[vectorIntrinsics] RFR: 8254826: Add reduction nodes implementation for Arm SVE

[Paul Sandoz]

On Fri, 16 Oct 2020 06:20:20 GMT, Ningsheng Jian <njian at openjdk.org> wrote:

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>
> Hi @PaulSandoz @adinn,
> 
> Since Vector API has been integrated into jdk mainline, do you think this kind of enhancements should go to
> vectorIntrinsics for now or jdk mainline directly? @XiaohongGong is also implementing some missing part (mask related)
> for vector api SVE codegen. So we could either:  1. Get this into vectorIntrinsics for now until we have full/mature
> SVE codegen support for Vector API. If there's any conflict with mainline, I can do the merge. So, the reviewers
> (@adinn ? :-) ) don't need to try and test every such patch, and we could also test the patches a bit more here in
> vectorIntrinsics before going to release.  2. Go to jdk mainline directly, as it does not (if no bug introduced) break
> anything, as we have op_sve_supported check to make sure no "bad AD file" error, with
> https://github.com/openjdk/panama-vector/pull/7 merged to jdk mainline.

@nsjian my sense is such enhancements are more appropriate in the panama-vector repository until the SVE implementation
is mature enough to your and other's satisfaction. In combination, perhaps there are API changes that are useful in
this regard. We have the opportunity to think a little more holistically, and AFAICT we have some luxury of time given
SVE is still in the early phases of production hardware compared to the other platforms.

Further, ramp down phase 1 of JDK 16 is not that far away. Focusing on the quality of the x86 and AArch64 Neon in the
mainline does seem a higher priority to me.

We will re-incubate the Vector API every release if there are changes, a new JEP with updated contents. When SVE
support is ready we could update the JEP contents to place it on the same level as x86 and AArch64 Neon.

Does that sound reasonable?

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PR: https://git.openjdk.java.net/panama-vector/pull/13