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From: jseigh <jseigh_es00@xemaps.com>
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Subject: Re: portable proxy collector test...
Date: Fri, 6 Dec 2024 17:43:33 -0500
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On 12/6/24 16:12, Chris M. Thomasson wrote:
> On 12/6/2024 11:55 AM, Brett wrote:
>> Chris M. Thomasson <chris.m.thomasson.1@gmail.com> wrote:
>>> I am wondering if anybody can try to compile and run this C++11 code of
>>> mine for a portable word-based proxy collector, a sort of poor mans RCU,
>>> on an ARM based system? I don't have access to one. I am interested in
>>> the resulting output.
>>
>> https://godbolt.org
>>
>>> https://pastebin.com/raw/CYZ78gVj
>>> (raw text link, no ads... :^)
> [...]
> 
> It seems that all of the atomics are LDREX/STREX wrt fetch_add/sub . 
> Even with relaxed memory order. Are the LDREX/STREX similar to the LOCK 
> prefix on an x86/64?
> 
> https://godbolt.org/z/EPGYWve71
> 
> It has loops for this in the ASM code. Adding a loop in there can change 
> things from wait-free to lock-free. Humm...
> 
Which compiler did you choose.  armv8?  Try ARM64.

The newer arms have new atomics
cas and atomic fetch ops.

LDREX/STREX is the older load store reserved.
On the newer stuff it's ldxr/stxr not ldrex/strex.