fix(qwen35): route non-tree GDN capture through persist buffer (src[7])#469
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Fast rollback / chain verify captures per-token SSM intermediate states for spec-decode rollback. Tree verify already wrote those states directly to a persistent buffer via src[7], while non-tree capture still copied from the GDN result tensor's embedded intermediate region. Route non-tree capture through the same persist mechanism: build the normal non-tree GDN op, attach cap->ssm_intermediate_states as result->src[7], and let the kernel write intermediates directly to external cache storage. This avoids allocator lifetime issues and removes the redundant result-region copy for the configured F16 capture path. The rebase keeps current main's can_skip_gdn_intermediate behavior intact, so pure chain prefill can still skip unused intermediates when no capture is requested.
With non-tree capture routed through src[7], the configured F16 intermediate buffer no longer needs the legacy result-region copy path. If a future caller requests capture with a type the persist path cannot handle, abort explicitly instead of leaving rollback state stale. This keeps fast-rollback failures obvious and prevents silent state corruption.
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Validated on lucebox2 (RTX 3090 CUDA, stability profile, Qwen3.6-27B-Q4_K_M + dflash-draft-3.6-q8_0, greedy):
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Summary
Route Qwen35 non-tree GDN capture through the persistent intermediate buffer (
src[7]) instead of copying from the GDN result tensor embedded intermediate region.This preserves the current main/#465 pure-chain prefill optimization:
ggml_gated_delta_net_set_skip_intermediate()is still applied only whencan_skip_gdn_intermediateis true. Capture paths keep intermediates available through the explicit persist buffer.Why
Fast rollback / chain verify needs per-token SSM intermediate states for rollback. Tree verify already had a persist path via
_tree_persist; non-tree capture was still using the legacy result-region copy path. Routing non-tree capture throughsrc[7]avoids allocator lifetime issues and keeps the rollback buffer populated directly.The unsupported fallback now fails loudly if capture is requested with a non-F32/F16 intermediate buffer, rather than leaving rollback state stale.
Relationship
mainincluding perf(qwen/qwen35moe): speed up prefill by skipping unused GDN writes #465.Validation
env CCACHE_DIR=/tmp/lucebox-ccache CCACHE_TEMPDIR=/tmp/lucebox-ccache-tmp cmake --build server/build --target test_server_unit -j 8server/build/test_server_unit2022 assertions, 0 failures