[eigen] Eigen on an arm core

[Daniel Stonier <d.stonier@xxxxxxxxx>]

Hi,

We threw eigen at an arm core recently running a SLAM algorithm and
ignorantly discovered we were completely unknowing about alignment
issues!

I narrowed down our code to a simple example to hopefully highlight the issue:

************************************************************************************************

#include <iostream>
#include <vector>
#include <Eigen/Array>
#include <Eigen/Core>

using std::vector;
using Eigen::Vector2d;
using Eigen::Vector3d;
using Eigen::VectorXd;

class A {
public:
    Vector2d v;

};

void f() {
    A a;
    double range = 3.0;
    double bearing = 4.0;
    a.v = Vector2d(range,bearing);
    std::cout << a.v << std::endl;
}

int main() {
    Vector3d v; // This is the spanner in the works, without this it runs fine.
    f();

    return 0;
}

************************************************************************************************

It then promptly drops out at the assert:

************************************************************************************************

Eigen/src/Core/MatrixStorage.h:44: Eigen::ei_matrix_array<T, Size,
MatrixOptions, Align>::ei_matrix_array() [with T = double, int Size =
2, int MatrixOptions = 2, bool Align = true]: Assertion
`(reinterpret_cast<size_t>(array) & 0xf) == 0 && "this assertion is
explained here:
http://eigen.tuxfamily.org/dox/UnalignedArrayAssert.html  **** READ
THIS WEB PAGE !!! ****"' failed.

************************************************************************************************

I suspect...what we have is similar to the problem described for
windows on mingw -
http://eigen.tuxfamily.org/dox/WrongStackAlignment.html . I've been
reading that the ARM series has some different alignment rules. Now
I'm a bit dumbstruck about where to go with this or how to find out
more about it. Do you have any tips?

Regards,
Daniel Stonier (Yujin Robot).