RFR: 8256431: [PPC64] Implement Base64 encodeBlock() for Power64-LE [v4]

[Corey Ashford]

On Wed, 16 Dec 2020 00:20:29 GMT, Corey Ashford <github.com+51754783+CoreyAshford at openjdk.org> wrote:

>> It's your choice. I'm also fine with your current version.
>> Btw. I had also thought a bit about the division by constants. It's only used outside the loop. Otherwise I'd have suggested to use mulhd (e.g. positive long can get divided by 3 by mulhd with 0x5555555555555556, division by 12 by mulhd with 0x2aaaaaaaaaaaaaab + right shift by one). Such fast code gets generated by C2 compiler.
>
>> Btw. I had also thought a bit about the division by constants. It's only used outside the loop. Otherwise I'd have suggested to use mulhd (e.g. positive long can get divided by 3 by mulhd with 0x5555555555555556, division by 12 by mulhd with 0x2aaaaaaaaaaaaaab + right shift by one). Such fast code gets generated by C2 compiler.
> 
> That's very interesting.  I'm not clear how that math works, but I tried it out in C using  the__int128 type, and it works as advertised.  However, since the division takes place outside the loop, it's hard to justify putting something this obtuse in the code.  Thanks for the thought, though.  I'll do some google searches to find out why this works for my own education.
> 
> I will put the struct change in tomorrow.  I think that's much cleaner.

Ah, I see now.  It's basically multiplying by the binary-point reciprocal, and then accounting for the number of number of binary point digits, which in this case effectively means take the answer from the top 64 bits of the 128-bit result.

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PR: https://git.openjdk.java.net/jdk/pull/1245