginnings of it at <
>> https://reviews.llvm.org/D12358>
>>
>> Devin
>>
>>
>
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