Use the new `loadIntoBitSet` API to speed up dense conjunctions. (#14080)

Now that loading doc IDs into a bit set is much more efficient thanks to
auto-vectorization, it has become tempting to evaluate dense conjunctions by
and-ing bit sets.

lucene/CHANGES.txt

@@ -55,6 +55,9 @@ Optimizations
 
 * GITHUB#14052: Speed up DisjunctionDISIApproximation#advance. (Adrien Grand)
 
+* GITHUB#14080: Use the `DocIdSetIterator#loadIntoBitSet` API to speed up dense
+  conjunctions. (Adrien Grand)
+
 Bug Fixes
 ---------------------
 (No changes)

lucene/core/src/java/org/apache/lucene/search/BooleanScorerSupplier.java

@@ -304,9 +304,9 @@ final class BooleanScorerSupplier extends ScorerSupplier {
   BulkScorer filteredOptionalBulkScorer() throws IOException {
     if (subs.get(Occur.MUST).isEmpty() == false
         || subs.get(Occur.FILTER).isEmpty()
-        || scoreMode != ScoreMode.TOP_SCORES
+        || (scoreMode.needsScores() && scoreMode != ScoreMode.TOP_SCORES)
         || subs.get(Occur.SHOULD).size() <= 1
-        || minShouldMatch > 1) {
+        || minShouldMatch != 1) {
       return null;
     }
     long cost = cost();
@@ -318,13 +318,28 @@ final class BooleanScorerSupplier extends ScorerSupplier {
     for (ScorerSupplier ss : subs.get(Occur.FILTER)) {
       filters.add(ss.get(cost));
     }
-    Scorer filterScorer;
+    if (scoreMode == ScoreMode.TOP_SCORES) {
-    if (filters.size() == 1) {
+      Scorer filterScorer;
-      filterScorer = filters.iterator().next();
+      if (filters.size() == 1) {
+        filterScorer = filters.iterator().next();
+      } else {
+        filterScorer = new ConjunctionScorer(filters, Collections.emptySet());
+      }
+      return new MaxScoreBulkScorer(maxDoc, optionalScorers, filterScorer);
     } else {
-      filterScorer = new ConjunctionScorer(filters, Collections.emptySet());
+      // In the beginning of this method, we exited early if the score mode is not either TOP_SCORES
+      // or a score mode that doesn't need scores.
+      assert scoreMode.needsScores() == false;
+      filters.add(new DisjunctionSumScorer(optionalScorers, scoreMode, cost));
+
+      if (filters.stream().map(Scorer::twoPhaseIterator).allMatch(Objects::isNull)
+          && maxDoc >= DenseConjunctionBulkScorer.WINDOW_SIZE
+          && cost >= maxDoc / DenseConjunctionBulkScorer.DENSITY_THRESHOLD_INVERSE) {
+        return new DenseConjunctionBulkScorer(filters.stream().map(Scorer::iterator).toList());
+      }
+
+      return new DefaultBulkScorer(new ConjunctionScorer(filters, Collections.emptyList()));
     }
-    return new MaxScoreBulkScorer(maxDoc, optionalScorers, filterScorer);
   }
 
   // Return a BulkScorer for the required clauses only
@@ -378,7 +393,14 @@ final class BooleanScorerSupplier extends ScorerSupplier {
         && requiredScoring.size() + requiredNoScoring.size() >= 2
         && requiredScoring.stream().map(Scorer::twoPhaseIterator).allMatch(Objects::isNull)
         && requiredNoScoring.stream().map(Scorer::twoPhaseIterator).allMatch(Objects::isNull)) {
-      return new ConjunctionBulkScorer(requiredScoring, requiredNoScoring);
+      if (requiredScoring.isEmpty()
+          && maxDoc >= DenseConjunctionBulkScorer.WINDOW_SIZE
+          && leadCost >= maxDoc / DenseConjunctionBulkScorer.DENSITY_THRESHOLD_INVERSE) {
+        return new DenseConjunctionBulkScorer(
+            requiredNoScoring.stream().map(Scorer::iterator).toList());
+      } else {
+        return new ConjunctionBulkScorer(requiredScoring, requiredNoScoring);
+      }
     }
     if (scoreMode == ScoreMode.TOP_SCORES && requiredScoring.size() > 1) {
       requiredScoring = Collections.singletonList(new BlockMaxConjunctionScorer(requiredScoring));

