64 bit Value Limitation

[David Alayachew]

To my understanding, they just haven't gotten to that part yet.

Multiple folks on the OpenJDK team (especially Brian Goetz) have expressed
desire to put that in ASAP, with the caveat that ASAP is going to take
time. Considering that they are just now providing EA Access to JEP 401.

On Sun, Nov 2, 2025, 10:53 PM Danny Thomas <dannyt at netflix.com> wrote:

> Hi folks,
>
> I caught Frederic's excellent JVMLS talk over the weekend and was most
> interested in the updates on tearing, and appreciate the pragmatism of not
> allowing tearing of flattened values.
>
> At 13:15[1] it's mentioned that there's no flattening for value types
> larger than 64 bits, as most platforms don't have support for larger atomic
> operations. My understanding is that both Intel and AMD have guaranteed the
> atomicity of 128 bit SSE loads and stores on CPUs with AVX[2] and aarch64
> has had support for 128 bit atomic instructions for some time (and without
> any performance limitations w/ LSE).
>
> JEP 401 only has this to say:
>
> In the future, 128-bit flattened references may be possible on platforms
> that support atomic reads and writes of that size, or in special cases like
> final fields.
>
> Is there a misunderstanding of the state of platform support for 128-bit
> atomics or have I missed details omitted for brevity?
>
> Cheers,
> Danny
>
> 1. https://youtu.be/NF4CpL_EWFI?t=795
> 2. https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104688
>
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