RFR: 8282664: Unroll by hand StringUTF16 and StringLatin1 polynomial hash loops [v13]

[Sandhya Viswanathan]

On Tue, 20 Dec 2022 19:52:34 GMT, Claes Redestad <redestad at openjdk.org> wrote:

>> src/java.base/share/classes/java/lang/StringUTF16.java line 418:
>> 
>>> 416:             return 0;
>>> 417:         } else {
>>> 418:             return ArraysSupport.vectorizedHashCode(value, ArraysSupport.UTF16);
>> 
>> Special case for 1 missing here.
>
> Intentionally left out. Array length is always even for `UTF16` arrays, but we could add a case for `2` that'd return `getChar(bytes, 0)` but I didn't see much of a win when I tested this.

I do see a 1.5x gain with this special case added:
       return switch (value.length) {
            case 0 -> 0;
            case 2 -> getChar(value, 0);
            default -> ArraysSupport.vectorizedHashCode(value, ArraysSupport.UTF16);
        };
before: 0.987 ns/op 
after: 0.640 ns/op

-------------

PR: https://git.openjdk.org/jdk/pull/10847