Add byte-packed IVF RaBitQ fast-scan layout for 4+ bit indexes#5084
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lyang24 wants to merge 1 commit intofacebookresearch:mainfrom
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Add byte-packed IVF RaBitQ fast-scan layout for 4+ bit indexes#5084lyang24 wants to merge 1 commit intofacebookresearch:mainfrom
lyang24 wants to merge 1 commit intofacebookresearch:mainfrom
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Introduce a new Iwrp serialization format for IndexIVFRaBitQFastScan and a byte-packed ex-code path for nb_bits >= 4. This switches multibit refinement to dedicated byte-packed SIMD kernels while leaving the existing bit-packed layout in place for lower-bit configurations. The new path preserves recall, keeps nbits=2 behavior unchanged, and continues to read older Iwrn/Iwrf indexes. It also guards unsupported distance_to_code usage on byte-packed indexes so the optimization stays confined to the FastScan batch search path.
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Summary
This PR adds an optional byte-packed multibit layout for
IndexIVFRaBitQFastScanwhennb_bits >= 4.The new layout stores
ex_codeas one byte per dimension and uses dedicated byte-packed multibit distance kernels in the FastScan refinement path. Lower-bit configurations keep the existing bit-packed layout, sonbits=1/2/3behavior is unchanged.Format / compatibility
IwrnandIwrfcontinue to read through the old bit-packed pathIwrpfourccIwrproundtrip serialization was verifiedPerformance
On Cohere 1M, single-threaded search,
nbits=4:Recall is unchanged:
nprobe=32:0.8847nprobe=64:0.9281nprobe=128:0.9501nbits=2is unchanged.Notes
nbits >= 4distance_to_code()is explicitly rejected for byte-packed IVF FastScan indexes, rather than silently using the wrong multibit decoder