Extracting ancillary information during tape sweeps

[perrydv]

Hi @bradbell

Is there a suggested way to have ancillary (not central to obtaining derivatives) access to values during tape execution?

Say I have a function like

std::vector foo(std::vector<double> x) {
  // some math creates y from x
  do_something_else_without_math(y); // ancillary, like maybe generating output or storing in some other object
  // more math creates result
  return result;
}

I want to CppAD-ize the function, and it would be ideal if I can still do the ancillary step and have it invoked during tape sweeps, at least forward0.

A workaround I've used is to embed the ancillary step in a derived atomic class with a dummy output. Then I invoke the atomic so it's in the tape. The dummy output is a CppAD::AD<double> that I add to result. The value of the dummy output is always 0, and by adding that to the result I think I am tricking the tape into not optimizing away the use of the atomic. (If the dummy output of the atomic was not added to the result, I think it would look like a dangling node in the computation graph recorded on the tape and could in theory be optimized away, although I don't know exactly what CppAD's optimize does.)

This workaround seems to work well, but the whole situation I'm doing this in is highly configurable, and I'm not sure I can always easily protect against having the atomic optimized out of the tape or if that is really something to worry about.

So I'm wondering if there is already a solution for this, or if the need makes sense and would be easy to support, or if there is a tweak on the solution with atomics that you would suggest. Thanks.