RFR: 8307523: [vectorapi] Optimize MaskFromLongBenchmark.java

[Chang Peng]

To avoid dead code elimination, a use-point laneIsSet() is added in each benchmark method in MaskFromLongBenchmark.java.

However, currently laneIsSet() [1] is implemented by toLong(). So it may generate a fromLong-toLong pair [2], making this benchmark to be noneffective after inlining laneIsSet() into the outer method. The assembly of maskFromLong_byte128 benchmark on SVE2 is shown in [3]. We cannot see the bdep instruction used by fromLong on AArch64 [4]. So, in this case, we cannot measure fromLong()'s performance by using this benchmark.

This patch uses trueCount() [5] instead of toLong() to measure the fromLong()'s performance effectively. After this patch, we can see the bdep instruction in the hot loop [6] of maskFromLong_byte128 benchmark.

[1]: https://github.com/openjdk/jdk/blob/96fa2751e8bbc05d6d064d80c07720cc9db05c54/src/jdk.incubator.vector/share/classes/jdk/incubator/vector/AbstractMask.java#L70
[2]: https://github.com/openjdk/jdk/blob/ff368d504e9101e11c7182185f56255f429d31e3/src/hotspot/share/opto/vectornode.cpp#L1736
[3]: https://gist.github.com/changpeng1997/467f6056f78d99c055030fa5888b6baa
[4]: https://github.com/openjdk/jdk/blob/787832a58677205c9a11ae100dd8a2fbddb30a4a/src/hotspot/cpu/aarch64/c2_MacroAssembler_aarch64.cpp#L1099
[5]: https://docs.oracle.com/en/java/javase/16/docs/api/jdk.incubator.vector/jdk/incubator/vector/VectorMask.html#trueCount()
[6]: https://gist.github.com/changpeng1997/79bea0a9f80530bec89978950897000d

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Commit messages:
 - 8307523: [vectorapi] Optimize MaskFromLongBenchmark.java

Changes: https://git.openjdk.org/jdk/pull/13851/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13851&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307523
  Stats: 136 lines in 1 file changed: 80 ins; 6 del; 50 mod
  Patch: https://git.openjdk.org/jdk/pull/13851.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13851/head:pull/13851

PR: https://git.openjdk.org/jdk/pull/13851