# Issue 001 — VP9 LPF wd=16 cycle (prediction validation)

**Status**: open, not blocking
**Type**: kernel-cycle (cycle 5 candidate)
**Predicted verdict**: RED (M4 likely negative, per cycle 4 lesson 4)
**Priority**: low (incremental; trend prediction)
**Filed**: 2026-05-18

## Background

Cycle 4 (LPF wd=8) closed PASS with M4 delta +4.1 % vs cycle 2 wd=4's
+6.9 %. The downward trend prompted Phase 9 lesson: "wd=16 would
probably show further R degradation; M4 may flip negative based on
the trend line." See `docs/k4_lpf8_phase4_7.md §"Phase 9 lessons"`.

This issue tracks the experiment to validate (or invalidate) that
prediction.

## What to do

Cycle 5 LPF wd=16, mirroring cycle 4's compact structure:

1. **Phase 3**: build `tests/bench_neon_lpf16.c` modelled on
   `bench_neon_lpf8.c`. NEON symbol: `ff_vp9_loop_filter_h_16_16_neon`
   (already in vendored `vp9lpf_neon.S`). Capture M3.
2. **Phase 4-7**: write `src/v3d_lpf_h_16_16.comp` extending the
   wd=8 kernel with the wd=16 outer-flat path (`flat8out` test, 14
   writes per row when both flat8out and flat8in pass). New
   contract: `dst_stride_u8 ≥ 14` (vs cycle 4's ≥ 6) because the
   flat8out path writes at `base-7..base+6` (14 contiguous bytes).
3. **Phase 5 review**: mandatory — wd=16 is not as incremental as
   wd=8 (much larger conditional logic, new contract bound).
4. **Phase 7**: measure M2, R; if M4 negative as predicted, document
   trend confirmation and close kernel as "CPU-only" in deployment
   recipe.

## Expected outcome (per prediction)

| Quantity | Predicted |
|---|---|
| M1 bit-exact | 100 % (same pattern as cycles 2/4) |
| M3 NEON | ~55 Medge/s (slightly faster than wd=8) |
| M2 QPU isolation | ~12-15 Medge/s |
| R isolation | 0.22-0.27 (ORANGE, downward) |
| M4 mixed vs NEON-4 | -2 % to +1 % (borderline; likely negative) |
| 30fps margin | still 5×+ (user-facing PASS regardless) |

## Acceptance criteria (issue closed when)

- Cycle 5 phases 1-7 complete, committed
- `docs/k5_lpf16_phase*.md` produced
- Phase 7 verdict documented, deployment recipe updated either way
- Phase 9 lesson 4 trend prediction validated or refuted

## Why deferred (not done in current session)

The session goal was "continue until user intervention necessary."
User directed: file as issue, progress to cycle 5 CDEF instead.
The trend prediction is interesting but the project's deployment
recipe is already locked through cycle 4; cycle 5 wd=16 result
would update at most one row of the recipe table.

## Related

- `docs/k4_lpf8_phase4_7.md §"Phase 9 lessons"` lesson 4 (the
  prediction this validates)
- `external/ffmpeg-snapshot/libavcodec/aarch64/vp9lpf_neon.S`
  (NEON ref already vendored — symbol `ff_vp9_loop_filter_h_16_16_neon`)
- `docs/k2_deblock_phase4.md` (cycle 2 template)
- `docs/k4_lpf8_phase4_7.md` (cycle 4 template, the most direct
  reference)