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From: "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com>
Newsgroups: comp.arch
Subject: Strange asm generated by GCC...
Date: Thu, 19 Dec 2024 16:43:46 -0800
Organization: A noiseless patient Spider
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Why in the world would GCC use an XCHG instruction for the following 
code. The damn XCHG has an implied LOCK prefix! Yikes!

https://godbolt.org/z/Thxchdcr8
_______________________
#include <atomic>

int main()
{

     std::atomic<unsigned long> m_state = 0;

     m_state.store(std::memory_order_relaxed);

     return 0;
}
_______________________


Here is the generated ASM:
_______________________
main:
         mov     QWORD PTR [rsp-8], 0
         xor     eax, eax
         xchg    rax, QWORD PTR [rsp-8]
         xor     eax, eax
         ret
_______________________


WTF!!!!!!!!