[GitHub] [lucene] msokolov commented on a change in pull request #18: LUCENE-9838: simd version of VectorUtil.dotProduct

[GitBox <gi...@apache.org>]

msokolov commented on a change in pull request #18:
URL: https://github.com/apache/lucene/pull/18#discussion_r594289302



##########
File path: lucene/core/src/java/org/apache/lucene/util/VectorUtil.java
##########
@@ -30,66 +36,26 @@ public static float dotProduct(float[] a, float[] b) {
     if (a.length != b.length) {
       throw new IllegalArgumentException("vector dimensions differ: " + a.length + "!=" + b.length);
     }
-    float res = 0f;
-    /*
-     * If length of vector is larger than 8, we use unrolled dot product to accelerate the
-     * calculation.
-     */
-    int i;
-    for (i = 0; i < a.length % 8; i++) {
-      res += b[i] * a[i];
-    }
-    if (a.length < 8) {
-      return res;
-    }
-    for (; i + 31 < a.length; i += 32) {
-      res +=
-          b[i + 0] * a[i + 0]
-              + b[i + 1] * a[i + 1]
-              + b[i + 2] * a[i + 2]
-              + b[i + 3] * a[i + 3]
-              + b[i + 4] * a[i + 4]
-              + b[i + 5] * a[i + 5]
-              + b[i + 6] * a[i + 6]
-              + b[i + 7] * a[i + 7];
-      res +=
-          b[i + 8] * a[i + 8]
-              + b[i + 9] * a[i + 9]
-              + b[i + 10] * a[i + 10]
-              + b[i + 11] * a[i + 11]
-              + b[i + 12] * a[i + 12]
-              + b[i + 13] * a[i + 13]
-              + b[i + 14] * a[i + 14]
-              + b[i + 15] * a[i + 15];
-      res +=
-          b[i + 16] * a[i + 16]
-              + b[i + 17] * a[i + 17]
-              + b[i + 18] * a[i + 18]
-              + b[i + 19] * a[i + 19]
-              + b[i + 20] * a[i + 20]
-              + b[i + 21] * a[i + 21]
-              + b[i + 22] * a[i + 22]
-              + b[i + 23] * a[i + 23];
-      res +=
-          b[i + 24] * a[i + 24]
-              + b[i + 25] * a[i + 25]
-              + b[i + 26] * a[i + 26]
-              + b[i + 27] * a[i + 27]
-              + b[i + 28] * a[i + 28]
-              + b[i + 29] * a[i + 29]
-              + b[i + 30] * a[i + 30]
-              + b[i + 31] * a[i + 31];
+    int i = 0;
+    float res = 0;
+    // if the array size is large (2x platform vector size), its worth the overhead to vectorize
+    // vector loop is unrolled a single time (2 accumulators in parallel)
+    if (a.length >= 2 * SPECIES.length()) {

Review comment:
       very elegant! We had played around with this API last summer and failed to achieve gains. Maybe it has incubated! Or maybe we fumbled the attempt; anyway this is awesome gains for a very small amount of code which is right at the hot spot for vector search. I'll leave some thoughts about what strategy to pursue for making this available on the JIRA, but as for the code, it's great




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