Cycle 3 (MC interpolation) closure: M1'''=100%, R'''=0.067 RED, M4=-19.5%
Third daedalus-fourier kernel — VP9 8-tap regular subpel filter,
horizontal direction, 8-wide output. Multiply-heavy by design to
stress V3D's no-DP4A deficit. Full cycle Phase 1-7 + M4'''.
Phase 5''' second-model review delivered cleanly — caught 1 RED
bug pre-implementation (src_off off-by-3 indexing convention) and
2 YELLOW gaps (assert MUST language, shaderdb filter-LUT gate).
Without the review, M1''' would have failed silently on first run
with cryptic "high-index source pixels wrong" symptoms.
Phase 6 v1 first-light: M1''' 100.0000% bit-exact (65536/65536
blocks across all 16 mx phases). Phase 5''' filter-LUT prediction
materialised exactly: 197 uniforms (gate was 144), 2 threads (down
from cycle-2's 4 due to register pressure).
Performance:
M2''' = 1.413 Mblock/s (707.9 ns/block)
M3''' = 20.997 Mblock/s (NEON baseline phase3)
R''' = 0.067 (RED band — structural mismatch)
shaderdb: 488 inst, 2 threads, 197 uniforms, 25 max-temps, 0 spills
M4''' concurrent matrix (8s windows):
NEON 1-core 14.479 Mblock/s
NEON 4-core 15.248 Mblock/s <- baseline (compute-bound,
not bandwidth-saturated
like cycles 1+2!)
QPU only 1.380 Mblock/s
MIXED NEON-3 + QPU 12.277 Mblock/s <- -19.5% (FAIL gate)
MIXED NEON-4 + QPU 12.158 Mblock/s <- -20.3%
NEW cross-cycle finding (Phase 9 lesson 2): compute-bound CPU
workloads make the QPU-offload story collapse. Cycles 1+2 were
bandwidth-saturated (4-core scaling 0.56-0.82x of 1-core), so
freeing a CPU core via QPU offload added throughput. Cycle 3 MC
is compute-bound (4-core scaling 1.05x of 1-core — near-linear),
no free cycles to free. QPU contribution (0.45 Mblock/s in
contention) doesn't compensate for losing 1 NEON core delivering
~3.8 Mblock/s.
But 30fps@1080p floor: PASS in every config (1.4x to 15.7x
isolation margin). Per project_30fps_floor_is_fine.md, user-facing
test never fails — daily YouTube playback works fine on any CPU/QPU
split.
DEPLOYMENT RECIPE for higgs (cycle 3 confirmed split):
IDCT (k1) -> QPU (R=0.92, +7% mixed, frees CPU core)
LPF (k2) -> QPU (R=0.41, +7% mixed, frees CPU core)
MC (k3) -> CPU (R=0.067, -19.5% mixed — stays on CPU)
Entropy -> CPU (structurally serial)
Mixed-substrate deployment, not "QPU does everything". Realistic for
higgs: entropy + MC on 2-3 ARM cores; IDCT + LPF dispatched to QPU
concurrently; 1-2 ARM cores left for vscode etc.
New artifacts:
- src/v3d_mc_8h.comp — GLSL kernel
- tests/vp9_mc_ref.c — standalone C ref (REGULAR filter
embedded; clean transcription)
- tests/bench_neon_mc.c — M1'''_c + M3''' bench
- tests/bench_v3d_mc.c — M1''' + M2''' bench with contract
asserts + 30fps margin display
- tests/bench_concurrent_mc.c — M4''' pthread bench
- external/ffmpeg-snapshot/libavcodec/aarch64/vp9mc_neon.S (vendored)
- external/ffmpeg-snapshot/libavcodec/vp9_subpel_filters_table.c
(hand-extracted; provides
ff_vp9_subpel_filters symbol
without dragging in full vp9dsp.c)
- docs/k3_mc_phase{1,2,3,4,5,7}.md — full cycle documentation
Memory updates: project_30fps_floor_is_fine.md (user's 30fps target
recalibration), MEMORY.md index updated.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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---
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cycle: 3
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phase: 2
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status: closed 2026-05-18
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date_opened: 2026-05-18
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parent: k3_mc_phase1.md
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---
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# Cycle 3, Phase 2 — MC situation analysis
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## 1. C reference
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- **Source**: `external/ffmpeg-snapshot/libavcodec/vp9dsp_template.c`
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(already vendored from cycle 1).
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- **Function**: `put_8tap_regular_8h_c` generated by
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`filter_fn_1d(8, h, mx, regular, FILTER_8TAP_REGULAR, put)` —
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expands to call `do_8tap_1d_c` with `ds=1` (horizontal) and the
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REGULAR filter bank.
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- **Underlying primitive**: `do_8tap_1d_c` iterates `h` rows;
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per row, iterates `w=8` columns; per column, computes the
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`FILTER_8TAP` macro: `clip((sum_{k=0..7} F[k] * src[x+k-3]
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+ 64) >> 7, 0, 255)`.
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- **Spec**: VP9 specification § 8.5.1 (subpel motion compensation).
