From: Qian Cai <cai@lca.pw>
To: David Miller <davem@davemloft.net>
Cc: sathya.perla@broadcom.com, ajit.khaparde@broadcom.com,
sriharsha.basavapatna@broadcom.com, somnath.kotur@broadcom.com,
arnd@arndb.de, dhowells@redhat.com, hpa@zytor.com,
netdev@vger.kernel.org, linux-arch@vger.kernel.org,
LKML <linux-kernel@vger.kernel.org>,
Nick Desaulniers <ndesaulniers@google.com>,
natechancellor@gmail.com
Subject: Re: [PATCH] be2net: fix adapter->big_page_size miscaculation
Date: Thu, 18 Jul 2019 17:01:27 -0400 [thread overview]
Message-ID: <D7E57421-A6F4-4453-878A-8F173A856296@lca.pw> (raw)
In-Reply-To: <20190712.175038.755685144649934618.davem@davemloft.net>
> On Jul 12, 2019, at 8:50 PM, David Miller <davem@davemloft.net> wrote:
>
> From: Qian Cai <cai@lca.pw>
> Date: Fri, 12 Jul 2019 20:27:09 -0400
>
>> Actually, GCC would consider it a const with -O2 optimized level because it found that it was never modified and it does not understand it is a module parameter. Considering the following code.
>>
>> # cat const.c
>> #include <stdio.h>
>>
>> static int a = 1;
>>
>> int main(void)
>> {
>> if (__builtin_constant_p(a))
>> printf("a is a const.\n");
>>
>> return 0;
>> }
>>
>> # gcc -O2 const.c -o const
>
> That's not a complete test case, and with a proper test case that
> shows the externalization of the address of &a done by the module
> parameter macros, gcc should not make this optimization or we should
> define the module parameter macros in a way that makes this properly
> clear to the compiler.
>
> It makes no sense to hack around this locally in drivers and other
> modules.
If you see the warning in the original patch,
https://lore.kernel.org/netdev/1562959401-19815-1-git-send-email-cai@lca.pw/
GCC definitely optimize rx_frag_size to be a constant while I just confirmed clang
-O2 does not. The problem is that I have no clue about how to let GCC not to
optimize a module parameter.
Though, I have added a few people who might know more of compilers than myself.
next prev parent reply other threads:[~2019-07-18 21:01 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-12 19:23 [PATCH] be2net: fix adapter->big_page_size miscaculation Qian Cai
2019-07-12 22:46 ` David Miller
2019-07-13 0:27 ` Qian Cai
2019-07-13 0:50 ` David Miller
2019-07-18 21:01 ` Qian Cai [this message]
2019-07-18 21:10 ` Nick Desaulniers
2019-07-18 21:21 ` Bill Wendling
2019-07-18 23:26 ` Qian Cai
2019-07-18 23:29 ` David Miller
2019-07-19 21:47 ` Qian Cai
2019-07-22 21:13 ` Qian Cai
2019-07-22 22:58 ` James Y Knight
2019-07-23 3:08 ` Qian Cai
[not found] ` <CAGG=3QWkgm+YhC=TWEWwt585Lbm8ZPG-uFre-kBRv+roPzZFbA@mail.gmail.com>
2019-07-18 21:22 ` Nick Desaulniers
2019-07-18 21:28 ` Bill Wendling
2019-07-19 10:32 ` kbuild test robot
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