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

[Chang Peng]

On Mon, 19 Jun 2023 09:52:22 GMT, Andrew Haley <aph at openjdk.org> wrote:

> > Sorry for the delay. Original performance was measured by a simple benchmark only measuring firstTrue()'s performance written by myself. When I wanted to add it to JDK I found an existing benchmark used to measure different mask operations' performance ([jdk/test/micro/org/openjdk/bench/jdk/incubator/vector/MaskQueryOperationsBenchmark.java at master · openjdk/jdk · GitHub](https://github.com/openjdk/jdk/blob/master/test/micro/org/openjdk/bench/jdk/incubator/vector/MaskQueryOperationsBenchmark.java)). I tried to measure firstTrue()'s performance by this benchmark, but I found Blackhole‘s proportion of hottest region is too high, like following:
> 
> Can you please send the entire output of JMH? Blackhole should not appear at all in the output because it's been intrinsified. I'd like to know why the intrinsic isn't working for you.

Output before this patch: https://gist.github.com/changpeng1997/734aa176577bfff56f5a87db9c8db69a
Output after this patch: https://gist.github.com/changpeng1997/73098069b8f814310d6606dfd7dc56c5

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PR Comment: https://git.openjdk.org/jdk/pull/14373#issuecomment-1596894355