RFR: 8370527: Memory leak after 8316694: Implement relocation of nmethod within CodeCache [v2]

[Leonid Mesnik]

On Wed, 29 Oct 2025 00:49:06 GMT, Chad Rakoczy <duke at openjdk.org> wrote:

>> Ah, so _there_! I confused myself. This one is readable: the counter `0` means we can free. It would be even better if you did `inc_immutable_data_refcount()` and `dec_immutable_data_refcount()`, and did e.g.:
>> 
>> 
>> if (dec_immutable_data_refcount() == 0) {
>>   os::free(_immutable_data);
>> }
>> 
>> int dec_immutable_data_refcount() {
>>   int refcount = get(...);
>>   assert(refcount > 0, "Must be positive");
>>   set(refcount - 1);
>>   return refcount - 1;
>> }
>> 
>> 
>> Because the next thing you know this would need to be replaced with Atomics a year later.
>
>> Ah, so _there_! I confused myself. This one is readable: the counter `0` means we can free. It would be even better if you did `inc_immutable_data_refcount()` and `dec_immutable_data_refcount()`, and did e.g.:
>> 
>> ```
>> if (dec_immutable_data_refcount() == 0) {
>>   os::free(_immutable_data);
>> }
>> 
>> int dec_immutable_data_refcount() {
>>   int refcount = get(...);
>>   assert(refcount > 0, "Must be positive");
>>   set(refcount - 1);
>>   return refcount - 1;
>> }
>> ```
>> 
>> Because the next thing you know this would need to be replaced with Atomics a year later.
> 
> I agree this makes the code cleaner.
> 
> I replaced the getter and setter for the counter with `init_immutable_data_ref_count`, `inc_immutable_data_ref_count`, and `dec_immutable_data_ref_count`. I also shortened the counter name from `immutable_data_references_counter` to `immutable_data_ref_count`
> 
> I modified `NMethod.java` to calculate the offsets that same way as is done in the JVM. I missed this in [JDK-8369642](https://bugs.openjdk.org/browse/JDK-8369642)
> 
> The last notable change is that I modified the [immutable data size calculation](https://github.com/chadrako/jdk/blob/26bdc3ceb4ab9ad9cb9a4218bb87ce2d7546fa22/src/hotspot/share/code/nmethod.cpp#L1155) to only include a reference counter if there is immutable data

@chadrako The testing pass now.

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

PR Comment: https://git.openjdk.org/jdk/pull/28008#issuecomment-3462037452