RFR: 8284932: [Vector API] Incorrect implementation of LSHR operator for negative byte/short elements

[Jie Fu]

On Mon, 18 Apr 2022 04:25:23 GMT, Quan Anh Mai <duke at openjdk.java.net> wrote:

> > However, just image that someone would like to optimize some code segments of bytes/shorts `>>>`
> 
> Then that person can just use signed shift (`VectorOperators.ASHR`), right? Shifting on masked shift counts means that the shift count cannot be greater than 8 for bytes and 16 for shorts, which means that `(byte)(src[i] >>> 3)` is exactly the same as `(byte)(src[i] >> 3)`. Please correct me if I misunderstood something here.

Yes, you're right that's why I said `LSHR` can be replaced with `ASHR`.

However, not all the people are clever enough to do this source code level replacement.
To be honest, I would never think of that `>>>` can be auto-vectorized by this idea proposed by https://github.com/openjdk/jdk/pull/7979 .

> 
> Your proposed changes make unsigned shifts for subwords behave exactly the same as signed shifts, which is both redundant (we have 2 operations doing exactly the same thing) and inadequate (we lack the operation to do the proper unsigned shifts)

`LSHR` following the behavior of scalar `>>>` is very important for Java developers to rewrite the code with vector api.
Maybe, we should add one more operator to support what you called `the proper unsigned shifts`, right?
But that's another topic which can be done in a separate issue.

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PR: https://git.openjdk.java.net/jdk/pull/8276