netdev - Re: [PATCH] be2net: fix adapter->big_page

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Date:   Thu, 18 Jul 2019 19:26:47 -0400
From:   Qian Cai <cai@....pw>
To:     Bill Wendling <morbo@...gle.com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        James Y Knight <jyknight@...gle.com>,
        David Miller <davem@...emloft.net>, sathya.perla@...adcom.com,
        ajit.khaparde@...adcom.com, sriharsha.basavapatna@...adcom.com,
        somnath.kotur@...adcom.com, Arnd Bergmann <arnd@...db.de>,
        David Howells <dhowells@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, netdev@...r.kernel.org,
        linux-arch <linux-arch@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Nathan Chancellor <natechancellor@...il.com>
Subject: Re: [PATCH] be2net: fix adapter->big_page_size miscaculation



> On Jul 18, 2019, at 5:21 PM, Bill Wendling <morbo@...gle.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@...gle.com> wrote:
>> 
>> On Thu, Jul 18, 2019 at 2:01 PM Qian Cai <cai@....pw> wrote:
>>> 
>>> 
>>> 
>>>> On Jul 12, 2019, at 8:50 PM, David Miller <davem@...emloft.net> wrote:
>>>> 
>>>> From: Qian Cai <cai@....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