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From: jseigh <jseigh_es00@xemaps.com>
Newsgroups: comp.arch
Subject: Re: Strange asm generated by GCC...
Date: Fri, 20 Dec 2024 15:17:19 -0500
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On 12/20/24 14:59, jseigh wrote:
> On 12/20/24 12:46, jseigh wrote:
>> On 12/19/24 19:43, 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!
>>>
>>
>> Speaking of strange code
>>
> That should be a ref paramter.  I though I updated the pasted code.
> 
> #include <atomic>
> 
> bool test1(std::atomic<int>& var, int addend)
> {
>      int expected = var.load(std::memory_order_relaxed);
>      int update = expected + addend;
>      return var.compare_exchange_weak(expected, update, 
> std::memory_order_acq_rel, std::memory_order_seq_cst);
> }

Alright, my bad.  I should have double checked the docs.
It's undefined behavior in this case.

It seems like the success/failure form of compare_exchange
is redundant unless you want it for self documentation.

Joe Seigh