Express Language Modeling: turning a non-causal attention approximation into a causal one
AspireAI community · arXiv
Express converts a non-causal attention approximation into a causal one while keeping the approximation guarantees. The authors report that combined with the Thinformer approximation it delivers log^(3/2)(n)/s approximation error with O(s) memory and O(s^2 log^2(n)) compression overhead for a sequence of length n, improving on the best known causal attention guarantees. They pair this with an I/O-aware Triton implementation and report substantial speedups over FlashAttention 2, and use the result to address four pipeline bottlenecks: long-context prefill, KV cache compression, long-form memory-constrained decoding and long-form compute-constrained decoding. The paper's v3 adds a Reflections and Conclusions section and a long-form decoding configuration with BalanceKV; the abstract itself is unchanged from earlier versions.
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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.