Re: [vector]About Graceful degradation feature in JEP 414

[Wang Zhuo(Zhuoren)]

Thank you Paul. 
I still have some questions.
1. Since we have not put any effort into an performant solution for graceful degradation, is it better to print warnings when intrinsic failures happen?
2.About Platform agnostic. I am not clear with the goal of Platform agnostic, because current APIs are already architecture independent. What will the Platform agnostic feature bring to us?

Regards,
Zhuoren


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From:Paul Sandoz <paul.sandoz at oracle.com>
Sent At:2021 Jul. 22 (Thu.) 23:37
To:Sandler <zhuoren.wz at alibaba-inc.com>
Cc:panama-dev <panama-dev at openjdk.java.net>
Subject:Re: [vector]About Graceful degradation feature in JEP 414

Hi Zhuoren,

Each vector operation goes through an intrinsic method. If a call to such a method cannot be intrinsified, such as when there is no suitable hardware instruction, then it falls back to the pure Java implementation. The last parameter of the intrinsic is a lambda that provides the pure Java implementation, and the intrinsic method body calls that lambda.

It’s sort of “graceful” :-) but its not as performant as I would like. Ideally it should be as performant as equivalent scalar code, that is optimizable by the auto-vectorizer. That is not currently the case and we have not put any effort into this area as of now.

—

One area developers struggle with is determining “does this vector expression optimize?”. In part this is because not all operations were made intrinsic, but we have made good progress filling in the gaps on x64 and ARM. There are still a few operational gaps left, and there are a few gaps left with regards to composition of methods with vector parameters and ensuring vector instances are not boxed (I hope in the latter two cases Project Valhalla will help consolidate and improve on those areas). 

We have not yet added a way to query what operations might be intrinsic, or say a way to get more details of the CPU properties. The API is designed such that a developer can write their algorithm without depending on the vector size. However, there might be cases that require slightly different expressions of the algorithm based on the CPU architecture.

Paul.

> On Jul 22, 2021, at 12:45 AM, Wang Zhuo(Zhuoren) <zhuoren.wz at alibaba-inc.com> wrote:
> 
> 
> Hi, 
> I am very interested in the Graceful degradation feature in JEP 414. We met several performance regression cases due to unsupported vector.
> How will this feature be implemented?  Using plain instructions instead of SIMD instructions when unsupported hardware is detected, or some other approaches?
> 
> Regards,
> Zhuoren