RFR: 8315716: RISC-V: implement ChaCha20 intrinsic

[Hamlin Li]

On Mon, 25 Sep 2023 18:02:56 GMT, Robbin Ehn <rehn at openjdk.org> wrote:

>> Anyway, I think your suggestion make more sense, I've modified as you suggested.
>> Thanks Robbin for the detailed reviewing!
>
> It's the paragraph above:
> 
> The tail elements during a vector instruction’s execution are the elements past the current vector length setting specified in vl.
> 
> 
> This means they are outside of your working set (body).
> 
> 
> When a set is marked agnostic, the corresponding set of destination elements in any vector destination operand
> can either retain the value they previously held, or are overwritten with 1s. Within a single vector instruction, each
> destination element can be either left undisturbed or overwritten with 1s, in any combination, and the pattern of
> undisturbed or overwritten with 1s is not required to be deterministic when the instruction is executed with the same
> inputs.
> 
> 
> Maybe this can't happen here since you work with such nice size.
> But the arrays result and startState will be initialized to zero when allocated.
> We don't want anything changing bytes which the VM didn't write to and we must be sure never to write outside of these. 
> 
> So tail is what comes after you data, in our case that will be Java heap, so we must always be sure only to write to the body.
> 
> Did that make sense?

Thanks for discussion. Yes, I see your point.
What I mean above is that the java heap can only be touched when store back to memory, but store `only access memory or raise exceptions for active elements.`, that seems mean that a vectore operation might touch tail elements in vector group register, but a vector store will not touch the memory corresponding to the tail elements.
Am I understanding the spec correctly?

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PR Review Comment: https://git.openjdk.org/jdk/pull/15899#discussion_r1337573111