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An AI coding agent ported 140,000 lines of Fortran climate code to Kokkos C++ and the port was 1.71x faster

AspireAI community · arXiv (cs.DC)
NICAM, a nonhydrostatic icosahedral atmospheric model written in Fortran with OpenACC, was ported to C++ with Kokkos by Claude Code under human review. About 140,000 lines of Fortran were analysed and ported into 35,000 lines of C++/Kokkos, covering the dynamical core and seven physics packages. A 100-member ensemble comparison with the original Fortran model showed the port is statistically consistent with it. Measured on an NVIDIA GH200 cluster and an AMD MI300A cluster, the port achieved application speedups of 1.71x on GH200 and 1.54x on MI300A over the Fortran/OpenACC baseline, and the CUDA build reached a GPU kernel-time speedup of 1.38x on GH200. After normalisation by peak memory bandwidth the HIP build on MI300A reached 0.96x the baseline's kernel performance, so the gain is backend-dependent.
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One distance function matched sparse-autoencoder features across layers and compressed feature circuits into supernodes
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