RFR: 8282664: Unroll by hand StringUTF16 and StringLatin1 polynomial hash loops [v2]

[Patrick Doyle]

On Mon, 7 Mar 2022 21:41:05 GMT, Richard Startin <duke at openjdk.java.net> wrote:

>> Ludovic Henry has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Add UTF-16 benchmarks
>
> Great to see this taken up. As it’s implemented here, it’s still scalar, but the unroll prevents a strength reduction of the multiplication in the loop from
> 
> result = 31 * result + element;
> 
> to:
> 
> result = (result << 5) - result + element
> 
> which creates a data dependency and slows the loop down.
> 
> This was first reported by Peter Levart here: http://mail.openjdk.java.net/pipermail/core-libs-dev/2014-September/028898.html

@richardstartin - does that strength reduction actually happen? The bit-shift transformation valid only if the original `result` is known to be non-negative.

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PR: https://git.openjdk.java.net/jdk/pull/7700