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

[Claes Redestad]

On Mon, 31 Oct 2022 02:15:35 GMT, Quan Anh Mai <qamai at openjdk.org> wrote:

>> src/hotspot/cpu/x86/c2_MacroAssembler_x86.cpp line 3387:
>> 
>>> 3385:   for (int idx = 0; idx < 4; idx++) {
>>> 3386:     // h = (31 * h) or (h << 5 - h);
>>> 3387:     movl(tmp, result);
>> 
>> If you are unrolling this, maybe break the dependency chain, `h = h * 31**4 + x[i] * 31**3 + x[i + 1] * 31**2 + x[i + 2] * 31 + x[i + 3]`
>
> A 256-bit vector is only 8 ints so this loop seems redundant, maybe running with the stride of 2 instead, in which case the single scalar calculation does also not need a loop.

Working on this..

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