linux-kernel - Re: [RFC PATCH v3 11/12] powerpc: Remove unreachable() from WARN

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Message-ID: <314d34d9-efe1-fd10-222f-3f9b3bce82da@csgroup.eu>
Date:   Mon, 27 Jun 2022 15:46:29 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Sathvika Vasireddy <sv@...ux.vnet.ibm.com>,
        Sathvika Vasireddy <sv@...ux.ibm.com>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
CC:     "jpoimboe@...hat.com" <jpoimboe@...hat.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "aik@...abs.ru" <aik@...abs.ru>,
        "mpe@...erman.id.au" <mpe@...erman.id.au>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "naveen.n.rao@...ux.vnet.ibm.com" <naveen.n.rao@...ux.vnet.ibm.com>,
        "mbenes@...e.cz" <mbenes@...e.cz>,
        "benh@...nel.crashing.org" <benh@...nel.crashing.org>,
        "paulus@...ba.org" <paulus@...ba.org>
Subject: Re: [RFC PATCH v3 11/12] powerpc: Remove unreachable() from WARN_ON()



Le 27/06/2022 à 17:35, Sathvika Vasireddy a écrit :
> 
> On 25/06/22 12:16, Christophe Leroy wrote:
>>
>> Le 24/06/2022 à 20:32, Sathvika Vasireddy a écrit :
>>> objtool is throwing *unannotated intra-function call*
>>> warnings with a few instructions that are marked
>>> unreachable. Remove unreachable() from WARN_ON()
>>> to fix these warnings, as the codegen remains same
>>> with and without unreachable() in WARN_ON().
>> Did you try the two exemples described in commit 1e688dd2a3d6
>> ("powerpc/bug: Provide better flexibility to WARN_ON/__WARN_FLAGS() with
>> asm goto") ?
>>
>> Without your patch:
>>
>> 00000640 <test>:
>>    640:    81 23 00 84     lwz     r9,132(r3)
>>    644:    71 29 40 00     andi.   r9,r9,16384
>>    648:    40 82 00 0c     bne     654 <test+0x14>
>>    64c:    80 63 00 0c     lwz     r3,12(r3)
>>    650:    4e 80 00 20     blr
>>    654:    0f e0 00 00     twui    r0,0
>>
>> 00000658 <test9w>:
>>    658:    2c 04 00 00     cmpwi   r4,0
>>    65c:    41 82 00 0c     beq     668 <test9w+0x10>
>>    660:    7c 63 23 96     divwu   r3,r3,r4
>>    664:    4e 80 00 20     blr
>>    668:    0f e0 00 00     twui    r0,0
>>    66c:    38 60 00 00     li      r3,0
>>    670:    4e 80 00 20     blr
>>
>>
>> With your patch:
>>
>> 00000640 <test>:
>>    640:    81 23 00 84     lwz     r9,132(r3)
>>    644:    71 29 40 00     andi.   r9,r9,16384
>>    648:    40 82 00 0c     bne     654 <test+0x14>
>>    64c:    80 63 00 0c     lwz     r3,12(r3)
>>    650:    4e 80 00 20     blr
>>    654:    0f e0 00 00     twui    r0,0
>>    658:    4b ff ff f4     b       64c <test+0xc>        <==
>>
>> 0000065c <test9w>:
>>    65c:    2c 04 00 00     cmpwi   r4,0
>>    660:    41 82 00 0c     beq     66c <test9w+0x10>
>>    664:    7c 63 23 96     divwu   r3,r3,r4
>>    668:    4e 80 00 20     blr
>>    66c:    0f e0 00 00     twui    r0,0
>>    670:    38 60 00 00     li      r3,0            <==
>>    674:    4e 80 00 20     blr                <==
>>    678:    38 60 00 00     li      r3,0
>>    67c:    4e 80 00 20     blr
>>
> The builtin variant of unreachable (__builtin_unreachable()) works.
> 
> How about using that instead of unreachable() ?
> 

That seems odd.

Look at linux/compiler.h

It seems like unreachable() exists to help objtool.

Christophe