RFR: 8284992: Fix misleading Vector API doc for LSHR operator

[Paul Sandoz]

On Tue, 19 Apr 2022 08:41:50 GMT, Jie Fu <jiefu at openjdk.org> wrote:

> Hi all,
> 
> The Current Vector API doc for `LSHR` is
> 
> Produce a>>>(n&(ESIZE*8-1)). Integral only.
> 
> 
> This is misleading which may lead to bugs for Java developers.
> This is because for negative byte/short elements, the results computed by `LSHR` will be different from that of `>>>`.
> For more details, please see https://github.com/openjdk/jdk/pull/8276#issue-1206391831 .
> 
> After the patch, the doc for `LSHR` is
> 
> Produce zero-extended right shift of a by (n&(ESIZE*8-1)) bits. Integral only.
> 
> 
> Thanks.
> Best regards,
> Jie

The intended pattern for the operator tokens is to present a short symbolic description using Java operators and common methods. It would be good to try and keep with this pattern, and clarify for the extra cases. Here's what i had in mind:

    /** Produce {@code a>>>(n&(ESIZE*8-1))}.  Integral only. 
      * <p>
      * For operand types {@code byte} and {@code short} the operation behaves as if the operand is first implicitly widened  
      * to an {@code int} value with {@code (a & ((1 << ESIZE) - 1))} the result of which is then applied as the operand to this 
      * operation, the result of the operation is then narrowed from {@code int} to the operand type using an explicit cast.
      */
    public static final /*bitwise*/ Binary LSHR;

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PR: https://git.openjdk.java.net/jdk/pull/8291