Path: ...!eternal-september.org!feeder2.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: "Chris M. Thomasson" Newsgroups: comp.arch Subject: Re: Arm ldaxr / stxr loop question Date: Tue, 12 Nov 2024 17:50:11 -0800 Organization: A noiseless patient Spider Lines: 27 Message-ID: References: <3lGdnVvGQIAq2676nZ2dnZfqnPGdnZ2d@supernews.com> <0TmdnaP6ecXoQ676nZ2dnZfqn_adnZ2d@supernews.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Injection-Date: Wed, 13 Nov 2024 02:50:11 +0100 (CET) Injection-Info: dont-email.me; posting-host="6a1ff3e9645d38e0c84409251fc3c409"; logging-data="1958449"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX18NwedVdAibAKKOx1ymnuKkfrDQtrTAFRQ=" User-Agent: Mozilla Thunderbird Cancel-Lock: sha1:OtrYytS46JyqqpmmzAL/1e9sKZg= In-Reply-To: <0TmdnaP6ecXoQ676nZ2dnZfqn_adnZ2d@supernews.com> Content-Language: en-US Bytes: 2805 On 11/12/2024 3:02 PM, aph@littlepinkcloud.invalid wrote: > Chris M. Thomasson 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. Iirc, a sequential membar was strange on SPARC. I have seen things like this before wrt RMO mode: membar #StoreLoad | #LoadStore | #LoadLoad | #StoreStore shit. It's been a while! damn.