Section: New Results
Algorithmic Differentiation and Dynamic Memory
Participants : Laurent Hascoët, Sri Hari Krishna Narayanan [Argonne National Lab. (Illinois, USA)] .
In the same framework as in section 6.5 , we made progress in the development of the adjoint AD model for programs that use dynamic memory. Adjoint differentiated code obtained by source transformation (OpenAD, Tapenade...) consists of a forward sweep that essentially copies the original code, and a backward sweep that computes the derivatives, These two sweeps must have the same control-flow shape, only reversed. The allocation and deallocation of dynamic memory inside the forward sweep requires a similar pattern in the backward sweep. However, allocations do not always return the same memory chunk, and therefore all memory addresses must be updated to preserve their consistency in the backward sweep.
This problem can only be solved at run-time. A compile-time analysis simply cannot extract the information needed. Our approach is thus to design a library that encapsulates all calls to memory allocation primitives (malloc, free ...) in order to register the allocated addresses and to restore consistency of pointers during the backward sweep. This strategy is similar to the one we use for MPI calls, cf 6.5 , and is actually an ingredient in our AMPI strategy.
This approach was tested with success on a medium-size industrial application in structural mechanics. For this unsteady simulation the C code allocates and frees memory repeatedly at each time step. The tangent and adjoint differentiated C codes, as produced by Tapenade, have been adapted by hand to run the new model, showing promising performance. Obviously, the next step is to update the Tapenade AD model to automate this hand adaptation.