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

[Yuri Gaevsky]

On Mon, 5 May 2025 18:10:02 GMT, Yuri Gaevsky <duke at openjdk.org> wrote:

>> Yuri Gaevsky has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   change slli+add sequence to shadd
>
> 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 ?

My apologies, just busy at the moment with other things, going to update the patch soon.

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PR Comment: https://git.openjdk.org/jdk/pull/17413#issuecomment-2943036356