[cfe-dev] [analyzer] exploration strategies and paths
George Karpenkov via cfe-dev
cfe-dev at lists.llvm.org
Wed Jan 31 17:00:03 PST 2018
The list did not like a posting with many images,
so I have posted an evaluation to phabricator: https://reviews.llvm.org/M1
The text part was:
After fixing a few bugs, another evaluation of the approach shows considerably better results.
On openssl:
9 reports added
1 report removed
On postgresql:
377 reports added
43 reports removed
On sqlite3 + a few other misc files:
239 reports added
1 report removed
Note on histograms (here and below)
-> Histograms only show the ratio for same bugs (compared using issue hash),
that is, if the histogram says “decrease by a factor of three”, it means the new approach finds the *same* bug
with a path size 1/3d of the original
-> Histograms omit data points where the path length has remained the same
(as otherwise they completely dominate the histogram)
-> Relative histograms are provided as both ratio and logarithm of the ratio.
Logarithms of the ratio are convenient as they are symmetric in case changes balance out
(e.g. log(1/2) = -log(2/1))
> On Jan 30, 2018, at 4:23 PM, George Karpenkov <ekarpenkov at apple.com> wrote:
>
> Preliminary evaluation of a patch which prefers exploring nodes associated with statements which weren’t seen before first:
>
> On openssl:
>
> - Adds four reports
> - Removes four reports
> - Path lengths before: 317, 75, 75, 72, 70, 58, 50, 50, 44, 36, 23, 23, 21, 21, 20, 20, 19, 19, 19, 19, 18, 18, 18, 16, 15, 15, 15, 14, 13, 13, 12, 11, 11, 9, 7, 7, 6, 4
> - Path lengths after: 72, 60, 59, 53, 53, 52, 46, 38, 37, 30, 29, 28, 23, 21, 20, 19, 19, 19, 19, 19, 18, 16, 15, 15, 15, 15, 13, 13, 12, 12, 11, 9, 8, 7, 7, 7, 6, 4
>
> The quality of the added reports seems higher, mainly due to the fact that report length is shorter.
>
> On postgresql:
>
> - Added 80 reports
> - Removed 154 reports
> -> Out of those, 72 are reports on the yacc/bison autogenerated files, so whatever the cause is, good thing they are gone
> - The overall number of reports is 1188
> - Path lengths are lower on overall, but not in such a dramatic way
> - For many reports, I am quite confused as to why they got removed
>
> On sqlite:
>
> - 7 inserted, 7 removed
>
>> On Jan 30, 2018, at 1:10 PM, Artem Dergachev <noqnoqneo at gmail.com <mailto:noqnoqneo at gmail.com>> wrote:
>>
>>
>>
>> On 30/01/2018 12:40 PM, Gábor Horváth via cfe-dev wrote:
>>> Hi George, Artem,
>>>
>>> I am glad that you are looking into this problem!
>>>
>>> On 30 January 2018 at 01:12, George Karpenkov via cfe-dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org> <mailto:cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>>> wrote:
>>>
>>> Hi All,
>>>
>>> I was investigating recently bug reports with very long analyzer
>>> paths (more than a few hundred nodes).
>>> In many of such cases the path is long for no good reason: namely,
>>> the analyzer would go 3 times around the loop before
>>> going further.
>>> The issue is surprisingly common, and it was exacerbated with a
>>> recent bump of analyzer thresholds.
>>>
>>> The problem is reproduced on the following file:
>>>
>>> ```
>>> extern int coin();
>>>
>>> int foo() {
>>> int *x = 0;
>>> while (coin()) {
>>> if (coin())
>>> return *x;
>>> }
>>> return 0;
>>> }
>>>
>>> void bar() {
>>> while(coin())
>>> if (coin())
>>> foo();
>>> }
>>> ```
>>>
>>> While a shortest path to the error does not loop around, the
>>> current version of the analyzer
>>> will go around the loop three times before going further.
>>> (and we are quite fortunate that the unrolling limit for loops is
>>> three, otherwise it would keep going
>>> until the unrolling limit is reached).
>>>
>>> Multiple issues were discovered during the investigation.
