RFR: 8309254: Implement fast-path for ASCII-compatible CharsetEncoders on RISC-V

[Ludovic Henry]

On Thu, 1 Jun 2023 05:45:14 GMT, Dingli Zhang <dzhang at openjdk.org> wrote:

> [JDK-8274242](https://bugs.openjdk.org/browse/JDK-8274242) propose to extend the x86 ISO-8859-1 encoding intrinsic to work 
> also for ASCII-compatible encodings, which helps speeding up various 
> CharsetEncoders. Implementing a similar intrinsic should be considered on 
> RISC-V as well.
> 
> The instruct log with -XX:+PrintOptoAssembly output looks like:
> 
> 06e +   Encode ISO array R12, R11, R13 -> R10 # KILL R12, R11, R13, R7, V0-V3
> 
> 
> ## Testing:
> qemu w/ UseRVV:
> - [x] Tier1 tests (release)
> - [x] Tier2 tests (release)
> - [x] Tier3 tests (release)
> - [x] test/hotspot/jtreg/compiler/intrinsics/string/TestEncodeIntrinsics.java

Marked as reviewed by luhenry (Committer).

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PR Review: https://git.openjdk.org/jdk/pull/14256#pullrequestreview-1454739659