64 bit Value Limitation

[Danny Thomas]

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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