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From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Boqun Feng <boqun.feng@gmail.com>, Olivier Dion <odion@efficios.com>
Cc: rnk@google.com, Alan Stern <stern@rowland.harvard.edu>,
	Andrea Parri <parri.andrea@gmail.com>,
	Will Deacon <will@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Nicholas Piggin <npiggin@gmail.com>,
	David Howells <dhowells@redhat.com>,
	Jade Alglave <j.alglave@ucl.ac.uk>,
	Luc Maranget <luc.maranget@inria.fr>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Nathan Chancellor <nathan@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Tom Rix <trix@redhat.com>,
	linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
	gcc@gcc.gnu.org, llvm@lists.linux.dev
Subject: Re: [RFC] Bridging the gap between the Linux Kernel Memory Consistency Model (LKMM) and C11/C++11 atomics
Date: Wed, 5 Jul 2023 09:16:50 -0400	[thread overview]
Message-ID: <58fa5c39-1481-58f3-9309-aa03bd3344ce@efficios.com> (raw)
In-Reply-To: <ZKUWPKWAnjZvqsgL@Boquns-Mac-mini.local>

On 7/5/23 03:05, Boqun Feng wrote:
> On Mon, Jul 03, 2023 at 03:20:31PM -0400, Olivier Dion wrote:
> [...]
>> NOTE: On x86-64, we found at least one corner case [7] with Clang where
>> a RELEASE exchange is optimized to a RELEASE store, when the returned
>> value of the exchange is unused, breaking the above expectations.
>> Although this type of optimization respect the standard "as-if"
>> statement, we question its pertinence since a user should simply do a
>> RELEASE store instead of an exchange in that case.  With the
>> introduction of these new primitives, these type of optimizations should
>> be revisited.
>>
> 
> FWIW, this is actually a LLVM bug:
> 
> 	https://github.com/llvm/llvm-project/issues/60418

So it was more than a dubious optimization, it's actually broken as well.

I am worried about adding to the compiler's ability to optimize those 
atomics because of the subtle corner-cases/bugs that can creep up.

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com


  reply	other threads:[~2023-07-05 13:16 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-07-03 19:20 [RFC] Bridging the gap between the Linux Kernel Memory Consistency Model (LKMM) and C11/C++11 atomics Olivier Dion
2023-07-03 20:27 ` Alan Stern
2023-07-04 17:19   ` Olivier Dion
2023-07-04 20:25     ` Alan Stern
2023-07-04 21:25       ` Paul E. McKenney
2023-07-06 16:37       ` Olivier Dion
2023-07-04  9:46 ` Peter Zijlstra
2023-07-04 10:23   ` Jonathan Wakely
2023-07-07 15:31     ` Mathieu Desnoyers
2023-07-07 14:04   ` Olivier Dion
2023-07-07 15:45     ` Peter Zijlstra
2023-07-05  7:05 ` Boqun Feng
2023-07-05 13:16   ` Mathieu Desnoyers [this message]
2023-07-07 10:40 ` Jonas Oberhauser
2023-07-07 17:25   ` Olivier Dion
2023-07-10 14:32     ` Jonas Oberhauser
2023-08-16 14:31       ` Mathieu Desnoyers

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