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Option (a) of PREFILL_GEMM_SCOPE.md: route large-M (prefill) NVFP4 dense weight GEMMs off the decode-tuned FP4-MMQ kernel onto the dequant->bf16 cuBLAS (nvjet) tensor-core path, wired via an M-threshold in ggml_cuda_should_use_mmq. Lands the validated, bit-exact-gated mechanism and records the honest GB10 result: it is a regression, so it ships default-off (== stock), mirroring the patch-0017 default-off discipline. Three-edit scaffold (no new kernel): should_use_mmq routes NVFP4+Blackwell+dense M>LLAMA_FP4_PREFILL_M to cuBLAS; op_mul_mat_cublas gains an NVFP4 branch that dequants the FP4 weights to a transient bf16 pool buffer (not cached - stays FP4-resident) and runs cublasGemmEx CUDA_R_16BF/COMPUTE_32F; ggml_get_to_bf16_cuda gains the NVFP4 case. Bit-exact gate PASS (benign): test-backend-ops MUL_MAT 1146/1146 + MUL_MAT_ID 806/806; the forced path (LLAMA_FP4_PREFILL_M=64) is green CUDA-vs-CPU at NVFP4 large-M shapes; greedy md5 on q36-27b is byte-identical to FP4-MMQ both for short prefill (5951a5b4, decode untouched) and for a >threshold prefill that exercises the bf16 path (5f3967df - no greedy argmax flips). Performance REGRESSES on GB10 (S_PP, q36-27b dense, A/B via env): M=512 958.99 -> 486.65 (-49%), M=1024 1013.65 -> 587.27 (-42%), M=2048 918.46 -> 649.42 (-29%). The scope premise (FP4-MMQ ~3% of FP4 peak at large M) is false here: FP4-MMQ beats bf16-cuBLAS because bf16 peak is ~half FP4 peak and the per-step weight dequant + 4x bf16 weight traffic (~8x total vs the FP4 read) dominate, only partially amortizing as M grows. Default-off keeps stock S_PP (966.98). Phase 2 (MoE grouped large-M) not implemented: it inherits the same bf16-peak<FP4-peak ceiling plus a per-expert dequant, so grouped bf16-cuBLAS would regress for the same reason; a real prefill GEMM win needs option (b), a native FP4-MMA large-M kernel. Full A/B in docs/PREFILL_GEMM_RESULTS.md. Assisted-by: Claude:opus-4.8 [Claude Code] Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
