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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Date:   Tue, 5 Apr 2022 13:20:50 +0000
From:   Andrew Cooper <Andrew.Cooper3@...rix.com>
To:     Mark Rutland <mark.rutland@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "gcc@....gnu.org" <gcc@....gnu.org>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "will@...nel.org" <will@...nel.org>,
        "marcan@...can.st" <marcan@...can.st>,
        "maz@...nel.org" <maz@...nel.org>,
        "szabolcs.nagy@....com" <szabolcs.nagy@....com>,
        "f.fainelli@...il.com" <f.fainelli@...il.com>,
        "opendmb@...il.com" <opendmb@...il.com>,
        "Andrew Pinski" <pinskia@...il.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "x86@...nel.org" <x86@...nel.org>,
        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 05/04/2022 14:04, Mark Rutland wrote:
> On Tue, Apr 05, 2022 at 01:51:30PM +0100, Mark Rutland wrote:
> My x86_64 test case is:
>
> Per compiler explorer (https://godbolt.org/z/cveff9hq5) GCC trunk currently
> compiles this as:
>
> | msr_rmw_set_bits:
> |         mov     rcx, rdi
> |         rdmsr
> |         sal     rdx, 32
> |         mov     eax, eax
> |         or      rax, rsi
> |         or      rax, rdx
> |         mov     rdx, rax
> |         shr     rdx, 32
> |         wrmsr
> |         ret
> | func_with_msr_side_effects:
> |         ret
>

Yeah... that code gen is very broken for func_with_msr_side_effects().

~Andrew