lucene/core/src/java/org/apache/lucene/search/DenseConjunctionBulkScorer.java

@@ -0,0 +1,193 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.lucene.search;
+
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.Comparator;
+import java.util.List;
+import org.apache.lucene.util.Bits;
+import org.apache.lucene.util.FixedBitSet;
+
+/**
+ * BulkScorer implementation of {@link ConjunctionScorer} that is specialized for dense clauses.
+ * Whenever sensible, it intersects clauses by loading their matches into a bit set and computing
+ * the intersection of clauses by and-ing these bit sets.
+ */
+final class DenseConjunctionBulkScorer extends BulkScorer {
+
+  // Use a small-ish window size to make sure that we can take advantage of gaps in the postings of
+  // clauses that are not leading iteration.
+  static final int WINDOW_SIZE = 4096;
+  // Only use bit sets to compute the intersection if more than 1/32th of the docs are expected to
+  // match. Experiments suggested that values that are a bit higher than this would work better, but
+  // we're erring on the conservative side.
+  static final int DENSITY_THRESHOLD_INVERSE = Long.SIZE / 2;
+
+  private final DocIdSetIterator lead;
+  private final List<DocIdSetIterator> others;
+
+  private final FixedBitSet windowMatches = new FixedBitSet(WINDOW_SIZE);
+  private final FixedBitSet clauseWindowMatches = new FixedBitSet(WINDOW_SIZE);
+  private final DocIdStreamView docIdStreamView = new DocIdStreamView();
+
+  DenseConjunctionBulkScorer(List<DocIdSetIterator> iterators) {
+    if (iterators.size() <= 1) {
+      throw new IllegalArgumentException("Expected 2 or more clauses, got " + iterators.size());
+    }
+    iterators = new ArrayList<>(iterators);
+    iterators.sort(Comparator.comparingLong(DocIdSetIterator::cost));
+    lead = iterators.get(0);
+    others = List.copyOf(iterators.subList(1, iterators.size()));
+  }
+
+  @Override
+  public int score(LeafCollector collector, Bits acceptDocs, int min, int max) throws IOException {
+    for (DocIdSetIterator it : others) {
+      min = Math.max(min, it.docID());
+    }
+
+    if (lead.docID() < min) {
+      lead.advance(min);
+    }
+
+    if (lead.docID() >= max) {
+      return lead.docID();
+    }
+
+    List<DocIdSetIterator> otherIterators = this.others;
+    DocIdSetIterator collectorIterator = collector.competitiveIterator();
+    if (collectorIterator != null) {
+      otherIterators = new ArrayList<>(otherIterators);
+      otherIterators.add(collectorIterator);
+    }
+
+    final DocIdSetIterator[] others = otherIterators.toArray(DocIdSetIterator[]::new);
+
+    int windowMax;
+    do {
+      windowMax = (int) Math.min(max, (long) lead.docID() + WINDOW_SIZE);
+      scoreWindowUsingBitSet(collector, acceptDocs, others, windowMax);
+    } while (windowMax < max);
+
+    return lead.docID();
+  }
+
+  private static int advance(FixedBitSet set, int i) {
+    if (i >= WINDOW_SIZE) {
+      return DocIdSetIterator.NO_MORE_DOCS;
+    } else {
+      return set.nextSetBit(i);
+    }
+  }
+
+  private void scoreWindowUsingBitSet(
+      LeafCollector collector, Bits acceptDocs, DocIdSetIterator[] others, int max)
+      throws IOException {
+    assert windowMatches.scanIsEmpty();
+    assert clauseWindowMatches.scanIsEmpty();
+
+    int offset = lead.docID();
+    lead.intoBitSet(acceptDocs, max, windowMatches, offset);
+
+    int upTo = 0;
+    for (;
+        upTo < others.length
+            && windowMatches.cardinality() >= WINDOW_SIZE / DENSITY_THRESHOLD_INVERSE;
+        upTo++) {
+      DocIdSetIterator other = others[upTo];
+      if (other.docID() < offset) {
+        other.advance(offset);
+      }
+      // No need to apply acceptDocs on other clauses since we already applied live docs on the
+      // leading clause.
+      other.intoBitSet(null, max, clauseWindowMatches, offset);
+      windowMatches.and(clauseWindowMatches);
+      clauseWindowMatches.clear();
+    }
+
+    if (upTo < others.length) {
+      // If the leading clause is sparse on this doc ID range or if the intersection became sparse
+      // after applying a few clauses, we finish evaluating the intersection using the traditional
+      // leap-frog approach. This proved important with a query such as "+secretary +of +state" on
+      // wikibigall, where the intersection becomes sparse after intersecting "secretary" and
+      // "state".
+      advanceHead:
+      for (int windowMatch = windowMatches.nextSetBit(0);
+          windowMatch != DocIdSetIterator.NO_MORE_DOCS; ) {
+        int doc = offset + windowMatch;
+        for (int i = upTo; i < others.length; ++i) {
+          DocIdSetIterator other = others[i];
+          int otherDoc = other.docID();
+          if (otherDoc < doc) {
+            otherDoc = other.advance(doc);
+          }
+          if (doc != otherDoc) {
+            int clearUpTo = Math.min(WINDOW_SIZE, otherDoc - offset);
+            windowMatches.clear(windowMatch, clearUpTo);
+            windowMatch = advance(windowMatches, clearUpTo);
+            continue advanceHead;
+          }
+        }
+        windowMatch = advance(windowMatches, windowMatch + 1);
+      }
+    }
+
+    docIdStreamView.offset = offset;
+    collector.collect(docIdStreamView);
+    windowMatches.clear();
+
+    // If another clause is more advanced than lead1 then advance lead1, it's important to take
+    // advantage of large gaps in the postings lists of other clauses.
+    int maxOtherDocID = -1;
+    for (DocIdSetIterator other : others) {
+      maxOtherDocID = Math.max(maxOtherDocID, other.docID());
+    }
+    if (lead.docID() < maxOtherDocID) {
+      lead.advance(maxOtherDocID);
+    }
+  }
+
+  @Override
+  public long cost() {
+    return lead.cost();
+  }
+
+  final class DocIdStreamView extends DocIdStream {
+
+    int offset;
+
+    @Override
+    public void forEach(CheckedIntConsumer<IOException> consumer) throws IOException {
+      int offset = this.offset;
+      long[] bitArray = windowMatches.getBits();
+      for (int idx = 0; idx < bitArray.length; idx++) {
+        long bits = bitArray[idx];
+        while (bits != 0L) {
+          int ntz = Long.numberOfTrailingZeros(bits);
+          consumer.accept(offset + ((idx << 6) | ntz));
+          bits ^= 1L << ntz;
+        }
+      }
+    }
+
+    @Override
+    public int count() throws IOException {
+      return windowMatches.cardinality();
+    }
+  }
+}

