[csmith-dev] Alignment of struct members cause target dependent checksum

[Kees Bakker]

Hi John,

Thank you for the explanation. We're grateful for the tool, so for sure we don't
want to complain too much :-)

We're using GCC4.7 as a reference to generate the checksum. The compilers
that we test are very diverse. And alignment of structure members can be
different from GCC-x86. When we hit a problem we first look at the checksum
of a few other compilers. When the difference is caused by structure alignment
it may be that 4 out of 5 give the same checksum. Then one might conclude that
the fifth compiler has a bug. And in the end you find out that it wasn't a problem.

Analyzing bugs in Csmith programs can be time consuming. And Creduce isn't always
helpful.

Kind regards,
Kees Bakker


On 05-06-13 18:22, John Regehr wrote:
> Hi Kees,
>
> It is not generally expected that Csmith programs will have the same results across compilers or platforms except when those compilers/platforms make the same choices for implementation-defined 
> characteristics such as alignment, integer width and representation, etc.
>
> When creating Csmith we had a choice between generating more portable code, which would permit differential testing across more platforms, and less portable code, which (we think) finds more 
> compiler bugs.  We decided to do the second one.  Basically this means that you will need to choose your compilers carefully when doing differential testing.
>
> Most compiler bugs can be found simply by comparing different optimization levels of the same compiler.  We've seen a few bugs where a compiler produces that same wrong result at all optimization 
> levels, but this is quite rare.
>
> Hopefully this is helpful?
>
> Thanks,
>
> John
>
>
>
> On 06/05/2013 06:27 AM, Kees Bakker wrote:
>> Hi,
>>
>> Csmith can generate tests with unions and struct where the result
>> is target dependent.
>>
>> Here is a small example.
>>
>>    struct S1 {
>>       const volatile uint8_t  f10;
>>       int32_t  f11;
>>    };
>>
>>    union U4 {
>>       const uint32_t  f0;
>>       struct S1  f2;
>>    };
>>    static union U4 g_1126 = {4294967294UL};
>>
>> One of the tests I created was writing in g_1126.f2.f11 and reading
>> from g_1126.f0 for the checksum.
>>
>> However, the struct member offset of f11 is not defined. For example GCC
>> (i386)
>> uses an offset 4, but other (compliant C99 compilers) may use offset 2
>> or even 1.
>> (And that is without specifying "pack".)
>>
>> In this case the checksum created with GCC (assuming GCC is correct) is not
>> valid for all other compilers.
>>
>> What options do I have to let Csmith avoid such a construct?
>>
>> Kind regards,
>> Kees Bakker
>>