Memory Corruption in JDK-21 on x86_64 with AVX-512

[Carter Kozak]

Hello,

In an attempt to use JDK-21, we have discovered what appears to be a memory corruption issue where a byte-array is missing data in the middle (unexpectedly filled with zeros). This only occurs when AVX-512 is enabled (`-XX:UseAVX=3`), and cannot be reproduced with `-XX:UseAVX=2`.

I have pushed a reproducer here: https://github.com/carterkozak/avx512-jit-reproducer

This impacts generally available releases of JDK-20 and JDK-21. I have bisected the failure to this change, which I wouldn’t expect to be problematic, yet does seem to trigger the failure reliably: https://github.com/openjdk/jdk/pull/9037.

While I have some experience replaying compiler failures, this bug doesn’t cause a crash or otherwise trip assertions in `fastdebug` or `slowdebug` builds that would give me a good place to start. I haven’t been able to find a great deal of literature on the best practices for constructing a reproducer/reporting the problem in detail — it took me more time than I’d like to admit to build what I have, and I’d be grateful for pointers that might help narrow things down next time.

Best,
Carter Kozak
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