Re: [eigen] can't reproduce gcc 4.3.4 errors

[Gael Guennebaud <gael.guennebaud@xxxxxxxxx>]

On Tue, Jun 29, 2010 at 2:46 PM, Benoit Jacob <jacob.benoit.1@xxxxxxxxx> wrote:
> 2010/6/29 Gael Guennebaud <gael.guennebaud@xxxxxxxxx>:
>> More news:
>>
>> in util/XprHelper.h we have:
>>
>> // just a workaround because GCC seems to not really like empty structs
>> #ifdef __GNUG__
>>  #define EIGEN_EMPTY_STRUCT_CTOR(X) \
>>    EIGEN_STRONG_INLINE X() {} \
>>    EIGEN_STRONG_INLINE X(const X&) {}
>> #else
>>  #define EIGEN_EMPTY_STRUCT_CTOR(X)
>> #endif
>>
>> and all our empty functors are declared with it:
>>
>> template<typename Scalar> struct ei_scalar_opposite_op {
>>  EIGEN_EMPTY_STRUCT_CTOR(ei_scalar_opposite_op)
>>  ...
>> };
>>
>> If I remove this trick, i.e., by defining EIGEN_EMPTY_STRUCT_CTOR(X)
>> to nothing, then I get only 2 errors (with -O2):
>>
>> guenneba@devel10:~/eigen/build/test> ./adjoint_1
>> Initializing random number generator with seed 1277814460
>> Repeating each test 10 times
>> Test adjoint(Matrix<float, 1, 1>()) failed in
>> "/home/guenneba/eigen/test/adjoint.cpp" (71)
>>    ei_isApprox((s1 * v1 + s2 * v2).dot(v3), ei_conj(s1) * v1.dot(v3)
>> + ei_conj(s2) * v2.dot(v3), largerEps)
>>
>> guenneba@devel10:~/eigen/build/test> ./eigensolver_complex_3
>> Initializing random number generator with seed 1277814463
>> Repeating each test 10 times
>> Test eigensolver(Matrix<std::complex<float>, 1, 1>()) failed in
>> "/home/guenneba/eigen/test/eigensolver_complex.cpp" (87)
>>    test_ei_isApprox(id.operatorNorm(), RealScalar(1))
>>
>> I don't remember exactly why I've introduced this trick, probably
>> because I found GCC produced poor code without it.
>
> Yes, that was the reason.
>
>> This is still very
>> strange.
>
> Perhaps G++ devs will be able to make sense of that.
>
> I have a question. In this line:
>
>>    EIGEN_STRONG_INLINE X(const X&) {}
>
> you are basically telling the compiler that the empty struct X in
> question has a copy constructor doing nothing. But at the same time,
> empty structs have nonzero sizeof() (typically sizeof==1, but check
> it). So if this empty struct (which is never exactly empty) aliases
> other data, which it totally does if the EBCO (empty base class
> optimization) happened, then boom! That could explain it.


I'm not sure about that because here the program is so simple that it
compiles to:

main:
..LFB3926:
	movq	-96(%rsp), %rax
	movl	$0x00000000, -40(%rsp)
	movss	.LC1(%rip), %xmm0
	movss	16(%rax), %xmm1
	xorps	%xmm0, %xmm1
	cvttss2si	%xmm1, %eax
	ret

Here are some explanation:

movl	$0x00000000, -40(%rsp)

set n1(2) to 0 (only one move because on 64bits one complex of float
fit into one word)

then,

movss	16(%rax), %xmm1

is supposed to moves n1(2) to xmm1, but 16(%rax) == -80(%rsp) that is
different than the -40(%rsp) above....

