RFR: 8248188: Add IntrinsicCandidate and API for Base64 decoding [v8]

[CoreyAshford]

On Thu, 22 Oct 2020 18:10:02 GMT, Martin Doerr <mdoerr at openjdk.org> wrote:

>> src/hotspot/cpu/ppc/stubGenerator_ppc.cpp line 3820:
>> 
>>> 3818:         __ vcmpequb_(eq_special_case_char, input, vec_special_case_char);
>>> 3819:         //
>>> 3820:         // There's a (63/64)^16 = 77.7% chance that there are no special
>> 
>> I think that assumes uniformly randomized data, is that a good assumption?   Is it measurably faster to skip around the xxsel instead of doing it unconditionally?
>
> Thanks for this question. I also stumbled over it when reviewing. I guess a branch which gets mispredicted in ~22% of the cases leads to a big performance loss. (In addition, the branch target is not aligned.)

Yes, it assumes uniformly random data, but also recall that the unencoded data bytes get shifted by 2, 4, 6 bits into the encoded bytes, which I'm guessing would tend to make the data somewhat more uniform, even if the source data has low entropy.

That said, I didn't actually benchmark it.  I will do that to make sure there is a gain, and if there isn't I will remove the conditional branch.

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PR: https://git.openjdk.java.net/jdk/pull/293