RFR: 8274242: Implement fast-path for ASCII-compatible CharsetEncoders on x86

[Sandhya Viswanathan]

On Tue, 21 Sep 2021 21:58:48 GMT, Claes Redestad <redestad at openjdk.org> wrote:

> This patch extends the `ISO_8859_1.implEncodeISOArray` intrinsic on x86 to work also for ASCII encoding, which makes for example the `UTF_8$Encoder` perform on par with (or outperform) similarly getting charset encoded bytes from a String. The former took a small performance hit in JDK 9, and the latter improved greatly in the same release.
> 
> Extending the `EncodeIsoArray` intrinsics on other platforms should be possible, but I'm unfamiliar with the macro assembler in general and unlike the x86 intrinsic they don't use a simple vectorized mask to implement the latin-1 check. For example aarch64 seem to filter out the low bytes and then check if there's any bits set in the high bytes. Clever, but very different to the 0xFF80 2-byte mask that an ASCII test wants.

x86 part of changes look good.

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PR: https://git.openjdk.java.net/jdk/pull/5621