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: <YlQDfC+J8BXHcvpk@FVFF77S0Q05N>
Date:   Mon, 11 Apr 2022 11:31:24 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Jeremy Linton <jeremy.linton@....com>
Cc:     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>,
        Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        andrew.cooper3@...rix.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.

Jan Hubicka fixed this in GCC commit:

  aabb9a261ef060cf ("Propagate nondeterministic and side_effects flags in modref summary after inlining")

... and all my local tests look good with that applied.

Compiler explorer's trunk build now has that fix, so the examples from before
now look good:

  aarch64: https://godbolt.org/z/vMczqjYvs

  x86_64: https://godbolt.org/z/cveff9hq5

Jeremy, now that the real issue has been identified and fixed, I assume you'll
send a revert for commit:

  8d3ea3d402db94b6 ("net: bcmgenet: Use stronger register read/writes to assure ordering")

... ?

Thanks,
Mark.