RFR: 8309583: AArch64: Optimize firstTrue() when amount of elements < 8 [v3]

[Chang Peng]

> This patch optimizes VectorMask.firstTrue() on Neon when there are 2 or 4 elements in vector registers.
> 
> VectorMask.firstTrue() should return VLEGNTH when vector mask is all false [1]. Current implementation uses rbit and then clz [2] to count leading zeros, then uses csel [3] (conditional select) to get the smaller value between VLENGTH and the number of unset lanes to ensure correctness.
> 
> This patch sets the 16th or 32nd bit as 1, when there are only 2 or 4 elements in boolean masks, before rbit and clz. With this trick, maximum value calculated in such case will be VLENGTH (2 or 4).
> 
> Test:
> All vector and vectorapi test passed.
> 
> Performance:
> The benchmark functions are in MaskQueryOperationsBenchmark.java [4]. This patch also modifies above benchmark to measure mask operations' performance more effectively.
> 
> Following data is collected on a 128-bit Neon machine.
> 
> Benchmark                                                                        (inputs)   Mode  Before    After    Units
> MaskQueryOperationsBenchmark.testFirstTrueInt            1            thrpt  5952.670  7298.491 ops/ms
> MaskQueryOperationsBenchmark.testFirstTrueInt            2            thrpt  5951.513  7297.620 ops/ms
> MaskQueryOperationsBenchmark.testFirstTrueInt            3            thrpt  5953.048  7298.072 ops/ms
> MaskQueryOperationsBenchmark.testFirstTrueLong        1            thrpt  3496.990  4003.188 ops/ms
> MaskQueryOperationsBenchmark.testFirstTrueLong        2            thrpt  3497.755  4002.577 ops/ms
> MaskQueryOperationsBenchmark.testFirstTrueLong        3            thrpt  3500.085  4002.471 ops/ms
> 
> [1]: https://docs.oracle.com/en/java/javase/20/docs/api/jdk.incubator.vector/jdk/incubator/vector/VectorMask.html#firstTrue()
> [2]: https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/aarch64/aarch64_vector.ad#L5540
> [3]: https://developer.arm.com/documentation/ddi0602/2021-12/Base-Instructions/CSEL--Conditional-Select-

Chang Peng has updated the pull request incrementally with one additional commit since the last revision:

  Update MaskQueryOperationsBenchmark.java

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/14373/files
  - new: https://git.openjdk.org/jdk/pull/14373/files/d8507105..62a6522c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14373&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14373&range=01-02

  Stats: 195 lines in 1 file changed: 103 ins; 34 del; 58 mod
  Patch: https://git.openjdk.org/jdk/pull/14373.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14373/head:pull/14373

PR: https://git.openjdk.org/jdk/pull/14373