Pull Request resolved: #20880

Fuse the three attention q/k/v `linear_q4gsw` projections into one multi-output GEMM that scatter-writes q/k/v, removing the occupancy-starved small k/v dispatches in prefill on Llama-3.2-1B. Auto-applied graph pass, perplexity-neutral.

Problem

Attention lowers q/k/v as three separate `linear_q4gsw` GEMMs; the two N=512 k/v projections each dispatch a grid too small to fill the GPU, so they run occupancy-starved.

Solution

- Before: q/k/v = 3 GEMM dispatches (N=2048/512/512).
- After: one [2048->3072] multi-output GEMM that scatter-writes q/k/v to their own buffers, M-gated so it fires only at prefill (M>1) and keeps the three coop4 GEMVs at decode (M=1, where the fused 64x64 tile would waste 63/64 rows).

Implementation

- New standalone kernel `q4gsw_linear_gemm_qkv_fused.wgsl` (+ generated `_wgsl.h`); build-time pattern detection and an M-gated dispatch/resize hook in `WebGPUGraph::build`.
- q/k/v are repointed to fresh scratch buffers so the memory planner's reuse-aliasing cannot overlap the simultaneous fused write with a still-live input.
- The fused dispatch owns its one-off compute pipeline (released in the destructor), mirroring `q4gsw_linear_impl`.

Constraints

Auto-applied when the pattern matches (no flag): the detection gates on shader-f16, a 256-thread workgroup, and the Llama-3.2 GQA projection shape {2048, 512, 512} with no bias, so any graph without that triple gets the byte-identical baseline lowering.

Co-authored-with: Claude Code.
ghstack-source-id: 405251962
@exported-using-ghexport

Differential Revision: [D111489253](https://our.internmc.facebook.com/intern/diff/D111489253/)

Pull Request resolved: #20881

Fold the SwiGLU `sigmoid` + two `mul`s into a single elementwise pass computing (gate * sigmoid(gate)) * up, cutting the MLP activation traffic from ~8 units to ~3 on Llama-3.2-1B. Auto-applied graph pass, bit-exact.

Problem

The SwiGLU MLP lowers `silu(gate)*up` as three memory-bound elementwise dispatches (`sigmoid` -> `mul` -> `mul`) over the [S, 8192] intermediate.

Solution

- Before: `sigmoid` + 2 `mul`s (~8 tensor-traffic units over [S, 8192]).
- After: one `silu_mul_fused` pass computing `(g*sigmoid(g))*up` with the sigmoid in-register (~3 traffic units); the folded sigmoid + 1st-mul dispatches are dropped.

Implementation

- New standalone kernel `silu_mul_fused.wgsl` (+ generated `_wgsl.h`); build-time triple detection (requiring single-consumer sig/silu intermediates) + an in-place fused dispatch at the 2nd-mul anchor in `WebGPUGraph::build`.
- gate is repointed to a private pooled scratch buffer at its producer op, and the fused output to another, so the memory planner's reuse-aliasing (which places up_proj's output onto gate's dead slot) cannot stomp gate before the fused dispatch reads it; both pooled buffers are released after last use for cross-layer reuse.

Constraints

Auto-applied to fp32 elementwise triples with identical dims and single-consumer sig/silu intermediates (no flag); a graph without a matching triple gets the byte-identical baseline lowering. Stacked on the QKV-concat fusion diff.

Co-authored-with: Claude Code.
ghstack-source-id: 405251982
@exported-using-ghexport

Differential Revision: [D111489277](https://our.internmc.facebook.com/intern/diff/D111489277/)