RFR: 8322174: RISC-V: C2 VectorizedHashCode RVV Version [v8]

[Yuri Gaevsky]

On Wed, 4 Jun 2025 06:04:46 GMT, Robbin Ehn <rehn at openjdk.org> wrote:

>> As you can expect I am trying to implement the following code with RVV:
>> 
>> for (; i + (N-1) < cnt; i += N) {
>>    h =   31^^N     * h 
>>        + 31^^(N-1) * val[i + 0] 
>>        + 31^^(N-2) * val[i + 1] 
>> 	   ...
>>        + 31^^1 * val[i + (N-2)] 
>>        + 31^^0 * val[i + (N-1)];
>> }
>> for (; i < cnt; i++) {
>>    h = 31 * h + val[i];
>> }
>> 
>> where `N` is a number of processing array elements in "chunk".
>> IIUC, the main issue with your approach is "reverse" order of array elements versus preloaded `31^^X` coeffs WHEN the remaining number of elems is less than `N`, say `M=N-1`.
>> 
>>    h =   31^^M     * h 
>>        + 31^^(M-1) * val[i + 0] 
>>        + 31^^(M-2) * val[i + 1] 
>> 	   ...
>>        + 31^^1 * val[i + (M-2)] 
>>        + 32^^0 * val[i + (M-1)];
>> 
>> or returning to our `N` for clarity
>> 
>>    h =   31^^(N-1)     * h 
>>        + 31^^(N-2) * val[i + 0] 
>>        + 31^^(N-3) * val[i + 1] 
>> 	   ...
>>        + 31^^1 * val[i + (N-3)] 
>>        + 31^^0 * val[i + (N-2)];
>> 
>> Now we need to "slide down" preloaded multiplier coeffs in designated vector register by one (as `M=N-1`) to be in "sync" with `val[i + X]` (may be move them into temporary VR in the process), and moreover, DO this operation IFF the remaining `cnt` is less than `N` (==>an additional check on every iteration). That's probably acceptable only at tail phase as one-time operation but NOT inside of main loop...
>
> @ygaevsky @RealFYang how can we procced ?

> Thanks for the update. Latest version LGTM. Please get approval from @robehn

Sure. Many thanks for your thorough review.

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PR Comment: https://git.openjdk.org/jdk/pull/17413#issuecomment-3199967971