RFR: 8367656: Refactor Constantpool's operand array into two [v8]

[David Holmes]

On Thu, 25 Sep 2025 12:23:11 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:
> 
>   Serguei's review

Updates look good. A couple of minor nits spotted.

src/hotspot/share/oops/constantPool.cpp line 761:

> 759:   if (!(which >= 0 && which < cpool->resolved_method_entries_length())) {
> 760:     // FIXME: should be an assert
> 761:     log_debug(class, resolve)("bad bsm %d in:", which); cpool->print();

Suggestion:

    log_debug(class, resolve)("bad BSM %d in:", which); cpool->print();

src/hotspot/share/oops/constantPool.cpp line 2303:

> 2301:   st->print("constant pool [%d]", length());
> 2302:   if (has_preresolution()) st->print("/preresolution");
> 2303:   if (!bsm_entries().is_empty())  st->print("/bsms[%d]", bsm_entries().bootstrap_methods()->length());

Suggestion:

  if (!bsm_entries().is_empty())  st->print("/BSMs[%d]", bsm_entries().bootstrap_methods()->length());

src/hotspot/share/oops/constantPool.cpp line 2358:

> 2356: BSMAttributeEntries::start_extension(const BSMAttributeEntries& other, ClassLoaderData* loader_data, TRAPS) {
> 2357:   InsertionIterator iter = start_extension(other.number_of_entries(), other.array_length(),
> 2358:                                             loader_data, CHECK_(BSMAttributeEntries::InsertionIterator()));

Suggestion:

  InsertionIterator iter = start_extension(other.number_of_entries(), other.array_length(),
                                           loader_data, CHECK_(BSMAttributeEntries::InsertionIterator()));

-------------

PR Review: https://git.openjdk.org/jdk/pull/27198#pullrequestreview-3277861284
PR Review Comment: https://git.openjdk.org/jdk/pull/27198#discussion_r2386675664
PR Review Comment: https://git.openjdk.org/jdk/pull/27198#discussion_r2386677067
PR Review Comment: https://git.openjdk.org/jdk/pull/27198#discussion_r2386677720