msfroh commented on PR #13521:
URL: https://github.com/apache/lucene/pull/13521#issuecomment-2313906920
I tried modifying the loop to process 4 longs per iteration and noticed no
difference on my Xeon host, which is unsurprising since there was no difference
between 1 and 3.
I also tried the following SIMD implementation of `decode`:
```
@Override
public void decode(IndexInput in, int start, int count, int[]
docIDs) throws IOException {
int i = 0;
long[] inputScratch = new long[LONG_SPECIES.length()];
long[] outputScratch = new long[LONG_SPECIES.length() * 3];
int bound = LONG_SPECIES.loopBound(count / 3) * 3;
for (; i < bound; i += outputScratch.length) {
for (int j = 0; j < LONG_SPECIES.length(); j++) {
inputScratch[j] = in.readLong();
}
LongVector longVector = LongVector.fromArray(LONG_SPECIES,
inputScratch, 0);
longVector.lanewise(VectorOperators.LSHR, 42)
.intoArray(outputScratch, 0);
longVector.lanewise(VectorOperators.AND, 0x000003FFFFE00000L)
.lanewise(VectorOperators.LSHR, 21)
.intoArray(outputScratch, LONG_SPECIES.length());
longVector.lanewise(VectorOperators.AND, 0x001FFFFFL)
.intoArray(outputScratch, LONG_SPECIES.length() * 2);
for (int j = 0; j < LONG_SPECIES.length(); j++) {
docIDs[i + j] = (int) outputScratch[j];
docIDs[i + j + 1] = (int) outputScratch[j +
LONG_SPECIES.length()];
docIDs[i + j + 2] = (int) outputScratch[j +
LONG_SPECIES.length() * 2];
}
}
for (; i < count - 2; i += 3) {
long packedLong = in.readLong();
docIDs[i] = (int) (packedLong >>> 42);
docIDs[i + 1] = (int) ((packedLong & 0x000003FFFFE00000L)
>>> 21);
docIDs[i + 2] = (int) (packedLong & 0x001FFFFFL);
}
for (; i < count; i++) {
docIDs[i] = in.readInt();
}
}
```
Unfortunately, it performs noticeably worse than the other implementations:
```
Benchmark (encoderName) Mode Cnt Score
Error Units
DocIdEncodingBenchmark.decode Bit21WithSimdEncoder avgt 5 2191.040 ±
14.913 ms/op
DocIdEncodingBenchmark.decode Bit21With3StepsEncoder avgt 5 850.331 ±
4.576 ms/op
DocIdEncodingBenchmark.decode Bit21With2StepsEncoder avgt 5 859.980 ±
4.567 ms/op
DocIdEncodingBenchmark.decode Bit24Encoder avgt 5 912.914 ±
5.488 ms/op
```
Maybe I'm doing it wrong 🤷
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