Automatic Differentiation

Gradient-based optimization methods in OptimLib (such as BFGS and gradient descent) require a user-defined function that returns a gradient vector at each function evaluation. While this is best achieved by knowing the gradient in closed form, OptimLib also provides experimental support for automatic differentiation via the autodiff library.

Requirements: an Eigen-based build of OptimLib, a copy of the autodiff header files, and a C++17 compatible compiler.

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Example

The example below uses forward-mode automatic differentiation to compute the gradient of the Sphere function, and the BFGS algorithm to find the input values that minimize the autodiff-enabled function.

/*
* Forward-mode autodiff test with Sphere function
*/

#define OPTIM_ENABLE_EIGEN_WRAPPERS
#include "optim.hpp"

#include <autodiff/forward/real.hpp>
#include <autodiff/forward/real/eigen.hpp>

//

autodiff::real
opt_fnd(const autodiff::ArrayXreal& x)
{
    return x.cwiseProduct(x).sum();
}

double
opt_fn(const Eigen::VectorXd& x, Eigen::VectorXd* grad_out, void* opt_data)
{
    autodiff::real u;
    autodiff::ArrayXreal xd = x.eval();

    if (grad_out) {
        Eigen::VectorXd grad_tmp = autodiff::gradient(opt_fnd, autodiff::wrt(xd), autodiff::at(xd), u);

        *grad_out = grad_tmp;
    } else {
        u = opt_fnd(xd);
    }

    return u.val();
}

int main()
{
    Eigen::VectorXd x(5);
    x << 1, 2, 3, 4, 5;

    bool success = optim::bfgs(x, opt_fn, nullptr);

    if (success) {
        std::cout << "bfgs: forward-mode autodiff test completed successfully.\n" << std::endl;
    } else {
        std::cout << "bfgs: forward-mode autodiff test completed unsuccessfully.\n" << std::endl;
    }

    std::cout << "solution: x = \n" << x << std::endl;

    return 0;
}

This example can be compiled using:

g++ -Wall -std=c++17 -O3 -march=native -ffp-contract=fast -I/path/to/eigen -I/path/to/autodiff -I/path/to/optim/include optim_autodiff_ex.cpp -o optim_autodiff_ex.out -L/path/to/optim/lib -loptim

See the examples/autodiff directory for an example using reverse-mode automatic differentiation.

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