[cfe-dev] GSoC 2018
George Karpenkov via cfe-dev
cfe-dev at lists.llvm.org
Mon Mar 26 15:13:54 PDT 2018
Great! Don’t forget to submit the PDF through the official website!
> On Mar 26, 2018, at 9:13 AM, Mikhail Ramalho <mikhail.ramalho at gmail.com> wrote:
>
> Hi all,
>
> I expanded the report quite a bit over the weekend, adding the suggested example, writing a bit more about the current state of the analyzer and implementing the suggestions (thank you George and Artem).
>
> Any feedback is welcome! I plan to submit the report tomorrow, mid-afternoon.
>
> Thank you,
>
> 2018-03-23 21:54 GMT+00:00 George Karpenkov <ekarpenkov at apple.com <mailto:ekarpenkov at apple.com>>:
> Hi Mikhail,
>
> Indeed, it seems that the constraint manager is more eager to throw out complex constraints than I’ve anticipated initially.
> It might be partly a good thing: now we need to know that evaluating disabling this heuristic and measuring a change in footprint
> on a large number of project needs to be a part of the schedule.
> An example where a change in this optimization is required would be also beneficial for proposal.
>
> Now onto the example.
>
> 1. Unlike SV-COMP, the analyzer treats “assert” as “assume”. So you would need to e.g. perform null pointer dereference to see whether there’s an error.
> (or use a debug checker and a special function, consult tests for those)
> 2. The following function seems to give the desired behavior:
>
> int foo(int x) {
> int *z = 0;
> if ((x & 1) && ((x & 1) ^ 1))
> return *z;
> return 0;
> }
>
> In a sense that the analyzer reports a false positive (if x & 1 is true, last bit is one, but then (x & 1) is the last bit and (x & 1) ^ 1 should be zero),
> it is non-obvious, and just using Z3 with constraints present in the graph should be sufficient.
>
> Regards,
> George
>
>> On Mar 23, 2018, at 12:48 PM, Mikhail Ramalho <mikhail.ramalho at gmail.com <mailto:mikhail.ramalho at gmail.com>> wrote:
>>
>> Hi,
>>
>> Just a quick update: I still haven't found a suitable case to add to the report but I tried to address every other comment there.
>>
>> Any feedback is welcome!
>>
>> Thank you,
>>
>>
>> 2018-03-23 18:39 GMT+00:00 Mikhail Ramalho <mikhail.ramalho at gmail.com <mailto:mikhail.ramalho at gmail.com>>:
>> Indeed the constraints start to appear when there are multiplications/divisions/remainders, but I noticed that it only prints the constraints when it's able to solve them.
>>
>> Maybe in a higher level it's dropping constraints without querying if the solver can handle it?
>>
>> I'll keep looking into it.
>>
>> 2018-03-23 18:25 GMT+00:00 Artem Dergachev <noqnoqneo at gmail.com <mailto:noqnoqneo at gmail.com>>:
>> Try making assumptions over 2 * x, these should work if i recall correctly.
>>
>>
>> On 3/23/18 10:51 AM, Mikhail Ramalho via cfe-dev wrote:
>>> Hi,
>>>
>>>
>>> We would have to find an easier example first, where the core modification are not necessary.
>>> For the easier example: I think it would have to be simple arithmetics over integers, even negation would work.
>>> The current solver can not handle any relational constraints.
>>>
>>>
>>> I'm having some problems finding a simple benchmark where the constraints are not dropped.
>>>
>>> For instance, consider the following (safe) program:
>>>
>>> void foo(unsigned x, unsigned y)
>>> {
>>> if (x > y)
>>> return;
>>>
>>> int base;
>>>
>>> if (x <= y)
>>> base = 1;
>>>
>>> assert(base == 1);
>>> }
>>>
>>> But the constraints are empty (both when I print the graph and the SMT formula). I'm calling the analyzer:
>>>
>>> $ ~/myclang/bin/clang --analyze -Xclang -analyzer-viz-egraph-graphviz -Xanalyzer -analyzer-checker=debug.DumpCFG main2.c
>>>
>>> I'm assuming that the constraints are being dropped somehow but is there any other way to check it?
>>>
>>> Btw, I'm using the head of the release_60 branch.
