rmuir commented on a change in pull request #18:
URL: https://github.com/apache/lucene/pull/18#discussion_r594337476
##########
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:
@msokolov here's a few notes of troubles i ran into in case it helps for
the next time:
* start with "full-performance" example in the JEP/javadocs and then iterate
from there.
* don't use "single masked vector op" at the end. don't use masked ops at
all :)
* don't put kernel "last" like current code in master. it must be "first"
(alignment restrictions i think).
* loop structure is important, otherwise whole loop gets slow (bounds checks
I think). See the tricks i had to do with upperBound: that's the only way i was
able to get a fast loop.
I may revisit the existing scalar code in master and see if things can be
improved there, too.
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