lucene/core/src/java/org/apache/lucene/search/DisjunctionDISIApproximation.java

@@ -21,6 +21,8 @@ import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Comparator;
 import java.util.List;
+import org.apache.lucene.util.Bits;
+import org.apache.lucene.util.FixedBitSet;
 
 /**
  * A {@link DocIdSetIterator} which is a disjunction of the approximations of the provided
@@ -141,6 +143,23 @@ public final class DisjunctionDISIApproximation extends DocIdSetIterator {
     return Math.min(leadTop.doc, minOtherDoc);
   }
 
+  @Override
+  public void intoBitSet(Bits acceptDocs, int upTo, FixedBitSet bitSet, int offset)
+      throws IOException {
+    while (leadTop.doc < upTo) {
+      leadTop.approximation.intoBitSet(acceptDocs, upTo, bitSet, offset);
+      leadTop.doc = leadTop.approximation.docID();
+      leadTop = leadIterators.updateTop();
+    }
+
+    minOtherDoc = Integer.MAX_VALUE;
+    for (DisiWrapper w : otherIterators) {
+      w.approximation.intoBitSet(acceptDocs, upTo, bitSet, offset);
+      w.doc = w.approximation.docID();
+      minOtherDoc = Math.min(minOtherDoc, w.doc);
+    }
+  }
+
   /** Return the linked list of iterators positioned on the current doc. */
   public DisiWrapper topList() {
     if (leadTop.doc < minOtherDoc) {

lucene/core/src/java/org/apache/lucene/util/FixedBitSet.java

@@ -33,6 +33,10 @@ public final class FixedBitSet extends BitSet {
   private static final long BASE_RAM_BYTES_USED =
       RamUsageEstimator.shallowSizeOfInstance(FixedBitSet.class);
 
+  // An array that is small enough to use reasonable amounts of RAM and large enough to allow
+  // Arrays#mismatch to use SIMD instructions and multiple registers under the hood.
+  private static long[] ZEROES = new long[32];
+
   private final long[] bits; // Array of longs holding the bits
   private final int numBits; // The number of bits in use
   private final int numWords; // The exact number of longs needed to hold numBits (<= bits.length)
@@ -470,8 +474,11 @@ public final class FixedBitSet extends BitSet {
     // Depends on the ghost bits being clear!
     final int count = numWords;
 
