Both vendored vectors carry three-byte Annex-B start codes throughout. The
real host emits four-byte ones on 100% of access units in both codecs —
1514/1514 H.264 and 1133/1133 HEVC, measured off the M0 NVENC corpus through
the capture hook's own .idx offsets. So every parity verdict this program has
recorded was taken on a prefix form that never ships, and the one form that
does ship was exercised by nothing.
That gap is not hypothetical. Submitting four-byte start codes to
vkCmdDecodeVideoKHR unchanged is exactly what made HEVC unplayable on every
driver tested: drivers are validated on the three-byte form, and a fixed
+3 + 2 skip into a four-byte-prefixed slice reads a nonsense pps_id — the
115 and 119 both NVIDIAs printed. H.264 was never safe here by structure,
only by its vendored encoder's convention, which is why the cure lives in the
shared ring layer and why this coverage is generic over both codecs.
Each codec's parity body now takes its access units as a parameter and runs
twice: once over the vector as it sits, once over the same vector rewritten
to four-byte prefixes. Prefix width carries no information, so both runs must
reproduce the same goldens — sharing one body is what makes that an equality
rather than two assertions that can drift.
The rewrite copies nalu.data[nalu.offset..], the same nal_size bytes the
parser hands the planner, so trailing_zero_8bits are dropped exactly where
the production parser drops them: the only difference between the two streams
is the width of every prefix.
Two CPU guards keep the new legs from passing vacuously, which is the failure
mode they are most exposed to — a rewrite that quietly returned its input
would make them trivially green and nothing on the fleet would notice. They
assert the original really does carry three-byte prefixes, that the rewritten
stream carries none, that the NAL count is preserved exactly, and that the
planner still yields 250 pictures.
Hardware: all four legs 250/250 bit-identical to libavcodec on two
independent driver stacks — AMD VanGogh on RADV/Mesa 26.0-devel (the Steam
Deck) and NVIDIA 610.43.03 on Linux. NVIDIA is the family that rejected the
four-byte form outright, so it is the meaningful witness for this regression.
enricobuehler
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