RFR: 8340710: Optimize DirectClassBuilder::build

[Shaojin Wen]

On Sun, 22 Sep 2024 20:48:08 GMT, Chen Liang <liach at openjdk.org> wrote:

>> src/java.base/share/classes/jdk/internal/classfile/impl/DirectClassBuilder.java line 207:
>> 
>>> 205: 
>>> 206:         // Now we can make the head
>>> 207:         head.writeLong(((long) ClassFile.MAGIC_NUMBER) << 32 | minorVersion << 16 | majorVersion);
>> 
>> `minorVersion` needs to be cast to a `long` first, as otherwise when the MSB is set after the shift, then it’ll overwrite the magic number with all 1s.
>> 
>> Suggestion:
>> 
>>         head.writeLong(((long) ClassFile.MAGIC_NUMBER) << 32 | ((long) minorVersion) << 16 | majorVersion);
>
> For clarity, I think using Integer.toUnsignedLong on the shift result is better

If minorVersion > 0xFFFF, the result of using Integer.toUnsignedLong will be wrong. I guess this is the reason why the previous version build failed

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PR Review Comment: https://git.openjdk.org/jdk/pull/21118#discussion_r1770667423