Fast Reverse-Mode Automatic Differentiation using Expression Templates in C++

Gradient-based optimization problems are encountered in many fields, but the associated task of differentiating large computer algorithms can be formidable. The operator-overloading approach to performing reverse-mode automatic differentiation is the most convenient for the user but current implementations are typically 10-35 times slower than the original algorithm. In this paper a fast new operator-overloading method is presented that uses the expression template programming technique in C++ to provide a compile-time representation of each mathematical expression as a computational graph that can be efficiently traversed in either direction. Benchmarking with four different numerical algorithms shows this approach to be 2.6--9 times faster than current operator-overloading libraries, and 1.3--7.7 times more efficient in memory usage. It is typically less than 4 times the computational cost of the original algorithm, although poorer performance is found for all libraries in the case of simple loops containing no mathematical functions. An implementation is freely available in the Adept C++ software library.