# opus_decode_fusion — Round 2 Track A: gemv_cat + gr_norm_comb wiring, mtp_encode barrier audit

Patch: `/tmp/opus_decode_fusion/opus_decode_fusion.patch` (applies cleanly with `git apply` at repo root against the
current working tree of `vendor/strata/metal/src/engine/window.cpp`; also extends `tests/fuse_parity.cpp` to ntok 1..8).
Work tree: `/tmp/opus_decode_fusion/strata` (patched), `/tmp/opus_decode_fusion/strata_orig` (baseline), both built.

## What changed (window.cpp)
1. **gemv_cat for ssm_alpha + ssm_beta** (36 GDN layers → −36 dispatches/window). gemv()'s tile choice factored out
   into `gemv_tile()` (pure code motion); `Window::gemv_cat()` uses gemvc_<ty>_<L>_<R>_<S> only when the tile gemv()
   would pick has KS = 1 and is instantiated (BF16/F32; a row's sums depend only on LPR, so bitwise). For the shipped
   GGUFs (alpha/beta BF16, 2560→48) the tile is 32_1_4_1 → `gemvc_30_32_1_4`. Writes alL_/beL_ at o_ab with y_stride 48:
   same layout fold() reads. Falls back to two gemvs otherwise. `STRATA_METAL_GEMV_CAT=0` disables. Prewarm needs no
   change: prewarm_pipelines() goes through layer(), so the gemvc#t pipelines are made there.
2. **gr_norm_comb**: moe_combine_sh is deferred (`comb_defer`) and folded into the next gr_norm that reads the same bo
   buffer with apply=1 (new `Window::gr_norm()` helper used by gr_read, the PLE gr_norm after layer 0, and gr_read_m).
   Any other gr_norm flushes the pending combine first (safety net; never hit on current paths). Audit of bo_: written
   only by ssm_out/attn_output gemv and the combine; read only by gr_norm. After every layer's combine the very next
   dispatch is that gr_norm (layer l+1 hc_attn, the PLE gr_norm for l=0, output_hc after l=47); nothing else reads bo_
   (ydump_ is only fold()'s gdn output, DUMP_MOE is prompt-chain only). Same for mbo_ in mtp_encode → mixer's gr_norm.
   Saves 48 dispatches + 48 barriers + bo_ round trip per window, 1 + 1 per MTP draft step. `STRATA_METAL_GR_COMB=0` disables.
3. **mtp_encode barriers**: the embedding chain (rms→xp→fc_embedding) and hidden chain (rms→xp→fc_hidden) now run
   side by side: 3 barriers instead of 6 (n==1); for n>1 stream 0 joins the pairs, streams 1..3 keep their barriers
   (XHn_ is reused). Buffers disjoint (men_/mhn_, XEn_/XHn_, me2_/mh2_).
4. **PLE key norm** (ple_gnorm on pkey_) moved ahead of layer 0, beside layer 0's first gr_norm: −1 barrier/window.
   Other nb_ sites in layer()/mtp_encode/mtp_pick audited: remaining barriers are true RAW/WAR deps.

## Verification
- `fuse_parity` (M5, extended to ntok 1..8): gemv_cat, router_route, gr_norm_comb all **bitwise PASS** (~6 s incl. compile).
- Engine builds clean (`ninja strata_window_cli`).
- End to end K512 IQ2_XS + PLE + MTP, deterministic config (see finding below): teacher-forced logits (`--logits-out`,
  22-token prompt + 24 T=1 steps) **byte-identical** orig vs patched (defaults) vs patched with both knobs =0.
  Greedy generation `--spec 3` (T=3 windows + MTP chain, catch-up n=1,2) and `--spec 4 --suffix-draft 3 --n-prompt 100`
  (prompt GEMM + T≤4 + MTP): identical GEN_IDS and identical draft/accept counts, orig vs patched, 2 runs each.
- Speed: with the served env the T=3 window time is within noise (orig 65.9–67.7 ms, patched 66.7–74.7 ms; I/O-
  dominated streaming). Expected gain ~84 fewer dispatches + ~50 fewer barriers per window, i.e. sub-ms; needs a
  quiet-machine benchmark (parent to run).

## Finding (not fixed, other track): served config is run-to-run nondeterministic
With `STRATA_METAL_EXPERT_SPLIT=1` (the served default with SHEXP_OVERLAP) the baseline binary gives different logits
on every run (all rows, |Δ| up to ~0.8 already at row 0), even with MOCE_BONUS=0 and PROMPT_GEMM=0. With
`STRATA_METAL_EXPERT_SPLIT=0` it is bitwise deterministic. The moe.metal comment claims the split is bit-for-bit
the unsplit plan, so this looks like a race (phase-1 resident experts running beside the host's turn — slot reuse or
plan/ye hazard). MOCE_BONUS>0 adds residency-dependent routing on top. Any A/B on the served config is therefore
noise-limited; use EXPERT_SPLIT=0, MOCE_BONUS=0 for bitwise checks.