RFR: 8256245: AArch64: Implement Base64 decoding intrinsic

[Dong Bo]

On Tue, 6 Apr 2021 09:44:28 GMT, Andrew Haley <aph at openjdk.org> wrote:

> > It would be no benefits to use SIMD for this case, so the stub use no-simd instructions for MIME encoded data now.
> 
> What is the reasoning here? Sure, there can be illegal characters at the start, but what if there are not? The generic logic uses decodeBlock() even in the MIME case, because we don't know that there certainly will be illegal characters.

This code block only process 80B of the inputs.
If no illegal characters were found, the stub will use the SIMD instructions to process the rest of the inputs if the data length is large enough, i.e. >= 64B, to form up at least one SIMD round.

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PR: https://git.openjdk.java.net/jdk/pull/3228