RFR: 8350468: x86: Improve implementation of vectorized numberOfLeadingZeros for int and long
Jasmine Karthikeyan
jkarthikeyan at openjdk.org
Mon Aug 4 02:27:14 UTC 2025
Hi all,
This is a patch that optimizes the x86 backend implementation of `CountLeadingZerosV` for int and long. In the review of [JDK-8349637)](https://bugs.openjdk.org/browse/JDK-8349637) an [optimized algorithm]( https://github.com/openjdk/jdk/pull/23579#issuecomment-2661332497) was proposed by @rgiulietti, which this PR implements. For integer operands, the optimized algorithm reduces the number of vector instructions from 19 to 13. The same algorithm does not work for long operands, however, since avx2 lacks a vectorized long->double conversion instruction. Instead, I found an optimized algorithm to reuse the code for int and compute the leading zeros for long with only 4 additional instructions. I added a benchmark and on my Zen 3 machine I get these results:
Baseline Patch
Benchmark Mode Cnt Score Error Units Score Error Units Improvement
LeadingZeros.testInt avgt 15 91.097 ± 3.276 ns/op 68.665 ± 1.740 ns/op (+ 28.1%)
LeadingZeros.testLong avgt 15 342.545 ± 4.470 ns/op 228.668 ± 5.994 ns/op (+ 39.9%)
I've updated the unit tests to more thoroughly test longs and they pass on my machine. Thoughts and reviews would be appreciated!
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Commit messages:
- Optimize numberOfLeadingZeros
Changes: https://git.openjdk.org/jdk/pull/26610/files
Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26610&range=00
Issue: https://bugs.openjdk.org/browse/JDK-8350468
Stats: 225 lines in 3 files changed: 160 ins; 17 del; 48 mod
Patch: https://git.openjdk.org/jdk/pull/26610.diff
Fetch: git fetch https://git.openjdk.org/jdk.git pull/26610/head:pull/26610
PR: https://git.openjdk.org/jdk/pull/26610
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