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: <CANpmjNNzx-zc_-ZPgwe5DoWLmexw5irT2n+XMUqxTrfwxTxTmg@mail.gmail.com>
Date: Wed, 4 Feb 2026 11:46:02 +0100
From: Marco Elver <elver@...gle.com>
To: Will Deacon <will@...nel.org>
Cc: David Laight <david.laight.linux@...il.com>, Peter Zijlstra <peterz@...radead.org>, 
	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>
Subject: Re: [PATCH v3 3/3] arm64, compiler-context-analysis: Permit alias
 analysis through __READ_ONCE() with CONFIG_LTO=y

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!