Source-transformation Differentiation of a C++-like Ice Sheet model
Résumé
Algorithmic Differentiation (AD) has become one of the most powerful tools to improve our understanding of the
Earth System. If AD has been used by the ocean and atmospheric circulation modeling
community for almost 20 years, it is relatively new in the ice sheet modeling community. The Ice Sheet
System Model (ISSM) is a C++, object-oriented, massively parallelized, new generation ice sheet model that recently
implemented AD to improve its data assimilation capabilities. ISSM currently relies on Object Overloading through
ADOL-C and AMPI. However, experience shows that Object Overloading AD on ISSM is significantly more memory
intensive compared to the primal code. We want to investigate other AD approaches to improve the performance of
the AD adjoint. Yet, to our knowledge, there is no source-to-source AD tool that supports C++.
To overcome this problem, we have developed a prototype of ISSM entirely in C, called Boreas, in order to test
source-to-source transformation and compare the performance of these two approaches to AD. Boreas is a clone of
ISSM, the main difference with ISSM is that all the objects are converted to C-structures and some function names
have been adapted in order to be unique, but the code architectures are identical. The programming style of Boreas is
a first attempt at defining a programming style of (or a sub-language of) C++ that source-transformation AD could
handle. We present here the first results of Source-Transformation AD of Boread with the AD tool Tapenade.
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