linux-kernel - Re: [PATCH] compiler-gcc.h: Remove ancient workaround for gcc PR 58670

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Message-ID: <fd71c136-4c1c-5c4d-003f-20cb8c7cf17f@csgroup.eu>
Date:   Thu, 25 Aug 2022 11:19:06 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Uros Bizjak <ubizjak@...il.com>,
        "x86@...nel.org" <x86@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
CC:     "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "sv@...ux.ibm.com" <sv@...ux.ibm.com>
Subject: Re: [PATCH] compiler-gcc.h: Remove ancient workaround for gcc PR
 58670



Le 25/08/2022 à 12:30, Naveen N. Rao a écrit :
> Uros Bizjak wrote:
>> The workaround for 'asm goto' miscompilation introduces a compiler
>> barrier quirk that inhibits many useful compiler optimizations. For
>> example, __try_cmpxchg_user compiles to:
>>
>>    11375:    41 8b 4d 00              mov    0x0(%r13),%ecx
>>    11379:    41 8b 02                 mov    (%r10),%eax
>>    1137c:    f0 0f b1 0a              lock cmpxchg %ecx,(%rdx)
>>    11380:    0f 94 c2                 sete   %dl
>>    11383:    84 d2                    test   %dl,%dl
>>    11385:    75 c4                    jne    1134b <...>
>>    11387:    41 89 02                 mov    %eax,(%r10)
>>
>> where the barrier inhibits flags propagation from asm when
>> compiled with gcc-12.
>>
>> When the mentioned quirk is removed, the following code is generated:
>>
>>    11553:    41 8b 4d 00              mov    0x0(%r13),%ecx
>>    11557:    41 8b 02                 mov    (%r10),%eax
>>    1155a:    f0 0f b1 0a              lock cmpxchg %ecx,(%rdx)
>>    1155e:    74 c9                    je     11529 <...>
>>    11560:    41 89 02                 mov    %eax,(%r10)
>>
>> The refered compiler bug:
>>
>> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58670
>>
>> was fixed for gcc-4.8.2.
>>
>> Current minimum required version of GCC is version 5.1 which has
>> the above 'asm goto' miscompilation fixed, so remove the workaround.
>>
>> Signed-off-by: Uros Bizjak <ubizjak@...il.com>
>> Cc: Andrew Morton <akpm@...ux-foundation.org>

It's unlikely that asm goto bug was completely fixed in 4.8 because it's 
been encountered with gcc 4.9.3 and 4.9.4, see commit 1344a232016d 
("powerpc: Use asm_goto_volatile for put_user()")

Christophe