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

[Chen Liang]

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

src/java.base/share/classes/java/lang/String.java line 569:

> 567:         }
> 568:         this.value = str.value;
> 569:         this.coder = str.coder;

Suggestion:

        this(str);

We have integrated flexible constructor bodies.

src/java.base/share/classes/java/lang/String.java line 646:

> 644:     }
> 645: 
> 646:     private static String create(Charset charset, byte[] bytes, int offset, int length) {

Should we call this "decode", as "create" feels too generic?

src/java.base/share/classes/java/lang/String.java line 703:

> 701:                 throw new Error(x);
> 702:             }
> 703:             if (COMPACT_STRINGS) {

Now we can call other constructors of String, so instead of having this manual handling of char[], we can call `return new String(ca, 0, caLen, null)`. Same for the `clen` version above.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25290#discussion_r2106082898
PR Review Comment: https://git.openjdk.org/jdk/pull/25290#discussion_r2106082727
PR Review Comment: https://git.openjdk.org/jdk/pull/25290#discussion_r2106082563