Avoid copying bitset that's subsequently cleared

lucene/CHANGES.txt

@@ -98,6 +98,8 @@ Optimizations
 
 * GITHUB#15085, GITHUB#15092: Hunspell suggestions: Ensure candidate roots are not worse before updating. (Ilia Permiashkin)
 
+* GITHUB#15582: Avoid copying bitset that's subsequently cleared
+
 Bug Fixes
 ---------------------
 * GITHUB#14049: Randomize KNN codec params in RandomCodec. Fixes scalar quantization div-by-zero

lucene/core/src/java/org/apache/lucene/index/ReadersAndUpdates.java

@@ -553,8 +553,7 @@ public void intoBitSet(int upTo, FixedBitSet bitSet, int offset) throws IOExcept
         scratch = new FixedBitSet(bitSet.length());
       } else {
         // It's OK even if bitset.length() == 0 according the contract.
-        scratch = FixedBitSet.ensureCapacity(scratch, bitSet.length() - 1);
-        scratch.clear();
+        scratch = FixedBitSet.ensureCapacityAndClear(scratch, bitSet.length() - 1);
       }
 
       onDiskDocValues.intoBitSet(upTo, scratch, offset);

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

@@ -42,25 +42,42 @@ public final class FixedBitSet extends BitSet {
   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)
 
-  /**
-   * If the given {@link FixedBitSet} is large enough to hold {@code numBits+1}, returns the given
-   * bits, otherwise returns a new {@link FixedBitSet} which can hold {@code numBits+1} bits. That
-   * means the bitset returned by this method can be safely called with {@code bits.set(numBits)}
-   *
-   * <p><b>NOTE:</b> the returned bitset reuses the underlying {@code long[]} of the given {@code
-   * bits} if possible. Also, calling {@link #length()} on the returned bits may return a value
-   * greater than {@code numBits+1}.
-   */
-  public static FixedBitSet ensureCapacity(FixedBitSet bits, int numBits) {
-    if (numBits < bits.numBits) {
+  /// Ensure the given `bits` can store a value at `desiredBit` index. If the current [#length()] is
+  /// sufficient, `bits` is simply returned. Otherwise, a new, larger bitset is allocated, with
+  /// contents of `bits` copied.
+  ///
+  /// @see #ensureCapacityAndClear(FixedBitSet, int)
+  public static FixedBitSet ensureCapacity(FixedBitSet bits, int desiredBit) {
+    return ensureCapacityInternal(bits, desiredBit, true);
+  }
+
+  /// Clear the given `bits` and ensure it can store a value at `desiredBit` index. If the current
+  /// [#length()] is sufficient, `bits` is simply cleared and returned. Otherwise, a new, larger
+  /// bitset is allocated.
+  ///
+  /// @see #ensureCapacity(FixedBitSet, int)
+  public static FixedBitSet ensureCapacityAndClear(FixedBitSet bits, int desiredBit) {
+    return ensureCapacityInternal(bits, desiredBit, false);
+  }
+
+  private static FixedBitSet ensureCapacityInternal(
+      FixedBitSet bits, int desiredBit, boolean preserveData) {
+    if (desiredBit < bits.numBits) {
+      if (!preserveData) {
+        bits.clear();
+      }
       return bits;
     } else {
       // Depends on the ghost bits being clear!
       // (Otherwise, they may become visible in the new instance)
-      int numWords = bits2words(numBits);
+      int numWords = bits2words(desiredBit);
       long[] arr = bits.getBits();
       if (numWords >= arr.length) {
-        arr = ArrayUtil.grow(arr, numWords + 1);
+        if (preserveData) {
+          arr = ArrayUtil.grow(arr, numWords + 1);
+        } else {
+          arr = new long[ArrayUtil.oversize(numWords + 1, Long.BYTES)];
+        }
       }
       return new FixedBitSet(arr, arr.length << 6);
     }

lucene/core/src/java/org/apache/lucene/util/hnsw/HnswGraphSearcher.java

@@ -369,9 +369,10 @@ void searchLevel(
   private void prepareScratchState(int capacity, int bulkScoreSize) {
     candidates.clear();
     if (visited.length() < capacity) {
-      visited = FixedBitSet.ensureCapacity((FixedBitSet) visited, capacity);
+      visited = FixedBitSet.ensureCapacityAndClear((FixedBitSet) visited, capacity);
+    } else {
+      visited.clear();
     }
-    visited.clear();
     if (bulkNodes == null || bulkNodes.length < bulkScoreSize) {
       bulkNodes = new int[bulkScoreSize];
       bulkScores = new float[bulkScoreSize];