Source-transformation Differentiation of a C++-like Ice Sheet model

Description

Algorithmic Differentiation (AD) has become one of the most powerful tools to improve our understanding of theEarth System. If AD has been used by the ocean and atmospheric circulation modelingcommunity for almost 20 years, it is relatively new in the ice sheet modeling community. The Ice SheetSystem Model (ISSM) is a C++, object-oriented, massively parallelized, new generation ice sheet model that recentlyimplemented AD to improve its data assimilation capabilities. ISSM currently relies on Object Overloading throughADOL-C and AMPI. However, experience shows that Object Overloading AD on ISSM is significantly more memoryintensive compared to the primal code. We want to investigate other AD approaches to improve the performance ofthe 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 testsource-to-source transformation and compare the performance of these two approaches to AD. Boreas is a clone ofISSM, the main difference with ISSM is that all the objects are converted to C-structures and some function nameshave been adapted in order to be unique, but the code architectures are identical. The programming style of Boreas isa first attempt at defining a programming style of (or a sub-language of) C++ that source-transformation AD couldhandle. We present here the first results of Source-Transformation AD of Boread with the AD tool Tapenade.

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