[Bug target/95750] [x86] Use dummy atomic insn instead of mfence in __atomic_thread_fence(seq_cst)

[josephcsible at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95750

Joseph C. Sible <josephcsible at gmail dot com> changed:

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                 CC|                            |josephcsible at gmail dot com

--- Comment #14 from Joseph C. Sible <josephcsible at gmail dot com> ---
I notice this change affects -Os too, even though "lock orq $0,(%rsp)" is 6
bytes and "mfence" is only 3 bytes.