Re: Strange asm generated by GCC...

On 12/19/2024 4:43 PM, Chris M. Thomasson wrote:

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
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#include <atomic>
 int main()
{
      std::atomic<unsigned long> m_state = 0;
      m_state.store(std::memory_order_relaxed);
      return 0;
}
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[...]
Ahhh! Actually, I am not sure what that even compiled! Now, this is much better:
https://godbolt.org/z/d46q9dd4h
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#include <atomic>
static std::atomic<unsigned long> m_state = 0;
int main()
{
     m_state.store(1, std::memory_order_relaxed);
     return 0;
}
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and I get:
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main:
         mov     QWORD PTR m_state[rip], 1
         xor     eax, eax
         ret
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Okay. This makes much more sense.