RFR: 8336856: Optimize String Concat [v10]

[Claes Redestad]

On Wed, 24 Jul 2024 10:18:03 GMT, Shaojin Wen <duke at openjdk.org> wrote:

>> The current implementation of StringConcat is to mix the coder and length into a long. This operation will have some overhead for int/long/boolean types. We can separate the calculation of the coder from the calculation of the length, which can improve the performance in the scenario of concat int/long/boolean.
>> 
>> This idea comes from the suggestion of @l4es in the discussion of PR https://github.com/openjdk/jdk/pull/20253#issuecomment-2240412866
>
> Shaojin Wen has updated the pull request incrementally with ten additional commits since the last revision:
> 
>  - minor refactor
>  - minor refactor
>  - reduce change
>  - copyright
>  - reduce change
>  - refactor based on 8335182
>  - use Float.toString & Double.toString
>  - remove ForceInline
>  - fix build error

This is looking much better!

An idea: evolve this PR to first and foremost be a replacement for the current `SimpleStringBuilderStrategy`. If you can prove the new strategy is always superior to the baseline `SimpleStringBuilderStrategy` (performance close to `generateMHInlineCopy` but without the problematic scaling JIT overheads for high-arity expressions) then we have a good, incremental improvement with relatively low risk.

We can then follow-up with making this better for low-arity expressions by implementing a sharing strategy
(need to build classes where we inject rather than embed constants and cache them, e.g. in a `ReferencedKeyMap<MethodType, Function<String[], MethodHandle>`) as a follow-up. 

One issue for both this and that future PR is that the hidden classes are generated into `java.lang` with `Lookup.ClassOption.STRONG`. This means such classes effectively can never be unloaded. That's a blocker. We need to get rid of that `STRONG` coupling, add tests to ensure generated classes can be unloaded and validate that this doesn't add too much overhead. 

For startup verification I added a `StringConcatStartup` JMH in #19927. You could extend that with a few tests that exercise high-arity expressions.

src/java.base/share/classes/java/lang/invoke/StringConcatFactory.java line 153:

> 151:     @SuppressWarnings("removal")
> 152:     private static boolean isGenerateInlineCopy() {
> 153:         return GENERATE_INLINE_COPY && System.getSecurityManager() == null;

This security manager hack is unfortunate. Likely some bootstrapping issue due use of the classfile API, which might be pulling in lambdas in the paths travelled by this patch.

src/java.base/share/classes/java/lang/invoke/StringConcatFactory.java line 1253:

> 1251:                     }
> 1252: 
> 1253:                     int lengthCoderSloat = paramSlotsTotalSize;

Suggestion:

                    int lengthCoderSlot = paramSlotsTotalSize;

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PR Review: https://git.openjdk.org/jdk/pull/20273#pullrequestreview-2196340289
PR Review Comment: https://git.openjdk.org/jdk/pull/20273#discussion_r1689583138
PR Review Comment: https://git.openjdk.org/jdk/pull/20273#discussion_r1689578966