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Re: std::atomic work as volatile, or not, I think not...

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On 5/11/2026 8:48 AM, Scott Lurndal wrote:
> David Brown <david.brown@hesbynett.no> writes:
>> On 11/05/2026 12:59, boltar@caprica.universe wrote:
> 
>>>>
>>>> Atomics are for shared information /without/ needing semaphores,
>>>
>>> The underlying implementation will be the same.
> 
> That depends entirely on the instruction set of the target
> processor.  Most of the modern architectures include
> instructions to atomically modify a location in memory
> (e.g. Fetch-and-add).   This is far more efficient
> than attempting to acquire a spinlock or posix mutex
> simply to update a single location in memory,
> particularly when there are a significant number
> of contenders (e.g. on a 128-core CPU).
> 
> This is essentially free in systems with
> processor memory caching, since only one
> processor (hardware thread) can own a cache line for write
> access at any point in time.  The processor
> simply sends the atomic op directly to the cache
> (or if caches are bypassed, the memory controller)
> for execution.
> 
> Those operations have been extended such that
> modern hardware supports that level of atomicity
> across PCI express and other interconnect
> technologies.
> 
> The advantage of the cache coherency algorithm
> efficiency vs LL/SC or LDEX/STREX is very signficiant.
> 
> ARM, for example, has atomic:
> 
>   - load and add (LDADD)
>   - load and exclusive or (LDEOR)
>   - load and clear bit (LDCLR)
>   - load and set bit (LDSET)
>   - load and return minimum value
>   - load and return maximum value

Is neat that ARM supports LL/SC _and_ the atomic RMW's.

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std::atomic work as volatile, or not, I think not... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2026-05-10 21:51 -0700
  Re: std::atomic work as volatile, or not, I think not... David Brown <david.brown@hesbynett.no> - 2026-05-11 08:25 +0200
    Re: std::atomic work as volatile, or not, I think not... boltar@caprica.universe - 2026-05-11 08:28 +0000
      Re: std::atomic work as volatile, or not, I think not... David Brown <david.brown@hesbynett.no> - 2026-05-11 11:02 +0200
        Re: std::atomic work as volatile, or not, I think not... boltar@caprica.universe - 2026-05-11 10:59 +0000
          Re: std::atomic work as volatile, or not, I think not... David Brown <david.brown@hesbynett.no> - 2026-05-11 13:20 +0200
            Re: std::atomic work as volatile, or not, I think not... Bonita Montero <Bonita.Montero@gmail.com> - 2026-05-11 13:22 +0200
              Re: std::atomic work as volatile, or not, I think not... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2026-05-11 13:00 -0700
            Re: std::atomic work as volatile, or not, I think not... boltar@caprica.universe - 2026-05-11 15:08 +0000
              Re: std::atomic work as volatile, or not, I think not... David Brown <david.brown@hesbynett.no> - 2026-05-11 22:19 +0200
                Re: std::atomic work as volatile, or not, I think not... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2026-05-11 13:25 -0700
            Re: std::atomic work as volatile, or not, I think not... scott@slp53.sl.home (Scott Lurndal) - 2026-05-11 15:48 +0000
              Re: std::atomic work as volatile, or not, I think not... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2026-05-11 13:09 -0700
          Re: std::atomic work as volatile, or not, I think not... Bonita Montero <Bonita.Montero@gmail.com> - 2026-05-11 13:21 +0200
          Re: std::atomic work as volatile, or not, I think not... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2026-05-11 12:48 -0700
            Re: std::atomic work as volatile, or not, I think not... red floyd <no.spam.here@its.invalid> - 2026-05-11 21:04 -0700
              Re: std::atomic work as volatile, or not, I think not... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2026-05-14 00:35 -0700
        Re: std::atomic work as volatile, or not, I think not... cross@spitfire.i.gajendra.net (Dan Cross) - 2026-05-11 11:53 +0000
          Re: std::atomic work as volatile, or not, I think not... David Brown <david.brown@hesbynett.no> - 2026-05-11 15:18 +0200
            Re: std::atomic work as volatile, or not, I think not... Bonita Montero <Bonita.Montero@gmail.com> - 2026-05-11 15:20 +0200
              Re: std::atomic work as volatile, or not, I think not... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2026-05-11 12:54 -0700
  Re: std::atomic work as volatile, or not, I think not... "Fred. Zwarts" <F.Zwarts@HetNet.nl> - 2026-05-11 11:46 +0200
    Re: std::atomic work as volatile, or not, I think not... David Brown <david.brown@hesbynett.no> - 2026-05-11 12:40 +0200
      Re: std::atomic work as volatile, or not, I think not... Chris Ahlstrom <OFeem1987@teleworm.us> - 2026-05-11 06:55 -0400
        Re: std::atomic work as volatile, or not, I think not... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2026-05-12 12:14 -0700
    Re: std::atomic work as volatile, or not, I think not... scott@slp53.sl.home (Scott Lurndal) - 2026-05-11 15:49 +0000
      Re: std::atomic work as volatile, or not, I think not... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2026-05-11 12:36 -0700

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