Differentiation fails if function is defined after the call to clad differentiation functions.

[parth-07]

Currently, differentiation fails if the function to differentiate is defined after the call to clad differentiation functions. For example,

#include <iostream>
#include "clad/Differentiator/Differentiator.h"

double fn(double, double);

int main() {
  auto d_fn = clad::differentiate(fn, "i");
  std::cout<<d_fn.execute(3, 5);
}

double fn(double i, double j) {
  return i*i*j;
}

This code gives error: attempted differentiation of function 'fn', which does not have a definition

The cause of the problem here is, clang is unaware of fn definition when DerivativeBuilder::Derive is called to differentiate fn and we need the function body to differentiate it.

[grimmmyshini]

For this issue, we might want to use getDefinition() here

DiffPlanner.cpp line 258

auto derivedFD = cast<FunctionDecl>(DRE->getDecl());
request.Function = derivedFD;

rather than getDecl(). Although, not entirely sure if just changing this works.

[parth-07]

Sorry for the late reply.

getDefinition will likely not help here, since we are already using getDefinition in DerivativeBuilder::Derive before differentiating functions.

The cause of the problem seems to be that the function body has not been parsed at the time when we are trying to differentiate the function.

[grimmmyshini]

Ah, well then we can't really fix this unless the gradient generation is deferred to after the whole code has been parsed, which I am not sure of the feasibility of...