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Date:   Mon, 11 Apr 2022 11:22:51 +0100
From:   Mark Rutland <mark.rutland@arm.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-arch@vger.kernel.org, gcc@gcc.gnu.org,
        catalin.marinas@arm.com, will@kernel.org, marcan@marcan.st,
        maz@kernel.org, szabolcs.nagy@arm.com, f.fainelli@gmail.com,
        opendmb@gmail.com, Andrew Pinski <pinskia@gmail.com>,
        Ard Biesheuvel <ardb@kernel.org>, x86@kernel.org,
        andrew.cooper3@citrix.com, Jeremy Linton <jeremy.linton@arm.com>
Subject: Re: GCC 12 miscompilation of volatile asm (was: Re: [PATCH]
 arm64/io: Remind compiler that there is a memory side effect)
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On Tue, Apr 05, 2022 at 04:05:22PM +0200, Peter Zijlstra wrote:
> On Tue, Apr 05, 2022 at 01:51:30PM +0100, Mark Rutland wrote:
> > Hi all,
> > 
> > [adding kernel folk who work on asm stuff]
> > 
> > As a heads-up, GCC 12 (not yet released) appears to erroneously optimize away
> > calls to functions with volatile asm. Szabolcs has raised an issue on the GCC
> > bugzilla:  
> > 
> >   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105160
> > 
> > ... which is a P1 release blocker, and is currently being investigated.
> > 
> > Jemery originally reported this as an issue with {readl,writel}_relaxed(), but
> > the underlying problem doesn't have anything to do with those specifically.
> > 
> > I'm dumping a bunch of info here largely for posterity / archival, and to find
> > out who (from the kernel side) is willing and able to test proposed compiler
> > fixes, once those are available.
> > 
> > I'm happy to do so for aarch64; Peter, I assume you'd be happy to look at the
> > x86 side?
> 
> Sure..

FWIW, compiler explorer now have a trunk build with the fix, and my x86-64
example now gets compiled to something which looks correct:

  https://godbolt.org/z/cveff9hq5

Thanks,
Mark.