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Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com>
Newsgroups: comp.arch
Subject: Re: Strange asm generated by GCC...
Date: Fri, 20 Dec 2024 12:12:46 -0800
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On 12/20/2024 3:55 AM, jseigh wrote:
> On 12/19/24 20:02, Chris M. Thomasson wrote:
>> 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
>>> _______________________
>>> #include <atomic>
>>>
>>> int main()
>>> {
>>>
>>>      std::atomic<unsigned long> m_state = 0;
>>>
>>>      m_state.store(std::memory_order_relaxed);
>> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>
>> Strange to me that this even compiled at all. I clearly forgot to put 
>> in a value. aka:
>>
>> m_state.store(1, std::memory_order_release);
>>
>> Sorry about that. ;^o
> 
> The 2nd parameter has a default value, std::memory_order_cst.
> Your 1st argument was a std::memory_order value which is an
> integer value.  So basically
> 
>    m.state.store(std::memory_order_relaxed, std::memory_order_cst);
> 

https://en.cppreference.com/w/cpp/atomic/atomic/store

The first param is the value we want to store. The second one is memory 
order.