The first device decoder walked 256-byte tiles, one thread each, straight from global memory: on a
PILATUS 6M frame (~6 MB of stream) that is ~24k threads walking alone, and the two walks took
~0.5 ms of the card per frame - with the CBF decode gone from the workers the card is what bounds
the 7qij loops (kernels 67-78 % busy). Now each thread walks a 68-byte part that its block has read
into shared memory together (68, not 64, so the threads of a warp do not read the same banks), the
scan runs over tiles of 8 parts whose maps are composed first, and the write kernel stages a block's
pixels in shared memory and writes them out coalesced.
Measured on 36 real frames of 9 CBF sets (nsys, average per frame): 0.41 ms against ~0.6 ms before.
Output unchanged: the same tests, and the same 36 frames identical to the host decoder.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EizBKhTqrAYago9KJG3dA3
The CBF reader decoded every frame's x-CBF_BYTE_OFFSET stream on the worker thread - 19-30 % of the
image-loop worker CPU on CBF sets (7qij, 4mtk) - and the 4-byte image was then uploaded. ReadRawImage
now hands over the stream as the file stores it, in the image's own buffer, as a new read-only
CompressionAlgorithm::CBF_BYTE_OFFSET (mode Int32, what it decodes to), and ImagePreprocessorGPU
decodes it on the card into the same int32 image and runs the same preprocessing kernel on it.
The device decode (CBFDecoderGPU) cuts the stream into 256-byte tiles. A token is 1, 3, 7 or 15 bytes,
so a tile can be entered at 15 offsets; for each one thread works out where the decode leaves the
tile, how many pixels it emitted and their sum modulo 2^32 (the walks from different entries meet
within a few bytes, after which they are one walk). An exclusive scan composing these maps gives each
tile its true entry, first pixel and running value, and a second walk writes the pixels. A stream
that ends before the image is refused as on the host.
The host decoder moves to compression/ (DecodeCBFByteOffset) and is reached through
JFJochDecompressPtr, so every host consumer - the CPU build, the azimuthal-integration worker, the
pre-scan's shadow and gain passes - decodes exactly as before, now from the stream. It is the CPU
path, not a fallback: a frame the device refuses is not retried on the host. A process file written
from CBF input stores the decoded int32 image, as it always did. .cbf.gz is still inflated on the host.
Tests: CBFDecoderGPU_MatchesHost (every token size, 0x80 inside payloads, escape runs across tiles,
trailing CIF text), CBFDecoderGPU_ShortStreamIsRefused, ImagePreprocessorGPU_StoredEncodingsMatchCPU.
Checked locally against the host decoder on 36 real frames of 9 open-arm CBF sets (signed 32-bit,
signed and unsigned 16-bit, plain and gzip): identical.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EizBKhTqrAYago9KJG3dA3
leonarski_fandClaude Opus 5.5