kaivalnp commented on code in PR #15742:
URL: https://github.com/apache/lucene/pull/15742#discussion_r2835272749
##########
lucene/core/src/java25/org/apache/lucene/internal/vectorization/PanamaVectorUtilSupport.java:
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@@ -1010,21 +1018,28 @@ private static int int4SquareDistanceSinglePackedBody(
// upper
ByteVector va8 = unpacked.load(Int4Constants.BYTE_SPECIES, i + j +
packed.length());
- ByteVector diff8 = vb8.and((byte) 0x0F).sub(va8);
- Vector<Short> diff16 = diff8.convertShape(B2S,
Int4Constants.SHORT_SPECIES, 0);
- acc0 = acc0.add(diff16.mul(diff16));
// lower
ByteVector vc8 = unpacked.load(Int4Constants.BYTE_SPECIES, i + j);
Review Comment:
Something I found interesting: we see a performance drop after a few warmup
iterations if we operate on `upper` (multiply with self and add to accumulator)
_before_ loading `lower`:
```
# Warmup Iteration 1: 12.736 ops/us
# Warmup Iteration 2: 16.919 ops/us
# Warmup Iteration 3: 4.171 ops/us
# Warmup Iteration 4: 4.174 ops/us
Iteration 1: 4.179 ops/us
Iteration 2: 4.188 ops/us
Iteration 3: 4.193 ops/us
Iteration 4: 4.201 ops/us
Iteration 5: 4.198 ops/us
```
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