Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Mon, 05 Sep 2022 15:48:31 +0530
From:   "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>
Subject: Re: [PATCH v2 01/16] powerpc: Replace unreachable() with it's builtin
 variant in __WARN_FLAGS()
To:     Christophe Leroy <christophe.leroy@csgroup.eu>,
        "linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>,
        Sathvika Vasireddy <sv@linux.ibm.com>
Cc:     "aik@ozlabs.ru" <aik@ozlabs.ru>,
        "chenzhongjin@huawei.com" <chenzhongjin@huawei.com>,
        "jpoimboe@redhat.com" <jpoimboe@redhat.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "mbenes@suse.cz" <mbenes@suse.cz>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "mpe@ellerman.id.au" <mpe@ellerman.id.au>,
        "npiggin@gmail.com" <npiggin@gmail.com>,
        "peterz@infradead.org" <peterz@infradead.org>,
        "rostedt@goodmis.org" <rostedt@goodmis.org>
References: <20220829055223.24767-1-sv@linux.ibm.com>
        <20220829055223.24767-2-sv@linux.ibm.com>
        <a3fc4685-9e8d-ebf6-62ca-2e9028753ce8@csgroup.eu>
In-Reply-To: <a3fc4685-9e8d-ebf6-62ca-2e9028753ce8@csgroup.eu>
User-Agent: astroid/4d6b06ad (https://github.com/astroidmail/astroid)
Message-Id: <1662371888.dwl4kym6qm.naveen@linux.ibm.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: quoted-printable
MIME-Version: 1.0
Precedence: bulk

Christophe Leroy wrote:
>=20
>=20
> Le 29/08/2022 =C3=A0 07:52, Sathvika Vasireddy a =C3=A9crit=C2=A0:
>> objtool is throwing *unannotated intra-function call* warnings in
>> .c files with a few instructions that are marked unreachable. The
>> problem comes from the annotate_unreachable() macro that is
>> called by unreachable(). This annotation is adding a call to a
>> function with size 0, and objtool does not add such symbols
>> to the rbtree. Due to this reason, find_call_destination() function
>> is not able to find the destination symbol for that call.
>>=20
>> With the annotation (annotate_unreachable()), gcc seems to
>> generate a 'bl' to unreachable symbol with size 0. But with
>> the builtin variant of unreachable (__builtin_unreachable()),
>> gcc does not emit calls to such symbols and the warnings
>> go away. Given that the codegen remains same, and that
>> there are no 'bl' instructions to such symbols emitted, fix
>> these warnings by replacing unreachable() with it's builtin
>> variant in __WARN_FLAGS().
>=20
> How can you say that the codegen remains the same if with the original=20
> you get stale 'bl' instructions and with the alternative you don't ?

I guess the reference to codegen remaining the same is more to do with=20
unreachable vs. __builtin_unreachable() in the absence of=20
CONFIG_OBJTOOL. But yeah, the changelog needs to be reworked to clarify=20
that.

>=20
>>=20
>> Also, add barrier_before_unreachable() before __builtin_unreachable()
>> to work around a gcc bug [1], for the problem reported at [2].
>=20
> Here my comment was not related to the gcc bug [1] but to gcc bug=20
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D82365 , which was worked=20
> around by commit 173a3efd3edb ("bug.h: work around GCC PR82365 in BUG()")
>=20
> By chance it also solve the problem [1] as you mention.

That's a good commit to reference, but please also retain a link to the=20
new PR.


- Naveen