RFR: 8357289: Break down the String constructor into smaller methods [v3]

[Per Minborg]

On Tue, 20 May 2025 02:11:34 GMT, Shaojin Wen <swen at openjdk.org> wrote:

>> Through JVM Option +PrintInlining, we found that String has a constructor codeSize of 852, which is too large. This caused failed to inline.
>> 
>> The following is the output information of PrintInlining:
>> 
>>                 @ 9   java.lang.String::<init> (12 bytes)   inline (hot)
>> !m                 @ 1   java.nio.charset.Charset::defaultCharset (52 bytes)   inline (hot)
>> !                  @ 8   java.lang.String::<init> (852 bytes)   failed to inline: hot method too big
>> 
>> 
>> In Java code, the big method that cannot be inlined is the following constructor
>> 
>> 
>> String(Charset charset, byte[] bytes, int offset, int length) {}
>> 
>> The above String constructor is too large; break it down into smaller methods with a codeSize under 325 to allow them to be inlined by the C2.
>
> Shaojin Wen has updated the pull request incrementally with one additional commit since the last revision:
> 
>   create method share variant val & coder

I wonder if it would be better to first check `COMPACT_STRINGS` in a first-level `if` and then branch off to separate support methods? Looking at the comments near the declaration of `COMPACT_STRINGS`, this might provide additional benefits.

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PR Comment: https://git.openjdk.org/jdk/pull/25290#issuecomment-2893426860