About flattening a nullable j.l.Long

[Quân Anh Mai]

Hi,

I have been agonizing over this issue for the past few days. In the end, I
may have come up with a way to make flattening work. In exchange, we need
to lock the field so the stores are serialized. My aim is that the loads
keep being lock-free, and for the stores, the supposedly common case of
storing a non-null into a non-null does not require locking. At the moment,
I am starting to implement a prototype and doing some benchmarking to see
if locking would be prohibitively expensive. My proposal and reasoning can
be found in the description of the JBS issue
https://bugs.openjdk.org/browse/JDK-8371199. Please take a look and let me
know what you think.

Cheers,
Quan Anh

On Sat, 8 Nov 2025 at 00:06, Quân Anh Mai <anhmdq at gmail.com> wrote:

> Hi,
>
> Thanks a lot for your answer. After thinking about it for a while, I think
> it is true that this approach will not work. While it is true that a field
> load must act as if it observes 1 of the concurrent stores, I forgot to
> take into consideration the possibility that such store may be rendered
> invalid due to a store of null happens after it. It seems that for
> atomicity to hold, the store must be atomic, and there is no way around
> that. How unfortunate!
>
> Regards,
> Quan Anh
>
>
> On Thu, 6 Nov 2025 at 20:02, Remi Forax <forax at univ-mlv.fr> wrote:
>
>> Hello,
>> as far as i remember, the problem is that you can resurrect a value,
>> for example with a Long
>>
>> Thread1
>>   a.x = new Long(12);
>>   a.x = null;
>>   IO.println(a.x); //12
>>
>> Thread2
>>   IO.println(a.x);  // null
>>   a.x = new Long(77);   // descheduled in between writing the maker and
>> writing the long
>>
>>
>> The thread1 can see the Long(12) to be resurrected, something that is not
>> possible with references.
>>
>> regards,
>> Rémi
>>
>> ------------------------------
>>
>> *From: *"Quân Anh Mai" <anhmdq at gmail.com>
>> *To: *"David Alayachew" <davidalayachew at gmail.com>
>> *Cc: *"valhalla-dev" <valhalla-dev at openjdk.org>
>> *Sent: *Thursday, November 6, 2025 3:52:41 AM
>> *Subject: *Re: About flattening a nullable j.l.Long
>>
>> Ah yes, my bad, it is https://github.com/openjdk/valhalla/pull/1720
>>
>> On Thu, 6 Nov 2025 at 07:26, David Alayachew <davidalayachew at gmail.com>
>> wrote:
>>
>>> Just a heads up, your [1] is a dead link. Can you try to put the url in
>>> again?
>>>
>>> On Wed, Nov 5, 2025, 2:06 PM Quân Anh Mai <anhmdq at gmail.com> wrote:
>>>
>>>> Hi,
>>>> Currently, I'm having a PR that allows flattening of a nullable
>>>> value class with a 64-bit payload [1]. Dan Smith told me that some ideas
>>>> along these lines were discussed a few years ago, and the consensus was
>>>> that it was a dead end. May I ask what the concerns were, and whether they
>>>> are still relevant now?
>>>>
>>>> Thanks a lot,
>>>> Quan Anh
>>>>
>>>
>>
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