[lworld] RFR: 8371199: [lworld] Flattening of nullable elements of classes similar to j.l.Long [v4]

[Dan Smith]

On Wed, 5 Nov 2025 04:25:55 GMT, Quan Anh Mai <qamai at openjdk.org> wrote:

>> Hi,
>> 
>> Currently, the layout of a nullable `j.l.Long`, if flattened, must be at least 65 bits. This exceeds the maximum size a GPR can generally hold, as well as the default object alignment of Hotspot. As a result, accessing such elements atomically require 128-bit atomic accesses, as well as mechanism from the GC to allocate overaligned objects. And even then, we will encounter the same issue, just with larger objects.
>> 
>> We can observe that, a nullable element does not really have an additional bit of information, it just has an additional state (e.g. a nullable `j.l.Long` has `2^64 + 1` states, not `2^65`), and it is just unfortunate that we need a whole bit to be able to represent such an element.  However, we can rely on the fact that all payload bits are irrelevant when the null marker is 0 to make a sequence of more than 1 memory access instructions look atomic.
>> 
>> C1 has not been implemented yet, we will bailout the compilation when encountering such an access. I will implement this functionality later.
>> 
>> Please take a look and leave your reviews, thanks a lot.
>
> Quan Anh Mai has updated the pull request incrementally with one additional commit since the last revision:
> 
>   missing barrier

FYI, we explored some ideas along these lines a couple of years ago. Ended up concluding that it was a dead end—I'd suggest discussing on valhalla-dev at openjdk.org if you'd like to understand what the concerns were.

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

PR Comment: https://git.openjdk.org/valhalla/pull/1720#issuecomment-3492524355