From: Peter Zijlstra <peterz@infradead.org>
To: David Laight <david.laight.linux@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
Marco Elver <elver@google.com>, Will Deacon <will@kernel.org>,
Ingo Molnar <mingo@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Boqun Feng <boqun.feng@gmail.com>,
Waiman Long <longman@redhat.com>,
Bart Van Assche <bvanassche@acm.org>,
llvm@lists.linux.dev, Catalin Marinas <catalin.marinas@arm.com>,
Arnd Bergmann <arnd@arndb.de>,
linux-arm-kernel@lists.infradead.org,
linux-kernel@vger.kernel.org, kernel test robot <lkp@intel.com>,
Boqun Feng <boqun@kernel.org>, Al Viro <viro@zeniv.linux.org.uk>
Subject: Re: [PATCH v3 3/3] arm64, compiler-context-analysis: Permit alias analysis through __READ_ONCE() with CONFIG_LTO=y
Date: Tue, 17 Feb 2026 13:16:19 +0100 [thread overview]
Message-ID: <20260217121619.GA1395266@noisy.programming.kicks-ass.net> (raw)
In-Reply-To: <20260216174324.75d47e37@pumpkin>
On Mon, Feb 16, 2026 at 05:43:24PM +0000, David Laight wrote:
> On Mon, 16 Feb 2026 07:32:53 -0800
> Linus Torvalds <torvalds@linux-foundation.org> wrote:
>
> > On Mon, 16 Feb 2026 at 03:09, David Laight <david.laight.linux@gmail.com> wrote:
> > >
> > > volatile structure members are almost free
> >
> > No, gcc does absolutely horrible things with volatiles. It disables a
> > lot of very basic stuff.
> >
> > Try doing something as simple as a "var++" on a volatile, and cry.
>
> On x86 I just see a load, inc, store - not that surprising really.
> (clang did do 'inc memory'.)
>
> It's not as though 'inc memory' is atomic (without a lock prefix).
>
> Also var++ will be 3 u-ops the same as the read, inc, write so the
> underlying execution is much the same (ok you might save on the
> address generation and the compiler doesn't have to find a register name,
> but I don't remember anything modern being limited by instruction retirement).
> You might save a bit of I-cache.
Interrupts can tell the difference, and that matters.
next prev parent reply other threads:[~2026-02-17 12:16 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-01-30 13:28 [PATCH v3 0/3] arm64: Fixes for __READ_ONCE() with CONFIG_LTO=y Marco Elver
2026-01-30 13:28 ` [PATCH v3 1/3] arm64: Fix non-atomic " Marco Elver
2026-01-30 15:06 ` David Laight
2026-01-30 13:28 ` [PATCH v3 2/3] arm64: Optimize " Marco Elver
2026-01-30 15:11 ` David Laight
2026-02-02 15:36 ` Will Deacon
2026-02-02 16:01 ` Peter Zijlstra
2026-02-02 16:05 ` Will Deacon
2026-02-02 17:48 ` Marco Elver
2026-02-02 19:28 ` David Laight
2026-01-30 13:28 ` [PATCH v3 3/3] arm64, compiler-context-analysis: Permit alias analysis through " Marco Elver
2026-01-30 15:13 ` David Laight
2026-02-02 15:39 ` Will Deacon
2026-02-02 19:29 ` David Laight
2026-02-03 11:47 ` Will Deacon
2026-02-04 10:46 ` Marco Elver
2026-02-04 13:14 ` Peter Zijlstra
2026-02-04 14:15 ` Will Deacon
2026-02-06 15:09 ` Marco Elver
2026-02-06 18:26 ` David Laight
2026-02-15 21:55 ` Marco Elver
2026-02-15 22:16 ` David Laight
2026-02-15 22:43 ` Marco Elver
2026-02-15 23:18 ` David Laight
2026-02-15 23:40 ` Linus Torvalds
2026-02-16 11:09 ` David Laight
2026-02-16 15:32 ` Linus Torvalds
2026-02-16 17:43 ` David Laight
2026-02-17 12:16 ` Peter Zijlstra [this message]
2026-02-17 14:25 ` David Laight
2026-02-17 16:23 ` Linus Torvalds
2026-02-17 16:32 ` Linus Torvalds
2026-02-18 19:34 ` Boqun Feng
2026-02-18 20:18 ` Linus Torvalds
2026-02-19 15:21 ` Gary Guo
2026-02-19 18:36 ` Linus Torvalds
2026-02-02 19:13 ` [PATCH v3 0/3] arm64: Fixes for " Will Deacon
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