RFR: 8287522: StringConcatFactory: Add in prependers and mixers in batches [v4]

[Mandy Chung]

On Wed, 1 Jun 2022 13:31:27 GMT, Claes Redestad <redestad at openjdk.org> wrote:

>> When generating `MethodHandle`-based concatenation expressions in `StringConcatFactory` we can reduce the number of classes generated at runtime by creating small batches of prependers and mixers before binding them into the root expression tree. 
>> 
>> Improvements on one-off tests are modest, while the improvement on bootstrapping stress tests can be substantial ([MixedStringCombinations.java](https://gist.github.com/cl4es/08fb581dece3a73e89bfa52337bc4248)):
>> 
>> | Build      |    # classes    |   Runtime   |
>> | ----------- | ----------------- |  --------------- |
>> | Baseline |     31119       |     2.942s     |
>> | Patch      |     16208       |     1.958s     |
>> 
>> An earlier version of this patch saw a regression in the `StringConcatFactoryBootstraps` microbenchmark. After some refactoring along with the optimizations in #8881 and #8923 that is no longer the case, and allocation pressure is down slightly compared to the baseline on such a repeat-the-same-bootstrap stress test:
>> 
>> Baseline:
>> 
>> Benchmark                                                      Mode  Cnt     Score      Error   Units
>> SCFB.makeConcatWithConstants                                   avgt    5  2170.039 ?  117.441   ns/op
>> SCFB.makeConcatWithConstants:?gc.alloc.rate.norm               avgt    5  3538.020 ?    4.558    B/op
>> 
>> This patch:
>> 
>> Benchmark                                                      Mode  Cnt     Score      Error   Units
>> SCFB.makeConcatWithConstants                                   avgt    5  2144.807 ?  162.685   ns/op
>> SCFB.makeConcatWithConstants:?gc.alloc.rate.norm               avgt    5  3184.943 ?    4.495    B/op
>
> Claes Redestad has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Address review comments from @ExE-Boss

src/java.base/share/classes/java/lang/invoke/StringConcatFactory.java line 459:

> 457:             String prefix = constants.get(0);
> 458:             if (prefix == null) {
> 459:                 if (suffix == null) {

Is it possible for prefix or suffix an empty string (non-null)?   The original code checks `if (constant.isEmpty())`

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PR: https://git.openjdk.java.net/jdk/pull/8855