From: Alan Stern <stern@rowland.harvard.edu>
To: Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>
Cc: paulmck@kernel.org, parri.andrea@gmail.com, will@kernel.org,
peterz@infradead.org, boqun.feng@gmail.com, npiggin@gmail.com,
dhowells@redhat.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr,
akiyks@gmail.com, dlustig@nvidia.com, joel@joelfernandes.org,
urezki@gmail.com, quic_neeraju@quicinc.com, frederic@kernel.org,
linux-kernel@vger.kernel.org, lkmm@lists.linux.dev,
hernan.poncedeleon@huaweicloud.com
Subject: Re: [RFC] tools/memory-model: Rule out OOTA
Date: Wed, 22 Jan 2025 14:11:26 -0500 [thread overview]
Message-ID: <b77902dd-d17f-4e09-ac52-35a6ab1e78e5@rowland.harvard.edu> (raw)
In-Reply-To: <a2057858-fd7c-4458-96fc-0c3f52eae5fb@huaweicloud.com>
On Wed, Jan 22, 2025 at 04:46:03AM +0100, Jonas Oberhauser wrote:
> > > > > Yes. Actually I wonder if you put this "all loads are volatile" restriction,
> > > > > can a globally analysing compiler still have any optimizations that a
> > > > > locally analysing compiler can not?
> > > >
> > > > Yes, although whether they are pertinent is open to question. For
> > > > example, a globally optimizing compiler may observe that no thread ever
> > > > reads the value of a particular shared variable and then eliminate that
> > > > variable.
> > >
> > > Oh, I meant the "all atomic objects is volatile" restriction, not just the
> > > loads. In that case, all stores the object - even if never read - still need
> > > to be generated.
> > >
> > > Are there still any optimizations?
> >
> > Perhaps not any that affect shared variable accesses. In a way, that
> > was the intention.
>
> Yes, but it becomes a bit strange then to treat the "globally analyzing
> compiler" as a harder problem. You have made a globally analyzing compiler
> that can not globally analyze. I understand that makes the argument
> feasible, but I am not sure if this is how compilers really behave (or at
> least will still behave in the future).
There's still an important difference. A globally optimizing compiler
is allowed to generate different object code for a thread (containing
the same source code) in different programs, depending on the source for
the other threads. A locally analyzing compiler is not allowed to do
this; it will always generate the same object code for threads
containing the same source code, regardless of what the rest of the
program does.
Alan
next prev parent reply other threads:[~2025-01-22 19:11 UTC|newest]
Thread overview: 59+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-01-06 21:40 [RFC] tools/memory-model: Rule out OOTA Jonas Oberhauser
2025-01-07 10:06 ` Peter Zijlstra
2025-01-07 11:02 ` Jonas Oberhauser
2025-01-07 15:46 ` Jonas Oberhauser
2025-01-07 16:09 ` Alan Stern
2025-01-07 18:47 ` Paul E. McKenney
2025-01-08 17:39 ` Jonas Oberhauser
2025-01-08 18:09 ` Paul E. McKenney
2025-01-08 19:17 ` Jonas Oberhauser
2025-01-09 17:54 ` Paul E. McKenney
2025-01-09 18:35 ` Jonas Oberhauser
2025-01-10 14:54 ` Paul E. McKenney
2025-01-10 16:21 ` Jonas Oberhauser
2025-01-13 22:04 ` Paul E. McKenney
2025-01-16 18:40 ` Paul E. McKenney
2025-01-16 19:13 ` Jonas Oberhauser
2025-01-16 19:31 ` Paul E. McKenney
2025-01-16 20:21 ` Jonas Oberhauser
2025-01-16 19:28 ` Jonas Oberhauser
2025-01-16 19:39 ` Paul E. McKenney
2025-01-17 12:08 ` Jonas Oberhauser
2025-01-16 19:08 ` Jonas Oberhauser
2025-01-16 23:02 ` Alan Stern
2025-01-17 8:34 ` Hernan Ponce de Leon
2025-01-17 11:29 ` Jonas Oberhauser
2025-01-17 20:01 ` Alan Stern
2025-01-21 10:36 ` Jonas Oberhauser
2025-01-21 16:39 ` Alan Stern
2025-01-22 3:46 ` Jonas Oberhauser
2025-01-22 19:11 ` Alan Stern [this message]
2025-01-17 15:52 ` Alan Stern
2025-01-17 16:45 ` Jonas Oberhauser
2025-01-17 19:02 ` Alan Stern
2025-01-09 20:37 ` Peter Zijlstra
2025-01-09 21:13 ` Paul E. McKenney
2025-01-08 17:33 ` Jonas Oberhauser
2025-01-08 18:47 ` Alan Stern
2025-01-08 19:22 ` Jonas Oberhauser
2025-01-09 16:17 ` Alan Stern
2025-01-09 16:44 ` Jonas Oberhauser
2025-01-09 19:27 ` Alan Stern
2025-01-09 20:09 ` Jonas Oberhauser
2025-01-10 3:12 ` Alan Stern
2025-01-10 12:21 ` Jonas Oberhauser
2025-01-10 21:51 ` Alan Stern
2025-01-11 12:46 ` Jonas Oberhauser
2025-01-11 21:19 ` Alan Stern
2025-01-12 15:55 ` Jonas Oberhauser
2025-01-13 19:43 ` Alan Stern
2025-07-23 0:43 ` Paul E. McKenney
2025-07-23 7:26 ` Hernan Ponce de Leon
2025-07-23 16:39 ` Paul E. McKenney
2025-07-24 14:14 ` Paul E. McKenney
2025-07-25 5:23 ` Hernan Ponce de Leon
2025-07-29 20:34 ` Paul E. McKenney
2025-07-23 17:13 ` Alan Stern
2025-07-23 17:27 ` Paul E. McKenney
2025-07-23 19:25 ` Alan Stern
2025-07-23 19:57 ` Paul E. McKenney
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