LUCENE-10453: Speed up euclidean distances. (#725)

lucene/CHANGES.txt

@@ -252,6 +252,9 @@ Optimizations
 
 * LUCENE-10450: IndexSortSortedNumericDocValuesRangeQuery could be rewrite to MatchAllDocsQuery. (Lu Xugang)
 
+* LUCENE-10453: Indexing and search speedup with KNN vectors when using
+  euclidean distance. (Adrien Grand)
+
 Changes in runtime behavior
 ---------------------
 

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

@@ -133,13 +133,36 @@ public final class VectorUtil {
     }
     float squareSum = 0.0f;
     int dim = v1.length;
-    for (int i = 0; i < dim; i++) {
+    int i;
+    for (i = 0; i + 8 <= dim; i += 8) {
+      squareSum += squareDistanceUnrolled8(v1, v2, i);
+    }
+    for (; i < dim; i++) {
       float diff = v1[i] - v2[i];
       squareSum += diff * diff;
     }
     return squareSum;
   }
 
+  private static float squareDistanceUnrolled8(float[] v1, float[] v2, int index) {
+    float diff0 = v1[index + 0] - v2[index + 0];
+    float diff1 = v1[index + 1] - v2[index + 1];
+    float diff2 = v1[index + 2] - v2[index + 2];
+    float diff3 = v1[index + 3] - v2[index + 3];
+    float diff4 = v1[index + 4] - v2[index + 4];
+    float diff5 = v1[index + 5] - v2[index + 5];
+    float diff6 = v1[index + 6] - v2[index + 6];
+    float diff7 = v1[index + 7] - v2[index + 7];
+    return diff0 * diff0
+        + diff1 * diff1
+        + diff2 * diff2
+        + diff3 * diff3
+        + diff4 * diff4
+        + diff5 * diff5
+        + diff6 * diff6
+        + diff7 * diff7;
+  }
+
   /**
    * Modifies the argument to be unit length, dividing by its l2-norm. IllegalArgumentException is
    * thrown for zero vectors.