linux-kernel - 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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Message-ID: <YkxMov3qpHxFa/n3@hirez.programming.kicks-ass.net>
Date:   Tue, 5 Apr 2022 16:05:22 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, gcc@....gnu.org,
        catalin.marinas@....com, will@...nel.org, marcan@...can.st,
        maz@...nel.org, szabolcs.nagy@....com, f.fainelli@...il.com,
        opendmb@...il.com, Andrew Pinski <pinskia@...il.com>,
        Ard Biesheuvel <ardb@...nel.org>, x86@...nel.org,
        andrew.cooper3@...rix.com, Jeremy Linton <jeremy.linton@....com>
Subject: Re: GCC 12 miscompilation of volatile asm (was: Re: [PATCH]
 arm64/io: Remind compiler that there is a memory side effect)

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..