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Cycle 6 (H.264) opened — IDCT 4x4 Phase 1+3, M3 = 175 Mblock/s

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H.264 scope added 2026-05-18 per user direction. Pi 5's VideoCore
VII has no hardware H.264 decoder block (only HEVC), so a
QPU-accelerated H.264 path fills the most impactful codec gap.
Cycle 6 = first H.264 kernel (4x4 IDCT + add, smallest H.264
transform, simplest first cycle).

Phase 1: goal doc + 1080p30 floor analysis (5.85 Mblock/s
worst-case, 2.0 Mblock/s realistic since most MBs use 8x8 or
P-skip).

Phase 3: NEON M3 baseline captured. ff_h264_idct_add_neon on
hertz delivers 175 Mblock/s (5.7 ns per block) = 30x worst-case
floor margin. H.264 IDCT 4x4 is dramatically lighter than VP9
IDCT 8x8 (21x faster per block).

Phase 3 closure also caught the key Phase 9 lesson: H.264/FFmpeg
blocks are COLUMN-MAJOR (block[c*4 + r] = (row=r, col=c)). NEON
ld1 with 4 registers interleaves loading, and the FFmpeg C ref
indexing makes this convention explicit. Initial C ref assumed
row-major, M1 was 5% bit-exact; after fix, M1 = 100%.

Convention encoded for all subsequent H.264 cycles (cycle 7+).

- external/ffmpeg-snapshot/libavcodec/aarch64/h264idct_neon.S
  (vendored verbatim from FFmpeg n7.1.3, 415 lines)
- external/ffmpeg-snapshot/PROVENANCE.md: updated
- tests/h264_idct4_ref.c: column-major C ref
- tests/bench_neon_h264idct4.c: M1 + M3 bench
- CMakeLists.txt: cycle 6 NEON bench wiring
- docs/k6_h264idct4_phase1.md, phase3.md

Phase 4 next: QPU shader for cycle 6. Predicted R6 = 0.01 (deep
RED — kernel too small relative to QPU dispatch overhead) but
worth building for cycle-completeness + the opportunistic-helper
hypothesis (cycle 6 may stay CPU per recipe).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
This commit is contained in:
Markus Fritsche
2026-05-18 14:14:43 +00:00
parent eb5cfb34c4
commit f92dc40f43
7 changed files with 974 additions and 0 deletions
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external/ffmpeg-snapshot/PROVENANCE.md
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@@ -26,6 +26,7 @@ tagged commit, no modifications.
| `libavcodec/aarch64/vp9itxfm_neon.S` | 1580 | 63534 | `82ee3ceed4735c63576bafdcee28e2215652743ade55a9eab46a16d9530369f6` |
| `libavcodec/aarch64/vp9lpf_neon.S` | 1334 | — | `384e49e7a6e838d9e38aedc00838ed4aebfa6c5bdb343ecaf23ef639bc10fbb7` |
| `libavcodec/aarch64/vp9mc_neon.S` | 665 | — | `6b1d50f9821742584fdd47758057f810644aff3a008faaa774ff5b9cac4d1fef` |
| `libavcodec/aarch64/h264idct_neon.S` | 415 | 16269 | `963ffe5f31b5a6a422e13b0d394cf5630126927abfb23aa214f7cbe83d60683f` — H.264 IDCT 4×4/8×8/DC NEON kernels for cycle 6+ |
| `libavcodec/vp9_subpel_filters_table.c` | — | — | hand-extracted from `libavcodec/vp9dsp.c` at same n7.1.3 pin — provides `ff_vp9_subpel_filters` for `vp9mc_neon.S` to link against without dragging in vp9dsp.c's full init machinery |
| `libavcodec/aarch64/neon.S` | 173 | 7496 | `72d36ce6c3fcc5e53de869cfe10fda16225ebe580c32891bccc240a30a85a538` |
| `libavutil/aarch64/asm.S` | 260 | 8069 | `c0d03143b1bc5a9e358222d08d2d449d595271844fe7a3dc23bffb91abe8b0e3` |