-    for (int i = 0; i < count; i++) {
+    for (int i = 0; i < count; i += ZEROES.length) {
-      if (bits[i] != 0) return false;
+      int cmpLen = Math.min(ZEROES.length, bits.length - i);
+      if (Arrays.equals(bits, i, i + cmpLen, ZEROES, 0, cmpLen) == false) {
+        return false;
+      }
     }
 
     return true;

lucene/core/src/test/org/apache/lucene/search/TestDenseConjunctionBulkScorer.java

@@ -0,0 +1,204 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.lucene.search;
+
+import java.io.IOException;
+import java.util.Arrays;
+import org.apache.lucene.tests.util.LuceneTestCase;
+import org.apache.lucene.util.BitSetIterator;
+import org.apache.lucene.util.FixedBitSet;
+
+public class TestDenseConjunctionBulkScorer extends LuceneTestCase {
+
+  public void testSameMatches() throws IOException {
+    int maxDoc = 100_000;
+    FixedBitSet clause1 = new FixedBitSet(maxDoc);
+    FixedBitSet clause2 = new FixedBitSet(maxDoc);
+    FixedBitSet clause3 = new FixedBitSet(maxDoc);
+    for (int i = 0; i < maxDoc; i += 2) {
+      clause1.set(i);
+      clause2.set(i);
+      clause3.set(i);
+    }
+    DenseConjunctionBulkScorer scorer =
+        new DenseConjunctionBulkScorer(
+            Arrays.asList(
+                new BitSetIterator(clause1, clause1.approximateCardinality()),
+                new BitSetIterator(clause2, clause2.approximateCardinality()),
+                new BitSetIterator(clause3, clause3.approximateCardinality())));
+    FixedBitSet result = new FixedBitSet(maxDoc);
+    scorer.score(
+        new LeafCollector() {
+          @Override
+          public void setScorer(Scorable scorer) throws IOException {}
+
+          @Override
+          public void collect(int doc) throws IOException {
+            result.set(doc);
+          }
+        },
+        null,
+        0,
+        DocIdSetIterator.NO_MORE_DOCS);
+
+    assertEquals(clause1, result);
+  }
+
+  public void testApplyAcceptDocs() throws IOException {
+    int maxDoc = 100_000;
+    FixedBitSet clause1 = new FixedBitSet(maxDoc);
+    FixedBitSet clause2 = new FixedBitSet(maxDoc);
+    clause1.set(0, maxDoc);
+    clause2.set(0, maxDoc);
+    FixedBitSet acceptDocs = new FixedBitSet(maxDoc);
+    for (int i = 0; i < maxDoc; i += 2) {
+      acceptDocs.set(i);
+    }
+    DenseConjunctionBulkScorer scorer =
+        new DenseConjunctionBulkScorer(
+            Arrays.asList(
+                new BitSetIterator(clause1, clause1.approximateCardinality()),
+                new BitSetIterator(clause2, clause2.approximateCardinality())));
+    FixedBitSet result = new FixedBitSet(maxDoc);
+    scorer.score(
+        new LeafCollector() {
+          @Override
+          public void setScorer(Scorable scorer) throws IOException {}
+
+          @Override
+          public void collect(int doc) throws IOException {
+            result.set(doc);
+          }
+        },
+        acceptDocs,
+        0,
+        DocIdSetIterator.NO_MORE_DOCS);
+
+    assertEquals(acceptDocs, result);
+  }
+
+  public void testEmptyIntersection() throws IOException {
+    int maxDoc = 100_000;
+    FixedBitSet clause1 = new FixedBitSet(maxDoc);
+    FixedBitSet clause2 = new FixedBitSet(maxDoc);
+    for (int i = 0; i < maxDoc - 1; i += 2) {
+      clause1.set(i);
+      clause2.set(i + 1);
+    }
+    DenseConjunctionBulkScorer scorer =
+        new DenseConjunctionBulkScorer(
+            Arrays.asList(
+                new BitSetIterator(clause1, clause1.approximateCardinality()),
+                new BitSetIterator(clause2, clause2.approximateCardinality())));
+    FixedBitSet result = new FixedBitSet(maxDoc);
+    scorer.score(
+        new LeafCollector() {
+          @Override
+          public void setScorer(Scorable scorer) throws IOException {}
+
+          @Override
+          public void collect(int doc) throws IOException {
+            result.set(doc);
