RFR: 8367656: Refactor Constantpool's operand array into two [v7]

[Serguei Spitsyn]

On Mon, 22 Sep 2025 08:02:57 GMT, Johan Sjölen <jsjolen at openjdk.org> wrote:

>> Hi,
>> 
>> This is a refactoring of the way that we store the Bootstrap method attribute in the ConstantPool class. We used to have a single `Array<u2>` which was divided into a section of `u4` offsets and a section which was the actual data. In this refactoring we make this split more clear, by actually allocating an `Array<u4>` to store the offsets in and an `Array<u2>` to store the data in. These arrays are then put into a `BSMAttributeEntries` class, which allows us to separate out the API from that of the rest of the `ConstantPool`.
>> 
>> We had multiple instances of the code knowing the layout of the operands array and using this to do 'clever' ways of copying and inserting data into it. See `ConstantPool::copy_operands` and `ConstantPool::resize_operands`. I felt like we could do things in a simpler way, so I added the `start_/end_extension` protocol and added the `InsertionIterator` for this. See `ClassFileParser::parse_classfile_bootstrap_methods_attribute` for how this works. I put several relevant definitions into the inline file in hopes of encouraging the compiler to optimize these appropriately.
>> 
>> For the Java SA code, I had to add a `U4Array` class. I also had to fix the vmstructs definitions, etc.
>> 
>> On the whole, while this code is a bit less terse, I think it's a good API improvement and the underlying implementation of splitting up the operands array is also an improvement.
>> 
>> Testing: Oracle Tier1-Tier5 has been run succesfully multiple times. Before integration, I will merge with master and run these tiers again.
>
> Johan Sjölen has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Merge remote-tracking branch 'origin/operands-again' into operands-again
>  - Fix BSM naming

src/hotspot/share/prims/jvmtiClassFileReconstituter.cpp line 393:

> 391: void JvmtiClassFileReconstituter::write_bootstrapmethod_attribute() {
> 392:   write_attribute_name_index("BootstrapMethods");
> 393:   u4 length = sizeof(u2) + // Size of num_bootstrap_methods

Nit: The comment update is confusing. Should it remain as it was? :

   u4 length = sizeof(u2); // num_bootstrap_methods

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PR Review Comment: https://git.openjdk.org/jdk/pull/27198#discussion_r2371115648