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Re: Add support to trace comparison instructions and switch statements


On Sun, Sep 3, 2017 at 12:19 PM, Dmitry Vyukov <dvyukov@google.com> wrote:
> On Sun, Sep 3, 2017 at 12:01 PM, Jakub Jelinek <jakub@redhat.com> wrote:
>> On Sun, Sep 03, 2017 at 10:50:16AM +0200, Dmitry Vyukov wrote:
>>> What we instrument in LLVM is _comparisons_ rather than control
>>> structures. So that would be:
>>>     _4 = x_8(D) == 98;
>>> For example, result of the comparison can be stored into a bool struct
>>> field, and then used in branching long time after. We still want to
>>> intercept this comparison.
>>
>> Then we need to instrument not just GIMPLE_COND, which is the stmt
>> where the comparison decides to which of the two basic block successors to
>> jump, but also GIMPLE_ASSIGN with tcc_comparison class
>> gimple_assign_rhs_code (the comparison above), and maybe also
>> GIMPLE_ASSIGN with COND_EXPR comparison code (that is say
>>   _4 = x_1 == y_2 ? 23 : _3;
>> ).
>>
>>> > Perhaps for -fsanitize-coverage= it might be a good idea to force
>>> > LOGICAL_OP_NON_SHORT_CIRCUIT/BRANCH_COST or whatever affects GIMPLE
>>> > decisions mentioned above so that the IL is closer to what the user wrote.
>>>
>>> If we recurse down to comparison operations and instrument them, this
>>> will not be so important, right?
>>
>> Well, if you just handle tcc_comparison GIMPLE_ASSIGN and not GIMPLE_COND,
>> then you don't handle many comparisons from the source code.  And if you
>> handle both, some of the GIMPLE_CONDs might be just artificial comparisons.
>> By pretending small branch cost for the tracing case you get fewer
>> artificial comparisons.
>
>
> Are these artificial comparisons on BOOLEAN_TYPE? I think BOOLEAN_TYPE
> needs to be ignored entirely, there is just like 2 combinations of
> possible values.
> If not, then what it is? Is it a dup of previous comparisons?
>
> I am not saying these modes should not be enabled. You know much
> better. I just wanted to point that that integer comparisons is what
> we should be handling.
>
> Your example:
>
>   _1 = x_8(D) == 21;
>   _2 = x_8(D) == 64;
>   _3 = _1 | _2;
>   if (_3 != 0)
>
> raises another point. Most likely we don't want to see speculative
> comparisons. At least not yet (we will see them once we get through
> the first comparison). So that may be another reason to enable these
> modes (make compiler stick closer to original code).

Wait, it is not speculative in this case as branch is on _1 | _2. But
still, it just makes it harder for fuzzer to get through as it needs
to guess both values at the same time rather then doing incremental
progress.


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