Created attachment 783 [details]
Patch

SSE intrinsics can be called with unaligned pointers in quat_product::run, causing SegFault. The same applies to quat_conj::run.

$ cat qprod.cpp
#include <Eigen/Geometry>
int main() {
    Eigen::Quaterniond q1 = Eigen::Quaterniond(1,0,0,0);
    Eigen::Quaterniond q2 = Eigen::Quaterniond(1,0,0,0);
    Eigen::Quaterniond q = q1 * q2;
    return q.norm();
}

$ clang++ --version
clang version 3.7.0 (trunk 239477)
Target: x86_64-unknown-linux-gnu

$ clang++ -I. -DEIGEN_DONT_ALIGN_STATICALLY qprod.cpp

$ ./a.out 
Segmentation fault

Note that the above code may not cause SegFault if on-stack Quaternion objects happen to be 16-byte aligned. 

The issue seems to exist in the latest stable and development branches, and is solved by an attached patch. 

Thanks,
Shuntaro