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

[Vladimir Ivanov]

On Sun, 13 Nov 2022 21:08:53 GMT, Claes Redestad <redestad at openjdk.org> wrote:

> How far off is this ...?

Back then it looked way too constrained (tight constraints on code shapes). But I considered it as a generally applicable optimization. 

>  ... do you think it'll be able to match the efficiency we see here with a memoized coefficient table etc?

Yes, it is able to build the constant table at runtime when folding multiplications of constant coefficients produced during loop unrolling and then packing scalars into a constant vector.

Moreover, briefly looking at the code shape, the vectorizer would produce a more optimal loop shape (pre-loop would align vector accesses and would use 512-bit vectors when available; vector post-loop could help as well).

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