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

[Jie Fu]

On Mon, 18 Apr 2022 03:48:13 GMT, Quan Anh Mai <duke at openjdk.java.net> wrote:

> Because unsigned cast should operate on unsigned types, the more appropriate usage is `(src[i] & 0xFF) >>> 3`, with the `&` operation is the cast from unsigned byte to int. Actually, I fail to understand the intention of your example, why not use signed shift instead, what does unsigned shift provide here apart from extra cognitive load in reasoning the operation.


The fact is that you can't prevent developers from using `>>>` upon negative elements since neither the JVMS nor the JLS prevents it.


> May you provide a more concrete example to the utilisation of unsigned shift on signed subword types, please. The example provided by @fg1417 in #7979 seems to indicate the real intention is to right shifting unsigned bytes, with the unsigned cast sometimes omitted (changed to a signed cast) because the shift results are masked by a stricter mask immediately.

Sorry, I can't show the detail of our customer's code.
However, just image that someone would like to optimize some code segments of bytes/shorts `>>>`, how can you say there should be always non-negative operands?

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