eigen3: take a long time to compile and very slow when debug - eigen

Question

I'm using eigen to do some matrix operation, but the compile time for the src files using eigen is very slow, by slow I mean it take about 40s when the file is only 300 lines. I only use Matrix smaller than Matrix4f and not even use dynamic size matrix, for only some matrix multiplication and matrix decomposition(SVD and FullPivLU).
In my other project where a cpp file is 1000 lines, it take minutes to compile, and the output .so file is really big, about 100M. I have to open the -bigobj choice.
This happens both in debug(where I set the opimization to -O0) and release(-O3) mode. I've tried add #define NDEBUG and EIGEN_NO_DEBUG in the header, not help.
I write this piece of very small code test.cpp below:
#include </home/user/mywork/software/eigen-3.3.9/Eigen/Dense>
#include "iostream"
using namespace std;
using namespace Eigen;
void test()
{
Matrix3f A;
A << 1,2,3,4,2,8,5,4,9;
BDCSVD<Matrix3f> svd(A, ComputeFullU | ComputeFullV);
Matrix3f U = svd.matrixU();
Matrix3f V = svd.matrixV();
cout << "------Eigen------" << endl;
cout << "A" << A << endl;
cout << "U" << U << endl;
cout << "V" << V << endl;
}
int main()
{
test();
return 0;
}
compile command
g++ test.cpp -o test_eigen
it take 20s to compile, and the output 'test_eigen' is 5.6M!!!
my os is ubuntu 16.04 and I use cmake, in the CMakelist, only thing related to eigen is include its directory. eigen version is 3.3.9.
does anyone have any clue? Thanks a lot.

Answer 1

Eigen consists of 7.8MB of pure header files. So any .cpp including it will have a hard life compiling (uses giga-bytes of memory and extremely slow). In addition, due to extensive use of templates and inline functions, the DEBUG version will be so slow and unusable for debug purpose. (For example, google's Cartographer will warn about debug version) and I also made some tests.
So I think Eigen should be used as follow:
Encapsulate Eigen. Create simple interface according to your own needs. (For example, create matrix_4x4.h, vector4.h, standard_deviation_calculaor.h, etc.) Include Eigen only in .cpp files. The exposed .h files will be so simple and will not propagate to slow down compiling of other files.
Proper Modularization. Precompile the library into static/shared libraries. And use the simple headers in other parts of your program.
Restrain Use eigen only in performance-critical parts of my program. For example, the following function will not use eigen and should be inlined(exposed in header):
// vector3.h
inline Vector3 operator + (const Vector3& l, const Vector3& r) {
return Vector3(l.x + r.x, l.y + r.y, l.z + r.z);
}
But Matrix4x4::multiplyInplace(const Matrix4x4& r) will be
implemented in .cpp and use Eigen.