[cfe-dev] GSoC 2018

Mikhail Ramalho via cfe-dev cfe-dev at lists.llvm.org
Mon Mar 26 09:13:33 PDT 2018 

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>:

> 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>
> 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>:
>
>> 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>:
>>
>>> 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>:
>>>>
>>>>> 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> 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>:
>>>>>
>>>>>> 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> 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
>>>>>>
>>>>>> 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>:
>>>>>>
>>>>>>> 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
>>>>>>>
>>>>>>> (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>:
>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> > On Feb 23, 2018, at 9:29 AM, Mikhail Ramalho via cfe-dev <
>>>>>>>> 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
>>>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>
>>>>>>
>>>>>> --
>>>>>>
>>>>>> Mikhail Ramalho.
>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>>
>>>>> --
>>>>>
>>>>> Mikhail Ramalho.
>>>>>
>>>>>
>>>>>
>>>>
>>>>
>>>> --
>>>>
>>>> Mikhail Ramalho.
>>>>
>>>>
>>>>
>>>
>>>
>>> --
>>>
>>> Mikhail Ramalho.
>>>
>>> _______________________________________________
>>> cfe-dev mailing listcfe-dev at lists.llvm.orghttp://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>>
>>>
>>>
>>
>>
>> --
>>
>> Mikhail Ramalho.
>>
>
>
>
> --
>
> Mikhail Ramalho.
>
>
>


-- 

Mikhail Ramalho.
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