Example 3 - Positional tape evaluations

Goal: Evaluate only parts of CoDiPack tapes.

Prerequisite: Tutorial 2 - Reverse mode AD

Function:

template<typename Real>
Real func(Real a, Real b) {
  Real x = a + b;
  Real y = a - b;
  Real w = sqrt(x * x + y * y);
 
  return cos(w);
}

Full code:

 
#include <iostream>
#include <codi.hpp>
 
template<typename Real>
Real func(Real a, Real b) {
  Real x = a + b;
  Real y = a - b;
  Real w = sqrt(x * x + y * y);
 
  return cos(w);
}
 
using Real = codi::RealReverse;
using Gradient = typename Real::Gradient;
using Tape = typename Real::Tape;
using Position = typename Tape::Position;
 
Real funcInner(Real x, Real y) {
  return sqrt(x * x + y * y);
}
 
void positionalExample() {
 
  Tape& tape = Real::getTape();
 
  // Recording
  Real a = 10.0;
  Real b = 4.0;
 
  tape.setActive();
  tape.registerInput(a);
  tape.registerInput(b);
 
  Real u1 = a + b;
  Real u2 = a - b;
 
  // Now comes a really long and complicated function. Tape it, reverse it and then store the result on the tape.
  // Step 1: Store the position
  Position begin = tape.getPosition();
 
  // Record the function
  Real w = funcInner(u1, u2);
 
  // Step 2: Reverse part of the tape
  w.gradient() = 1.0;
  tape.evaluate(tape.getPosition(), begin);
  Gradient u1_d = u1.gradient();
  Gradient u2_d = u2.gradient();
 
  // Step 3: Cleanup the reversal
  tape.resetTo(begin);
  u1.gradient() = Gradient();
  u2.gradient() = Gradient();
 
  // Now store the computed gradient data
  tape.storeManual(w.value(), w.getIdentifier(), 2);
  tape.pushJacobianManual(u1_d, u1.value(), u1.getIdentifier());
  tape.pushJacobianManual(u2_d, u2.value(), u2.getIdentifier());
 
  Real y = cos(w);
 
  tape.registerOutput(y);
  tape.setPassive();
 
  // Reverse evaluation
  y.setGradient(1.0);
  tape.evaluate();
 
  std::cout << "Positional example:" << std::endl;
  std::cout << "Gradient of dy/da: " << a.getGradient() << std::endl;
  std::cout << "Gradient of dy/db: " << b.getGradient() << std::endl;
 
  tape.reset();
}
 
void validation() {
 
  Tape& tape = Real::getTape();
 
  // Recording
  Real a = 10.0;
  Real b = 4.0;
 
  tape.setActive();
  tape.registerInput(a);
  tape.registerInput(b);
 
  Real y = func(a, b);
 
  tape.registerOutput(y);
  tape.setPassive();
 
  y.setGradient(1.0);
  tape.evaluate();
 
  std::cout << "Validation:" << std::endl;
  std::cout << "Gradient of dy/da: " << a.getGradient() << std::endl;
  std::cout << "Gradient of dy/db: " << b.getGradient() << std::endl;
 
  tape.reset();
}
 
int main(int nargs, char** args) {
  positionalExample();
  std::cout << std::endl;
  validation();
}
 

In this example, some part of the function func() can be replaced by another function funcInner. The tape recording and evaluation for func() can either be done in the usual way or by using funcInner through storing its position during the recording process and manually providing its derivatives (u1_d, u2_d).

In general, positional evaluation can have multiple use cases:

• Record independent parts of the application and access them on a need basis
• Directly evaluate recorded functions to compute the Jacobians and store them (See also Example 15 - Preaccumulation of code parts)
• etc.

For a partly evaluated/reset tape the following should always be kept in mind:

• Is the adjoint vector cleared properly
• Are all active variables still valid after the reset.