Derivatives By-Address for Fortran 77

FIXME Automatic differentiation tools use 1 of 2 strategies to access derivative values. These strategies are: 1. By-address 2. By-name The by-address method is typically implemented by introducing structured types for each active scalar type. For example, scalar type ’real’ will have an associated structure type ’active-real’. Using this strategy, all active variables types are changed to the associated structured type. On the other hand, the by-name method introduces an associated new variable for each active variable. For example, the derivatives associated with ’pressure’ would be ’d_pressure’. Since the by-address strategy employs structured types, AD tools for Fortran 77 have not employed that strategy. In this paper, we show how to use array access to implement the by-address strategy for Fortran 77. We discuss the canonicalization issues, outline our Adifor3.0 implementation of this technique, and give a few sample performance comparisons of the by-address vs the standard Adifor3.0 by-name.