The tiled-transpose fast path in ggml_vk_concat is a large prefill win but a
loss at decode. On a hybrid model like Qwen3.6-35B (gated delta-net on 30 of
40 layers) build_conv_state emits a transposed concat per recurrent layer, so
the fast path runs 30 times per decoded token.
At one token the transposed source is a single column, so there is no
uncoalesced stride left to fix, but the path still costs two dispatches and an
unconditional ggml_vk_sync_buffers - a full pipeline barrier that serializes
the command stream. Measured -17% tg256 on RX 580; the barrier, not the
half-empty transpose dispatch, is nearly all of it.
Require the transposed source to be at least one TILE_DIM wide. That source is
qkv_mixed transposed, so ne[0] is the ubatch token count: prefill keeps the
fast path, decode and small speculative drafts take the generic one.
pp8192 +11% retained, tg256 back to parity, generation byte-identical.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PZz44SLQvTXMyWGio6t9DZ
LumpiastyandClaude Opus 5
c233ce9b51