# round 4 track b: mtp input and hyper-connection fusion

## deliverables

- patch: `/tmp/sol_mtp_fuse/sol_mtp_fuse.patch`
- apply: `git -C vendor/strata apply /tmp/sol_mtp_fuse/sol_mtp_fuse.patch`
- patch sha256: `4316deb086fa556ffd5328f60637d123dc1a2e5539c0eb2dc888c783f10803ae`
- isolated sources, reference harness, command and logs: `/tmp/sol_mtp_fuse/`
- no tracked file under `vendor/strata/` was edited. no server access, gpu dispatch, or gpu/ssd benchmark was run.

## design and invariants

owner: `mtp.metal` owns the fused arithmetic; `Window::mtp_encode` owns layouts and dispatch dependencies. this runs on every draft step and mtp prefill. existing prepared-activation allocations, weights, norm epsilon, residual layout, injection state and gemv call structure are unchanged.

`mtp_rms_xprep` uses the original 256-thread reduction, the same `fma` accumulation, simd sums, eight-part summation, and `xr[d] * rs * w[d]` order. the common `mtp_prepare` helper preserves all four planes and separate even/odd s16 sums exactly. the generic `units.metal` helper cannot be called directly: this mtp library is compiled standalone, without quant/units dependencies.

- embedding: one 2560-wide norm per token, direct prepared output, no `men_` writes.
- hidden, `n == 1`: one 10240-wide norm, split into four 2560-wide prepared rows, with row count/plane stride still `MAXT=8`. no `mhn_` writes.
- hidden, `n > 1`: one 10240-wide norm per token, prepared stream 0 directly; retain normalized `mhn_` for the existing stream 1..3 xprep/gemv sequence. deleting that scratch would be incorrect because `XHn_` only holds eight prepared 2560-wide rows, not all four streams of eight tokens. the retained scratch is intentional, not a full-prefill intermediate-elimination claim.

`mtp_combine_gnorm` launches `(HC, n)` groups of 256 threads. each thread retains its ten combined values across the same reduction as `gr_norm(apply=false)`, so neither rereading `mR_` nor recomputing/reloading h/e is necessary. it writes residual `mR_` and `xn_`, preserving `v * w[d] * rs` (not the other rms kernel's multiply order). a host static assertion binds the ten-register-value layout to `N=2560`.

`gr_read_m(..., already_normed=true)` skips only the initial norm. `apply=true` always runs the original norm regardless of the flag. other callers keep their original default behavior. the normal `push()` dependency barrier remains before gr_down reads `xn_`; removing the intermediate norm dispatch removes one barrier, not the producer-consumer dependency. embedding/hidden dispatches remain independent, followed by a barrier before gemv.

obsolete `mtp_rms` / `mtp_combine` entry points and their unused argument structs were removed. original snapshots remain available through the temporary repository's baseline commit.

## checks actually run

all final checks exited 0:

1. `clang++ -fsyntax-only` on the isolated modified `window.cpp`, using existing build.ninja defines/includes/flags and the original engine header directory. exact command: `/tmp/sol_mtp_fuse/syntax_command.txt`; output: `cpp_syntax.log` (empty).
2. `clang++ -std=c++20 -O2 -ffp-contract=off -fsanitize=address,undefined /tmp/sol_mtp_fuse/reference.cpp -o /tmp/sol_mtp_fuse/reference`, then the binary. **40 cases, n=1..8, five input patterns; 21,388,320 float bit-pattern checks passed.** checks cover all xp planes/s16 including untouched padding and trailing guards, embedding and hidden nonzero source offsets, untouched draft scratch, retained prefill scratch and later stream preparations, combined residuals and normalized residuals. patterns: seeded random, signed zeros, mixed powers of two, alternating signs, and tiny inputs. no sanitizer failures. output: `reference.log`.
3. actual safe-math Metal source compilation using `newLibraryWithSource`, then compute pipeline creation for both fused entry points and retained xprep. all have SIMD width 32 and max threads 1024, compatible with 256-thread groups. **no command encoder or gpu dispatch.** source: `compile_metal.mm`; output: `metal_compile.log`. the helper build has one expected deprecation warning for its pre-macos-15 `fastMathEnabled` fallback.
4. `git -C vendor/strata apply --check /tmp/sol_mtp_fuse/sol_mtp_fuse.patch`: passed (`apply_check.log`).
5. final visual diff review and context review performed on isolated sources and reference harness.

`xcrun -sdk macosx metal -c ...` is unavailable with this machine's command-line tools (utility not found); the real runtime Metal compiler above was used instead.

## what the evidence does and does not establish

cpu reference results establish exact bit-pattern equivalence of the tested arithmetic/layout transformations. both fused reductions retain the baseline arithmetic order under the engine's default safe-math setting. **this is not a gpu-executed bitwise-parity claim**: GPU `simd_sum` / `rsqrt` were not executed, per task restriction. the reference shares a deterministic simulated SIMD tree and uses host reciprocal square root; neither claims to emulate the device's exact numerical implementation. `STRATA_METAL_MATH=fast` reassociation is outside the bitwise-equivalence claim.

structurally: **three fewer dispatches and two fewer barriers** per encode call. for a single-token draft, the eliminated intermediate traffic is nominally 100 kib from normalized embedding/hidden write+read pairs plus 80 kib from the two gr_norm residual reads, or **180 kib/token** of explicit device-buffer traffic. caches, register spills, occupancy, and wall-clock speed are unmeasured. scratch allocations remain unchanged. multi-token prefill still writes hidden scratch for later streams, so its traffic reduction is smaller (nominally 30 kib/token from rms/xprep plus 80 kib/token from combine/norm).

recommendation: ready for parent integration/review, then an authorized standalone gpu bitwise-parity test before claiming 100% device equivalence, and a separately authorized benchmark before claiming a speedup.

## baseline identity

vendor revision: `296b59d2f73c8f04a805df29c2fa97655adace78`; snapshot included the working-tree source state, not just the revision.

- original `mtp.metal` sha256: `564c849df5c36a22baae10b3f11396a5b45cb5835a67a3ee37325e418195fd4c`
- original `window.cpp` sha256: `8c892efe7441a604695da7bf1badaba09738aa1b5d482e4a46bc6991c6450d31`