>>>
>>> Thank you,
>>>
>>> BTW instead of looking into the Z3ConstraintManager I think it would be easier to look at the exploded graph (using the option I have previously described)
>>> and see what formulas are mentioned there.
>>>
>>>>
>>>> Thank you,
>>>>
>>>>
>>>> 2018-03-22 20:34 GMT+00:00 George Karpenkov <ekarpenkov at apple.com <mailto:ekarpenkov at apple.com>>:
>>>> Hi Mikhail,
>>>>
>>>> That’s a good improvement!
>>>>
>>>> I think an awesome next step would be to see whether the analyzer already has the formula required to solve your motivational example.
>>>> This would be a preliminary feasibility study: if the formula is there, it’s just a matter of converting it and giving it to Z3, and otherwise,
>>>> the exercise is much harder and might require substantial changes.
>>>>
>>>> Perhaps an easiest way to see what formulas the analyzer has is to launch it with an extra flag
>>>> `-Xclang -analyzer-viz-egraph-graphviz` which would dump a graph in a GraphViz format containing all the information analyzer has along all the states.
>>>>
>>>> This is important for judging feasibility, as it might be the case that analyzer at some point decides to get rid of the “complex” constraint.
>>>> While it would be possible to change that, that would be a second step of the project,
>>>> and for preliminary evaluation a simpler example would be needed.
>>>>
>>>> Also, the information above could be helpful for structuring the project: a first stage would be checking most trivial examples, a second stage would be seeing how far
>>>> can we get with only minimal modifications to the core.
>>>>
>>>> Regards,
>>>> George
>>>>
>>>>> On Mar 22, 2018, at 1:19 PM, Mikhail Ramalho <mikhail.ramalho at gmail.com <mailto:mikhail.ramalho at gmail.com>> wrote:
>>>>>
>>>>> Hi all,
>>>>>
>>>>> Thank you for the feedback, George and Dominic.
>>>>>
>>>>> I updated my proposal with an example, showing the encoded SMT formula for the program and a brief explanation of the verification process. I used a simplified program from a bug report in Bugzilla.
>>>>>
>>>>> May I ask for some feedback in this section?
>>>>>
>>>>> ~
>>>>>
>>>>> I addressed most of the comments, except for:
>>>>>
>>>>> George: stretch goals are great, but for now I think it would be better to focus on writing a considerably more detailed proposal on how and why the main goal would be implemented.
>>>>>
>>>>> I tried to explain the motivation in the Overview section, do you think a motivation section would be better?
>>>>>
>>>>> Regarding the how, I'll have another look in the BugReportVisitor and update the proposal with a more concrete solution.
>>>>>
>>>>> Thank you,
>>>>>
>>>>>
>>>>> 2018-03-21 17:54 GMT+00:00 George Karpenkov <ekarpenkov at apple.com <mailto:ekarpenkov at apple.com>>:
>>>>> Hi Mikhail,
>>>>>
>>>>> I’ve added some feedback.
>>>>> Overall, I think we should be aiming for something more low-level and concrete:
>>>>> adding examples with explanations would be a great improvement.
>>>>>
>>>>> Regards,
>>>>> George
>>>>>
>>>>>
>>>>>> On Mar 21, 2018, at 10:12 AM, Mikhail Ramalho <mikhail.ramalho at gmail.com <mailto:mikhail.ramalho at gmail.com>> wrote:
>>>>>>
>>>>>> Hi all,
>>>>>>
>>>>>> I've written a first draft of my proposal:
>>>>>>
>>>>>> https://docs.google.com/document/d/1-zNSv0l4WyoxYpJUAw8LFnQq_TY4AGjIpPu1VPkmO-g/edit?usp=sharing <https://docs.google.com/document/d/1-zNSv0l4WyoxYpJUAw8LFnQq_TY4AGjIpPu1VPkmO-g/edit?usp=sharing>
>>>>>>
>>>>>> I've added a few comments in places I think need improvement.
>>>>>>
>>>>>> May I ask the community to have a look and give some feedback?
>>>>>>
>>>>>> Thank you,
>>>>>>
>>>>>>
>>>>>> 2018-03-12 18:24 GMT+00:00 George Karpenkov <ekarpenkov at apple.com <mailto:ekarpenkov at apple.com>>:
>>>>>> Hi Mikhail,
>>>>>>
>>>>>> I’m assuming Dominic have answered your questions regarding the point (3).
