[Issue 20838] on modern (x86_64) CPUs, dmd emit cmpxchg8b instead of CMPXCHG16B

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20838

--- Comment #1 from mw <mingwu at gmail.com> ---
BTW, I only tested on x86_64 Linux, I think this bug on other platform too
MacOS, Windows (on modern x86_64 CPUs with CX16 support).

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