RFR: 8314124: RISC-V: implement Base64 intrinsic - decoding [v4]

[Fei Yang]

On Mon, 26 Aug 2024 16:46:23 GMT, Hamlin Li <mli at openjdk.org> wrote:

>> Sorry for not being clear on this. The java code snippet of decodeBlock:
>> 
>>  795                 int b1 = base64[src[sp++] & 0xff];
>>  796                 int b2 = base64[src[sp++] & 0xff];
>>  797                 int b3 = base64[src[sp++] & 0xff];
>>  798                 int b4 = base64[src[sp++] & 0xff];
>>  799                 if ((b1 | b2 | b3 | b4) < 0) {    // non base64 byte
>>  800                     return new_dp - dp;
>>  801                 }
>>  802                 int bits0 = b1 << 18 | b2 << 12 | b3 << 6 | b4;
>> 
>> L799 simply OR-ed all the initial values of `b1`-`b4` and compare the result with zero. I think this should be reflected on the value of `combined32Bits` when it is used to do following error check. Correspondingly, It should be the OR-ed result of the initial loaded values in `byte0` - `byte3`.
>> 
>>       // error check
>>       __ bltz(combined32Bits, Exit);
>> 
>> 
>> Anything I missed?
>
> `(b1 | b2 | b3 | b4) < 0`, this java code is to tell if any of b1-b4 is < 0.
> On the other side, `bltz(combined32Bits, Exit)` is doing the similar thing (same effect, but different way), as when loading byte1-4, `lb` will sign-extend it, if any of byte1-4 < 0, then the top bit will be 1, and after shift left, top bit will still be 1, so as a result, we can use combined32Bits to tell if any of byte1-4 < 0, and at the same time, we can use combined32Bits to decode the final data (from 4 bytes to 3 bytes). Or to put it another way, combined32Bits is constructed for 2 purposes at the same time.
> Hope this answers your question?

Yeah. Interesting. Could you please add a small code comment for this? Thanks.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20026#discussion_r1732043633