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## 2. NEON reference
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- **Source**: `external/ffmpeg-snapshot/libavcodec/aarch64/vp9mc_neon.S`
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(vendored 2026-05-18, FFmpeg n7.1.3, SHA-256
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`6b1d50f9821742584fdd47758057f810644aff3a008faaa774ff5b9cac4d1fef`).
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- **Symbol**: `ff_vp9_put_regular8_h_neon` (note: filter type baked
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into name, width=8 baked in, h-direction baked in)
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- **Signature** (VP9 `vp9_mc_func` typedef):
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```c
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void ff_vp9_put_regular8_h_neon(uint8_t *dst, ptrdiff_t dst_stride,
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const uint8_t *src, ptrdiff_t src_stride,
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int h, int mx, int my);
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```
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Registers: `x0=dst, x1=dst_stride, x2=src, x3=src_stride, w4=h, w5=mx, w6=my`.
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- **Dependencies**:
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- `libavutil/aarch64/asm.S` ✓ (already vendored)
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- `ff_vp9_subpel_filters[3][16][8]` symbol — provided by
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`external/ffmpeg-snapshot/libavcodec/vp9_subpel_filters_table.c`
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(hand-extracted from `libavcodec/vp9dsp.c` of the same n7.1.3
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pin; copying just the constant data avoids dragging in the
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rest of `vp9dsp.c` which would require linking the entire VP9
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decoder).
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## 3. Workload model
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Per 8×8 block output:
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- 8 multiplies × 8 columns × 8 rows = **512 multiplies**
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- 7 additions × 8 columns × 8 rows = 448 additions
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- 1 round (+64), 1 shift (>>7), 1 clip per pixel × 64 = 192 ops
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- Total ~1150 integer ops per block
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Per-block memory (horizontal-only filter, 8-pixel-wide output):
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- Read: 8 rows × (8 output cols + 7 tap overhang) = 8 × 15 = **120 source bytes**
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- Write: 8 rows × 8 cols = **64 dst bytes**
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- Total: **~184 bytes / block**
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Per 1080p frame (32 400 8×8 blocks, worst case all-MC):
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- ~5.9 MB total memory traffic
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- ~37 Mops compute
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- At GPU 4 GB/s share: 1.48 ms / frame = 675 FPS = 21.9 Mblock/s
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- At V3D 92 GFLOPS theoretical scalar (SMUL24 throughput ≈ FP MUL): 0.4 ms compute / frame = 2500 FPS theoretical → **compute is NOT the bottleneck** at this shape
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So MC is **bandwidth-bound on the QPU**, similar to LPF cycle 2.
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## 4. Per-row workload diversity (vs cycle 1+2)
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| | IDCT (k1) | LPF (k2) | MC (k3) |
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|---|---|---|---|
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| Per-block math | Heavy butterflies (~60 ops/block via separable transform) | Light: 0-30 ops per edge × 8 rows | 8-tap convolution: 1150 ops per block |
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| Per-block memory | ~320 B in + 64 B out | ~64 B in + ~24 B out per edge | 120 B in + 64 B out |
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| Compute / memory ratio | High | Low (memory-bound, lots of skipping) | Medium (compute-rich but bandwidth-bound at GPU) |
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| Conditional? | No (always-execute) | Yes (fm/hev divergence per row) | No (deterministic per pixel) |
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| QPU mult intensity | Q14 16b×16b mults | Light (compares, small clips) | 16b×8b mults (filter × pixel) |
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MC is interesting because it's **compute-rich AND bandwidth-bound** —
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the closest match in workload shape to a real-world GPU compute kernel
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the V3D was designed for (graphics filtering).
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## 5. Constraints carried from cycle 1+2
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Same V3D 7.1 device profile (vulkaninfo unchanged). The relevant
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specifics for MC:
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- No DP4A → 8-tap convolution must be 8 separate SMUL24 + ADDs
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(the typical GPU "dot4" packing is not available)
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- shaderInt16 = false → filter coefficients widened to int32 in
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registers; the filter table itself can be a uint16-storage SSBO
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- shaderInt8 = false → source pixels widened to int32 in registers
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- 1024-byte (16 KiB / 16) shared mem per WG is ample for MC source
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staging if useful (15 cols × 8 rows × 1 byte per block-row × 32
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blocks per WG = 3 840 B per row); for v1 we skip shared-mem
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staging and let TMU handle reads directly
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## 6. What Phase 2 does *not* close
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- Per-block (block_y, block_x) layout / meta format. Phase 4 picks.
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Likely same shape as cycle 2 (uvec4 per block: dst_offset,
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src_offset, mx, _pad).
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- Filter table residency: as SSBO load every row, push-constants
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per dispatch (different mx per dispatch), or constant baked into
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shader (one filter per shader = 16 specialised shaders for the 16
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mx phases). Phase 4 picks; v1 likely SSBO for simplicity.
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- Vertical / "hv" / "avg" / 4-pixel / 16-pixel / 32-pixel / 64-pixel
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variants — out of cycle 3 scope; cycle 4+ if needed.
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Phase 3 next: build `tests/bench_neon_mc.c`, capture M3'''.
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