>>>>>>
>>>>>> On point (1) I have recently sent an email on the list answering, I believe, to essentially the same question:
>>>>>> http://lists.llvm.org/pipermail/cfe-dev/2018-March/057064.html <http://lists.llvm.org/pipermail/cfe-dev/2018-March/057064.html>
>>>>>>
>>>>>> (yes, unfortunately we do not have better archives, so messages might be often hard to track)
>>>>>>
>>>>>>> 2. I still don't quite understand how dynamic memory track works in the analyzer, is the double checker expected to work for pointers and dynamic memory as well? I'm assuming yes here and that Z3ConstraintManager might need to be extended somehow (a plan will be added to the proposal).
>>>>>>
>>>>>> I think here we should get the extra precision for free by adding a bug reporter visitor, as described in the email thread I have linked to.
>>>>>>
>>>>>> Please feel free to ask any further questions, bug reporter visitors are quite messy in the analyzer.
>>>>>>
>>>>>> Regards,
>>>>>> George
>>>>>>
>>>>>>>
>>>>>>> ~
>>>>>>>
>>>>>>> 3. This is a list of the TODOs in Z3ConstraintManager, from more important to less important, in my opinion. I just want to know if the analyzer's developers (and the project mentor) agree with this list, as it might go into my proposal:
>>>>>>>
>>>>>>> 3.1. Don't assume nearest ties to even rounding mode: currently, only rounding to even is supported, even if the code changes the rounding mode using fesetround.
>>>>>>>
>>>>>>> 3.2. Don't add all the constraints, only the relevant ones: adding unnecessary constraints will slowdown the solver.
>>>>>>>
>>>>>>> 3.3. Refactor doTypeConversion to use built-in conversion functions (Refactor to Sema::FindCompositePointerType(), and Sema::CheckCompareOperands(); Refine behavior for invalid type casts)
>>>>>>> 3.4. Refactor doIntTypeConversion to use Sema::handleIntegerConversion()
>>>>>>> 3.5. Refactor doFloatTypeConversion to use Sema::handleFloatConversion()
>>>>>>>
>>>>>>> I bundled this together because, although the conversion seems incomplete (based on the comments), it's about removing duplicated code.
>>>>>>>
>>>>>>> 3.6. Refactor getAPSIntType(const llvm::APSInt &Int) const to put elsewhere.
>>>>>>>
>>>>>>> ~
>>>>>>>
>>>>>>> Thank you,
>>>>>>>
>>>>>>>
>>>>>>> 2018-02-24 1:03 GMT+00:00 Devin Coughlin <dcoughlin at apple.com <mailto:dcoughlin at apple.com>>:
>>>>>>>
>>>>>>>
>>>>>>> > On Feb 23, 2018, at 9:29 AM, Mikhail Ramalho via cfe-dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote:
>>>>>>> >
>>>>>>> > I also have a question about the proposal. I understand that ideas about the project will be discussed in the mailing list. However, once that's settled and I write my first draft proposal, should I send it to the mailing list for discussion again or should I send it only to the mentor?
>>>>>>>
>>>>>>> Please make sure to keep email discussions on the mailing list rather than just personal email. This is a topic that members of the community will be interested in and will have valuable feedback on.
>>>>>>>
>>>>>>> Devin
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> --
>>>>>>>
>>>>>>> Mikhail Ramalho.
>>>>>>> _______________________________________________
>>>>>>> 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>
>>>>>>
>>>>>>
>>>>>>
>>>>>> --
>>>>>>
>>>>>> Mikhail Ramalho.
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> --
>>>>>
>>>>> Mikhail Ramalho.
>>>>
>>>>
>>>>
>>>>
>>>> --
>>>>
>>>> Mikhail Ramalho.
>>>
>>>
>>>
>>>
>>> --
>>>
>>> Mikhail Ramalho.
>>>
>>>
>>> _______________________________________________
>>> 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>
>>
>>
>>
>>
>> --
>>
>> Mikhail Ramalho.
>>
>>
>>
>> --
>>
>> Mikhail Ramalho.
>
>
>
>
> --
>
> Mikhail Ramalho.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-dev/attachments/20180326/c634239d/attachment.html>
More information about the cfe-dev
mailing list