RFR: 8367656: Refactor Constantpool's operand array into two [v3]

[David Holmes]

On Tue, 16 Sep 2025 10:11:09 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 one additional commit since the last revision:
> 
>   Fix type name

Updates look good - thanks. A couple of follow on nits and comments.

Still need to take full second pass through the code.

src/hotspot/share/oops/constantPool.cpp line 1830:

> 1828: bool ConstantPool::compare_bootstrap_entry_to(int idx1, const constantPoolHandle& cp2, int idx2) {
> 1829:   const BSMAttributeEntry* const bsmae1 = bsm_attribute_entry(idx1);
> 1830:   const BSMAttributeEntry* const bsmae2 = cp2->bsm_attribute_entry(idx2);

Suggestion:

  const BSMAttributeEntry* const e1 = bsm_attribute_entry(idx1);
  const BSMAttributeEntry* const e2 = cp2->bsm_attribute_entry(idx2);

Simple names can be used throughout.

src/hotspot/share/prims/jvmtiRedefineClasses.hpp line 371:

> 369:   intArray *                  _bsm_index_map_p;
> 370: 
> 371:   // After merge_constant_pools pass 0, the BSMAttribute entries of merge_cp_p will have been expanded to fit

Suggestion:

  // After merge_constant_pools "Pass 0", the BSMAttribute entries of merge_cp_p will have been expanded to fit

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PR Review: https://git.openjdk.org/jdk/pull/27198#pullrequestreview-3232321618
PR Review Comment: https://git.openjdk.org/jdk/pull/27198#discussion_r2354096048
PR Review Comment: https://git.openjdk.org/jdk/pull/27198#discussion_r2354103465