RFR: 8357289: Break down the String constructor into smaller methods [v8]

[Shaojin Wen]

On Wed, 25 Jun 2025 00:13:56 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:
> 
>   code format, from @RogerRiggs

Scalar  293  CheckCastPP  === 290 288  [[ 430 367 367 430 357 357 ]]  #java/lang/String (java/io/Serializable,java/lang/Comparable,java/lang/CharSequence,java/lang/constant/Constable,java/lang/constant/ConstantDesc):NotNull:exact *,iid=276  Oop:java/lang/String (java/io/Serializable,java/lang/Comparable,java/lang/CharSequence,java/lang/constant/Constable,java/lang/constant/ConstantDesc):NotNull:exact *,iid=276 !jvms: String::iso88591 @ bci:6 (line 624) String::<init> @ bci:42 (line 562) String::<init> @ bci:12 (line 1463) StringConstructor::newStringFromBytesCharSet @ bci:12 (line 19) StringConstructorTest::newStringFromBytesCharSet @ bci:25 (line 27)
++++ Eliminated: 276 Allocate

By configuring the JVM startup parameters `-XX:+UnlockDiagnosticVMOptions -XX:+PrintEliminateAllocations` to print the log, the allocated String temporary object escapes successfully

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PR Comment: https://git.openjdk.org/jdk/pull/25290#issuecomment-3011550441