# round 30 track b: no shippable improvement

## decision

keep the existing kernels. nine isolated source variants were screened; none established a repeatable 2% improvement. all valid a/b runs were bitwise identical. `/tmp/sol_r30_prompt/sol_r30_prompt.patch` is deliberately empty; `git -C vendor/strata apply --allow-empty --check /tmp/sol_r30_prompt/sol_r30_prompt.patch` exits 0. plain `apply --check` rejects an empty patch, so there is nothing to apply. no tracked vendor files were edited, and no test changes are proposed.

## experiment contract and provenance

question: can redundant accumulator state, smaller reduction storage, gate-load/unroll changes, or hc epilogue address simplification reduce gpu time without changing a bit?

acceptance set before runs: bitwise equality and repeatable >2% target-kernel improvement without material regression across 512/1024/2048. no candidate cleared screening, so the planned two-repeat confirmation of a winner was unnecessary. the accumulator/gate candidates were nevertheless checked at all three sizes, and up-hoist at 2048 too.

- device: apple m5; macos version 27.0 (build 26a428).
- vendor head: `296b59d2f73c8f04a805df29c2fa97655adace78`; working source snapshot, not assumed clean.
- immutable baseline: `/tmp/sol_r30_prompt/base_prompt.metal`, sha256 `86adff9cfdbdf1af34ecfbdab0bef8b9e0668b3884a577469ac7bc58140f13a3`.
- executable: existing `vendor/strata/build-metal/comb_fold_parity`; sha256 `ba877816cf682d912ebdcd5fcd30e83dfcdcb9ff024b06ff58c8ad3d9547bf30`. no rebuild needed: tests compile the selected msl at runtime.
- scratch tree: `/tmp/sol_r30_prompt/strata`, with `STRATA_METAL_KERNELS=/tmp/sol_r30_prompt/strata/metal/kernels`.
- evaluator: existing ab_parity, 12 alternating paired samples of 8 dispatches, rotated output buffers; medians of paired ratios. safe metal math, explicit fma order retained.
- source selection independently verified by injecting an invalid type into scratch gr_norm_h4: compilation failed as expected (exit 3), then source was restored. no silent fallback to live kernels.
- gpu invocations serialized; existing live server left alone. no port access or full-model benchmark. every measured invocation completed in <5s (20s cold-compile timeout cap).
- metadata and immutable candidate `.metal` sources alongside each `.log` under `/tmp/sol_r30_prompt/`.

## screens

ratios are candidate/baseline; lower is faster. each row is one source variant, not an additive stack.

| variant | t | moec | h4 apply 1 | h4 apply 0 | result |
|---|---:|---:|---:|---:|---|
| self/self baseline | 1024 | 0.9996 | 0.9998 | 0.9985 | control |
| direct bv accumulation, no s array | 512 | 1.0000 | 1.0066 | 0.9861 | no target improvement |
| direct bv accumulation | 1024 | 0.9960 | 0.9999 | 1.0032 | noise-scale |
| direct bv accumulation | 2048 | 0.9999 | 1.0043 | 0.9997 | no improvement |
| float4 gates, exact quarter when hc=4 | 512 | 0.9995 | 0.9983 | 0.9936 | noise-scale |
| float4 gates | 1024 | 1.0014 | 0.9989 | 0.9955 | noise-scale |
| float4 gates | 2048 | 0.9996 | 1.0006 | 0.9975 | no improvement |
| ping-pong part[2][8], scoped to owned kernels | 1024 | 0.9975 | 0.9976 | 0.9960 | noise-scale |
| fully unrolled four-stream loop, hc guard | 1024 | 0.9988 | 0.9989 | 0.9978 | noise-scale |
| first simdgroup returns register slot count | 1024 | 0.9988 | 1.0013 | 1.0040 | no improvement |
| all residual streams live, one reduction barrier | 1024 | 1.0021 | 1.0011 | 0.9929 | no target improvement |

all bitwise pass. the earlier `pingpong` exploratory snapshot accidentally also replaced an unowned helper declaration *in scratch only*; it is excluded from the decision and superseded by `pingpong_fixed`, whose prefix is unchanged. nothing from either is shipped.

| epilogue variant | t | hc-down | hc-up reg 32/128/4 | result |
|---|---:|---:|---:|---|
| self/self | 1024 | 1.0014 | 0.9935 | control; up noisier in early run |
| hc-up tile-base address hoist | 1024 | 1.0104 (unchanged source) | 1.0004 | no improvement |
| hc-up tile-base address hoist | 2048 | 1.0006 (unchanged source) | 1.0039 | slight loss |
| hc-down hardcode documented hc=4 precondition | 1024 | 0.9973 | 1.0010 (unchanged source) | noise-scale |

no measured register-occupancy evidence supports the task's proposed “saves ten vgprs” claim: compiler lifetime/coalescing may already remove the distinction. similarly, source-level arithmetic simplification alone did not improve measured throughput. near-linear scaling of gr latency is consistent with bandwidth pressure, but this is not a hardware-counter diagnosis.

## final baseline controls

scratch restored byte-for-byte to the original baseline. independent `ab ... gr` and `ab ... gemm` modes exercise the same cases as unfiltered ab, while keeping each invocation short. all cases pass at all requested sizes.

| t | moec ms | h4 apply 1 ms | h4 apply 0 ms | hc-down ms | hc-up ms |
|---:|---:|---:|---:|---:|---:|
| 512 | 0.8743 | 0.4548 | 0.2247 | 0.2899 | 0.2945 |
| 1024 | 1.7317 | 0.9049 | 0.4691 | 0.6718 | 0.5732 |
| 2048 | 3.4438 | 1.8054 | 0.9586 | 1.2101 | 1.1294 |

hc-down tiles follow the evaluator's production-size selection: 32/128/4, 64/128/4, 32/128/2 respectively. these are standalone latencies, not promised full-model savings.

additional requested regressions, all exit 0:

- `comb_fold_parity 1024`: all 20 bitwise checks pass at t=1,7,64,1024; 4.32s.
- `comb_fold_parity 0 epi`: all tested tiles/sizes through 2048 bitwise pass; 0.60s.
- `comb_fold_parity 256 hcup`: all enumerated tiles, mixed bound and unbound, bitwise pass; 0.05s. this mode's existing tile list does not include reg 32/128/4; the a/b evaluator explicitly covers that production tile instead.

logs: `/tmp/sol_r30_prompt/{variant}_{t}_{gr|gemm}.log`, `parity1024.log`, `epi.log`, `hcup.log`. final control logs use `final_control_...`.

## stop reason

valid negative result: all high-leverage suggestions tested, no qualifying performance winner. do not ship cosmetic kernel rewrites on this evidence. a larger gain would need a different memory-traffic or dispatch decomposition, not the source-level state/address rewrites tested here.