[GitHub] [lucene] jpountz commented on issue #12396: Make ForUtil Vectorized

["jpountz (via GitHub)" <gi...@apache.org>]

jpountz commented on issue #12396:
URL: https://github.com/apache/lucene/issues/12396#issuecomment-1611459853

   Thanks for looking into this! For reference, I've been separately looking into whether we could vectorize prefix sums, which is one bottleneck of postings decoding today as we managed to get auto-vectorization to help with bit unpacking, but not with computing a prefix sum of doc IDs: https://github.com/jpountz/vectorized-prefix-sum. Unfortunately, Java's vector API doesn't offer a way to perform a shift across lanes efficiently (e.g. `IntVector#unslice` doesn't compile to [vpslldq](https://www.felixcloutier.com/x86/pslldq) with SPECIES_128) while it's the traditional way how prefix sum is vectorized. I've looked into other approaches, but they're all slower than a scalar prefix sum. @ChrisHegarty looked into it and had a few interesting observations, in particular vectorized prefix sums suddenly become several times faster when bound checks are disabled: https://github.com/jpountz/vectorized-prefix-sum/issues, so there might be room for improvement in the JDK that would allow us 
 to eventually take advantage of vectorization for prefix sums?


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