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

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Date:   Mon, 27 Jun 2022 21:05:02 +0530
From:   Sathvika Vasireddy <sv@...ux.vnet.ibm.com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        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()


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() ?


- Sathvika