From: Qian Cai <cai@lca.pw>
To: Bill Wendling <morbo@google.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>,
James Y Knight <jyknight@google.com>,
David Miller <davem@davemloft.net>,
sathya.perla@broadcom.com, ajit.khaparde@broadcom.com,
sriharsha.basavapatna@broadcom.com, somnath.kotur@broadcom.com,
Arnd Bergmann <arnd@arndb.de>,
David Howells <dhowells@redhat.com>,
"H. Peter Anvin" <hpa@zytor.com>,
netdev@vger.kernel.org, linux-arch <linux-arch@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
Nathan Chancellor <natechancellor@gmail.com>
Subject: Re: [PATCH] be2net: fix adapter->big_page_size miscaculation
Date: Thu, 18 Jul 2019 19:26:47 -0400 [thread overview]
Message-ID: <75B428FC-734C-4B15-B1A7-A3FC5F9F2FE5@lca.pw> (raw)
In-Reply-To: <CAGG=3QUvdwJs1wW1w+5Mord-qFLa=_WkjTsiZuwGfcjkoEJGNQ@mail.gmail.com>
> On Jul 18, 2019, at 5:21 PM, Bill Wendling <morbo@google.com> wrote:
>
> [My previous response was marked as spam...]
>
> Top-of-tree clang says that it's const:
>
> $ gcc a.c -O2 && ./a.out
> a is a const.
>
> $ clang a.c -O2 && ./a.out
> a is a const.
I used clang-7.0.1. So, this is getting worse where both GCC and clang will start to suffer the
same problem.
>
>
> On Thu, Jul 18, 2019 at 2:10 PM Nick Desaulniers
> <ndesaulniers@google.com> wrote:
>>
>> On Thu, Jul 18, 2019 at 2:01 PM Qian Cai <cai@lca.pw> wrote:
>>>
>>>
>>>
>>>> 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.
>>
>> + Bill and James, who probably knows more than they'd like to about
>> __builtin_constant_p and more than other LLVM folks at this point.
>>
>> --
>> Thanks,
>> ~Nick Desaulniers
next prev parent reply other threads:[~2019-07-18 23:26 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
2019-07-18 21:10 ` Nick Desaulniers
2019-07-18 21:21 ` Bill Wendling
2019-07-18 23:26 ` Qian Cai [this message]
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=75B428FC-734C-4B15-B1A7-A3FC5F9F2FE5@lca.pw \
--to=cai@lca.pw \
--cc=ajit.khaparde@broadcom.com \
--cc=arnd@arndb.de \
--cc=davem@davemloft.net \
--cc=dhowells@redhat.com \
--cc=hpa@zytor.com \
--cc=jyknight@google.com \
--cc=linux-arch@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=morbo@google.com \
--cc=natechancellor@gmail.com \
--cc=ndesaulniers@google.com \
--cc=netdev@vger.kernel.org \
--cc=sathya.perla@broadcom.com \
--cc=somnath.kotur@broadcom.com \
--cc=sriharsha.basavapatna@broadcom.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).