Re: [eigen] Problem with GCC<4.5+amd64+vectorization (Was: On lazy evaluation and temporary matrices)

[Benoit Jacob <jacob.benoit.1@xxxxxxxxx>]

Oh by the way --- is this GCC on Windows?

GCC/Windows sure had stack alignment problems that were solved in GCC 4.5:
http://gcc.gnu.org/gcc-4.5/changes.html

We even have a dox page about them, linked from the unaligned arrays
assert page:
http://eigen.tuxfamily.org/dox-devel/TopicWrongStackAlignment.html

Benoit

2011/1/12 Benoit Jacob <jacob.benoit.1@xxxxxxxxx>:
> Have you made a GCC bug report about this?
>
> http://gcc.gnu.org/bugs/#need
>
> If you follow these instructions (produce .ii selfcontained
> preprocessed test case, probably compress it as it's going to be
> large) they should be able to fix it.
>
> Of course this only works if you can produce a small test case...
>
> Benoit
>
> 2011/1/10 Jose Luis Blanco <joseluisblancoc@xxxxxxxxx>:
>> I don't think many would find this problem, but after two month I
>> finally fixed this issue and wanted to leave here a (dirty) solution
>> for the joy of future readers of the list archive ;-)
>>
>> It turned out that just for one specific part of my code, an Eigen
>> matrix wasn't computed OK. This only happened with GCC older than 4.5
>> and for the amd64 architecture. In all other cases, everthing works
>> nicely. Since valgrind doesn't complain of anything, I wasn't able to
>> generate an isolated test case and the code actually works for all
>> other situations, I'm forced to think it may be some intrincated bug
>> in GCC 4.4 (?).
>>
>> My solution has been to:
>>
>> 1) Disable vectorization for GCC <4.5 with EIGEN_DONT_VECTORIZE
>> (thanks Gael!) in the file with the problematic code, and
>> 2) move to GCC 4.5 !!
>>
>> JL
>>
>> On Thu, Nov 25, 2010 at 8:25 AM, Gael Guennebaud
>> <gael.guennebaud@xxxxxxxxx> wrote:
>>> Hi,
>>>
>>> first make sure to test it in "debug mode", i.e., without NDEBUG or
>>> EIGEN_DEBUG defined to enable the detection of some some alignment
>>> issues. If that does not help, try to disable vectorization by
>>> defining EIGEN_DONT_VECTORIZE.
>>>
>>> Is your code multithreaded?
>>>
>>> Also in your case the .eval() is superfluous.
>>>
>>> gael
>>>
>>> On Thu, Nov 25, 2010 at 2:45 AM, Jose Luis Blanco
>>> <joseluisblancoc@xxxxxxxxx> wrote:
>>>> Hi Thomas,
>>>>
>>>> Thanks, I added the EIGEN_ALIGN16 macro to all the number arrays, it
>>>> definitively makes sense... but it behaves exactly the same :-(
>>>>
>>>> Best,
>>>> JL
>>>>
>>>> PS: And yes, it's beta3*, actually an hg checkout from a few days ago...
>>>>
>>>> On Thu, Nov 25, 2010 at 2:22 AM, Thomas Capricelli
>>>> <orzel@xxxxxxxxxxxxxxx> wrote:
>>>>> It looks like an alignement problem. You should ensure "numbers" is aligned,
>>>>> see bottom of http://eigen.tuxfamily.org/dox-devel/
>>>>>
>>>>> (I assume you use the devel branch.. 3.0beta*)
>>>>>
>>>>> regards,
>>>>>
>>>>> --
>>>>>
>>>>> Thomas Capricelli <orzel@xxxxxxxxxxxxxxx>
>>>>>
>>>>> http://www.freehackers.org/thomas
>>>>>
>>>>> On Thursday 25 November 2010 02:01:48 Jose Luis Blanco wrote:
>>>>>
>>>>>> Hi guys,
>>>>>
>>>>>>
>>>>>
>>>>>> I need the help of someone knowing the inner workings of Eigen. I've
>>>>>
>>>>>> spent a few days stuck debugging an error in some silly computations
>>>>>
>>>>>> with Eigen matrices, and want to rule out potential errors...
>>>>>
>>>>>> Consider this code:
>>>>>
>>>>>>
>>>>>
>>>>>> ======================
>>>>>
>>>>>> void func( Matrix<...> &m_out )
>>>>>
>>>>>> {
>>>>>
>>>>>> Matrix<...> I = ....
>>>>>
>>>>>> const double numbers[] = { 1.0, x(), -y()*x() , whatever, .... };
>>>>>
>>>>>> m_out = I * Matrix<...>(numbers);
>>>>>
>>>>>> }
>>>>>
>>>>>>
>>>>>
>>>>>> int main()
>>>>>
>>>>>> {
>>>>>
>>>>>> Matrix<...> M;
>>>>>
>>>>>> func(M);
>>>>>
>>>>>> cout << M << endl;
>>>>>
>>>>>> }
>>>>>
>>>>>> ======================
>>>>>
>>>>>>
>>>>>
>>>>>> Is it a problem for the temporary object "Matrix<...>(numbers)" to be
>>>>>
>>>>>> created with a pointer to memory on the stack, while the evaluation of
>>>>>
>>>>>> the object for the matrix product is actually evaluated *out* of that
>>>>>
>>>>>> function (where obviously that pointer is invalid)??
>>>>>
>>>>>>
>>>>>
>>>>>> I ask because I'm having a very erratic behavior in code like above
>>>>>
>>>>>> (on GCC doesn't work, it does on MSVC; adding a "cout << ..." for an
>>>>>
>>>>>> unrelated matrix changes the result, etc...).
>>>>>
>>>>>> Is it necessary to use instead:
>>>>>
>>>>>>
>>>>>
>>>>>> ( I * Matrix<...>(numbers) ).eval();
>>>>>
>>>>>>
>>>>>
>>>>>> or not even with this .eval() is my code correct?
>>>>>
>>>>>> I'm still having random wrong matrix elements with and without eval(),
>>>>>
>>>>>> so I wanted to be sure on this part before keep on debugging...
>>>>>
>>>>>>
>>>>>
>>>>>> Thanks for your time!
>>>>>
>>>>>> JL
>>>>>
>>
>>
>>
>