[vectorIntrinsics] RFR: 8265817: Add LoadVectorGather nodes implementation for Arm SVE

[Xiaohong Gong]

On Fri, 23 Apr 2021 06:54:50 GMT, Wang Huang <whuang at openjdk.org> wrote:

> * Add LoadVectorGather nodes implementation for Arm SVE
> * add new instruction `sve_ld1w_gather`[1] & `sve_ld1d_gather`[2]
> * move a small code into `op_sve_supported` as @nsjian suggested[3]
> * some code styles fix 
> [1] https://developer.arm.com/documentation/ddi0596/2020-12/SVE-Instructions/LD1W--scalar-plus-vector---Gather-load-unsigned-words-to-vector--vector-index--
> [2] https://developer.arm.com/documentation/ddi0596/2020-12/SVE-Instructions/LD1D--scalar-plus-vector---Gather-load-doublewords-to-vector--vector-index--
> [3]https://github.com/openjdk/panama-vector/pull/70#discussion_r618240204

src/hotspot/cpu/aarch64/aarch64_sve.ad line 4115:

> 4113:   %}
> 4114:   ins_pipe(pipe_slow);
> 4115: %}

Thanks for your contribution! This patch looks good to me. I just have one question: since currently the SVE supports the vector operations for vector length smaller than the max vector size, does it need to  consider the partial implementation for the `LoadVectorGather `op? Thanks!

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PR: https://git.openjdk.java.net/panama-vector/pull/72