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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Two sparse-autoencoder analyses have resisted scaling: matching semantically similar features across layers, and compressing large feature circuits into interpretable supernodes. This paper argues both are the same underlying problem, estimating semantic distances between SAE features that sit on different activation manifolds. Its representation treats each feature not as a single decoder vector but as an activation-weighted distribution over the hidden states expressing it, projects those distributions into a shared reference space, and compares them with Wasserstein distance. The representation is proved invariant to activation rescaling and stable under perturbation, and recovers true matches under finite-sample margin conditions. Empirically it outperformed decoder-vector and LLM-based baselines and captured subtle functional distinctions between related features, and it compressed large feature circuits into interpretable supernodes automatically. The limitation is that this is interpretability tooling: the paper demonstrates better feature matching and circuit compression, not that downstream interpretability claims become more accurate.