Fix saveloadstate ov - #309
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Assisted-by: Claude Sonnet
…state The stateful path seeds its KV state from ggml's cache when the decode position is ahead of what the state holds. That only works when ggml's cache is a plain prefix, where cell i holds position i. A sliding-window layer keeps just the last n_swa positions and drops the rest, so past the window cell i no longer holds position i and the seeded state is wrong. Slicing the state to the decode position also had no bounds check, so a position past the end surfaced as a bare ov::Exception from the ROI constructor (llama_decode ret = -3, with no reason given at default verbosity). Refuse both cases with a clear message instead, and refuse on the compile path too, where a new model starts with an empty state and so can only serve a sequence from its beginning. Reproducible with llama-bench -d, which restores a saved sequence state rather than recomputing the depth prefill. Assisted-by: Claude Opus 5
The stateful path reinterprets ggml's KV buffer [1, 1, seq, n_heads_kv * head_size] as [1, seq, n_heads_kv, head_size]. The head size is already taken from the tensor's own combined dim, because gemma-4 varies it per layer type, but the head count still came from a model-level scalar that compute_llm_params() overwrites per attention node, so it ended up holding whatever the last layer said. gemma-4 varies the head count per layer too: 12B has 8 x 256 sliding layers and 1 x 512 full layers, 31B has 16 x 256 and 4 x 512. So 40 of 12B's 48 layers were split as 1 x 2048 instead of 8 x 256, and attention read the state with the wrong head split - both models decoded garbage on CPU and GPU. E2B is unaffected, its head count is 1 everywhere. Record the count per layer instead and look it up by the cache_k_l<N> leaf name. Key it by layer, not by layer type: the sliding/full classification comes from cache extents, which tie at a small -c, while the head count does not. The stateful state trim now derives its sequence axis per state for the same reason, since pass::KVStateSeqAxis matches per state on the head count. Assisted-by: Claude Opus 5
pass::KVStateSeqAxis was limited to states with a single KV head, where moving the sequence axis from dim 1 to dim 2 is a pure metadata change. The limit was also based on a measurement showing no gain for a multi-head model, but that was taken at depth 0, which is the one depth where this change does nothing. With several heads the pass does more than move metadata: it drops the reader side transpose of the whole accumulated state, which the graph otherwise redoes every token at a cost that grows with the context length, and replaces it with a transpose of the single new row. Measured on GPU, tg128, alternating arms: gemma-4-12B 6.27 -> 9.11 t/s at depth 8192 (stateless is 7.69, so stateful now wins at depth instead of losing), Llama-3.2-1B 47.8 -> 59.6 t/s. Both are within noise at depth 0, which is why the earlier check saw nothing. The state refill needs the rows copied rather than reinterpreted now: ggml stores [seq][n_heads_kv * head_size], and a relayout state with several heads is a different element order. Without that, a refill would seed wrong data - it is reachable today through llama-bench -d. Assisted-by: Claude Opus 5
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