AD in Fortran, Part 2: Implementation via Prepreprocessor

TL;DR

This paper presents a prepreprocessor implementation for Farfel Fortran AD extensions, enabling seamless integration of forward and reverse AD into Fortran programs by generating code compatible with existing AD tools.

Contribution

It introduces a prepreprocessor that transforms Farfel AD constructs into subprograms suitable for existing AD preprocessors, enhancing flexibility and modularity.

Findings

Enables nested AD in Fortran with existing tools

Improves modularity and ease of use for AD in Fortran

Facilitates integration of AD into Fortran programming workflow

Abstract

We describe an implementation of the Farfel Fortran AD extensions. These extensions integrate forward and reverse AD directly into the programming model, with attendant benefits to flexibility, modularity, and ease of use. The implementation we describe is a "prepreprocessor" that generates input to existing Fortran-based AD tools. In essence, blocks of code which are targeted for AD by Farfel constructs are put into subprograms which capture their lexical variable context, and these are closure-converted into top-level subprograms and specialized to eliminate EXTERNAL arguments, rendering them amenable to existing AD preprocessors, which are then invoked, possibly repeatedly if the AD is nested.