RFR: 8321468: Remove StringUTF16::equals

[Vladimir Kozlov]

On Wed, 6 Dec 2023 14:20:14 GMT, Claes Redestad <redestad at openjdk.org> wrote:

> https://bugs.openjdk.org/browse/JDK-8215017 removed the only use of `StringUTF16::equals`. At the time I did some performance verification focused on x86 showing that simplifying and only using `StringLatin1::equals` was either neutral or a win. 
> 
> I repeated this experiment recently, adding some focused tests on aarch64 where the code generation actually tries to take advantage and generate slightly more efficient code for `StringUTF16::equals`: 
> https://github.com/openjdk/jdk/pull/16933#discussion_r1414118658
> 
> The indication here is that disabling use of `StringUTF16::equals` was the right choice: any effect from the low-level optimization (one less branch at the tail end) was offset by the `isLatin1()` branch and added code generation (that all gets inlined).
> 
> In a `-XX:-CompactStrings` configuration the slightly improved code generation in `StringUTF16::equals` might help, since the `isLatin1()` test and subsequent call to `StringLatin1::equals` would be DCEd. To get the best of both worlds the code in `String::equals` _could_ be sharpened so that we statically pick the best implementation based on `CompactStrings` mode (see comment below). This shows a tiny win when testing with `-XX:-CompactStrings` on M1 (up to -0.2ns/op per `String::equals`; neutral on x86). But is all this complexity worth it for a gain that will get lost in the noise on anything realistic?
> 
> This PR instead proposes removing `StringUTF16::equals` and simplifying the mechanisms to support the `StringLatin1/UTF16::equals` pair of intrinsics in hotspot.

I don't see x86 changes. Why?

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Changes requested by kvn (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/16995#pullrequestreview-1860078647