gsmiller commented on PR #12417:
URL: https://github.com/apache/lucene/pull/12417#issuecomment-1627487242
[Disclaimer: I'm pretty new to working with the incubator vector code, so
please excuse wonky terminology or silly ideas]
@ChrisHegarty have you experimented at all with narrower vector lanes for
lower bpv? I poked around your bitpacking repo a bit this morning and
experimented a bit with the 2bpv unpacking case. If I'm understanding
correctly, you essentially have four 32-bit lanes involved in the shift/mask
operations, so you're getting 4x parallelism right? (i.e., you're "generating"
four output ints in each vectorized op?). I wonder if there's any way to use
8bit lanes and get 16x parallelism in each shift/mask op? I messed about with
this a little bit, and—if I unpack into a byte[] instead of int[]—this produced
a pretty nice improvement in the benchmarks[1]. What I'm not sure of is how to
efficiently get into an int[] for output. Maybe that's the killer? Anyway,
wanted to just float this question out there as a newbie to the space but
curious and trying to learn more.
[1] here's what I experimented with:
```
private static final VectorSpecies<Byte> BYTE_SPECIES_128 =
ByteVector.SPECIES_128;
[...]
byte[] input = [...] // input is in bytes in this case, not int[]
byte[] byteOut = new byte[128];
ByteVector inVec = ByteVector.fromArray(BYTE_SPECIES_128, input, 0);
ByteVector outVec;
int outOff = 0;
final byte mask = (1 << 2) - 1;
outVec = inVec.and(mask);
outVec.intoArray(byteOut, outOff);
outVec = inVec.lanewise(VectorOperators.LSHR, 2).and(mask);
outVec.intoArray(byteOut, outOff+=16);
outVec = inVec.lanewise(VectorOperators.LSHR, 4).and(mask);
outVec.intoArray(byteOut, outOff+=16);
[... you get the idea ...]
```
```
Benchmark Mode Cnt Score Error Units
Benchmark.decode2SimdPack thrpt 5 58.689 ± 0.191 ops/us
Benchmark.decode2SimdPack2 thrpt 5 85.847 ± 1.800 ops/us
```
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