gael

> Benoit
>
>>
>> Marking these empty constructors with EIGEN_DONT_INLINE also fix the errors.
>>
>>
>> Here is the short program I used to debug this:
>>
>> #include <Eigen/Core>
>>
>> typedef std::complex<float> T;
>> template <typename A> T foo(const A& a) { return (-a-a)(2); }
>>
>> int main()
>> {
>>  Eigen::Matrix<T,3,1> n1(0,0,0);
>>  return int(Eigen::ei_real(foo(n1)));
>> }
>>
>> (You need to run it under valgrind to see the issue)
>>
>> if foo is marked with EIGEN_DONT_INLINE, then no error
>> if I manually inline foo, s/foo(n1)/(-n1-n1)(2), then no error
>> if I do only a+a or -a, then no error
>> if I output (-a-a)(2) at the beginning of foo, then the error still
>> occurs after the output.
>>
>>
>> gael
>>
>> On Tue, Jun 29, 2010 at 1:14 PM, Benoit Jacob <jacob.benoit.1@xxxxxxxxx> wrote:
>>> anyway, it sounds like either Eigen or some of its unit tests are
>>> unsafe wrt strict aliasing!!
>>>
>>> If it's Eigen itself, we have no choice but to fix that, since strict
>>> aliasing is part of -O2 in gcc...
>>>
>>> Benoit
>>>
>>> 2010/6/29 Benoit Jacob <jacob.benoit.1@xxxxxxxxx>:
>>>> hehe, this sounds like the problem that Sune had the other day on IRC. CC'ing.
>>>>
>>>> Benoit
>>>>
>>>> 2010/6/29 Gael Guennebaud <gael.guennebaud@xxxxxxxxx>:
>>>>> hm,
>>>>>
>>>>> -O2 -fno-strict-aliasing (and removing the hardcoded -fstrict-aliasing
>>>>> from our cmakefile)
>>>>>
>>>>> 100% tests passed, 0 tests failed out of 453
>>>>>
>>>>> much better.
>>>>>
>>>>> gael
>>>>>
>>>>> On Tue, Jun 29, 2010 at 10:55 AM, Gael Guennebaud
>>>>> <gael.guennebaud@xxxxxxxxx> wrote:
>>>>>> ok here,  with -O2 -fno-guess-branch-probability, 4 tests fail as follow:
>>>>>>
>>>>>> guenneba@devel10:~/eigen/build/test> ./array_2
>>>>>> Initializing random number generator with seed 1277801661
>>>>>> Repeating each test 10 times
>>>>>> Test comparisons(Array22f()) failed in
>>>>>> "/home/guenneba/eigen/test/array.cpp" (136)
>>>>>>    test_ei_isApprox(((m1.abs()+1)>RealScalar(0.1)).colwise().count(),
>>>>>> ArrayOfIndices::Constant(cols,rows).transpose())
>>>>>>
>>>>>> guenneba@devel10:~/eigen/build/test> ./array_3
>>>>>> Initializing random number generator with seed 1277801662
>>>>>> Repeating each test 10 times
>>>>>>
>>>>>>    actual   = 2.66729
>>>>>>    expected = 2.53391
>>>>>>
>>>>>>
>>>>>>    actual   = 1.17816
>>>>>>    expected = 1.93492
>>>>>>
>>>>>>
>>>>>>    actual   = -2.79358
>>>>>>    expected = -0.758141
>>>>>>
>>>>>>
>>>>>>    actual   = -2.06046
>>>>>>    expected = -2.54951
>>>>>>
>>>>>>
>>>>>>    actual   = -1.27569
>>>>>>    expected = 0.919771
>>>>>>
>>>>>>
>>>>>>    actual   = 5.67633
>>>>>>    expected = 4.85775
>>>>>>
>>>>>>
>>>>>>    actual   = -5.2517
>>>>>>    expected = -3.85892
>>>>>>
>>>>>>
>>>>>>    actual   = -1.33353
>>>>>>    expected = -0.700732
>>>>>>
>>>>>>
>>>>>>    actual   = -3.33972
>>>>>>    expected = 0.721723
>>>>>>
>>>>>>
>>>>>>    actual   = -1.61675
>>>>>>    expected = 0.256261
>>>>>>
>>>>>> Segmentation fault
>>>>>>
>>>>>> guenneba@devel10:~/eigen/build/test> ./geo_hyperplane_4
>>>>>> Initializing random number generator with seed 1277801668
>>>>>> Repeating each test 10 times
>>>>>> Segmentation fault
>>>>>>
>>>>>> guenneba@devel10:~/eigen/build/test> ./geo_parametrizedline_4
>>>>>> Initializing random number generator with seed 1277801673
>>>>>> Repeating each test 10 times
>>>>>> Segmentation fault
>>>>>>
>>>>>>
>>>>>> gael
>>>>>>
>>>>>> On Tue, Jun 29, 2010 at 10:29 AM, Hauke Heibel
>>>>>> <hauke.heibel@xxxxxxxxxxxxxx> wrote:
>>>>>>> Ok, I can try to run the complete tests too. Here are also the flags I
>>>>>>> used to run (so far just a few of) the failing tests.
>>>>>>>
>>>>>>> CXX:               /usr/bin/c++-4.exe
>>>>>>> CXX_VERSION:       c++-4 (GCC) 4.3.4 20090804 (release) 1
>>>>>>> CXX_FLAGS:          -Wnon-virtual-dtor -Wno-long-long -ansi -Wundef
>>>>>>> -Wcast-align -Wchar-subscripts -Wall -W -Wpointer-arith
>>>>>>> -Wwrite-strings -Wformat-s
>>>>>>> ecurity -fexceptions -fno-check-new -fno-common -fstrict-aliasing
>>>>>>> -Wno-variadic-macros -Wextra -pedantic
>>>>>>>
>>>>>>> - Hauke
>>>>>>>
>>>>>>> On Tue, Jun 29, 2010 at 10:25 AM, Gael Guennebaud
>>>>>>> <gael.guennebaud@xxxxxxxxx> wrote:
>>>>>>>> On Tue, Jun 29, 2010 at 10:14 AM, Hauke Heibel
>>>>>>>> <hauke.heibel@xxxxxxxxxxxxxx> wrote:
>>>>>>>>> On Mon, Jun 28, 2010 at 11:15 PM, Gael Guennebaud
>>>>>>>>> <gael.guennebaud@xxxxxxxxx> wrote:
>>>>>>>>>> I believe there is a bug in gcc, here is the related bug report:
>>>>>>>>>> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42255
>>>>>>>>>> but no fix yet.
>>>>>>>>>
>>>>>>>>> That means we can get rid of the errors by disabling -fprofile-arcs?
>>>>>>>>
>>>>>>>> A while ago, such errors appeared only with  -fprofile-arcs, i.e.,
>>>>>>>> when coverage testing was enabled. But now, even without
>>>>>>>> -fprofile-arcs we hit these weird errors. After some investigations, I
>>>>>>>> found that disabling the guess-branch-probability optimization that is
>>>>>>>> automatically enabled with -O2 solved a couple of errors:
>>>>>>>>
>>>>>>>> -O2 -fno-guess-branch-probability
>>>>>>>>
>>>>>>>> Actually, guess-branch-probability and -fprofile-arcs seems to be
>>>>>>>> quite related. I've updated the bug report with that respect, but news
>>>>>>>> so far.
>>>>>>>>
>>>>>>>> I'm currently compiling with -O2 -fno-guess-branch-probability to see
>>>>>>>> if there remain other errors....
>>>>>>>>
>>>>>>>>
>>>>>>>> gael
>>>>>>>>
>>>>>>>>> - Hauke
>>>>>>>>>
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