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NNTP-Posting-Date: Tue, 12 Nov 2024 23:02:12 +0000
Sender: Andrew Haley <aph@zarquon.pink>
From: aph@littlepinkcloud.invalid
Subject: Re: Arm ldaxr / stxr loop question
Newsgroups: comp.arch
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Chris M. Thomasson <chris.m.thomasson.1@gmail.com> wrote:
> On 11/12/2024 4:14 AM, aph@littlepinkcloud.invalid wrote:
>> 
>> One other thing to be aware of is that the StoreLoad barrier needed
>> for sequential consistency is logically part of an LDAR, not part of a
>> STLR. This is an optimization, because the purpose of a StoreLoad in
>> that situation is to prevent you from seeing your own stores to a
>> location before everyone else sees them.
> 
> Fwiw, even x86/x64 needs StoreLoad when an algorithm depends on a
> store followed by a load to another location to hold. LoadStore is
> not strong enough. The SMR algorithm needs that. Iirc, Peterson's
> algorithms needs it as well.

That's right, but my point about LDAR on AArch64 is that you can get
sequential consistency without needing a StoreLoad. LDAR can peek
inside the store buffer and, much of the time, determine that it isn't
necessary to do a flush. I don't know if Arm were the first to do
this, but I don't recall seeing it before. It is a brilliant idea.

Andrew.