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Re: Add support to trace comparison instructions and switch statements


On Sun, Sep 3, 2017 at 12:38 PM, 吴潍浠(此彼) <weixi.wwx@antfin.com> wrote:
> Hi
> I will update the patch according to your requirements, and with some my suggestions.
> It will take me one or two days.

Thanks! No hurry, just wanted to make sure you still want to pursue this.

> Wish Wu
>
> ------------------------------------------------------------------
> From:Dmitry Vyukov <dvyukov@google.com>
> Time:2017 Sep 3 (Sun) 18:21
> To:Jakub Jelinek <jakub@redhat.com>
> Cc:Wish Wu <weixi.wwx@antfin.com>; gcc <gcc@gcc.gnu.org>; gcc-patches <gcc-patches@gcc.gnu.org>; Jeff Law <law@redhat.com>; wishwu007 <wishwu007@gmail.com>
> Subject: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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