Per the QPU-default substrate decree 2026-05-23, cycle 6 (H.264
IDCT 4x4 + add) was the highest-priority H.264 kernel to flip
from NEON-only to QPU-capable. The same shape as VP9 IDCT 8x8
(cycle 1) — two-pass butterfly with shared-memory transpose —
but at 4x4 scale: 4 lanes per block, 16 blocks per WG.
What's added:
- src/v3d_h264_idct4.comp: GLSL compute shader implementing
the H.264 §8.5.12.1 1D butterfly twice (row pass then column
pass), with (val + 32) >> 6 rounding and clip-to-u8 add to
dst. Block memory layout is column-major (matches FFmpeg
`ff_h264_idct_add_neon` convention).
- CMakeLists: glslang rule + install entry for v3d_h264_idct4.spv.
- dispatch_h264_idct4_qpu() in daedalus_core.c: lazy pipeline
init, 3 SSBOs (coeffs / dst / meta as uvec4), push-constant
(n_blocks, dst_stride), 16 blocks per WG dispatch. Matches
the existing dispatch_*_qpu patterns; uses
v3d_runner_create_buffer / destroy_buffer (will swap to
pool API once PR #6 lands).
- daedalus_dispatch_h264_idct4() replaces ROUTE_CPU_ONLY with
the same CPU/QPU substrate switch the deblock dispatch uses.
- daedalus_recipe_substrate_for(H264_IDCT4) returns QPU now
that the shader exists.
Verification on hertz (Pi 5 + V3D 7.1):
$ ./test_api_h264
=== Phase 8a API smoke: H.264 kernels via recipe dispatch ===
H264_IDCT4 recipe substrate: 2 (1=CPU, 2=QPU)
H264_IDCT8 recipe substrate: 1
H264_DEBLOCK_LV recipe substrate: 2
H264_QPEL_MC20 recipe substrate: 1
H.264 IDCT 4x4: 2048/2048 bytes bit-exact (100.0000%) ← QPU
H.264 IDCT 8x8: 2048/2048 bytes bit-exact
H.264 deblock luma v: 2048/2048 bytes bit-exact
H.264 qpel mc20: 1024/1024 bytes bit-exact
The AUTO-substrate path now picks QPU for H.264 IDCT 4x4, and
the output is bit-exact against the C reference (which is
identical to the NEON .S code by construction — same FFmpeg
upstream).
Remaining cycle-6/7/9 work in task #165:
- cycle 7: H.264 IDCT 8x8 (template same shape; 8 lanes per
block, fewer blocks per WG)
- cycle 9: H.264 luma qpel mc20 (different shape — 6-tap MC
not a transform)
This commit lands the cycle-6 piece of task #165.
claude-noether
65bd5c3fe3