[Metal:Perf] Fused-Q4 M8 tile and K-split for speculative-block decode - #4772
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The fused Q4 GEMM stage kernel uses an M32 matmul2d tile. A speculative
block is 8 rows, so it runs at quarter occupancy, and for projections
with few output tiles it also dispatches too few threadgroups to
saturate bandwidth.
Add two variants behind FUSED_Q4_REAL_UNPACK:
- conv1x1_fused_q4_gemm_stage_m8: descriptor sized to M=8.
- conv1x1_fused_q4_gemm_stage_ksplit{,_m8} + _ksplit_reduce: split K
four ways across gid.z into fp32 partials, then sum + bias +
activation in a second pass.
Both are gated on KVMeta::spec_block so nothing outside speculative
decoding changes path.
New switches, all default to auto (unset):
MNN_METAL_FUSED_Q4_KSPLIT =1 force on, =0 force off
MNN_METAL_FUSED_Q4_M8 =0 force off (=1 same as unset, the tile is
MNN_METAL_FUSED_Q4_KSPLIT_M8 only correct for area <= 8)
K-split reassociates the K accumulation, so output is not
bit-identical: on an iPad tier-9 sweep 8 of 9 prompts match the
baseline verbatim and one diverges on a near-tie greedy argmax.
The area-gate part of this work already landed with the fused-Q4
spec-block gate.
Discussed-in: Merge-Request 29266048 , URL: https://code.alibaba-inc.com/AliNN/AliNNPrivate/codereview/29266048
GitOrigin-RevId: c465bb4195f3722161c2338440b301f43e052b01
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456032377d[Metal:Perf] Fused-Q4 M8 tile and K-split for speculative-block decode — 花熊 zhanghuaxiang.zhx@taobao.com