Mixing Scalar Types in Eigen

Question

#include <iostream>

#include <Eigen/Core>

namespace Eigen {

// float op double -> double
template <typename BinaryOp>
struct ScalarBinaryOpTraits<float, double, BinaryOp> {
  enum { Defined = 1 };
  typedef double ReturnType;
};

// double op float -> double
template <typename BinaryOp>
struct ScalarBinaryOpTraits<double, float, BinaryOp> {
  enum { Defined = 1 };
  typedef double ReturnType;
};

}

int main() {
    Eigen::Matrix<float, Eigen::Dynamic, Eigen::Dynamic> m1(2, 2);
    m1 << 1, 2, 3, 4;

    Eigen::Matrix<double, Eigen::Dynamic, Eigen::Dynamic> m2(2, 2);
    m2 << 1, 2, 3, 4;

    std::cerr << m1 * m2 <<std::endl;  // <- boom!!
}

I'd like to know why the above code does not compile. Here is the full error messages. Please note that if I define m1 and m2 to have fixed sizes, it works fine.

I'm using Eigen3.3.1. It's tested on a Mac running OSX-10.12 with Apple's clang-800.0.42.1.

Answer 1

This is because the general matrix-matrix product is highly optimized with aggressive manual vectorization, pipelining, multi-level caching, etc. This part does not support mixing float and double. You can bypass this heavily optimized implementation with m1.lazyProduct(m2) that corresponds to the implementations used fro small fixed-size matrices, but there is only disadvantages of doing so: the ALUs does not support mixing float and double, so float values have to be promoted to double anyway and you will loose vectorization. Better cast the float to double explicitly:

m1.cast<double>() * m2