RFR: 8307795: AArch64: Optimize VectorMask.truecount() on Neon

[Chang Peng]

On Mon, 15 May 2023 08:56:37 GMT, Andrew Haley <aph at openjdk.org> wrote:

> That makes sense. Is it likely that there are more of these combined operations on vector masks that could be matched? if so, it might make sense to do the job earlier, in the C2 optimizer.

Thanks for your review.

I have tried to optimize ```VectorMask.firstTrue()``` [1] and ```VectorMask.lastTrue()``` [2] in the same way as this patch, but these two operations are strong correlated with xtn, we cannot simply remove it.

I didn't find a way to optimize these two operations, they are hignly optimized in C2 backend and there are not extra instructions.

[1]: https://github.com/openjdk/jdk/blob/8d49ba9e8d3095f850b3007b56488a0c0cf8ddff/src/hotspot/cpu/aarch64/aarch64_vector.ad#L5517
[2]: https://github.com/openjdk/jdk/blob/8d49ba9e8d3095f850b3007b56488a0c0cf8ddff/src/hotspot/cpu/aarch64/aarch64_vector.ad#L5624

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

PR Comment: https://git.openjdk.org/jdk/pull/13974#issuecomment-1547484838