Re: [eigen] ei_redux_impl & ei_alignmentOffset

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

2009/12/16 Hauke Heibel <hauke.heibel@xxxxxxxxxxxxxx>:
> Hi,
>
> I am still every now and then looking at the inlining warnings created by
> MSVC (+1500). Here are some findings regarding a whole bunch of those
> warnings.
>
> Many many warnings we observe on MSVC (W4) are caused by this call here
> (taken from ei_redux_impl<Func, Derived, LinearVectorizedTraversal,
> NoUnrolling>::run)
>
> &mat.const_cast_derived().coeffRef(0)
>
> My most simple example to trigger this warning is
>
> #include <Eigen/Core>
> #include <Eigen/Array>
>
> using namespace Eigen;
>
> void main ()
> {
>   typedef Matrix<float, Eigen::Dynamic, Eigen::Dynamic, Eigen::AutoAlign>
> MatrixType;
>   MatrixType a = MatrixType::Random(50,50);
>   a.cwise().abs2().sum();
> }
>
> This call brings us to
>
> ei_redux_impl<Func, Derived, LinearVectorizedTraversal,
> NoUnrolling>::run(...)
>
> triggering the warning while the variable alignedStart is zero. I am
> wondering how I can create an example that actually causes alignedStart != 0
> ?? When I set the DontAlign option, I am ending up in a completely different
> branch.

To produce alignedStart!=0, you could do:

VectorXf v(1000);
v.end(999).sum();

>
> For testing purposes I added this code at the very beginning of the function
>
> const Scalar& s = mat.const_cast_derived().coeffRef(0);
>
> resulting in warning telling me that everything beyond this line is
> unreachable code. That is the case since derived() will end up in an
> infinite loop.

Do you mean const_cast_derived() ?

> Here is why:
>
> - mat is a CwiseUnaryOp< ei_scalar_abs2_op<float>, MatrixXf >

yes,

> - mat.const_cast_derived() calls MatrixBase< CwiseUnaryOp<
> ei_scalar_abs2_op<float>, MatrixXf > >

Yes, it calls MatrixBase::const_cast_derived(),

> and returns a CwiseUnaryOp<
> ei_scalar_abs2_op<float>, MatrixXf >
>
> and here we go - infinite loop.

Why ? Here, MatrixBase::const_cast_derived() is implemented like this:

    inline Derived& const_cast_derived() const
    { return *static_cast<Derived*>(const_cast<MatrixBase*>(this)); }

So this call doesn't call anything itself, so where is the infinite recursion?

If you have an infinite recursion, you get a stack overflow. Is that
happening? Can you get a backtrace then? If the stack is damaged by
the overflow, try doing an assert() on the call depth using a static
int variable, then you'll get a good stack...

Benoit


> So there is something odd going on and I can
> understand that MSVC complains about inlining. Infinite inlining is rather
> tricky.
>
> Any ideas?
>
> - Hauke
>