Muliple tapes in CoDiPack

[MaxSagebaum]

@benno90

Since I'm already here, i have a suggestion for a new feature:
Is it possible to have several different tapes in memory at the same time with CoDiPack?
By simply switching the tapes one could differentiate different functions repeatedly without having to re-tape again.
I'm not yet familiar enough with the architecture of CoDiPack to judge if this is even possible, but it would be very useful.

This feature is already there:

using Real = codi::RealReverse;
using Tape = typename Real::Tape;

Tape& globalTape = Real::getGlobalTape();

// Do some recording

Tape tape1;
tape1.swap(globalTape);
// tape one contains now the recorded tape and can be evaluated 
tape1.evaluate()
// You should store the identifiers of your inputs and outputs, since x.gradient() always access the global tape

// Do some recording

Tape* tape2 = new Tape();
tape2->swap(globalTape);

[benno90]

Thanks for the example. I will test it as soon as possible.

[benno90]

I managed to create a simple example. Do you have any comments or spot any mistakes?
Should this also work with RealReverseIndex?

using Real = codi::RealReverse;
using Tape = typename Real::Tape;
Tape& tape = Real::getTape();

Real f(Real a, Real b)
{
    return (a + b) * a;
}

Real g(Real a, Real b)
{
    return a / b;
}

Tape tape1;
Tape tape2;

int main()
{
    Real a = 2.0;
    int ai1, ai2;
    Real b = 3.0;
    int bi1, bi2;
    Real y1;
    Real y2;

    tape.setActive();
    tape.registerInput(a);
    tape.registerInput(b);

    y1 = f(a, b);
    tape.registerOutput(y1);
    y1.setGradient(1.0);
    tape.setPassive();
    ai1 = a.getIdentifier();
    bi1 = b.getIdentifier();
    tape1.swap(tape);

    //tape.deactivateValue(a);
    //tape.deactivateValue(b);
    // ----------------------------------
    tape.setActive();
    tape.registerInput(a);
    tape.registerInput(b);

    y2 = g(a, b);
    tape.registerOutput(y2);
    y2.setGradient(1.0);
    tape.setPassive();
    ai2 = a.getIdentifier();
    bi2 = b.getIdentifier();
    tape2.swap(tape);
    


    for(int I = 0; I < 2; ++I)
    {
        cout << "=============================== outer iteration " << I << "\n\n";
        cout << "-------------- tape 1\n";
        tape1.evaluate();
        a.getIdentifier() = ai1;
        b.getIdentifier() = bi1;
        tape.swap(tape1);
        cout << "a.getGradient() = " << a.getGradient() << endl;
        cout << "b.getGradient() = " << b.getGradient() << endl;
        cout << "\n-------------- tape 2\n";
        tape1.swap(tape);
        tape2.evaluate();
        a.getIdentifier() = ai2;
        b.getIdentifier() = bi2;
        tape.swap(tape2);
        cout << "a.getGradient() = " << a.getGradient() << endl;
        cout << "b.getGradient() = " << b.getGradient() << endl;
        tape2.swap(tape);
    }
}