>>>
>>> 1. Analyzer queue does not have a concept of priority, and
>>> performs a simple DFS by default.
>>> Thus if the successor of the if-branch under the loop in “bar"
>>> containing the desired destination is generated second,
>>> it will never be evaluated until the loop exploration limit is
>>> exhausted.
>>>
>>> 2. The previous issue slows down the exploration, but is not
>>> enough to get a pathological behavior of ultra-long paths.
>>> The second problem is a combination of:
>>> a) Block counter is not a part of a node's identity, and node A
>>> with a small block counter can be merged into a node B with a
>>> large block counter,
>>> and the resulting node will have a block counter associated with B.
>>>
>>>
>>> Sorry for the questions, just wanted to clarify some things. You mean ExplodedNodes? By merge, you mean the same thing as "caching-out"?
>>
>> Yeah, George notices that if we construct the same ExplodedNode on two different paths that have different block counts, we'd cache-out on the latter path, while the worklist element of the first path would still possess the original block count.
>>
>> Which happens a lot when we're evaluating foo() conservatively in this example.
>>
>> This isn't directly related to our problem though, as i noticed in http://lists.llvm.org/pipermail/cfe-dev/2018-January/056719.html <http://lists.llvm.org/pipermail/cfe-dev/2018-January/056719.html> .
>>
>>
>>> b) The issue in (a) is triggered due to our heuristic to abandon
>>> the function’s exploration and switch to conservative evaluation
>>> if we are already *inside* the function and the block limit has
>>> been reached.
>>>
>>> Issue (1) combined with (2-b) causes the problematic behavior: the
>>> issue is discovered on the longest path first,
>>> and by the time the shortest path gets to “bar”, the block limit
>>> is already reached, and the switch to conservative evaluation is
>>> performed.
>>>
>>> Thus there are two mitigation strategies currently being evaluated:
>>>
>>> i) Remove the heuristic in (2-b)
>>> ii) Use a priority queue to hold nodes which should be explored;
>>> prefer nodes which give new source code coverage over others
>>> (or alternatively prefer nodes with least depth of loop stack)
>>>
>>> Me and Artem have evaluated the option (i) and the results were
>>> surprisingly good: some reports disappear, and slightly more
>>> reports reappear.
>>> The quality of the new reports seems to be slightly better, and I
>>> am still trying to figure out exact reasons.
>>> I suspect merges resulting from heuristic (2-b) cause us to lose
>>> some actually valid reports.
>>>
>>>
>>> I also find the results surprising. If you have more information about the reasons please do not forget to follow up this thread. We are curious :)
>>>
>>>
>>> Option (ii) has not been evaluated fully yet, but current
>>> experiments show slightly more reports (5-10%), and a radical
>>> decline in report lengths
>>> (e.g. from 400+ to <100 for largest reports)
>>>
>>> Are there any thoughts on the matter?
>>>
>>> Personally I think we should do both (i) and (ii), even if they
>>> would shake up the results.
>>> - The original idea for heuristics (2-b) was to be able to produce
>>> a report even if we are out of budget, but since it actually
>>> results in less reports,
>>> I think the data does not validate the approach.
>>>
>>> - Option (ii) is AFAIK how most similar engines work, and should
>>> get us much larger coverage (and shorter paths) for the same node
>>> budget,
>>> even at the cost of log(N) overhead of the priority queue.
>>> Moreover, not having the priority queue will bite us later if we
>>> ever decide to further
>>> increase the analyzer budget or to increase the unroll limit.
>>>
>>>
>>> I wonder what will the performance implication be. But I also like the idea of having a priority queue. If we find that we get more and better report
>>> but also have worse performance, we can also consider reducing the analysis budget slightly.
>>>
>>> Regards,
>>> Gábor
>>>
>>>
>>> George
>>>
>>>
>>>
>>> _______________________________________________
>>> cfe-dev mailing list
>>> cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org> <mailto:cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>>
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev <http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev>
>>> <http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev <http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev>>
>>>
>>>
>>>
>>>
>>> _______________________________________________
>>> cfe-dev mailing list
>>> cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev <http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-dev/attachments/20180131/96a9da14/attachment.html>
More information about the cfe-dev
mailing list