Wire RaBitQ search through FastScanCodeScanner dispatch (facebookresearch#4903)

Summary:
Pull Request resolved: facebookresearch#4903

Add make_knn_scanner() virtual to IndexFastScan and IndexIVFFastScan
that returns a SIMD-dispatched FastScanCodeScanner. Override in
RaBitQ classes to produce rabitq-specific scanners via per-SIMD TU
factories (rabitq_dispatching.h). Move RaBitQ handler method bodies
into headers, add context pointer, and update rabitq_result_handler.h
for SL template param. Wire IVFRaBitQFastScanScanner::scan_codes to
use the new scanner. Update build files and SWIG ignores.

Callers (search_implem_12/14, search_dispatch_implem) are unchanged
and still use make_knn_handler.

Reviewed By: mdouze

Differential Revision: D96116557

Author: algoriddle

faiss/CMakeLists.txt

@@ -271,6 +271,7 @@ set(FAISS_HEADERS
   impl/fast_scan/decompose_qbs.h
   impl/fast_scan/kernels_simd256.h
   impl/fast_scan/kernels_simd512.h
+  impl/fast_scan/rabitq_dispatching.h
   impl/fast_scan/rabitq_result_handler.h
   impl/residual_quantizer_encode_steps.h
   impl/simd_dispatch.h

faiss/IndexFastScan.cpp

@@ -211,6 +211,19 @@ void estimators_from_tables_generic(
 
 } // anonymous namespace
 
+std::unique_ptr<FastScanCodeScanner> IndexFastScan::make_knn_scanner(
+        bool is_max,
+        idx_t n,
+        idx_t k,
+        size_t ntotal,
+        float* distances,
+        idx_t* labels,
+        const IDSelector* sel,
+        const FastScanDistancePostProcessing&) const {
+    return make_fast_scan_knn_scanner(
+            is_max, 0, n, ntotal, k, distances, labels, sel);
+}
+
 // Default implementation of make_knn_handler with centralized fallback logic
 SIMDResultHandlerToFloat* IndexFastScan::make_knn_handler(
         bool is_max,

faiss/IndexFastScan.h

@@ -7,8 +7,11 @@
 
 #pragma once
 
+#include <memory>
+
 #include <faiss/Index.h>
 #include <faiss/impl/fast_scan/FastScanDistancePostProcessing.h>
+#include <faiss/impl/fast_scan/fast_scan.h>
 #include <faiss/utils/AlignedTable.h>
 
 namespace faiss {
@@ -121,6 +124,22 @@ struct IndexFastScan : Index {
             const float* x,
             const FastScanDistancePostProcessing& context) const = 0;
 
+    /** Create a SIMD-dispatched scanner for knn search.
+     *
+     * Returns a FastScanCodeScanner that bundles handler + accumulation
+     * kernel behind the SIMD dispatch boundary.
+     * The scanner's accumulate methods dispatch to the optimal SIMD level.
+     */
+    virtual std::unique_ptr<FastScanCodeScanner> make_knn_scanner(
+            bool is_max,
+            idx_t n,
+            idx_t k,
+            size_t ntotal,
+            float* distances,
+            idx_t* labels,
+            const IDSelector* sel,
+            const FastScanDistancePostProcessing& context = {}) const;
+
     /** Create a KNN handler for this index type
      *
      * This method can be overridden by derived classes to provide

faiss/IndexIVFFastScan.cpp

@@ -495,6 +495,26 @@ int compute_search_nslice(
 
 } // namespace
 
+std::unique_ptr<FastScanCodeScanner> IndexIVFFastScan::make_knn_scanner(
+        bool is_max,
+        idx_t n,
+        idx_t k,
+        float* distances,
+        idx_t* labels,
+        const IDSelector* sel,
+        const FastScanDistancePostProcessing&) const {
+    return make_fast_scan_knn_scanner(
+            is_max,
+            0,
+            n,
+            0,
+            k,
+            distances,
+            labels,
+            sel,
+            /*with_id_map=*/true);
+}
+
 SIMDResultHandlerToFloat* IndexIVFFastScan::make_knn_handler(
         bool is_max,
         int impl,

faiss/IndexIVFFastScan.h

@@ -9,6 +9,7 @@
 
 #include <faiss/IndexIVF.h>
 #include <faiss/impl/fast_scan/FastScanDistancePostProcessing.h>
+#include <faiss/impl/fast_scan/fast_scan.h>
 #include <faiss/utils/AlignedTable.h>
 
 namespace faiss {
@@ -221,6 +222,31 @@ struct IndexIVFFastScan : IndexIVF {
             RangeSearchResult* result,
             const SearchParameters* params = nullptr) const override;
 
+    /** Create a SIMD-dispatched scanner for knn search (IVF variant).
+     *
+     * Returns a FastScanCodeScanner that bundles handler + accumulation
+     * kernel behind the SIMD dispatch boundary. ntotal is not passed
+     * because IVF sets it per-list via handler->ntotal.
+     * Derived classes that need custom handlers (e.g. RaBitQ) override
+     * this to return nullptr, falling back to make_knn_handler.
+     *
+     * @param is_max       whether to use CMax comparator (true) or CMin
+     * @param n            number of queries
+     * @param k            number of neighbors to find
+     * @param distances    output distances array
+     * @param labels       output labels array
+     * @param sel          optional ID selector
+     * @return             scanner, or nullptr if unsupported
+     */
+    virtual std::unique_ptr<FastScanCodeScanner> make_knn_scanner(
+            bool is_max,
+            idx_t n,
+            idx_t k,
+            float* distances,
+            idx_t* labels,
+            const IDSelector* sel,
+            const FastScanDistancePostProcessing& context = {}) const;
+
     /** Create a KNN handler for this index type
      *
      * This method can be overridden by derived classes to provide