Re: Memory ordering

On 11/16/2024 1:21 PM, Chris M. Thomasson wrote:

On 11/15/2024 12:42 PM, Chris M. Thomasson wrote:

On 11/15/2024 5:24 AM, Michael S wrote:

On Fri, 15 Nov 2024 03:17:22 -0800
"Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> wrote:
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On 11/14/2024 11:25 PM, Anton Ertl wrote:

aph@littlepinkcloud.invalid writes:

Yes. That Alpha behaviour was a historic error. No one wants to do
that again.

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Was it an actual behaviour of any Alpha for public sale, or was it
just the Alpha specification?  I certainly think that Alpha's lack
of guarantees in memory ordering is a bad idea, and so is ARM's:
"It's only 32 pages" <YfxXO.384093$EEm7.56154@fx16.iad>.  Seriously?
Sequential consistency can be specified in one sentence: "The result
of any execution is the same as if the operations of all the
processors were executed in some sequential order, and the
operations of each individual processor appear in this sequence in
the order specified by its program."

[...]
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Well, iirc, the Alpha is the only system that requires an explicit
membar for a RCU based algorithm. Even SPARC in RMO mode does not
need this. Iirc, akin to memory_order_consume in C++:
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https://en.cppreference.com/w/cpp/atomic/memory_order
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data dependent loads
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You response does not answer Anton's question.
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I guess not. Shit happens. ;^o

 Fwiw, in C++ std::memory_order_consume is useful for traversing a node based stack of something in RCU. In most systems it only acts like a compiler barrier. On the Alpha, it must emit a membar instruction. Iirc, mb for alpha? Cannot remember that one right now.

I think, iirc, there is a way to use an acquire membar on the loading of the initial node of a collection to iterate it without using memory_order_consume for every node. I might be wrong on that. It's been a while!