+          }
+        },
+        null,
+        0,
+        DocIdSetIterator.NO_MORE_DOCS);
+
+    assertTrue(result.scanIsEmpty());
+  }
+
+  public void testClustered() throws IOException {
+    int maxDoc = 100_000;
+    FixedBitSet clause1 = new FixedBitSet(maxDoc);
+    FixedBitSet clause2 = new FixedBitSet(maxDoc);
+    FixedBitSet clause3 = new FixedBitSet(maxDoc);
+    clause1.set(10_000, 90_000);
+    clause2.set(0, 80_000);
+    clause3.set(20_000, 100_000);
+    DenseConjunctionBulkScorer scorer =
+        new DenseConjunctionBulkScorer(
+            Arrays.asList(
+                new BitSetIterator(clause1, clause1.approximateCardinality()),
+                new BitSetIterator(clause2, clause2.approximateCardinality()),
+                new BitSetIterator(clause3, clause3.approximateCardinality())));
+    FixedBitSet result = new FixedBitSet(maxDoc);
+    scorer.score(
+        new LeafCollector() {
+          @Override
+          public void setScorer(Scorable scorer) throws IOException {}
+
+          @Override
+          public void collect(int doc) throws IOException {
+            result.set(doc);
+          }
+        },
+        null,
+        0,
+        DocIdSetIterator.NO_MORE_DOCS);
+
+    FixedBitSet expected = new FixedBitSet(maxDoc);
+    expected.set(20_000, 80_000);
+    assertArrayEquals(expected.getBits(), result.getBits());
+    assertEquals(expected, result);
+  }
+
+  public void testSparseAfter2ndClause() throws IOException {
+    int maxDoc = 100_000;
+    FixedBitSet clause1 = new FixedBitSet(maxDoc);
+    FixedBitSet clause2 = new FixedBitSet(maxDoc);
+    FixedBitSet clause3 = new FixedBitSet(maxDoc);
+    // 13 and 17 are primes, so their only intersection is on multiples of both 13 and 17
+    // Likewise, 19 is prime, so the only intersection of the conjunction is on multiples of 13, 17
+    // and 19
+    for (int i = 0; i < maxDoc; i += 13) {
+      clause1.set(i);
+    }
+    for (int i = 0; i < maxDoc; i += 17) {
+      clause2.set(i);
+    }
+    for (int i = 0; i < maxDoc; i += 19) {
+      clause3.set(i);
+    }
+    DenseConjunctionBulkScorer scorer =
+        new DenseConjunctionBulkScorer(
+            Arrays.asList(
+                new BitSetIterator(clause1, clause1.approximateCardinality()),
+                new BitSetIterator(clause2, clause2.approximateCardinality()),
+                new BitSetIterator(clause3, clause3.approximateCardinality())));
+    FixedBitSet result = new FixedBitSet(maxDoc);
+    scorer.score(
+        new LeafCollector() {
+          @Override
+          public void setScorer(Scorable scorer) throws IOException {}
+
+          @Override
+          public void collect(int doc) throws IOException {
+            result.set(doc);
+          }
+        },
+        null,
+        0,
+        DocIdSetIterator.NO_MORE_DOCS);
+
+    FixedBitSet expected = new FixedBitSet(maxDoc);
+    for (int i = 0; i < maxDoc; i += 13 * 17 * 19) {
+      expected.set(i);
+    }
+    assertEquals(expected, result);
+  }
+}

lucene/core/src/test/org/apache/lucene/util/TestFixedBitSet.java

@@ -616,4 +616,30 @@ public class TestFixedBitSet extends BaseBitSetTestCase<FixedBitSet> {
     set.set(5);
     assertTrue(bits.get(5));
   }
+
+  public void testScanIsEmpty() {
+    FixedBitSet set = new FixedBitSet(0);
+    assertTrue(set.scanIsEmpty());
+
+    set = new FixedBitSet(13);
+    assertTrue(set.scanIsEmpty());
+    set.set(10);
+    assertFalse(set.scanIsEmpty());
+
+    set = new FixedBitSet(1024);
+    assertTrue(set.scanIsEmpty());
+    set.set(3);
+    assertFalse(set.scanIsEmpty());
+    set.clear(3);
+    set.set(1020);
+    assertFalse(set.scanIsEmpty());
+
+    set = new FixedBitSet(1030);
+    assertTrue(set.scanIsEmpty());
+    set.set(3);
+    assertFalse(set.scanIsEmpty());
+    set.clear(3);
+    set.set(1028);
+    assertFalse(set.scanIsEmpty());
+  }
 }