linux-kernel - Re: [PATCH v3 3/3] arm64, compiler-context-analysis: Permit alias analysis through __READ_ONCE() with CONFIG

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Message-ID: <20260204131400.GI2995752@noisy.programming.kicks-ass.net>
Date: Wed, 4 Feb 2026 14:14:00 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Marco Elver <elver@...gle.com>
Cc: Will Deacon <will@...nel.org>,
	David Laight <david.laight.linux@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Boqun Feng <boqun.feng@...il.com>, Waiman Long <longman@...hat.com>,
	Bart Van Assche <bvanassche@....org>, llvm@...ts.linux.dev,
	Catalin Marinas <catalin.marinas@....com>,
	Arnd Bergmann <arnd@...db.de>, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, kernel test robot <lkp@...el.com>,
	Boqun Feng <boqun@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v3 3/3] arm64, compiler-context-analysis: Permit alias
 analysis through __READ_ONCE() with CONFIG_LTO=y

On Wed, Feb 04, 2026 at 11:46:02AM +0100, Marco Elver wrote:
> On Tue, 3 Feb 2026 at 12:47, Will Deacon <will@...nel.org> wrote:
> [...]
> > > > What does GCC do with this? :/
> > >
> > > GCC currently doesn't see it, LTO is clang only.
> >
> > LTO is just one way that a compiler could end up breaking dependency
> > chains, so I really want to maintain the option to enable this path for
> > GCC in case we run into problems caused by other optimisations in future.
> 
> It will work for GCC, but only from GCC 11. Before that __auto_type
> does not drop qualifiers:
> https://godbolt.org/z/sc5bcnzKd (switch to GCC 11 to see it compile)
> 
> So to summarize, all supported Clang versions deal with __auto_type
> correctly for the fallback; GCC from version 11 does (kernel currently
> supports GCC 8 and above). From GCC 14 and Clang 19 we have
> __typeof_unqual__.
> 
> I really don't see another way forward; there's no other good way to
> solve this issue. I would advise against pessimizing new compilers and
> features because maybe one day we might still want to enable this
> version of READ_ONCE() for GCC 8-10.
> 
> Should we one day choose to enable this READ_ONCE() version for GCC,
> we will (a) either have bumped the minimum GCC version to 11+, or (b)
> we can only do so from GCC 11. At this point GCC 11 was released 5
> years ago!

There is, from this thread:

  https://lkml.kernel.org/r/20260111182010.GH3634291@ZenIV

another trick to strip qualifiers:

  #define unqual_non_array(T) __typeof__(((T(*)(void))0)())

which will work from GCC-8.4 onwards. Arguably, it should be possible to
raise the minimum from 8 to 8.4 (IMO).

But yes; in general I think it is fine to have 'old' compilers generate
suboptimal code.